Download photorefractive keratectomy

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PHOTOREFRACTIVE KERATECTOMY
SEND CORRESPONDANCE TO:
W. Bruce Jackson, MD, FRCSC
University of Ottawa Eye Institute, The Ottawa Hospital, General Campus
501 Smyth
Ottawa, Ontario, Canada
K1H 7 B2
OFFICE PHONE: 613-737-8759
OFFICE FAX: 613-737-8374
E-MAIL: [email protected]
ASSOCIATE AUTHOR:
Abdulrahman Al-Muammar, MD, FRCSC
University of Ottawa Eye Institute, The Ottawa Hospital, General Campus
501 Smyth
Ottawa, Ontario, Canada
K1H 7 B2
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PHOTOREFRACTIVE KERATECTOMY
OUTLINE
I.
INTRODUCTION
II.
INDICATIONS FOR PRK
III.
PATIENT SELECTION
IV.
PREOPERATIVE MANAGEMENT
V.
SURGICAL TECHNIQUE
A. PREOPERATIVE MEDICATIONS
B. EPITHELIAL REMOVAL
1. MECHANICAL
2. CHEMICAL
3. LASER
i. LASER/SCRAPE
ii. TRANSEPITHELIAL
C. STROMAL TREATMENT
VI.
POSTOPERATIVE MANAGEMENT
A. MEDICATIONS
B. EPITHELIAL HEALING
VII.
RESULTS
A. MYOPIC PRK
B. HYPEROPIC PRK
C. WAVEFRONT-GUIDED PRK
D. PRK FOR MONOVISION
PHOTOREFRACTIVE KERATECTOMY
E. PRK FOR PRESBYOPIA
E. PRK IN CHILDREN
F. PRK AFTER REFRACTIVE SURGERY
1. LASIK
2. RADIAL KERATOTOMY
3. INTRAOCULAR LENS
4. PRK FOLLOWING PENETRATING KERATOPLASTY
VIII.
COMPLICATONS
A. RECURRENT CORNEAL EROSIONS
B. CORNEAL INFILTRATES AND INFECTIONS KERATITIS
C. CENTRAL ISLANDS
D. ECCENTRIC ABLATIONS AND DECENTRATIONS
E. IRREGULAR ASTIGMATISM
F. LATE COMPLICATIONS
1. UNDER-CORRECTION
2. OVER-CORRECTION
3. HAZE, SCARRING, AND REGRESSION
i. TREATMENT OF HAZE AND REGRESSION
4. QUALITY OF VISION
5. DRY EYES
6. OTHER COMPLICATIONS
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PHOTOREFRACTIVE KERATECTOMY
W. Bruce Jackson, MD, FRCSC and Abdulrahman Al-Muammar, MD, FRCSC
INTRODUCTION
Photorefractive keratectomy (PRK) was the first widely accepted surgical procedure to
ablate corneal tissue for the correction of refractive errors. Beginning in the late 1980s, surface
stromal PRK using the 193 nm argon fluoride excimer laser quickly became the procedure of
choice for correcting low to moderate myopia.1 and, in the mid 1990s, hyperopia2, 3 During the
nineties, increasingly successful outcomes with few complications were the result of
improvement in laser design, surgical technique, and postoperative management.4 Despite being
a relatively safe and effective procedure its major limitations were discomfort or pain in the first
few days, prolonged wound healing and visual rehabilitation, stromal haze, and the side effects
from the use of topical steroids.5 To circumvent these problems combined with the prospect of
treating higher degrees of myopia with better predictability led to the development and
introduction of lamellar techniques of which, laser in situ keratomileusis (LASIK) has now
become the dominant refractive procedure worldwide.6 With the introduction of high volume,
discount laser centers in many countries, new refractive surgeons learn only LASIK because of
the minimal discomfort, rapid corneal healing, good uncorrected visual acuity within twenty-four
hours, rapid stabilization of refraction, and minimal stromal haze, gaining little or no experience
with PRK. However, for those who have experience with PRK, it has proven to be an excellent
technique with few serious complications for refractive errors up to -10 of myopia and +4 of
hyperopia.7-11 With the increasing volume of patients treated with LASIK, many of the potential
complications of LASIK began to appear including microkeratome and flap complications (eg
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small or incomplete flaps, diffuse lamellar keratitis, epithelial ingrowth and infectious keratitis,
as well as dry eyes and iatrogenic keratectasia).12, 13
In 1999, Massimo Camellin (M. Cimberle, “LASEK May Offer Advantages of Both
LASIK and PRK,” Ocular Surgery News, March 1999, page 28) introduced a modified surface
ablation technique named LASEK (laser assisted sub-epithelial keratomileusis) now referred to
as laser assisted sub-epithelial keratectomy in which an epithelial flap is created and then
following stromal ablation is re-positioned. This procedure which has the theoretical advantage
of less postoperative discomfort, faster visual rehabilitation, and less stromal haze14-19 combined
with the potential of better results with Wavefront-guided custom laser surface ablation20-22 has
resulted in a resurgence of interest for surface treatment (PRK and LASEK).
Despite PRK being over thirteen years old, it is safe, and effective, with sight-threatening
complications occurring in as few as 1% or less, and maintains an important place in the
refractive surgical armamentarium when LASIK or other procedures are not possible or desired
by the patient. Besides improved surgical techniques, the excimer lasers have undergone
extensive modifications aimed at smoother beams, improved ablation profiles with increased
optical zone size, transition zones to over 8 mm, active eye trackers, and less corneal temperature
rise and central island formation with small spots. The prospect that Wavefront-guided excimer
laser ablations may yield better outcomes along with techniques to improve epithelial healing
and discomfort will see surface ablation (PRK and LASEK) regain popularity as a corneal
refractive procedure.23 At this time, LASEK, has not consistently been demonstrated to be better
than PRK.17, 24 However, most of the discussion of PRK in this chapter will also apply to LASEK
both procedures being referred to as surface ablation.
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INDICATIONS FOR PRK
Despite the popularity of LASIK, PRK has been well accepted by patients and a number of
comparative studies have failed to demonstrate any real superiority in outcome for LASIK over
PRK except in the first few weeks.25-29 Many patients will prefer surface ablation when they
understand the two procedures and their potential complications, especially those related to the
microkeratome and later flap complications.12, 30-32 Those choosing PRK tend to be well
informed, plan to have enough time for recovery of visual acuity, more tolerant of pain, available
for more frequent follow-up examinations, and willing to comply with the regiment of
postoperative medications. After a thorough understanding of both procedures, patients often
have more realistic expectations and understand that the discomfort can be minimized with the
use of a bandage contact lens and non-steroidals, and Mitomycin-C (MMC) has proven effective
in those patients who do get significant haze. Many of the patients choosing PRK have low
refractive errors – myopia of -1 to -4 and hyperopia up to +4, and are concerned about overall
safety, especially those engaged in contact sports, hobbies, or occupations (military, law
enforcement) where the risk of ocular injury is high with the potential risk of flap loss or
dislocation following blunt trauma.33-37 PRK is the preferred technique in the presence of
anterior membrane dystrophy or a history of recurrent corneal erosions because of the increased
risk of epithelial erosions and defects at the time of surgery resulting in an increased risk of
diffuse lamellar keratitis38 postoperatively and epithelial ingrowth.13, 39 These patients do well
with few recurrences of their corneal erosions postoperatively, especially when the treatment can
be extended to 8 or 9 mm using large transition zones or a combination of myopic and hyperopic
treatments to increase the diameter of the phototherapeutic keratectomy (PTK).40, 41
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PRK should be seriously considered in other situations including the inability to obtain
good suction at the time of surgery because of poor exposure (deep orbits or anterior placed
scleral buckles, and thin corneas where the residual bed thickness will be less than 250-300
µm).13, 32 Patients with flat corneas, 40 D or less, are at greater risk for free caps and those with
steep corneas, over 48 D, button-holes and can be treated with less risk with surface ablation.
With more cases of keratectasia following LASIK being reported with only minimal evidence of
forme fruste keratoconus, patients with any degree of asymmetric astigmatism are best managed
with PRK.42-48 Good results have been reported in this group without the risk of late
keratectasia.2, 49 In certain patients, PRK may be the preferred option following previous
refractive surgery including RK, LASIK, and intraocular lens procedures.50 In mild to moderate
dry eyed patients, PRK may be preferable avoiding some of the symptoms of fluctuating vision
and increased symptoms due to the exacerbation of the dry eye following LASIK where corneal
sensation and tear function may be depressed for six months or greater.51-56 Finally, for the
occasional refractive surgeon, PRK may be a safer procedure, avoiding many of the potential
complications associated with the increased complexity of LASIK12, 57
PATIENT SELECTION
Patient selection and evaluation are the most important aspects of elective refractive
surgery. Patients should be at least eighteen years of age with a stable refractive error and
realistic goals for surgery. The young engineer with moderate myopic astigmatism, who requires
20/15 vision OU, without visual symptoms, might better delay the surgery. Patients with
unrealistic expectations, even if they meet all the criteria for the procedure, should still be
advised against elective refractive surgery. Today, the refractive surgery patient has heard and
read more as well as searching the internet, and is aware of many of the possible complications
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compared to patients five years ago. However, it is time well spent to ensure the patient has
realistic expectations as well as a thorough knowledge of the complications and will be more
accepting of a less than ideal result if complications should occur.58, 59 Many pre-presbyopic
myopes and presbyopic hyperopes do not realize they will still need reading glasses after the
surgery unless treated with monovision or a presbyopic ablation profile. Despite improved
results with the new laser designs, and ablation profiles patients should be thoroughly counselled
on the possibility of halos and glare postoperatively as well as night vision and driving
difficulties.60
Similar to any other ophthalmic surgical procedure, detailed history taking (including
past and present medical and ophthalmic history, family history, medications, social and
occupational history) thorough examination, and specialized testing are important for a
successful outcome. Patients with a history of keloid formation of the skin, may still have PRK
since there does not appear to be an increased risk of corneal haze.5 Patients with connective
tissue disease61, herpetic keratitis62-64, corneal scarring, cataracts, uveitis, and immunocompromised disease (HIV infection) are not good candidates for refractive surgery. PRK
during pregnancy and lactation is contra-indicated because it can cause unstable refractive
outcome owing to hormonal changes.65-67 Visually significant lens changes are a contraindication because a refractive error can be corrected with cataract surgery and an intraocular
lens implantation. Relative exclusions include large pupils greater than 7-8 mm, diabetes
mellitus, prior ocular surgery, and open-angle glaucoma, if not well controlled medically.
Surgery is not advised if the patient is using medications such as Amiodarone, Isotretinoin,
Sumatriptan, topical or oral corticosteroids, or other immunosuppressive medication.
PREOPERATIVE MANAGEMENT
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A thorough ocular examination is critical to determine patient eligibility and includes
uncorrected and best corrected visual acuity, measurement of pupil size in bright and dim
illumination,68 manifest and cycloplegic refraction,69 autorefraction, keratometry, pachymetry,
intraocular pressure, eye dominance, and dilated fundus examination. Corneal topography is
essential in all cases to rule out contact lens induced corneal warpage, asymmetric and irregular
astigmatism, and early keratoconus. Wavefront aberrometry is quickly becoming an important
tool to measure the refractive error, higher order (spherical, coma, trefoil) aberrations and
determine preoperative quality of vision and point spread function. Comparison with the
manifest and cycloplegic refraction will be important but, in many cases, treatment will be
completely derived from the aberrometry readings and topography subject to any personal
nomogram changes.70
A thorough slit-lamp examination is critical to rule out any significant corneal
abnormalities such as scarring, corneal vascularization, as well as the presence of a cataract.
Careful attention must be paid to detect the presence of ocular rosacea, anterior or posterior
blepharitis, atopic appearing conjunctiva, and tear film abnormalities. Since PRK can cause a
symptomatic decrease in tear flow and tear film stability71, 72, probably because of decreased
corneal sensation, a Schirmer test and tear break-up time should be documented especially in any
patient with a history of dry eye or contact lens wear.55 Hyperopic patients with dry eyes and
steep corneas may develop persistent epithelial keratitis and corneal scarring due to poor
lubrication.73 Patients with symptomatic blepharitis should be treated with good lid hygiene and
topical antibiotics such as Erythromycin while meibomitis associated with ocular rosacea and dry
eye, will frequently improve on oral Doxycycline 100 mg bid and frequent artificial tears.
Ocular surface disease, not responding to copious lubrication or poor compliance, should be
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treated with punctal plugs preoperatively and refractive surgery delayed until the cornea is free
of any fluorescein or rose bengal staining.
Contact lens wearers using rigid and gas permeable lenses should discontinue wearing
their lenses for two to four weeks, while soft lens wearers may stop their lenses two weeks
before the preoperative examination to ensure stability of refraction and corneal topography.
Before proceeding with refractive surgery, the patient must read and fully understand the
risks and benefits of the procedure, attend information sessions and sign an informed consent.59
Thoroughly discussing the surgery with the surgeon will decrease the likelihood of medicolegal
action.74 It is important to document clearly the exact parameters that will be used for the
surgery including optical and transition zone size, monovision and any nomogram adjustment.
The depth of the ablation should be calculated and compared with the preoperative pachymetry
readings, and the resulting degree of corneal flattening or steepening should be determinded such
that the final K readings are not flatter than 35 or steeper than 50 D to avoid symptoms of
decreased visual performance.
SURGICAL TECHNIQUE
On the day of surgery, the surgeon may wish to verify that the refractive error has not
changed (if a few months have elapsed since the preoperative examination) and the slit-lamp
examination is normal. Surgery may be planned on each eye separated by a few days to one or
two weeks or bilateral simultaneous surgery may be performed. A delay for one week or more
will ensure epithelial healing and allow for any nomogram changes for the second eye depending
on the refractive result. For patients and surgeons favouring simultaneous surgery there are few
additional risks and a shorter rehabilitation time.75, 76
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It is the surgeon’s responsibility to ensure that the laser is carefully maintained with
calibration and centration checked daily. For best results, the temperature in the laser room
should be maintained between 70-75 F° with humidity between 35-50%. The patient’s
information must be entered into the laser correctly and verified.
The first two steps of PRK, removal of the epithelium and stromal treatment, include a
variety of options and techniques which are discussed in the following section.
PREOPERATIVE MEDICATIONS
Most surgeons will use one or two drops of an antibiotic, ofloxacin [Ocuflox 0.3%,
Allergan] or ciprofloxacin [Ciloxan 0.3%, Alcon] and a non-steroidal anti-inflammatory
(NSAID) ketorolac tromethamine, [Acular, Allergan] or diclofenac sodium [Voltaren, Novartis]
10 minutes preoperatively. In very anxious patients, mild sedation with Valium may be helpful.
Before the patient is positioned in the laser chair, the 3 and 9 or 6 and 12 meridians may be
marked at the limbus with a gentian violet pen at the slit-lamp and then used for precise
alignment prior to stromal ablation. The eye is anaesthetized with 3 drops of topical anesthetic
(proparacaine hydrochloride), prior to inserting the lid speculum. Some surgeons will irrigate
the cornea with chilled balanced salt solution (BSS) before epithelial removal to decrease the risk
of thermal damage with high repetition rate wide area lasers.77, 78
EPITHELIAL REMOVAL
The complete and rapid removal of the corneal epithelium leaving a smooth, undamaged
Bowman’s membrane within the 6.5 to 9.5 mm area is critical to maintain consistent hydration of
the bed and avoid uneven stromal ablation, minimize keratocyte apoptosis and promote rapid reepithelialization. Before commencing the stromal ablation it is critical to ensure that all the
epithelium has been removed within the ablation diameter.
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Epithelial removal may involve mechanical, chemical, or laser techniques.
MECHANICAL
The original technique of epithelial removal was scraping using a Paton spatula, scalpel
blade, Desmarres blade or blunt #67 blade. Experience is required to achieve consistent results
without uneven hydration, avoid nicks in Bowman’s membrane79, 80 and an irregular edge
delaying re-epithelialization.
A motorized brush such as that described by Pallikaris or the Amoils Epithelial Scrubber
(Innovative Excimer Solutions, Inc., Toronto, Canada) will uniformly remove the epithelium out
to 9.5 mm which is especially useful for large diameter myopic and hyperopic treatments.77, 79
The cornea and brush are both moistened and gentle pressure is applied to the brush quickly
removing the epithelium leaving a smooth edge. After either scraping or brush epithelial
removal, the stromal bed is wiped with a moistened sponge or spatula to remove any residual
epithelium or debris. Excess tears should be dried from the fornix especially for hyperopic PRK
before starting treatment.
CHEMICAL
An 18% to 20% ethanol solution (absolute alcohol mixed with BSS or distilled water )
applied to the corneal epithelium for 20 seconds in a well or on a corneal light shield and
thoroughly irrigated with BSS will allow easy debridement with a spatula or microsponge
leaving a smooth Bowman’s membrane.81-84 Less postoperative discomfort85, corneal haze83,
and quicker visual rehabilitation81, 82, have been reported with this method.
LASEK is an extension of this technique in which a corneal marker is used to trephine
through the epithelium and 20% alcohol solution prepared in BSS (preferred) or distilled water is
applied for 20 to 40 seconds to loosen the epithelium. An epithelial flap is then raised, hinged at
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the 12 o’clock meridian. Vinciguerra19 has proposed a butterfly LASEK technique aimed to
preserve the connection between the corneal flap and limbus which is felt essential for reestablishing corneal epithelial lesion and stratification. After stromal treatment, the flap is repositioned and a bandage contact lens is applied. Although a number of studies have compared
PRK and LASEK, it is unclear if there is a real difference in postoperative discomfort,
uncorrected visual acuity, risk of infection, or postoperative corneal haze.17, 24
LASER
LASER/SCRAPE
A rapid and consistent method of epithelial removal can be achieved by using the laser in
PTK mode followed by a gentle wipe with a spatula removing the remaining cells and fluid. The
PTK mode can be used to remove a fixed depth of epithelium (43-50 µm) at a maximum of 6.5
mm diameter (VISX Excimer Laser System). A precise epithelial edge is created which is
helpful for centration and enables faster healing. Although broad beam lasers start with a
slapping sound that may make the patient jump, active eye tracking will ensure that the full
treatment is delivered, centered on the pupil, even if the patient moves. This method of epithelial
removal has been reported effective for re-treatments as well.86 A modification of this technique
which has gained in popularity involves starting the ablation with a small PRK correction of -.50
to -1 D followed by a PTK of 32-35 µm. With this technique, one can visualize epithelial
breakthrough and stop the PTK, leaving a smooth bed. The initial small spherical treatment with
PRK prevents the central island of corneal epithelial remaining and pushes the fluid from the
center of the bed and, with some lasers such as VISX, may give an overall smoother epithelial
removal without breakthrough first in the periphery. The initial small diameter results in less
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laser crack, with less tendency of the patient to jump. Any remaining cells are removed with a
spatula and uniform hydration is achieved with the spatula acting as a windshield wiper.
TRANSEPITHELIAL
The laser in PTK mode (transepithelial) or Johnson’s “no-touch technique”87 sphere/PTK
uses the excimer laser to completely remove the corneal epithelium without the need for scraping
or wiping prior to the stromal ablation. Good results have been reported but the surgeon must be
very familiar with the technique and the appropriate nomograms. This technique has been
associated with less anterior keratocyte apoptosis in rabbits than the laser/scrape88 and better retreatment results than with PTK alone have been reported.89
STROMAL TREATMENT
Prior to starting the stromal ablation, the head should be positioned so that the alignment
marks made at the limbus coincide with the reticle to avoid cyclotorsion with the patient lying
down and the patient encouraged to fix on the fixation target. The reticle is centered on the pupil
center and the eye tracker is engaged. In some patients, a dilated pupil is required for the active
eye tracking. Miotics should be avoided as the can shift the pupil superonasally.13 Despite an
eye tracker, it is important that the patient maintain fixation throughout the procedure and, to
help, excessive illumination should be avoided. Occasionally, a ring fixator can be used if there
is excessive movement but this is rarely necessary with reliable eye trackers. Verbal
encouragement is appreciated by the patient and may ensure fixation throughout long treatments.
Improved results can be obtained with nomogram adjustments based on an analysis of the
refractive center’s data.90, 91 Initially, with broad beam lasers, multipass techniques versus single
pass were used to improve results90, 92 and decrease the incidence of central islands. As the
lasers now have smoother beams, flying spots or variable spot size and active eye tracking, this
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technique is used infrequently. Multizone treatments were introduced to bring about an increase
in the ablation zone size while minimizing central ablation depth and creating a more gradual
transition zone which should potentially reduce the incidence of adverse corneal wound healing
complications92, 93
Wavefront customized laser ablations treating, not only sphere and cylinder but also
higher order aberrations using a different ablation profile and variable or small spot scanning
with larger ablation diameters, promises to improve results and eliminate much of the need for
customization.21, 22, 94 However, small nomogram adjustments will be needed depending on the
laser epithelial removal technique84 and environmental conditions as well as age and targeted
refractive outcome. For broad beam lasers, treating high myopia, the treatment may be broken
into two or three sessions with a laser pause for 5-15 seconds to reduce the chance of corneal
heating.95 Some surgeons have used cold BSS on the cornea partway through the treatment and
sponge it off quickly to lower corneal temperature for potentially less corneal haze.96
At the completion of the stromal ablation, some surgeons will use a few pulses of PTK
along with artificial tears, BSS or hyaluronic acid which acts as a viscous masking agent to
reduce ablation irregularities and promote faster healing with less risk of haze and regression.97
This technique has also been used for the treatment of irregular astigmatism and using 0.25%
sodium hyaluronate a more regular corneal surface has been achieved.98
A cross-cylinder ablation has proven successful for myopic or hyperopic eyes with high
or mixed astigmatism. In this technique, the refraction is converted into two cross cylinders,
resulting in the steepening of the flat axis and flattening of the steep axis. Vinciguerra99 has
reported a modification of this technique in which the astigmatism is corrected first – half the
amount of cylinder in dioptres is ablated along the steepest meridian and the other half is next
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ablated along the flattest meridian followed by the spherical equivalent. This tissue-sparing
technique offers a smooth transition between the untreated and treated cornea. The use of
Wavefront customized treatments has the potential to improve the visual results in these difficult
cases without the limitation of defined spheres, slits, and ellipses.
Following the reports that topical MMC 0.02% prevented the recurrent of sub-epithelial
fibrosis after corneal refractive surgery, a number of authors have now used prophylactic MMC
after PRK for high myopia (8 D or higher).100 Initial reports indicate no toxicity and clear
corneas with follow-up up to one year (Majmudar, Ophthalmology 2002). Adjustments must be
made in the nomogram if MMC is used prophylactically to avoid over-corrections. Following
the application of a MMC-soaked disc for 90-120 seconds, the surface of the eye and the fornices
should be irrigated thoroughly.
Cool irrigation with BSS at the end of the procedure quickly lowers the stromal
temperature, rehydrates the cornea, and removes any debris from the surface. The fornices
should be wiped of any debris before the contact lens is inserted.
POSTOPERATIVE MANAGEMENT
MEDICATIONS
A broad spectrum antibiotic-fluoroquinolone, 0.3% Ofloxacin (Ocuflox, Allergan) or
0.3% Ciprofloxacin (Ciloxan, Alcon), should be instilled four times a day until the epithelium is
healed and the contact lens is removed.101 Ofloxacin plus Tears and Ofloxacin alone, had a more
positive effect on epithelial healing than Ciprofloxacin which may be prone to impair or delay
wound healing and development of corneal haze postoperatively.102 Ciprofloxacin crystals have
been seen within the epithelial defect under the BCL but resolved without sequelae.
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A topical NSAID (ketorolac tromethamine, diclofenac, fluriprofen) is used four times a
day for the first twenty-four to forty-eight hours to control pain.103, 104 Their use does not affect
corneal re-epithelialization nor inhibit corneal haze formation.105-107 Sterile infiltrates have been
reported when steroids were not used concomitantly.106, 108 Corneal melting with perforation has
been reported with the use of diclofenac sodium 0.1% (Alcon, Fort Worth, Texas) ophthalmic
solution, possibly due to the Vitamin E solubilizer that may have inhibited epithelial proliferation
and produced an aberrant up regulation of collagenase and gelatanase.109
Although the use of topical steroids was initially controversial, especially in low myopia,
most refractive surgeons begin treatment immediately following the procedure with
fluoromethalone 0.1% (FML, Allergan), four times a day for a few weeks to six months, tapering
slowly.106 Topical steroids are more effective than NSAIDs in decreasing inflammation within
the first forty-eight to seventy-two hours and is important in the prevention of long-term
complications such as haze and myopic regression especially in high myopic patients.110 There
does not appear to be any adverse effect on re-epithelialization.106 It is important that the patient
be followed closely as there is a very small risk of steroid induced cataract111 and increased
intraocular pressure112, 113 which is lower postoperatively,114-118 following both myopic and
hyperopic treatment.
For those few patients experiencing considerable discomfort in the first few postoperative
days, despite the use of NSAIDS and a bandage contact lens, 0.5% to 1% Tetracaine or 0.05%
topical proparacaine119, 120 can be used sparingly. Its use does not prolong re-epithelialization
and providing only a limited amount of Tetracaine to patients prevents abuse and toxicity to the
cornea.119, 121 However, prolonged use of a topical anaesthetic following PRK can result in
delayed epithelial healing and corneal stromal ring infiltrates.122 Orally, the patient may be given
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Tylenol #3 (Janssen-Ortho) or another oral analgesic for the first forty-eight to seventy-two
hours as needed. The application of ice packs or cold compresses helps relieve discomfort. For
the first six months postoperatively, non-preserved or minimally preserved artificial tears should
be used frequently to re-establish a smooth ocular surface. A decrease in tear break-up time may
occur following surgery with dry eye symptoms. This is probably caused by decreased corneal
sensation after PRK.72 In patients with moderate dry eye, punctal plugs may be inserted
postoperatively if not done prior to surgery to ensure a good tear film and avoid fluctuations in
vision and punctuate epithelial keratitis.
EPITHELIAL HEALING
Most surgeons and patients prefer the use of a bandage contact lens to promote reepithelialization, decreased pain and increased mobility.5 Too tight a contact lens may result in
conjunctival injection, peripheral corneal infiltrate, and increased discomfort while too loose a
lens may delay epithelial healing.123 Disadvantages include delayed epithelialization if the
contact lens falls out while attempts to re-insert the lens may cause a larger defect, sterile
infiltrates, and an increased risk of infectious keratitis.101, 124 With the use of a bandage contact
lens, myopic PRK patients with a 6.5 mm defect will heal in two to three days, while 8.5 to 9.5
mm defects, will heal in four to five days.3 We have observed that the epithelium is thinner,
fragile, and less firmly adherent to the underlying stroma under a BCL compared to an occlusive
patch. The lens must be left in place until the epithelium is healed since early removal can result
in a large epithelial defect, delayed epithelial healing, regression and haze.125, 126
In a small percentage of patients, under 5%, an occlusive patch with an antibiotic steroid
ointment is used to promote healing especially in patients who are intolerant of a contact lens.127
The patient is seen daily and the patch re-applied. In our experience, re-epithelialization after
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myopic and hyperopic PRK is faster with a thicker appearing epithelium with patching.3 The
patch may be discontinued when the patient is comfortable even in the presence of a small
epithelial defect as these close quickly in contrast to those seen with a BCL. The epithelial
surface may be rougher with increased tear film instability for the first few days to a week
compared to epithelialization under a BCL. Delays in healing can be the results of a lost contact
lens, anterior basement membrane dystrophy, dry eye, or toxicity to any of the topical
medications. Rarely the epithelium may heal with a pseudodendritic pattern in the first week and
should not be confused with a herpes simplex dendrite.
Occasionally, if epithelial healing is incomplete under a bandage contact lens, removal of
the contact and pressure patching from one to two days promotes epithelialization. It is
important to ensure the patient is not self-medicating with a topical anesthetic. Delay in
epithelial healing can result in haze and regression. The use of high dose Vitamin A and E
(25,000 retinol palmitate and 230 mg alpha-tocopheryl nicotinate, 1 capsule three times a day for
thirty days and twice a day for the following two months)128 reduces the incidence of haze
formation, and myopic regression in patients treated from -2 to -10 while promoting epithelial
healing.
RESULTS
MYOPIC PRK
Beginning in the early 90s, PRK was performed for the correction of a wide range of
myopia and myopic astigmatism. Although the results of low myopia were encouraging, poor
predictability, increased corneal healing response manifested by haze or scarring, regression, and
lines lost of BSCVA were seen with moderate to high dioptic correction. LASIK was then
introduced to manage the higher dioptic treatments while PRK was restricted, in most laser
PHOTOREFRACTIVE KERATECTOMY
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centers, to the treatment of low to moderate myopia. LASIK in the late 90’s began to loose
favour for the correction of high myopia because of the potential of keratectasia42, 129, 130,
irregular astigmatism31, 43 with a resulting decrease in contrast acuity especially under mesopic
conditions and poor visual performance. The introduction and success of phakic – refractive
intraocular lenses and refractive lensectomy for high myopia has many centers restricting
refractive corneal surgery to between -8 and -10 D and, above that, utilizing intraocular
lenses.131, 132 Within the last five years, many published reports in the literature compare PRK
and LASIK for low to moderate myopia with very similar outcomes.25, 26, 57 Even the most
recently published papers on the results of PRK are based on studies that were performed one to
two years ago and, since then, modifications to the lasers and to the ablation profiles, continue to
improve results. Now almost 100% of patients will gain 20/40 or better uncorrected visual
acuity, close to 85% 20/20, and almost two thirds 20/15 or better (Table 1). In low myopia, haze
peaks at three months, declines by six, and only a trace is present at twelve months in 10% of
patients.
The results of current data indicate as many patients will gain a line of best spectacle
corrected visual acuity, as loose a line and a two-line loss or greater is under 1%. The use of
prophylactic MMC for corrections above -8 D has been beneficial in reducing the incidence of
haze and lines lost of best spectacle corrected visual acuity, although there is controversy as to
whether this is needed in all cases.100
With the latest generation of lasers, lines lost of BSCVA is due in most cases to irregular
astigmatism induced by the ablation profile or irregularities in the wound healing response. PRK
for myopia has been shown to give good stability after three to six months and astigmatism is
well corrected by most lasers (80%)133. Predictability is less for low astigmatic corrections and
PHOTOREFRACTIVE KERATECTOMY
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small surgically induced astigmatism, with a trend toward with the rule axis shift, was noted after
PRK.133, 134 Nonetheless, visual performance can be below preoperative levels for high
corrections and under low light conditions.135 Even in the most recent studies, high contrast
acuity was preserved following PRK but low contrast acuity is reduced along with, an up to four
times increase in spherical aberration.136, 137 Despite the handicaps of daytime glare, 55.1%,
decrease in night vision, 31.7%, and increased difficulty driving at night because of the vision,
31.1%, reported by Brunette138 in 690 patients, 91.8% were very satisfied, and 95.7% would still
choose to have surgery again. However, patients who need optimal visual acuity under low
luminance or night illumination should be advised of the limitation of our current technology.
Studies currently in progress are using lasers with eye tracking, smoother beams, and
larger ablation diameters producing smoother transition zones which should reduce the
aberrations associated with large pupillary diameters.
Table 2 compares PRK and LASIK in a number of published reports.
HYPEROPIC PRK
The treatment of hyperopia has lagged considerably behind myopia with the earliest
treatments being performed by Dausch2 in 1991 on the Aesculap-Meditec MEL60 utilizing a
scanning slit and rotating mask. The ablation profile for the correction of hyperopia had been
suggested by L’Esperance139 and aims to create a smoothly transitioned annular ablation in the
stroma at the periphery of a defined optical zone that effectively steepens the central visual axis
while flattening the periphery. Early results using a 4 mm diameter optical zone which tapered
to 7 mm for corrections under 5 D were more stable and predictable than those above 5 D but the
major problems were regression, decentration, slow recovery, and loss of BSCVA.2 It soon
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became clear that the best results were obtained with low hyperopia up to 4 D with optical zones
5-7 mm with total ablation area extending to 8.5 to 9.5 mm.140-142
Different techniques were used by each laser manufacturer to develop this ablation
profile. The Summit Apex Plus Laser (Alcon, Summit Autonomous, Fort Worth, Texas, USA)
used an axicon lens system to treat the area between 6.5 and 9.5 mm.9, 143 An erodible disc is
also used which absorbs laser energy to produce a smooth transition zone. Other laser systems
utilize smaller spots or scanning slit to produce a circular pattern with an optical zone between 5
and 6 mm with an ablation zone out to 9 mm.10, 11
Results for low and moderate hyperopia – low, up to 4 D and moderate, up to 7.75 D are
given in table 2. Since the epithelium is removed beyond 9 mm, re-epithelialization takes an
average of four days, range 3-103, 10, and stability of refraction and vision takes six to twelve
months. Patients are frequently overcorrect by 1 D initially with drift to plano at 3-6 months.3, 10,
11
Uncorrected visual acuity at one year varies from 46% to 70%, 20/20 or better, depending
upon the refractive mean spherical equivalent. A one line loss of best spectacle corrected visual
acuity is seen in upwards of 30% in part due to the loss in image magnification but is rarely
appreciated by the patient. With higher corrections, the results are not as encouraging, with more
regression, loss of contrast acuity and BSCVA, and daytime visual problems.11, 144
Overall, hyperopic patients are extremely pleased with the results of their surgery with
many achieving a multi-focal cornea and are able to read much better than expected.10
Complications unique to hyperopic PRK include an iron-ring145, central nodule formation with
scarring73, 146 and relatively more loss of BSCVA than seen with myopic corrections, and acute
corneal necrosis.147 Because of the older age of the hyperopic population 50 years vs. 39 for
myopia and knowing that steepening the curve of the central cornea can lead to decreased tear
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film break-up time148 it is not unexpected that these patients may experience symptoms and signs
of dry eye. Decentration may occur more frequently because of the small optical zone and
longer treatments but the use of an eye tracker will help decrease the incidence of this
complication as well as the induced regular and irregular astigmatism which may limit BSCVA.
Lubrication is essential to prevent the corneal erosions and scarring.
Results for hyperopic LASIK149-152 are very similar to those for hyperopic PRK but
refractive stability occurs sooner with less regression, and there is less discomfort and followup153. It is for these reasons that hyperopic LASIK is the current preferred procedure. A review
of the literature by Nagy11 led him to conclude that the refractive results, efficacy and
predictability depend more on the type of excimer laser beam delivery system than on the
refractive procedure used (PRK or LASIK).
The treatment of spherical hyperopia does better in both low and high hyperopia groups
compared to eyes with a high hyperopic astigmatic component.11 Alessio154 reported good
results with flying-spot technology in eyes with irregular hyperopic astigmatism in the case of
topography driven PRK treatments. We have noted peripheral segmental haze to be more
prominent with astigmatic treatments than with spherical where haze is annular sparing the
corneal center and is minimal. Surgically induced astigmatism was no more frequent with
hyperopic than with myopic treatments.155
As with any refractive procedure, the patient must be selected carefully and, if in the
presbyopic range, realize they will still need glasses for reading. Hyperopia treated with PRK
should be no more than 4 to 6 D and the patient must be counselled that visual recovery may take
up to a year. Re-treatments can be performed but may result in central corneal erosions and scar
formation Figure associated with a steep cornea and poor ocular tear film.73
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WAVEFRONT-GUIDED PRK
The limited data available for Wavefront guided treatment suggests that PRK may give
better outcomes than LASIK by avoiding some of the challenges associated with the LASIK flap
including flap induced aberrations156 and microkeratome accuracy and profile, flap
biomechanics, positioning of the flap after ablation and fitting the flap on the bed following
treatment. However, PRK has its challenges related to the wound healing response that follows
the epithelialization and the stromal ablation. Standard treatment already achieves 20/15 or
better in 60% or more of cases10 and it is yet to be determined whether we can consistently
increase this percentage and for how long these patients will maintain better than 20/20
uncorrected visual acuity (157). However, re-treatments based on aberration sensing and
Wavefront guided treatment does hold significant promise for not only correcting defocus but
higher order aberrations thereby reducing subjective complaints of glare, halos, doubling, and
night vision disturbances.
Studies using wavefront-guided PRK for treatment of myopia, myopic astigmatism, and
hyperopia have shown early encouraging results as reviewed in Table 4. Nagy21, 22 found that the
result with wavefront-guided PRK were not only safe, effective and predictable but were slightly
superior to traditional PRK for myopia, myopia astigmatism, and hyperopia in terms of UCVA
and BSCVA, although additional studies with longer follow up and larger samples are needed.
Wavefront-guided PRK for hyperopia has been reported to result in less halo and night time
glare when compared to patients treated with traditional PRK. Also Nagy noted that with
wavefront guided PRK, patients had not lost contrast sensitivity after 6 months follow up as seen
with traditional PRK.3, 10
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Wavefront guided treatments still increased the root mean square value for higher order
aberrations postoperatively despite the improvement in UCVA, which raises the possibility that a
decrease in higher order aberrations is not a prerequisite for improvement in subjective visual
acuity following wavefront guided PRK treatments 21, 22. Panagopoulou 94 has demonstrated that
the higher order aberrations increase less at 3 months with PRK (1.3 vs. 1.8 times) compared to
LASIK using Wavefront Aberration Supported Cornea Ablation (WASCA).
PRK AND MONOVISION
It is important to discuss the option of monovision, correcting one eye for distance and
the other eye for intermediate (50 cm for computer use) or near vision (33 cm) in presbyopic or
pseudophakic patients. Patients already using monovision with contact lenses will want this
option. Occasionally myopes, but more commonly hyperopes, corrected in both eyes for
distance develop a multifocal cornea which allows for good distance vision but also reading of
moderately small print. Although a number of studies have been reported, with the intentional
creation of a multifocal cornea for the correction of presbyopia, loss of distance visual acuity,
insufficient near vision, and decreased image quality have resulted in new ablation profiles using
Wavefront guided ablations to be under investigation.
Among myopic patients who underwent refractive surgery for monovision there was over
85% satisfaction with the results. The amount of anisometropia does not correlate with the level
of satisfaction but should not exceed 3 D. It is recommended to trial patients with contact lenses
prior to refractive surgery but many argue this does not reliably mimic the postoperative result.
Some centres will routinely under-correct one eye in all presbyopic patients.
The trend today is to correct the dominant eye for distant, then either leave the other eye
untreated if the refractive error is between –1 to –3, or under-correct if myopic or over-correct if
PHOTOREFRACTIVE KERATECTOMY
-26-
hyperopic in order to achieve final spherical equivalten (SE) between –1 to –3, depend on patient
choice and type of near work. Inadvertent correction of the non dominant eye does not seem to
affect the degree of patient satisfaction. The potential disadvantage of monovision is the
decrease in stereo acuity (range between 100 to 150 seconds of arc), which correlate with the
degree of the anisometropia induced.
PRK FOR PRESBYOPIA
Although monovision is the easiest and time-tested correction for presbyopia158 excimer
laser multi-focal corneal ablations that can be customized for the patients refractive error and in
the future higher order aberrations offers potentially a solution to achieve good distance and near
vision. Early attempts to create an inferior sectorial or central near zone often led to
compromised distant vision, loss of BSCVA and monocular diplopia.140 Following the
widespread introduction of hyperopic PRK, a number of patients were found they could achieve
20/20 distance vision and J1 for reading because of the resultant corneal multi-focality.
Vinciguerra159 reported the use of zonal PRK for the presbyopic correction of three eyes with
some success but the ablation zone was only 15% of the total area of a 3.0 mm pupil. Inferior
off-center ablation with LASIK for hyperopia gave improved near vision when compared to
centered ablations.160 Other ablation patterns are being tested including high dioptic power in
the peripheral cornea. At the University of Ottawa Eye Institute, The Ottawa Hospital we have
used a VISX patented multi-focal ablation profile to steepen the central area to provide a myopic
region for near vision while the peripheral optical zone is targeted for emmetropia. Optical
modeling of the multi-focal optical system shows good image quality over the range from
distance to near. Hyperopic patients with a mean of +2.26 D were treated, most with LASIK but
some with PRK using this ablation profile in both eyes. Results at 6 months revealed 83%,
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20/25 or better for distance and 100% J1 for near without loss of BSCVA. Current studies in
progress are testing pupil size dependent ablation patterns and Wavefront-guided treatments.
PRK IN CHILDREN
Refractive surgery in the pediatric age group should be approached with caution and
remains controversial. PRK has been performed in children as young as 1 year of age but the
majority of cases are 5-12 years. The indications are a diverse group of conditions consisting of
myopic anisometropic amblyopia,161-164 bilateral high myopia, myopia after cataract surgery or
penetrating keratoplasty, and unequal hypermetropia with amblyopia,165 all having failed to
respond to standard measures such as spectacles and contact lenses. This is especially true in
patients who have associated corneal, retinal and optic nerve pathology in conjunction with
myopia. All the studies that have been reported have shown that PRK is a safe procedure and
can be a good alternative equalizing the refractive errors, decreasing the anisometropia and
reversing the amblyopia. To date we do not have long term follow up especially for cases of
myopia which has a tendency toward progression. Overall, there was positive improvement in
UCVA, BSCVA with no or minimal loss of BSCVA and good achievement of the targeted SE
(75% or better reduction). In a few cases of high myopia there was a tendency toward over
correction.163 Alio noticed that there was more haze in children than what he has seen with adult
PRK162 but others do not share this opinion.163 In the reported cases the incidence of haze varies
from 16 to 40%161-163 but only a few eyes have had visually significant haze at last follow-up. A
few cases have required repeat PRK for haze reduction.163but currently studies are in progress
using MMC to prevent PRK haze.
Pediatric PRK has the advantages over LASIK of no risk of ectasia or flap complications
but does have the disadvantages of risk of haze, long-term steroid use, epithelial defect with
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discomfort, more postoperative visits and more critical need for postoperative medication
compliance. Despite these potential risks many feel that PRK is safer especially since children
rub their eyes more than adults which can lead to striae and epithelial ingrowth.
Young children required general anaesthesia to have PRK to ensure stability while doing
the laser treatment, which includes inhalation induction and maintenance treatment (laryngeal
mask airway preferred or nasopharyngeal airway) throughout the case. There must not be any
anesthetic gases leak into the area of the excimer laser beam as the laser will shut down because
the wavelength of the excimer laser is within the absorption spectrum of nitrous oxide and
halothane. Older children can be sedated with IV ketamine or propofol 1% while still older
children (11 and older) may have the procedure performed as in adult cases.
Head positioning and eye stabilization using a fixation ring will ensure proper alignment
during the procedure. In one study despite using a suction ring there was decentration of 1 mm
in one patient out of 6, while in the other patients, decentration was less than 0.5mm.162
Preoperative pilocarpine is necessary in the pediatric population due to the excessive pupil
dilation even under low vacuum, making surgeon fixation difficult. Patient fixation is ideal but
not possible under general anesthesia. Although in most studies the center of the pupil was
chosen as the most appropriate center for the beam preoperative assessment of the angle kappa
and locating the laser beam closer to this point while staying within the pupil may help. With
active eye tracking maintaining centration may be less of a problem. The ablation and
postoperative care is similar to that in adults but regression and haze remain the two major areas
of concern. The excimer laser may be used as an adjunct to conventional amblyopic treatment
and patients must receive amblyopia treatment postoperatively. 166
PRK AFTER REFRACTIVE SURGERY
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LASIK
PRK has been performed after LASIK, for the treatment of under-correction, regression,
irregular astigmatism, or surface irregularities from a flap complication. For 0.5 to 0.75 D of
over or under-corrections epithelial PRK may provide significant visual improvement without
complications.167 Although Carones has reported severe haze following regular PRK after
LASIK and has strongly advised against using this technique for treating regression/under
correction others have not had this experience and have found it useful when repeat LASIK is
contraindicated because of a thin stromal bed or small flap.31, 168 MMC may be used following
the photo ablation which will decrease the likelihood of haze.
Transepithelial PRK has been performed for defective LASIK flaps intraoperatively with
good results but many feel a more conservative approach should be taken by replacing the flap
and waiting for 3-6 months before recutting the flap.13, 32, 169 Wilson has suggested using
transepithelial PRK, not later than 2 weeks, for thin or donut shaped flaps rather than waiting
although a central scar from surfacing of the blade can also be treated with PRK.31, 170
We have treated a 47 year old man who had hyperopic astigmatism of +1.00+2.50x25
with UCVA of 20/30 with monocular triplopia and distortion following myopic LASIK for
–7.75+2.75x85. Because of the thin cornea, pachymetry of 390 µm, the small flap of 6.5 mm
and striae, PRK was recommended. The epithelium was removed with 20% alcohol over the
flap without difficulty and the hyperopic PRK was performed followed by smoothing PTK and
MMC for 2 minutes. Postoperative he has done well with UCVA of 20/25 without distorsion at
6 months.
RADIAL KERATOTOMY (RK)
PHOTOREFRACTIVE KERATECTOMY
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PRK has been performed suceesfully over RK to correct residual myopia.171, 172 There is
concern of postoperative haze, and an increased risk of night vision difficulties and monocular
diplopia following these retreatments as well as loss of BSCVA due to irregular astigmatism and
haze.173}174-177 Patients with lower residual and initial amounts of myopia did better following
PRK treatment.178 Venter reported good results correcting a small group of patients with
residual hyperopia using PRK after RK with results except for 1 patient with haze.179 We have
also had good results without significant haze formation.
INTRAOCULAR LENS SURGERY
Following successful cataract surgery PRK has been performed to correct the residual
refractive error as well as following clear lens extraction.180-183 The results are encouraging but
the predictability maybe less than PRK performed in eyes that have not undergone intraocular
surgery and more likely to result in loss of BSCVA.180.
PRK FOLLOWING PENETRATING KERATOPLASTY (PKP)
PRK has been used to minimize residual refractive error after PKP in patients who are
unable to wear glasses due to anisometropia or are contact lenses intolerant.184 Many of these
patients are myopic with high postoperative astigmatism. The results have shown that PRK after
PKP is less predictable and less effective than PRK in naturally occurring myopia and
astigmatism.185-187 This might be due to the possibility that wound healing in a grafted cornea
may differ from the normal wound-healing response of a patient’s own cornea186 or the
difficulties in maintaining the correction in eyes that were grafted for keratoconus.
Our experience and that of others have demonstrated that PRK is effective in reducing the
spherical equivalent with reduction in astigmatism and improvement in UCVA. We have seen a
regression in the astigmatic component more than the spherical over 1 to 2 years. Corneal haze,
PHOTOREFRACTIVE KERATECTOMY
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regression and irregular astigmatism186, 188-192 limits the effectiveness of PRK. Allograft
rejection may follow PRK193, 194 and there is a higher incidence of 2 line loss of visual acuity
than one would expect in normal PRK eyes.180 The use of MMC has been shown to be safe and
effective in the treatment of corneal haze has been used after PRK for PKP195 The prophylactic
use of MMC may improve the efficacy of PRK following PKP. 196, 197
Customized ablation PRK to treat irregular astigmatism after PKP using combination of
topographic data with computer controlled flying spot excimer laser ablation has been found to
be an effective solution to treat irregular astigmatism and was associated with improvement in
UCVA, and BCVA with no haze after 8 months follow up as well as decreasing corneal
aberrations. 198, 199 Ablating this localized abnormality requires a smaller amount of tissue to be
removed than that required to correct a similar amount of regular astigmatism. Dausch has also
reported good results with improved vision in eyes with corneal irregularities after PRK using
videokeratography-controlled (Orbscan II,Orbtek) and the MEL-70 laser system. (Aesculap
Meditec).200
COMPLICATIONS
RECURRENT EROSIONS
Patients may complain of a foreign body sensation or gritty feeling upon awakening
during the first year after PRK. Rarely, the patient will present with spontaneous corneal erosion
or following minimal trauma. The defect usually heals quickly with patching or a BCL and the
use of hypertonic ointment – Muro 128 or a gel lubricant at night along with lubricants during
the day will control symptoms. Occasionally, a short course of a weak topical corticosteroid will
be of benefit to patients suffering recurring attacks due to incomplete hemidesmosome
development.
PHOTOREFRACTIVE KERATECTOMY
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CORNEAL INFILTRATES AND INFECTIOUS KERATITIS
Sterile or immune corneal infiltrates occur uncommonly following PRK.201 They may be
single or multiple usually in the peripheral cornea starting on postoperative day 2 associated with
increase discomfort and redness and a bandage contact lens. A higher frequency has been noted
in patients with ocular rosacea and blepharitis. Infiltrates have been described when a bandage
contact lens and NSAIDs were used in combination without the addition of topical steroids108 but
concurrent treatment with topical steroids and limiting the NSAID use to 24-48 hours, which is
now routinely used, will avoid this problem.106
The incidence of postoperative infectious keratitis is extremely rare (0.1% to 0.2%.
The major risk factors are an epithelial defect and the use of a bandage contact lens. Patients
should be followed closely on a daily basis until the epithelium is healed. The appearance of an
infiltrate should be treated as a possible corneal infection and the BCL should be discontinued.
If the infiltrate is small, peripheral, with little or no epithelial defect in the absence of an anterior
chamber reaction or discharge, topical antibiotics and steroids can be increased and the patient
followed up in twenty-four hours.201 Usually contact lens induced or toxic infiltrates resolve
quickly without permanent sequelae. If an infectious etiology is suspected, the BCL and cornea
should be cultured for bacteria and fungi as well as a scraping for gram stain. An infectious
corneal ulcer is potentially the most serious complication of PRK and usually appears in the first
few days and involves Gram +ve organisms202, 203 – coagulase negative Staphylococcus,
Staphylococcus aureus204, Streptococcus pneumoniae but organisms such as Pseudomonas205 or
atypical Mycobacterium206, fungi207, 208, herpes simplex209, and Acanthamoeba210 have been
reported. Frequent administration of a topical fluoroquinolone either alone or combined with
fortified Cefazolin or Vancomycin and Tobramycin should be started. Failure to respond to
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treatment should be managed by reassessing compliance to current treatment, reculturing with
special media for unusual organisms, and modifying therapy based on culture results. Early
recognition and aggressive treatment usually results in a favourable outcome but a corneal
transplant may be necessary because of scarring.203 At least one PTK may be tried to reduce the
corneal scar.211
CENTRAL ISLANDS
A central island is a localized elevated area within the corneal treatment ablation zone
greater than 1.5 mm in diameter and over 3 D in height on corneal topographical mapping.212
This area of under-ablation may result in subjective complaints including ghosting, blurred
vision, halo formation around lights, and decreases in uncorrected and best corrected visual
acuity.
In the mid-nineties, central islands were seen frequently in the postoperative period,
especially as the diameter of broad beam lasers increased to 6 mm and beyond. 213 Many theories
have been proposed to explain the etiology, including the vortex (plume) of ablated debris
interfering with laser pulses, degradation of the optics, acoustic shock wave leading to stromal
hydration centrally, and non-homogeneous beam profile under-treating the central cornea.5, 214,
215
Risk factors for the development of central islands include high myopia, greater depth of
ablation, large optical zones, and a persistent epithelial defect. Steinert216 noted on confocal
microscopy in vivo that central islands appear to consist of persistent dense subepithelial
extracellular deposits.
Laser manufacturers of broad beam lasers developed strategies to prevent the problem by
either a pre-treatment protocol (Summit) or an ablation profile adjustment (VISX) and which
provides additional treatment to the central 2.5 mm optical zone. The use of flying spot and
PHOTOREFRACTIVE KERATECTOMY
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variable spot scanning lasers has almost eliminated this problem217 but fluid accumulation on the
stromal bed before or during photo ablation can lessen the effect of the additional pulses from the
excimer laser. Patients’ complaint of visual symptom following laser treatment may suggest this
complication and retinoscopy will reveal a small central shadow. The diagnosis is confirmed on
corneal topography and it is important to document an elevation in the central or peri-central
zone. Prior to elevation topography, islands were incorrectly diagnosed and re-treated when, in
fact, there was a central divot with epithelial hyperplasia and re-treatment aggravated the
problem.
Most central islands are transient and do not require treatment.213 Central islands may
take up to twelve months to resolve and early re-treatment may lead to an over-correction longterm. For persistent islands, elevation topography can be used to determine the location width
and height of the island, and following epithelial removal, a PTK of small diameter 2.5 to 3 mm
is used to flatten the island and a masking agent may be used for further smoothing.218-222
ECCENTRIC ABLATIONS AND DECENTRATIONS
This is now a rare complication, especially with refinements in patient fixation targets
and auto-centration eye trackers. Decentration is defined as a shift of the center of the ablation
pattern from the pupil or visual axis to a more eccentric location. It may occur as a result of poor
alignment of the patient’s head, poor patient fixation, significant eye movement, laser beam not
aligned to the surgeon’s eye piece, mis-alignment of the eye tracker, and failure of the surgeon to
realize the eye tracker is not properly engaged. Prolonged treatment times have a greater
probability of decentration without eye tracking owing to the patient’s greater difficulty in
maintaining fixation (intraoperative drift). A decentered area of flattening may be associated
with corneal astigmatism and with symptoms of monocular diploplia, glare, halos, and decreased
PHOTOREFRACTIVE KERATECTOMY
-35-
visual performance. Small decentrations may not affect postoperative Snellen visual acuity or
contrast sensitivity.223, 224 Although decentrations were originally seen with myopic ablations,
they are more often associated with hyperopic treatment due to the very small central zone of
unablated tissue with hyperopic ablation patterns. The surgeon may introduce less than perfect
centering on the cornea during the procedure which can be accentuated by poor patient
cooperation in the form of avoidance movements away from bright lights and the normal Bell’s
reflex to shift the eye upward beneath the upper eyelid. The treatment should be stopped and
proper fixation and centering achieved. In patients with pupils with large angle kappa, centering
might be best between the visual axis and the pupil center rather than directly on the pupil.11
Symptoms are usually worse under low lighting settings owing to the enlargement of the
pupil. The ablation zone may appear decentered due to an irregular wound healing response and
early corneal topography showing a decentered zone, allows distinction of these two entities.
Although the diagnosis can be made on corneal topography225, comparing the center of the
ablation to the pupillary center with aberometry sensing will readily disclose the irregular
astigmatism (the resulting spherical aberration and coma).226
Decentration is one of the most difficult laser ablation problems to correct and every
effort must be made to ensure the primary ablation is centered over the entrance pupil. Although
a number of treatment methods have been proposed surgery is more an art than science. A
equally decentered PRK 180o away from the original decentration can be perfomed after
transepithelial PTK with good results.222, 227 Topography planning software (VisionPro™, Carl
Zeiss Ophthalmic Systems Humphrey ATLAS® ) combined with custom programs such as the
VISX Custom-CAP ™ can substantially improve symptoms. Topography guided laser
treatment with a flying spot laser198 or wavefront-guided laser treatment with small spot size
PHOTOREFRACTIVE KERATECTOMY
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should prove to be able the best treatment for this complication. Encouraging results have been
obtained using LASIK228 but programs that utilize both topography and wavefront aberrometry
and under development.
IRREGULAR ASTIGMATISM
The causes of irregular astigmatism include preoperative asymmetric astigmatism (3243% of cases), decentration, central islands, forme fruste keratoconus and differential wound
healing with annular haze. This is the most common cause of loss of BSCVA and is associated
with symptoms of glare, halos, starburst, diploplia, and decreased contrast acuity. Although
elevation topography will make the diagnosis, wavefront sensing is becoming the preferred
diagnostic tool being able to define and quantify the higher order (spherical, coma and trefoil)
aberrations as well as display point spread function and quality of vision. In addition, the
surgeon should look for a posterior ectasia on Orbscan topography if the patient has undergone
PRK re-treatment for high myopia. A diagnostic contact lens trial will help confirm the
diagnosis when there is marked improvement in corrected visual acuity.
Topographically guided ablations show improvement200 but wavefront-guided ablations
are the future for the correction of this problem.
LATE COMPLICATIONS
UNDER-CORRECTIONS
Under-correction is defined as failure to achieve within 0.75 D of the intended correction
by 6-12 weeks postoperatively. Causes are multiple and include use of the wrong refractive
error, poor laser calibration, laser software version, room temperature and humidity, poor
PHOTOREFRACTIVE KERATECTOMY
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fixation, inadequate epithelial removal, excessive moisture on the stromal bed and variations in
corneal healing229. Usually, there is no associated corneal haze.
Early re-treatment within one to six months230 gives good results but most surgeons will
wait for the refraction to stabilize. It is important that the patient has discontinued topical
steroids for three months to ensure stability of the refraction before proceeding. Care must be
exercised in performing full corrections in older patients to avoid over-correction.
Epithelial removal may be manual or laser but many surgeons prefer not to touch the stromal bed
after epithelial removal except for drying to allow the epithelium to act as a masking agent
allowing for a smoother ablation.89
OVER-CORRECTIONS
Over-corrections are not uncommon initially and usually resolve with time which may
take from a few months to six months or longer. Persistent over-corrections are correlated with
older age, higher attempted correction, low humidity, and use of topical steroids. In many cases,
they are unexplained and may be due to the patient’s wound healing response229 in which
inadequate responders were seen in 11% of cases with minimal regression at six months.
Over-corrections are managed initially by cessation of corticosteroid drops and
administration of artificial tears. Pilocarpine 0.5% may be started to stimulate accommodation
and reduce the hyperopia and anisometropia if the other eye is close to plano. NSAID drops may
be instilled four times a day along with a soft contact lens which may improve visual acuity as
well as potentially stimulating stromal healing. Often, the over-correction will suddenly resolve.
Removal of the epithelium by scraping has been demonstrated to have only limited success
reducing the over-correction by 0.50 D but results are better if performed in the first 4 months
and may be accompanied by induction of haze.231-233 Small, over- and under-corrections, 0.25 to
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0.50 D may be treated by applying the PRK ablation directly into the epithelium. We have found
that approximately half the patients will show improvement of 0.25 to 0.50 D which can remain
stable out to one year. There is only minimal discomfort for a few hours with this procedure.
Although holmium Yag laser thermoplasty showed promising results in patients with
small persistent over-correction234, 235 the treatment was often accompanied by surgically induced
astigmatism and regression of effect over months to years. Conductive keratoplasty (Refractec,
ViewPoint CK System, Irvine, California ) potentially could correct postoperative overcorrections.
Hyperopic PRK has also been shown to be beneficial with a small over-correction noted
following treatment that regresses over three to six months. The full amount of the hyperopia is
corrected in young patients and reducing the correction by 10% in older patients will avoid
residual myopia.
Myopic or hyperopic LASIK can be used to re-treat residual PRK refractive errors
provided there is sufficient corneal thickness. Good results can be obtained with more rapid
stabilization and less chance of haze.
HAZE, SCARRING, AND REGRESSION
One of the major complications associated with PRK is the wound healing response
associated with stromal re-modelling and epithelial hyperplasia. In patients with high myopia
haze and regression related to stromal wound healing are major limitations for PRK. Excimer
laser ablation and extensive postoperative ultraviolet exposure induces apoptosis through a
mechanism of reactive oxygen species and this may be responsible for early death of keratocytes
immediately adjacent to the photo ablated area.236 Apoptosis plays a pivotal role in the wound
healing response that leads to the deposition of new materials such as collagen, proteoglycans,
PHOTOREFRACTIVE KERATECTOMY
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and hyaluronic acid.20, 237 These may cause haze, regression, and light scattering resulting in
visual impairment.
Many patients will develop a mild, anterior stromal haze or loss of corneal transparency
as a normal healing response to PRK which starts a few weeks postoperatively and reaches a
peak in three months.238 In most cases, the haze, measured at the slit lamp and graded clinically
on a scale 0 (clear cornea) to 4 (dense opacity), resolves over time leaving an absolutely clear
cornea or trace haze in over 95% of cases.239 The incidence of haze has decreased dramatically
over the past ten years due to improved lasers and ablation contours, surgical technique, and
postoperative follow-up. In our series 99% and 93% had clear to trace haze for -1 to -6 and -6 to
-10 D at one year. No patient had more than 1+ haze beyond 18 months.
In patients with significant haze, there is a fine reticulated appearance which often clears
centrally leaving more annular opacity.240 In a few cases, a permanent scar will result which
may interfere with vision and induce topographic abnormalities. Late onset corneal haze
(LOCH) has been reported to occur in 2% of patients who, at three months or later, had a clear
cornea and then developed haze with regression.241 This is most likely to occur in cases of high
myopia when steroids have been stopped and is not seen with this incidence today. I have seen
patients six to twelve months postoperatively with crystal-clear corneas, suddenly develop haze,
regression of 1-2 D with decreased vision following an acute systemic viral infection which was
completely reversed with a short course of topical steroids. The most likely results from
differential stromal hydration due to keratocyte activity and increased glycosaminoclycan
hyaluronic acid and (laminin).
Risk factors for the development of haze associated with keratocyte, activation,
migration, and newly sensitized collagen includes higher degrees of myopia (requiring deeper
PHOTOREFRACTIVE KERATECTOMY
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ablation),242 small ablation diameters with abrupt transition edges,243 pregnancy,66 oral
contraceptives,244 exposure to ultraviolet radiation,244-246 viral keratitis – epidemic
keratoconjunctivitis 247 and acute systemic viral illness, trauma,248 ocular surface disorders,244
increased corneal temperature during ablation95, 96, a rough stromal bed249 and dark coloured
irides.250 Although haze may occur without regression, the two often go together.126
Encouraging patients to wear sunglasses and a peak hat when they are outdoors for the first year
and to report any sudden change in vision may allow prompt treatment. Epidemic
keratoconjunctivitis or trauma may also induce regression.
Especially following hyperopic PRK, non-preserved artificial tears and, if necessary,
punctal plugs, may prevent haze and a central nodular scar.146 Although topical corticosteroids
have been shown effective in the prevention of haze of high myopia,106, 110 there is little evidence
of their efficacy in low myopia and low hyperopia.3, 251, 252 Intraoperative MMC has proven
helpful in the prophylaxis of haze for higher myopia100 as has postoperative vitamin A and E
supplementation128, 253 and amniotic membrane.254
TREATMENT OF HAZE AND REGRESSION
For the patient who experiences sudden regression of correction with or without haze, a
course of topical steroids should be initiated (Prednisolone acetate 1% instilled every one to two
hours for one to two weeks then tapered). The patient should be closely monitored for an
increase in intraocular pressure which may be falsely low on tonometry.115, 118, 255 NSAIDs do
not appear to be effective in managing haze.105 If there is no improvement in the haze or
regression after one month of intensive therapy, the steroids should be stopped; otherwise, with
improvement, slowly tapered. A few patients will regress again when the steroids are stopped
and a second course may be needed.
PHOTOREFRACTIVE KERATECTOMY
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In patients with regression and little to no haze, a transepithelial PTK without scraping is
perfomed86, 89 followed by the stomal ablation and a smoothing PTK.249 It is usually necessary to
over-correct 25-33%256, 257 but if there is significant haze, the scar may ablate at a different rate
than clear cornea, and under-correction is advised.258 Postoperative steroids should be
administered tapering over 3 months. Prophylactic MMC 0.02% for two minutes should be used
at the end of the stromal ablation for patients who have significant regression after the first
treatment, greater than 1.5 D. MMC 0.02% drops administered bid can be used following retreatment but a number of patients will develop a toxic or hypersensitivity reaction. This can be
used from two to eight weeks postoperatively.
Patients who developed significant haze such that it is affecting BSCVA, which makes an
accurate refraction impossible, should be treated first with mechanical epithelial removal and,
scar removal with a No. 64 Beaver blade then the application of MMC 0.02% on a 6.0 mm
circular sponge for two minutes followed by copious irrigation with BSS.195, 259, 260 Azar and
Jain have suggested application of MMC soaked rings instead of discs,197 may be a better
approach as Maldonado260 has noted reduced central corneal clarity in the absence of recurrent
central sub-epithelial fibrosis after the use of MMC-soaked disk. This would allow sparing of
the central cornea from drug exposure and treatment in the rabbit model with a soaked disk
shows more corneal light scatter than after soaked rings. It is important to note that some
patients, even though the haze is resolved, will be left with irregular astigmatism due to
incomplete scar removal and this may be best treated with Wavefront-guided ablations. Despite
the potential toxicity of MMC, the use of 0.02% for up to two minutes has proven to be safe and
without significant complications.
PHOTOREFRACTIVE KERATECTOMY
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The under-correction should not be treated since the refractive error may improve as the
scar resolves over the following six months. A BCL is used to heal the epithelium and the reepithelialization is similar to the primary procedure. Topical steroids should be continued and
slowly tapered over the next three to six months. Once the haze has resolved, re-treatment with
LASIK may be more successful, avoiding recurrence of the haze.261 If PRK is used, MMC may
be re-applied or the epithelial defect covered with amniotic membrane which prevents
polymorphonuclear cell infiltration and, thus, reduces perioxidation and prevents keratocyte
apoptosis but its routine use presents difficulties.254, 262 In rare cases, a superficial lamellar
keratectomy may be necessary to excise the corneal scarring.263
Following hyperopic PRK, regression may occur as part of the normal wound healing
response, extending out to six months with little change from six to twelve months. Regression
may occur over the next few years and re-treatment is usually effective provided there is an
adequate, stable tear film and, postoperatively, the Ks are not greater than 49 or 50 D. A central
nodule may occur73 in very steep corneas which if, recognized early, can be treated with
increased lubrication and steroids. In some cases, it may be necessary to do a therapeutic PTK
by first removing the epithelium overlying the nodule and, with masking solution, flatten the
nodule. MMC may be used prophylactically. Peripheral haze and irregular astigmatism have
been seen with higher hyperopic astigmatic corrections. MMC offers the best treatment at this
time to reduce the scarring.
A number of agents have been tried with little or limited success in reducing
postoperative haze. They include topical anti TGF beta antibodies264, topical interferon-alpha
2b,265 and synthetic inhibitor of metalloproteinase and cyclosporin A.266 The use of
PHOTOREFRACTIVE KERATECTOMY
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postoperative autologous blood has not proved helpful.254 In the future, tests may be available to
identify patients who have the potential for significant corneal haze after PRK.267
Although postoperative haze has been reported to be less with LASEK,24, 268 we have not
been able to demonstrate any difference between these two surface ablation techniques because
of the inherently low incidence of haze with modern lasers and surgical technique. Lee269 has
suggested that the lower grade of corneal haze seen in LASEK may come from a decreased
release of TGF- β1 measured in the tears which may have a role in wound healing. Long-term
experience with LASEK will be needed.
In patients who present with regression years following PRK, it is important to
distinguish between regression of the treatment (corresponding change in keratometry values),
progression of myopia due to nuclear sclerosis, posterior ectasia which occurs rarely in PRK but
may be seen with high myopic corrections with re-treatments and abnormal corneal
biomechanics associated with forme fruste or manifest keratoconus. Failure to recognize the lens
as the cause of the myopia may result in repeat PRK with subsequent ectasia. Cataract surgery
has been successful in post PRK eyes but careful calculation of the IOL power is important to
avoid hyperopia.270-272
QUALITY OF VISION
The outcome of refractive surgery is most frequently assessed by Snellen visual acuity.
Since the early 90’s there has been an increase in the percentage of patients that achieve
uncorrected visual acuity of 20/25, 20/20 and 20/15, maintain best corrected visual acuity and
even gain lines of vision with PRK. However, despite the impressive improvement patients
describe visual disturbances which may interfere with daily activities especially night driving.
PHOTOREFRACTIVE KERATECTOMY
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Fan-Paul has proposed that “quality of vision” is the most appropriate term to describe visual
disturbances including glare disability, contrast sensitivity, and image degradation. 60
Glare disability is the term that has been adapted to describe any subjective reduction of
visual performance due to a glare source and can cause annoyance, discomfort or rarely, loss in
visual performance or visibility. Although the Snellen chart is universally used, its characters are
pictured at 100% contrast while objects in the real world are rarely at such high contrast level
thus accounting for the patient complaints of poor vision despite good vision in the office setting.
Tests of contrast sensitivity are designed to determine the threshold of contrast required to
identify a target. Some of these (Pelli-Robson and Regan chart) can be combined with glare
sources (Brightness Acuty Tester (BAT)) to simulate real life situations such as a bright sunny
day or oncoming car headlights. Glare disability and reduced contrast sensitivity have been
referred to as “night vision disturbances”. The most common descriptions of night vision
disturbances are based on image degradation (altered object shape or size) such as starbursts and
halos. Starbursts can occur in nonsurgical patients especially when under-corrected while halos
are due to a pupil diameter that is greater than the optical zone diameter as seen following small
zone PRK. Factors that may increase the risk for post refractive night vision disturbances
include large pupils, large refractive error, thin corneas (limiting optical zone size) and low
neural ability of adaptation as well as factors related to the specific refractive procedure.60
In most PRK studies there were a substantial number of patients with vision disturbances
in the early postoperative period.273 After 6 months to 1 year most patients noted a significant
improvement in their complaints with many returned to preoperative levels while some have
reported disturbances persisting even after 2 years.274, 275 There are also a significant number of
patients that report improvement in their visual disturbances following PRK.276 High contrast
PHOTOREFRACTIVE KERATECTOMY
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visual acuity usually drops over the first six months and then recovers while low contrast visual
acuity may remain depressed at one year in some series, while others report a return to
preoperative levels.
10, 137, 277, 278
Although patients, when questioned, may complain of glare,
halos, and difficulty driving at night, many had similar preoperative symptoms, especially with
contact lenses, and most patients would have the procedure again.138, 279
Pupil size is an important factor in PRK visual success. Following PRK there is
substantial increase in higher order aberrations over preoperative aberrations.280 The numerically
calculated increase in the higher-order coma and spherical like aberration correlated with the
clinical results, and these increase with increasing pupil size.226, 281 Using corneal topography
Maeda has demonstrated that evaluation of the optical properties of the cornea can be useful for
assessing quality of vision.282 As well, ablation zone diameters smaller than the entrance pupil
diameter can account for an increase in glare disability, starbursts and halos. Despite the
correlation of visual disturbances with large pupils and higher refractive corrections, recent
reports indicate that pupil size is not that important as a risk factor.
Irregular wound healing and ablation decentration can produce monocular diplopia with a
defocused second ghost image. How much decentration is significant is controversial with some
showing that 1 mm can produce symptoms while others feel that there is no difference in snellen
visual acuity or contrast sensitivity in patients with less than or more than 0.5 mm of
decentation.223, 224 Mrochen has demonstrated that subclinical decentration (less than 1.0 mm)
was found to be a major factor in increased coma-like and spherical-like aberrations after corneal
laser surgery. It has been suggested that there is less chance of decentration with PRK than
LASIK. 283
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Holliday, on the other hand, has shown that functional vision changes do occur after
LASIK and this is associated with the reduced optical quality of the cornea and the oblate
asphericity. Changes in functional vision worsen as the target contrast diminishes and the pupil
size increases. These findings indicate that the oblate shape of the cornea following LASIK is the
predominant factor in the functional vision decrease.284
The incidence of significant visual complaints has decreased dramatically with the
introduction of larger optical zones with transition blend zones extending to 8.5 mm.281 It is
hoped that Wavefront-guided treatments will help reduce the incidence of this complication at
least by treating any pre-existing higher order aberrations while other investigators are working
to create a more prolate cornea following surgery.94 Patients complaining of persistent glare and
halos, and treated with smaller ablation zones, may be retreated successfully using Wavefrontguided ablations and larger treatment zones.285, 286 Brimonidine 0.2% ophthalmic drops
(Alphagan, Allergan) has been used to prevent pupil dilatation at night and has offered
symptomatic relief in patients with large pupils. A loss of effect (tachyphylaxis) may occur with
prolonged use. A rigid gas permeable lens may be used to improve visual disturbances for
aberrated eyes when retreatment is not possible by providing a smooth front optical surface. In
rare cases penetrating keratoplasy may be necessary usually with good results.
It is important to stress that minor amounts of defocus – sphere and cylinder (lower order
aberrations) are the major cause affecting quality of vision. Retreatments can be very successful
and may be combined with enlarging the optical zone. MMC can be helpful, used alone to
reduce significant haze, or used in conjunction with retreatment to prevent haze and regression.
Fluctuations in the quality of vision throughout the day suggests instability of the ocular tear
film.287
PHOTOREFRACTIVE KERATECTOMY
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DRY EYES
Although a small number of patients complained of dryness, foreign body sensation,
tearing, burning and blurred vision following PRK, the widespread acceptance of LASIK and the
finding that 80% of patients experienced variable symptoms of ocular surface dryness in the first
1 to 6 months drew attention to the importance of this problem.54, 55 A decrease in corneal
sensation,288 tear secretion and tear quality and stability51, 72, 289 occurs following PRK and
persists for up to 3-6 months although not as severe as seen with LASIK.51 This can result in
symptoms of dry eye, and punctate erosions and may be a factor contributing to a decrease in
image quality and contrast sensitivity especially in hyperopic patients.289
Patients need to be carefully screened for dry eye before having surgery and warned of
the possible complications. If symptoms or signs are present preoperatively they must be treated
before proceeding with surgery. Therapy consists of artificial tears, gels or ointments and/or
insertion of a punctal plug. Treatment of meibomian gland dysfunction with doxycycline 100
mg bid, topical antibiotics and warm compresses may improve comfort and reduce evaporation
of aqueous tears. The use of topical cyclosporin (Restasis, Allergan) has recently been improved
for the treatment of dry eye. The same strategies are used in the postoperative period.
Symptoms usually improve by 6 months but a few patients will require prolonged therapy.
OTHER COMPLICATIONS
Corticosteroids are routinely used following PRK for a few weeks to many months.
Intraocular pressure (IOP) rise may occur in 10-25% of patients depending on the potency
used.290 Flurometholone 0.1% (FML) is the most frequently used steroid with a steroid response
seen in less than 10% of our patients. Control of IOP is accomplished with the use of topical
antiglaucoma medications while the topical steroid is reduced as much as need to control IOP
PHOTOREFRACTIVE KERATECTOMY
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and stopped as soon as clinically indicated. Vetrugno291 has reported that 0.005% latanoprost is
as safe and effective as 0.50% timolol in the treatment of steroid induced pressure rise. Both
drugs provided a significant and stable IOP reduction in the majority of patients after short-term
treatment. Combination therapy of timolol 0.5% and dorzolamide 2% was reported by Nagy112
to also be effective in treating secondary IOP elevation while dorzolamide alone was not
effective. Permanent glaucomatous optic nerve damaged has been reported following PRK with
intensive corticosteroid therapy.292 Carefully monitoring of IOP is important until steroids have
been discontinued. Other complications of topical steroids have been reported including stromal
thinning, posterior subcapsular cataract,111 increased incidence of infection and rebound corneal
haze.241
The accurate measurement of intraocular pressure following refractive surgery has
important implications for the diagnosis and management of glaucoma. Studies have shown that
there is a decrease in measured IOP following myopic115, 293 and hyperopic PRK.118 Some
investigators have noted a direct correlation between the decrease in IOP and the magnitude of
treatment or the change in central corneal thickness (CCT).117 Difference in IOP measurements
with different tonometers has also been reported.255, 294, 295 Measurements made using a TonoPen temporally outside the treatment zone may be the most reliable.296 As the number of
patients treated with refractive surgery increases and with an aging cohort it will be critical for
IOP measurements to be adjusted for CCT as measured by pachymetry to avoid missing early
cases of glaucoma.
Although retinal detachments, cystoid macular edema and maculopathy have been
reported following PRK, there is no causal link between excimer laser surgery and retinal
pathology.297 Since most of the patients undergoing refractive surgery are myopic, retinal breaks
PHOTOREFRACTIVE KERATECTOMY
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and myopic maculopathy are characteristic of the natural history of the myopic eye and the
pathologic complications that exist before surgery is performed.
Anisocoria (relative papillary mydriasis) has been reported following unilateral PRK.298
with wide-field excimer laser ablation and subsequent application of topical corticosteroids.
Neither an altered corneal profile nor parasympathetic denervation is responsible for this.
Weakening of the pupillary sphincter of the treated eye may cause this phenomenon.
Ocular tenderness and sensitivity has been reported after PRK often continuing for some
time.5 In some patients the symptoms may be related to a dry eye or mild erosions, no pathology
is usually found on careful examination. Symptoms usually diminish with time.
PHOTOREFRACTIVE KERATECTOMY
-50REFERENCES
1. McDonnell PJ, Moreira H, Clapham TN, et al: Photorefractive keratectomy for astigmatism.
Initial clinical results. Arch Ophthalmol 109:1370-3, 1991.
2. Dausch D, Klein R, Schroder E: Excimer laser photorefractive keratectomy for hyperopia.
Refract Corneal Surg 9:20-8, 1993.
3. Jackson WB, Casson E, Hodge WG, et al: Laser vision correction for low hyperopia. An 18month assessment of safety and efficacy. Ophthalmology 105:1727-38; discussion 37-8, 1998.
4. Alio JL, Artola A, Claramonte PJ, et al: Complications of photorefractive keratectomy for
myopia: two year follow-up of 3000 cases. J Cataract Refract Surg 24:619-26, 1998.
5. Stein R: Photorefractive keratectomy. Int Ophthalmol Clin 40:35-56, 2000.
6. Carr JD, Stulting RD, Thompson KP, et al: Laser in situ keratomileusis: surgical technique.
Ophthalmol Clin North Am 14:285-94, vii, 2001.
7. Kapadia MS, Wilson SE: One-year results of PRK in low and moderate myopia: fewer than
0.5% of eyes lose two or more lines of vision. Cornea 19:180-4, 2000.
8. Tuunanen TH, Tervo TT: Results of photorefractive keratectomy for low, moderate, and high
myopia. J Refract Surg 14:437-46, 1998.
9. Haw WW, Manche EE: Prospective study of photorefractive keratectomy for hyperopia using
an axicon lens and erodible mask. J Refract Surg 16:724-30, 2000.
10. Stevens JD, Ficker LA: Results of photorefractive keratectomy for hyperopia using the VISX
star excimer laser system. J Refract Surg 18:30-6, 2002.
11. Nagy ZZ, Munkacsy G, Popper M: Photorefractive keratectomy using the meditec MEL 70
G-scan laser for hyperopia and hyperopic astigmatism. J Refract Surg 18:542-50, 2002.
PHOTOREFRACTIVE KERATECTOMY
-51-
12. Tham VM, Maloney RK: Microkeratome complications of laser in situ keratomileusis.
Ophthalmology 107:920-4, 2000.
13. Melki SA, Azar DT: LASIK complications: etiology, management, and prevention. Surv
Ophthalmol 46:95-116, 2001.
14. Anderson NJ, Beran RF, Schneider TL: Epi-LASEK for the correction of myopia and
myopic astigmatism. J Cataract Refract Surg 28:1343- 7, 2002.
15. Claringbold TV, 2nd: Laser-assisted subepithelial keratectomy for the correction of myopia.
J Cataract Refract Surg 28:18-22, 2002.
16. Lee JB, Choe CM, Seong GJ, et al: Laser subepithelial keratomileusis for low to moderate
myopia: 6-month follow-up. Jpn J Ophthalmol 46:299-304, 2002.
17. Litwak S, Zadok D, Garcia-de Quevedo V, et al: Laser-assisted subepithelial keratectomy
versus photorefractive keratectomy for the correction of myopia. A prospective comparative
study. J Cataract Refract Surg 28:1330-3, 2002.
18. Shahinian L, Jr.: Laser-assisted subepithelial keratectomy for low to high myopia and
astigmatism. J Cataract Refract Surg 28:1334-42, 2002.
19. Vinciguerra P, Camesasca FI: Butterfly laser epithelial keratomileusis for myopia. J Refract
Surg 18:S371-3, 2002.
20. Wilson SE, Mohan RR, Hong JW, et al: The wound healing response after laser in situ
keratomileusis and photorefractive keratectomy: elusive control of biological variability and
effect on custom laser vision correction. Arch Ophthalmol 119:889-96, 2001.
21. Nagy ZZ, Palagyi-Deak I, Kelemen E, et al: Wavefront-guided photorefractive keratectomy
for myopia and myopic astigmatism. J Refract Surg 18:S615-9, 2002.
PHOTOREFRACTIVE KERATECTOMY
-52-
22. Nagy ZZ, Palagyi-Deak I, Kovacs A, et al: First results with wavefront-guided
photorefractive keratectomy for hyperopia. J Refract Surg 18:S620-3, 2002.
23. Oshika T, Klyce SD, Applegate RA, et al: Comparison of corneal wavefront aberrations after
photorefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol 127:1-7, 1999.
24. Lee JB, Seong GJ, Lee JH, et al: Comparison of laser epithelial keratomileusis and
photorefractive keratectomy for low to moderate myopia. J Cataract Refract Surg 27:565-70,
2001.
25. el Danasoury MA, el Maghraby A, Klyce SD, et al: Comparison of photorefractive
keratectomy with excimer laser in situ keratomileusis in correcting low myopia (from -2.00 to 5.50 diopters). A randomized study. Ophthalmology 106:411-20; discussion 20-1, 1999.
26. Pop M, Payette Y: Photorefractive keratectomy versus laser in situ keratomileusis: a controlmatched study. Ophthalmology 107:251-7, 2000.
27. Fraunfelder FW, Wilson SE: Laser in situ keratomileusis versus photorefractive keratectomy
in the correction of myopic astigmatism. Cornea 20:385-7, 2001.
28. Tole DM, McCarty DJ, Couper T, et al: Comparison of laser in situ keratomileusis and
photorefractive keratectomy for the correction of myopia of -6.00 diopters or less. Melbourne
Excimer Laser Group. J Refract Surg 17:46-54, 2001.
29. Van Gelder RN, Steger-May K, Yang SH, et al: Comparison of photorefractive keratectomy,
astigmatic PRK, laser in situ keratomileusis, and astigmatic LASIK in the treatment of myopia. J
Cataract Refract Surg 28:462-76, 2002.
30. Ambrosio R, Jr., Wilson SE: Complications of laser in situ keratomileusis: etiology,
prevention, and treatment. J Refract Surg 17:350-79, 2001.
PHOTOREFRACTIVE KERATECTOMY
-53-
31. Wilson SE: LASIK: management of common complications. Laser in situ keratomileusis.
Cornea 17:459-67, 1998.
32. Davis EA, Hardten DR, Lindstrom RL: LASIK complications. Int Ophthalmol Clin 40:6775, 2000.
33. Aldave AJ, Hollander DA, Abbott RL: Late-onset traumatic flap dislocation and diffuse
lamellar inflammation after laser in situ keratomileusis. Cornea 21:604-7, 2002.
34. Tumbocon JA, Paul R, Slomovic A, et al: Late traumatic displacement of laser in situ
keratomileusis flaps. Cornea 22:66-9, 2003.
35. Iskander NG, Peters NT, Anderson Penno E, et al: Late traumatic flap dislocation after laser
in situ keratomileusis. J Cataract Refract Surg 27:1111-4, 2001.
36. Norden RA, Perry HD, Donnenfeld ED, et al: Air bag-induced corneal flap folds after laser
in situ keratomileusis. Am J Ophthalmol 130:234-5, 2000.
37. Lemley HL, Chodosh J, Wolf TC, et al: Partial dislocation of laser in situ keratomileusis flap
by air bag injury. J Refract Surg 16:373-4, 2000.
38. Harrison DA, Periman LM: Diffuse lamellar keratitis associated with recurrent corneal
erosions after laser in situ keratomileusis. J Refract Surg 17:463-5, 2001.
39. Dastgheib KA, Clinch TE, Manche EE, et al: Sloughing of corneal epithelium and wound
healing complications associated with laser in situ keratomileusis in patients with epithelial
basement membrane dystrophy. Am J Ophthalmol 130:297-303, 2000.
40. Rashad KM, Hussein HA, El-Samadouny MA, et al: Phototherapeutic keratectomy in
patients with recurrent corneal epithelial erosions. J Refract Surg 17:511-8, 2001.
41. Cavanaugh TB, Lind DM, Cutarelli PE, et al: Phototherapeutic keratectomy for recurrent
erosion syndrome in anterior basement membrane dystrophy. Ophthalmology 106:971-6, 1999.
PHOTOREFRACTIVE KERATECTOMY
-54-
42. Amoils SP, Deist MB, Gous P, et al: Iatrogenic keratectasia after laser in situ keratomileusis
for less than -4.0 to -7.0 diopters of myopia. J Cataract Refract Surg 26:967-77, 2000.
43. Holland SP, Srivannaboon S, Reinstein DZ: Avoiding serious corneal complications of laser
assisted in situ keratomileusis and photorefractive keratectomy. Ophthalmology 107:640-52,
2000.
44. Rao SN, Epstein RJ: Early onset ectasia following laser in situ keratomileusus: case report
and literature review. J Refract Surg 18:177-84, 2002.
45. Bianchi C: LASIK and corneal ectasia. Ophthalmology 109:619-21; discussion 21-2, 2002.
46. Pallikaris IG, Kymionis GD, Astyrakakis NI: Corneal ectasia induced by laser in situ
keratomileusis. J Cataract Refract Surg 27:1796-802, 2001.
47. Vinciguerra P, Camesasca FI: Prevention of corneal ectasia in laser in situ keratomileusis. J
Refract Surg 17:S187-9, 2001.
48. McLeod SD, Kisla TA, Caro NC, et al: Iatrogenic keratoconus: corneal ectasia following
laser in situ keratomileusis for myopia. Arch Ophthalmol 118:282-4, 2000.
49. Sun R, Gimbel HV, Kaye GB: Photorefractive keratectomy in keratoconus suspects. J
Cataract Refract Surg 25:1461-6, 1999.
50. Kohnen T: Retreating residual refractive errors after excimer surgery of the cornea: PRK
versus LASIK. J Cataract Refract Surg 26:625-6, 2000.
51. Lee JB, Ryu CH, Kim J, et al: Comparison of tear secretion and tear film instability after
photorefractive keratectomy and laser in situ keratomileusis. J Cataract Refract Surg 26:1326-31,
2000.
52. Toda I, Asano-Kato N, Hori-Komai Y, et al: Laser-assisted in situ keratomileusis for patients
with dry eye. Arch Ophthalmol 120:1024-8, 2002.
PHOTOREFRACTIVE KERATECTOMY
-55-
53. Albietz JM, Lenton LM, McLennan SG. Effect of laser in situ keratomileusis for hyperopia
on tear film and ocular surface. Vol. 18. 2002, 113-23.
54. Wilson SE, Ambrosio R: Laser in situ keratomileusis-induced neurotrophic epitheliopathy.
Am J Ophthalmol 132:405-6, 2001.
55. Ang RT, Dartt DA, Tsubota K: Dry eye after refractive surgery. Curr Opin Ophthalmol
12:318-22, 2001.
56. Yu EY, Leung A, Rao S, et al: Effect of laser in situ keratomileusis on tear stability.
Ophthalmology 107:2131-5, 2000.
57. Yo C, Vroman C, Ma S, et al: Surgical outcomes of photorefractive keratectomy and laser in
situ keratomileusis by inexperienced surgeons. J Cataract Refract Surg 26:510-5, 2000.
58. Shah S, Perera S, Chatterjee A: Satisfaction after photorefractive keratectomy. J Refract Surg
14:S226-7, 1998.
59. Geerling G, Meyer C, Laqua H: Patient expectations and recollection of information about
photorefractive keratectomy. J Cataract Refract Surg 23:1311-6, 1997.
60. Fan-Paul NI, Li J, Miller JS, et al: Night vision disturbances after corneal refractive surgery.
Surv Ophthalmol 47:533-46, 2002.
61. Cua IY, Pepose JS: Late corneal scarring after photorefractive keratectomy concurrent with
development of systemic lupus erythematosus. J Refract Surg 18:750-2, 2002.
62. Pepose JS, Laycock KA, Miller JK, et al: Reactivation of latent herpes simplex virus by
excimer laser photokeratectomy. Am J Ophthalmol 114:45-50, 1992.
63. Wulff K, Fechner PU: Herpes simplex keratitis after photorefractive keratectomy. J Refract
Surg 13:613, 1997.
PHOTOREFRACTIVE KERATECTOMY
-56-
64. Asbell PA: Valacyclovir for the prevention of recurrent herpes simplex virus eye disease
after excimer laser photokeratectomy. Trans Am Ophthalmol Soc 98:285-303, 2000.
65. McCarty CA, Ng I, Waldron B, et al: Relation of hormone and menopausal status to
outcomes following excimer laser photorefractive keratectomy in women. Melbourne Excimer
Laser Group. Aust N Z J Ophthalmol 24:215-22, 1996.
66. Sharif K: Regression of myopia induced by pregnancy after photorefractive keratectomy. J
Refract Surg 13:S445-6, 1997.
67. Starr MB: Pregnancy-associated overcorrection following myopic excimer laser
photorefractive keratectomy. Arch Ophthalmol 116:1551, 1998.
68. Pop M, Payette Y, Santoriello E: Comparison of the pupil card and pupillometer in
measuring pupil size. J Cataract Refract Surg 28:283-8, 2002.
69. Weiss RA, Berke W, Gottlieb L, et al: Clinical importance of accurate refractor vertex
distance measurements prior to refractive surgery. J Refract Surg 18:444-8, 2002.
70. Wilson SE, Ambrosio R: Computerized corneal topography and its importance to wavefront
technology. Cornea 20:441-54, 2001.
71. Siganos DS, Popescu CN, Siganos CS, et al: Tear secretion following spherical and
astigmatic excimer laser photorefractive keratectomy. J Cataract Refract Surg 26:1585-9, 2000.
72. Ozdamar A, Aras C, Karakas N, et al: Changes in tear flow and tear film stability after
photorefractive keratectomy. Cornea 18:437-9, 1999.
73. Sener B, Ozdamar A, Aras C: Apical nodular subepithelial corneal scar after retreatment in
hyperopic photorefractive keratectomy. J Cataract Refract Surg 26:352-7, 2000.
74. Ellis JH, Abbott RL, Brick DC, et al: Liability issues associated with PRK and the excimer
laser. Surv Ophthalmol 42:279-82, 1997.
PHOTOREFRACTIVE KERATECTOMY
-57-
75. Vetrugno M, Maino A, Cardia L: Prospective randomized comparison of simultaneous and
sequential bilateral photorefractive keratectomy for the correction of myopia. Ophthalmic Surg
Lasers 31:400-10, 2000.
76. Pop M, Payette Y: Results of bilateral photorefractive keratectomy. Ophthalmology 107:4729, 2000.
77. Amoils SP: Photorefractive keratectomy using a scanning-slit laser, rotary epithelial brush,
and chilled balanced salt solution. J Cataract Refract Surg 26:1596-604, 2000.
78. Kitazawa Y, Maekawa E, Sasaki S, et al: Cooling effect on excimer laser photorefractive
keratectomy. J Cataract Refract Surg 25:1349-55, 1999.
79. Griffith M, Jackson WB, Lafontaine MD, et al: Evaluation of current techniques of corneal
epithelial removal in hyperopic photorefractive keratectomy. J Cataract Refract Surg 24:1070-8,
1998.
80. Weiss RA, Liaw LH, Berns M, et al: Scanning electron microscopy comparison of corneal
epithelial removal techniques before photorefractive keratectomy. J Cataract Refract Surg
25:1093-6, 1999.
81. Abad JC, Talamo JH, Vidaurri-Leal J, et al: Dilute ethanol versus mechanical debridement
before photorefractive keratectomy. J Cataract Refract Surg 22:1427-33, 1996.
82. Abad JC, An B, Power WJ, et al: A prospective evaluation of alcohol-assisted versus
mechanical epithelial removal before photorefractive keratectomy. Ophthalmology 104:1566-74;
discussion 74-5, 1997.
83. Carones F, Fiore T, Brancato R: Mechanical vs. alcohol epithelial removal during
photorefractive keratectomy. J Refract Surg 15:556-62, 1999.
PHOTOREFRACTIVE KERATECTOMY
-58-
84. Shah S, Doyle SJ, Chatterjee A, et al: Comparison of 18% ethanol and mechanical
debridement for epithelial removal before photorefractive keratectomy. J Refract Surg 14:S2124, 1998.
85. Kanitkar KD, Camp J, Humble H, et al: Pain after epithelial removal by ethanol-assisted
mechanical versus transepithelial excimer laser debridement. J Refract Surg 16:519-22, 2000.
86. Kapadia MS, Meisler DM, Wilson SE: Epithelial removal with the excimer laser (laserscrape) in photorefractive keratectomy retreatment. Ophthalmology 106:29-34, 1999.
87. Johnson DG, Kezirian GM, George SP, et al: Removal of corneal epithelium with
phototherapeutic technique during multizone, multipass photorefractive keratectomy. J Refract
Surg 14:38-48, 1998.
88. Kim WJ, Shah S, Wilson SE: Differences in keratocyte apoptosis following transepithelial
and laser-scrape photorefractive keratectomy in rabbits. J Refract Surg 14:526-33, 1998.
89. George SP, Johnson DG: Photorefractive keratectomy retreatments: comparison of two
methods of excimer laser epithelium removal. Ophthalmology 106:1469-79; discussion 79-80,
1999.
90. Pop M, Payette Y: Multipass versus single pass photorefractive keratectomy for high myopia
using a scanning laser. J Refract Surg 15:444-50, 1999.
91. Rao SN, Chuck RS, Chang AH, et al: Effect of age on the refractive outcome of myopic
photorefractive keratectomy. J Cataract Refract Surg 26:543-6, 2000.
92. Pop M, Aras M: Multizone/multipass photorefractive keratectomy: six month results. J
Cataract Refract Surg 21:633-43, 1995.
93. Rosa N, Lanza M, De Rosa G, et al: Anterior corneal surface after Nidek EC-5000 multipass
and multizone photorefractive keratectomy for myopia. J Refract Surg 18:460-2, 2002.
PHOTOREFRACTIVE KERATECTOMY
-59-
94. Panagopoulou SI, Pallikaris IG: Wavefront customized ablations with the WASCA
Asclepion workstation. J Refract Surg 17:S608-12, 2001.
95. Maldonado-Codina C, Morgan PB, Efron N: Thermal consequences of photorefractive
keratectomy. Cornea 20:509-15, 2001.
96. Stein HA, Salim AG, Stein RM, et al: Corneal cooling and rehydration during
photorefractive keratectomy to reduce postoperative corneal haze. J Refract Surg 15:S232-3,
1999.
97. Vinciguerra P, Azzolini M, Airaghi P, et al: Effect of decreasing surface and interface
irregularities after photorefractive keratectomy and laser in situ keratomileusis on optical and
functional outcomes. J Refract Surg 14:S199-203, 1998.
98. Alio JL, Belda JI, Shalaby AM: Correction of irregular astigmatism with excimer laser
assisted by sodium hyaluronate. Ophthalmology 108:1246-60, 2001.
99. Vinciguerra P, Sborgia M, Epstein D, et al: Photorefractive keratectomy to correct myopic or
hyperopic astigmatism with a cross-cylinder ablation. J Refract Surg 15:S183-5, 1999.
100. Carones F, Vigo L, Scandola E, et al: Evaluation of the prophylactic use of mitomycin-C to
inhibit haze formation after photorefractive keratectomy. J Cataract Refract Surg 28:2088-95,
2002.
101. Barequet IS, Jabbur NS, Barron Y, et al: Perioperative microbiologic profile of the
conjunctiva in photorefractive keratectomy. J Refract Surg 17:55-62, 2001.
102. Patel GM, Chuang AZ, Kiang E, et al: Epithelial healing rates with topical ciprofloxacin,
ofloxacin, and ofloxacin with artificial tears after photorefractive keratectomy. J Cataract Refract
Surg 26:690-4, 2000.
PHOTOREFRACTIVE KERATECTOMY
-60-
103. Vetrugno M, Maineo A, Quaranta GM, et al: A randomized, double-masked, clinical study
of the efficacy of four nonsteroidal anti-inflammatory drugs in pain control after excimer laser
photorefractive keratectomy. Clin Ther 22:719-31, 2000.
104. Arshinoff S, D'Addario D, Sadler C, et al: Use of topical nonsteroidal anti-inflammatory
drugs in excimer laser photorefractive keratectomy. J Cataract Refract Surg 20 Suppl:216-22,
1994.
105. Kaji Y, Amano S, Oshika T, et al: Effect of anti-inflammatory agents on corneal woundhealing process after surface excimer laser keratectomy. J Cataract Refract Surg 26:426-31,
2000.
106. Vetrugno M, Maino A, Quaranta GM, et al: The effect of early steroid treatment after PRK
on clinical and refractive outcomes. Acta Ophthalmol Scand 79:23-7, 2001.
107. Baek SH, Choi SY, Chang JH, et al: Short-term effects of flurbiprofen and diclofenac on
refractive outcome and corneal haze after photorefractive keratectomy. J Cataract Refract Surg
23:1317-23, 1997.
108. Teal P, Breslin C, Arshinoff S, et al: Corneal subepithelial infiltrates following excimer
laser photorefractive keratectomy. J Cataract Refract Surg 21:516-8, 1995.
109. Hargrave SL, Jung JC, Fini ME, et al: Possible role of the vitamin E solubilizer in topical
diclofenac on matrix metalloproteinase expression in corneal melting: an analysis of
postoperative keratolysis. Ophthalmology 109:343-50, 2002.
110. Baek SH, Chang JH, Choi SY, et al: The effect of topical corticosteroids on refractive
outcome and corneal haze after photorefractive keratectomy. J Refract Surg 13:644-52, 1997.
111. Bilgihan K, Gurelik G, Akata F, et al: Fluorometholone-induced cataract after
photorefractive keratectomy. Ophthalmologica 211:394-6, 1997.
PHOTOREFRACTIVE KERATECTOMY
-61-
112. Nagy ZZ, Szabo A, Krueger RR, et al: Treatment of intraocular pressure elevation after
photorefractive keratectomy. J Cataract Refract Surg 27:1018-24, 2001.
113. Kim JH, Sah WJ, Hahn TW, et al: Some problems after photorefractive keratectomy. J
Refract Corneal Surg 10:S226-30, 1994.
114. Chatterjee A, Shah S, Bessant DA, et al: Reduction in intraocular pressure after excimer
laser photorefractive keratectomy. Correlation with pretreatment myopia. Ophthalmology
104:355-9, 1997.
115. Damji KF, Munger R, Herndon LW, et al: Reduction of IOP after PRK. Ophthalmology
104:1525-6, 1997.
116. Garcia J, Sherry RC: Reduction of IOP after PRK. Ophthalmology 104:1526-7, 1997.
117. Munger R, Hodge WG, Mintsioulis G, et al: Correction of intraocular pressure for changes
in central corneal thickness following photorefractive keratectomy. Can J Ophthalmol 33:159-65,
1998.
118. Munger R, Dohadwala AA, Hodge WG, et al: Changes in measured intraocular pressure
after hyperopic photorefractive keratectomy. J Cataract Refract Surg 27:1254-62, 2001.
119. Brilakis HS, Deutsch TA: Topical tetracaine with bandage soft contact lens pain control
after photorefractive keratectomy. J Refract Surg 16:444-7, 2000.
120. Shahinian L, Jr., Jain S, Jager RD, et al: Dilute topical proparacaine for pain relief after
photorefractive keratectomy. Ophthalmology 104:1327-32, 1997.
121. Verma S, Corbett MC, Marshall J: A prospective, randomized, double-masked trial to
evaluate the role of topical anesthetics in controlling pain after photorefractive keratectomy.
Ophthalmology 102:1918-24, 1995.
PHOTOREFRACTIVE KERATECTOMY
-62-
122. Kim JY, Choi YS, Lee JH: Keratitis from corneal anesthetic abuse after photorefractive
keratectomy. J Cataract Refract Surg 23:447-9, 1997.
123. Detorakis ET, Siganos DS, Kozobolis VP, et al: Corneal epithelial wound healing after
excimer laser photorefractive and photoastigmatic keratectomy (PRK and PARK). Cornea 18:258, 1999.
124. Detorakis ET, Siganos DS, Houlakis VM, et al: Microbiological examination of bandage
soft contact lenses used in laser refractive surgery. J Refract Surg 14:631-5, 1998.
125. Nakamura K, Kurosaka D, Bissen-Miyajima H, et al: Intact corneal epithelium is essential
for the prevention of stromal haze after laser assisted in situ keratomileusis. Br J Ophthalmol
85:209-13, 2001.
126. Siganos DS, Katsanevaki VJ, Pallikaris IG: Correlation of subepithelial haze and refractive
regression 1 month after photorefractive keratectomy for myopia. J Refract Surg 15:338-42,
1999.
127. Demers P, Thompson P, Bernier RG, et al: Effect of occlusive pressure patching on the rate
of epithelial wound healing after photorefractive keratectomy. J Cataract Refract Surg 22:59-62,
1996.
128. Vetrugno M, Maino A, Cardia G, et al: A randomised, double masked, clinical trial of high
dose vitamin A and vitamin E supplementation after photorefractive keratectomy. Br J
Ophthalmol 85:537-9, 2001.
129. Seiler T, Quurke AW: Iatrogenic keratectasia after LASIK in a case of forme fruste
keratoconus. J Cataract Refract Surg 24:1007-9, 1998.
130. Seiler T, Koufala K, Richter G: Iatrogenic keratectasia after laser in situ keratomileusis. J
Refract Surg 14:312-7, 1998.
PHOTOREFRACTIVE KERATECTOMY
-63-
131. O'Brien TP, Awwad ST: Phakic intraocular lenses and refractory lensectomy for myopia.
Curr Opin Ophthalmol 13:264-70, 2002.
132. Uusitalo RJ, Aine E, Sen NH, et al: Implantable contact lens for high myopia. J Cataract
Refract Surg 28:29-36, 2002.
133. Shen EP, Yang CN, Hu FR: Corneal astigmatic change after photorefractive keratectomy
and photoastigmatic refractive keratectomy. J Cataract Refract Surg 28:491-8, 2002.
134. Yang SH, Van Gelder RN, Pepose JS: Astigmatic changes after excimer laser refractive
surgery. J Cataract Refract Surg 28:477-84, 2002.
135. Vetrugno M, Quaranta GM, Maino A, et al: Contrast sensitivity measured by 2 methods
after photorefractive keratectomy. J Cataract Refract Surg 26:847-52, 2000.
136. Montes-Mico R, Charman WN: Mesopic contrast sensitivity function after excimer laser
photorefractive keratectomy. J Refract Surg 18:9-13, 2002.
137. Stevens J, Giubilei M, Ficker L, et al: Prospective study of photorefractive keratectomy for
myopia using the VISX StarS2 excimer laser system. J Refract Surg 18:502-8, 2002.
138. Brunette I, Gresset J, Boivin JF, et al: Functional outcome and satisfaction after
photorefractive keratectomy. Part 2: survey of 690 patients. Ophthalmology 107:1790-6, 2000.
139. L'Esperance FA, Jr., Warner JW, Telfair WB, et al: Excimer laser instrumentation and
technique for human corneal surgery. Arch Ophthalmol 107:131-9, 1989.
140. Anschutz T: Laser correction of hyperopia and presbyopia. Int Ophthalmol Clin 34:107-37,
1994.
141. Dausch D, Smecka Z, Klein R, et al: Excimer laser photorefractive keratectomy for
hyperopia. J Cataract Refract Surg 23:169-76, 1997.
PHOTOREFRACTIVE KERATECTOMY
-64-
142. Jackson WB, Mintsioulis G, Agapitos PJ, et al: Excimer laser photorefractive keratectomy
for low hyperopia: safety and efficacy. J Cataract Refract Surg 23:480-7, 1997.
143. Carones F, Brancato R, Morico A, et al: Photorefractive keratectomy for hyperopia using an
erodible disc and axicon lens: 2-year results. J Refract Surg 14:504-11, 1998.
144. Nagy ZZ, Krueger RR, Hamberg-Nystrom H, et al: Photorefractive keratectomy for
hyperopia in 800 eyes with the Meditec MEL 60 laser. J Refract Surg 17:525-33, 2001.
145. Bilgihan K, Akata F, Gurelik G, et al: Corneal iron ring after hyperopic photorefractive
keratectomy. J Cataract Refract Surg 25:685-7, 1999.
146. Nagy ZZ, Krueger RR, Suveges I: Central bump-like opacity as a complication of high
hyperopic photorefractive keratectomy. Am J Ophthalmol 128:636-8, 1999.
147. Mietz H, Severin M, Seifert P, et al: Acute corneal necrosis after excimer laser keratectomy
for hyperopia. Ophthalmology 106:490-6, 1999.
148. Golding TR, Bruce AS, Mainstone JC: Relationship between tear-meniscus parameters and
tear-film breakup. Cornea 16:649-61, 1997.
149. Salz JJ, Stevens CA: LASIK correction of spherical hyperopia, hyperopic astigmatism, and
mixed astigmatism with the LADARVision excimer laser system. Ophthalmology 109:1647-56;
discussion 57-8, 2002.
150. Lian J, Ye W, Zhou D, et al: Laser in situ keratomileusis for correction of hyperopia and
hyperopic astigmatism with the Technolas 117C. J Refract Surg 18:435-8, 2002.
151. Ditzen K, Fiedler J, Pieger S: Laser in situ keratomileusis for hyperopia and hyperopic
astigmatism using the Meditec MEL 70 spot scanner. J Refract Surg 18:430-4, 2002.
PHOTOREFRACTIVE KERATECTOMY
-65-
152. Pineda-Fernandez A, Rueda L, Huang D, et al: Laser in situ keratomileusis for hyperopia
and hyperopic astigmatism with the Nidek EC-5000 Excimer laser. J Refract Surg 17:670-5,
2001.
153. el-Agha MS, Johnston EW, Bowman RW, et al: Excimer laser treatment of spherical
hyperopia: PRK or LASIK? Trans Am Ophthalmol Soc 98:59-66; discussion -9, 2000.
154. Alessio G, Boscia F, La Tegola MG, et al: Topography-driven photorefractive keratectomy:
results of corneal interactive programmed topographic ablation software. Ophthalmology
107:1578-87, 2000.
155. Yi DH, Petroll M, Bowman RW, et al: Surgically induced astigmatism after hyperopic and
myopic photorefractive keratectomy. J Cataract Refract Surg 27:396-403, 2001.
156. Pallikaris IG, Kymionis GD, Panagopoulou SI, et al: Induced optical aberrations following
formation of a laser in situ keratomileusis flap. J Cataract Refract Surg 28:1737-41, 2002.
157. Lipshitz I: Thirty-four challenges to meet before excimer laser technology can achieve
super vision. J Refract Surg 18:740-3, 2002.
158. Jain S, Ou R, Azar DT: Monovision outcomes in presbyopic individuals after refractive
surgery. Ophthalmology 108:1430-3, 2001.
159. Vinciguerra P, Nizzola GM, Bailo G, et al: Excimer laser photorefractive keratectomy for
presbyopia: 24-month follow-up in three eyes. J Refract Surg 14:31-7, 1998.
160. Bauerberg JM: Centered vs. inferior off-center ablation to correct hyperopia and
presbyopia. J Refract Surg 15:66-9, 1999.
161. Nano HD, Jr., Muzzin S, Irigaray F: Excimer laser photorefractive keratectomy in pediatric
patients. J Cataract Refract Surg 23:736-9, 1997.
PHOTOREFRACTIVE KERATECTOMY
-66-
162. Alio JL, Artola A, Claramonte P, et al: Photorefractive keratectomy for pediatric myopic
anisometropia. J Cataract Refract Surg 24:327-30, 1998.
163. Astle WF, Huang PT, Ells AL, et al: Photorefractive keratectomy in children. J Cataract
Refract Surg 28:932-41, 2002.
164. Nucci P, Drack AV: Refractive surgery for unilateral high myopia in children. J Aapos
5:348-51, 2001.
165. Singh D: Photorefractive keratectomy in pediatric patients. J Cataract Refract Surg 21:6302, 1995.
166. Haw WW, Alcorn DM, Manche EE: Excimer laser refractive surgery in the pediatric
population. J Pediatr Ophthalmol Strabismus 36:173-7; quiz 206-7, 1999.
167. Guell JL, Lohmann CP, Malecaze FA, et al: Intraepithelial photorefractive keratectomy for
regression after laser in situ keratomileusis. J Cataract Refract Surg 25:670-4, 1999.
168. Carones F, Vigo L, Carones AV, et al: Evaluation of photorefractive keratectomy
retreatments after regressed myopic laser in situ keratomileusis. Ophthalmology 108:1732-7,
2001.
169. Jain VK, Abell TG, Bond WI, et al: Immediate transepithelial photorefractive keratectomy
for treatment of laser in situ keratomileusis flap complications. J Refract Surg 18:109-12, 2002.
170. Kapadia MS, Wilson SE: Transepithelial photorefractive keratectomy for treatment of thin
flaps or caps after complicated laser in situ keratomileusis. Am J Ophthalmol 126:827-9, 1998.
171. Meza J, Perez-Santonja JJ, Moreno E, et al: Photorefractive keratectomy after radial
keratotomy. J Cataract Refract Surg 20:485-9, 1994.
172. John ME, Martines E, Cvintal T: Photorefractive keratectomy for residual myopia after
radial keratotomy. J Cataract Refract Surg 22:901-5, 1996.
PHOTOREFRACTIVE KERATECTOMY
-67-
173. Gimbel HV, Sun R, Chin PK, et al: Excimer laser photorefractive keratectomy for residual
myopia after radial keratotomy. Can J Ophthalmol 32:25-30, 1997.
174. Hersh PS, Shah SI, Durrie D: Monocular diplopia following excimer laser photorefractive
keratectomy after radial keratotomy. Ophthalmic Surg Lasers 27:315-7, 1996.
175. Burnstein Y, Hersh PS: Photorefractive keratectomy following radial keratotomy. J Refract
Surg 12:163-70, 1996.
176. Lee YC, Park CK, Sah WJ, et al: Photorefractive keratectomy for undercorrected myopia
after radial keratotomy: two-year follow up. J Refract Surg 11:S274-9, 1995.
177. Probst LE, Machat JJ: Conservative photorefractive keratectomy for residual myopia
following radial keratotomy. Can J Ophthalmol 33:20-7, 1998.
178. Azar DT, Tuli S, Benson RA, et al: Photorefractive keratectomy for residual myopia after
radial keratotomy. PRK After RK Study Group. J Cataract Refract Surg 24:303-11, 1998.
179. Venter JA: Photorefractive keratectomy for hyperopia after radial keratotomy. J Refract
Surg 13:S456, 1997.
180. Maloney RK, Chan WK, Steinert R, et al: A multicenter trial of photorefractive keratectomy
for residual myopia after previous ocular surgery. Summit Therapeutic Refractive Study Group.
Ophthalmology 102:1042-52; discussion 52-3, 1995.
181. Artola A, Ayala MJ, Claramonte P, et al: Photorefractive keratectomy for residual myopia
after cataract surgery. J Cataract Refract Surg 25:1456-60, 1999.
182. Pop M, Payette Y, Amyot M: Clear lens extraction with intraocular lens followed by
photorefractive keratectomy or laser in situ keratomileusis. Ophthalmology 108:104-11, 2001.
PHOTOREFRACTIVE KERATECTOMY
-68-
183. Sanchez-Galeana CA, Smith RJ, Rodriguez X, et al: Laser in situ keratomileusis and
photorefractive keratectomy for residual refractive error after phakic intraocular lens
implantation. J Refract Surg 17:299-304, 2001.
184. Nordan LT, Binder PS, Kassar BS, et al: Photorefractive keratectomy to treat myopia and
astigmatism after radial keratotomy and penetrating keratoplasty. J Cataract Refract Surg 21:26873, 1995.
185. Donnenfeld ED, Solomon R, Biser S: Laser in situ keratomileusis after penetrating
keratoplasty. Int Ophthalmol Clin 42:67-87, 2002.
186. Bilgihan K, Ozdek SC, Akata F, et al: Photorefractive keratectomy for post-penetrating
keratoplasty myopia and astigmatism. J Cataract Refract Surg 26:1590-5, 2000.
187. Yoshida K, Tazawa Y, Demong TT: Refractive results of post penetrating keratoplasty
photorefractive keratectomy. Ophthalmic Surg Lasers 30:354-9, 1999.
188. Danjoux JP, Fraenkel G, Wai D, et al: Corneal scarring and irregular astigmatism following
refractive surgery in a corneal transplant. Aust N Z J Ophthalmol 26:47-9, 1998.
189. Chan WK, Hunt KE, Glasgow BJ, et al: Corneal scarring after photorefractive keratectomy
in a penetrating keratoplasty. Am J Ophthalmol 121:570-1, 1996.
190. Tuunanen TH, Ruusuvaara PJ, Uusitalo RJ, et al: Photoastigmatic keratectomy for
correction of astigmatism in corneal grafts. Cornea 16:48-53, 1997.
191. Deschenes J, Jovkar S, Balazsi G, et al: Photorefractive keratectomy for correction of
astigmatism after penetrating keratoplasty. Int Ophthalmol Clin 36:113-8, 1996.
192. Bansal AK: Photoastigmatic refractive keratectomy for correction of astigmatism after
keratoplasty. J Refract Surg 15:S243-5, 1999.
PHOTOREFRACTIVE KERATECTOMY
-69-
193. Hersh PS, Jordan AJ, Mayers M: Corneal graft rejection episode after excimer laser
phototherapeutic keratectomy. Arch Ophthalmol 111:735-6, 1993.
194. Epstein RJ, Robin JB: Corneal graft rejection episode after excimer laser phototherapeutic
keratectomy. Arch Ophthalmol 112:157, 1994.
195. Majmudar PA, Forstot SL, Dennis RF, et al: Topical mitomycin-C for subepithelial fibrosis
after refractive corneal surgery. Ophthalmology 107:89-94, 2000.
196. Carones F, Vigo L, Scandola E, et al: Evaluation of prophylactic use of mitomycin-C to
inhibit haze formation after PRK. Abstact American Academy of Ophthalmology 248, 2001.
197. Azar DT, Jain S: Topical MMC for subepithelial fibrosis after refractive corneal surgery.
Ophthalmology 108:239-40, 2001.
198. Alessio G, Boscia F, La Tegola MG, et al: Topography-driven excimer laser for the
retreatment of decentralized myopic photorefractive keratectomy. Ophthalmology 108:1695-703,
2001.
199. Hjortdal JO, Ehlers N: Treatment of post-keratoplasty astigmatism by topography supported
customized laser ablation. Acta Ophthalmol Scand 79:376-80, 2001.
200. Dausch D, Schroder E, Dausch S: Topography-controlled excimer laser photorefractive
keratectomy. J Refract Surg 16:13-22, 2000.
201. Rao SK, Fogla R, Rajagopal R, et al: Bilateral corneal infiltrates after excimer laser
photorefractive keratectomy. J Cataract Refract Surg 26:456-9, 2000.
202. Heidemann DG, Clune M, Dunn SP, et al: Infectious keratitis after photorefractive
keratectomy in a comanaged setting. J Cataract Refract Surg 26:140-1, 2000.
203. Hill VE, Brownstein S, Jackson WB, et al: Infectious keratopathy complicating
photorefractive keratectomy. Arch Ophthalmol 116:1382-4, 1998.
PHOTOREFRACTIVE KERATECTOMY
-70-
204. Forster W, Becker K, Hungermann D, et al: Methicillin-resistant Staphylococcus aureus
keratitis after excimer laser photorefractive keratectomy1. J Cataract Refract Surg 28:722-4,
2002.
205. Wee WR, Kim JY, Choi YS, et al: Bacterial keratitis after photoreactive keratectomy in a
young, healthy man. J Cataract Refract Surg 23:954-6, 1997.
206. Brancato R, Carones F, Venturi E, et al: Mycobacterium chelonae keratitis after excimer
laser photorefractive keratectomy. Arch Ophthalmol 115:1316-8, 1997.
207. Dunphy D, Andrews D, Seamone C, et al: Fungal keratitis following excimer laser
photorefractive keratectomy. Can J Ophthalmol 34:286-9, 1999.
208. Kouyoumdjian GA, Forstot SL, Durairaj VD, et al: Infectious keratitis after laser refractive
surgery. Ophthalmology 108:1266-8, 2001.
209. Keskinbora HK: Long-term results of multizone photorefractive keratectomy for myopia of
-6.0 to -10.0 diopters. J Cataract Refract Surg 26:1484-91, 2000.
210. Kaldawy RM, Sutphin JE, Wagoner MD: Acanthamoeba keratitis after photorefractive
keratectomy. J Cataract Refract Surg 28:364-8, 2002.
211. Faschinger CW: Phototherapeutic keratectomy of a corneal scar due to presumed infection
after photorefractive keratectomy. J Cataract Refract Surg 26:296-300, 2000.
212. Krueger RR, Saedy NF, McDonnell PJ: Clinical analysis of steep central islands after
excimer laser photorefractive keratectomy. Arch Ophthalmol 114:377-81, 1996.
213. McGhee CN, Bryce IG: Natural history of central topographic islands following excimer
laser photorefractive keratectomy. J Cataract Refract Surg 22:1151-8, 1996.
214. Oshika T, Klyce SD, Smolek MK, et al: Corneal hydration and central islands after excimer
laser photorefractive keratectomy. J Cataract Refract Surg 24:1575-80, 1998.
PHOTOREFRACTIVE KERATECTOMY
-71-
215. Forster W, Clemens S, Bruning, et al: Steep central islands after myopic photorefractive
keratectomy. J Cataract Refract Surg 24:899-904, 1998.
216. Steinert RF: Wound healing anomalies after excimer laser photorefractive keratectomy:
correlation of clinical outcomes, corneal topography, and confocal microscopy. Trans Am
Ophthalmol Soc 95:629-714, 1997.
217. Fiore T, Carones F, Brancato R: Broad beam vs. flying spot excimer laser: refractive and
videokeratographic outcomes of two different ablation profiles after photorefractive keratectomy.
J Refract Surg 17:534-41, 2001.
218. Castillo A, Romero F, Martin-Valverde JA, et al: Management and treatment of central
steep islands after excimer laser photorefractive keratectomy. J Refract Surg 12:715-20, 1996.
219. Lafond G, Solomon L: Retreatment of central islands after photorefractive keratectomy. J
Cataract Refract Surg 25:188-96, 1999.
220. Krueger RR, Tersi I, Seiler T: Corneal iron line associated with steep central islands after
photorefractive keratectomy. J Refract Surg 13:401-3, 1997.
221. Manche EE, Maloney RK, Smith RJ: Treatment of topographic central islands following
refractive surgery. J Cataract Refract Surg 24:464-70, 1998.
222. Rachid MD, Yoo SH, Azar DT: Phototherapeutic keratectomy for decentration and central
islands after photorefractive keratectomy. Ophthalmology 108:545-52, 2001.
223. Doane JF, Cavanaugh TB, Durrie DS, et al: Relation of visual symptoms to topographic
ablation zone decentration after excimer laser photorefractive keratectomy. Ophthalmology
102:42-7, 1995.
224. Deitz MR, Piebenga LW, Matta CS, et al: Ablation zone centration after photorefractive
keratectomy and its effect on visual outcome. J Cataract Refract Surg 22:696-701, 1996.
PHOTOREFRACTIVE KERATECTOMY
-72-
225. Vinciguerra P, Camesasca FI: Decentration after refractive surgery. J Refract Surg 17:S1901, 2001.
226. Mrochen M, Kaemmerer M, Mierdel P, et al: Increased higher-order optical aberrations
after laser refractive surgery: a problem of subclinical decentration. J Cataract Refract Surg
27:362-9, 2001.
227. Alkara N, Genth U, Seiler T: Diametral ablation--a technique to manage decentered
photorefractive keratectomy for myopia. J Refract Surg 15:436-40, 1999.
228. Mrochen M, Krueger RR, Bueeler M, et al: Aberration-sensing and wavefront-guided laser
in situ keratomileusis: management of decentered ablation. J Refract Surg 18:418-29, 2002.
229. Durrie DS, Lesher MP, Cavanaugh TB: Classification of variable clinical response after
photorefractive keratectomy for myopia. J Refract Surg 11:341-7, 1995.
230. Pop M: Prompt re-treatment after photorefractive keratectomy. J Cataract Refract Surg
24:320-6, 1998.
231. Forster W, Ratkay I, Busse H: Corneal haze after mechanical debridement for
overcorrection after myopic photorefractive keratectomy. Graefes Arch Clin Exp Ophthalmol
234:278-9, 1996.
232. Kapadia MS, Genos JJ, Wilson SE: Epithelial scrape for photorefractive keratectomy
overcorrection associated with induced regression. Cornea 18:661-3, 1999.
233. Gauthier CA, Fagerholm P, Epstein D, et al: Failure of mechanical epithelial removal to
reverse persistent hyperopia after photorefractive keratectomy. J Refract Surg 12:601-6, 1996.
234. Pop M: Laser thermal keratoplasty for the treatment of photorefractive keratectomy
overcorrections: a 1-year follow-up. Ophthalmology 105:926-31, 1998.
PHOTOREFRACTIVE KERATECTOMY
-73-
235. Goggin M, Lavery F: Holmium laser thermokeratoplasty for the reversal of hyperopia after
myopic photorefractive keratectomy. Br J Ophthalmol 81:541-3, 1997.
236. Bilgihan K, Bilgihan A, Adiguzel U, et al: Keratocyte apoptosis and corneal antioxidant
enzyme activities after refractive corneal surgery. Eye 16:63-8, 2002.
237. Wilson SE: Role of apoptosis in wound healing in the cornea. Cornea 19:S7-12, 2000.
238. Moller-Pedersen T, Cavanagh HD, Petroll WM, et al: Stromal wound healing explains
refractive instability and haze development after photorefractive keratectomy: a 1-year confocal
microscopic study. Ophthalmology 107:1235-45, 2000.
239. Fantes FE, Hanna KD, Waring GO, 3rd, et al: Wound healing after excimer laser
keratomileusis (photorefractive keratectomy) in monkeys. Arch Ophthalmol 108:665-75, 1990.
240. Maldonado MJ, Arnau V, Navea A, et al: Direct objective quantification of corneal haze
after excimer laser photorefractive keratectomy for high myopia. Ophthalmology 103:1970-8,
1996.
241. Lipshitz I, Loewenstein A, Varssano D, et al: Late onset corneal haze after photorefractive
keratectomy for moderate and high myopia. Ophthalmology 104:369-73; discussion 73-4, 1997.
242. Moller-Pedersen T, Cavanagh HD, Petroll WM, et al: Corneal haze development after PRK
is regulated by volume of stromal tissue removal. Cornea 17:627-39, 1998.
243. O'Brart DP, Corbett MC, Verma S, et al: Effects of ablation diameter, depth, and edge
contour on the outcome of photorefractive keratectomy. J Refract Surg 12:50-60, 1996.
244. Corbett MC, O'Brart DP, Warburton FG, et al: Biologic and environmental risk factors for
regression after photorefractive keratectomy. Ophthalmology 103:1381-91, 1996.
PHOTOREFRACTIVE KERATECTOMY
-74-
245. Stojanovic A, Nitter TA: Correlation between ultraviolet radiation level and the incidence
of late-onset corneal haze after photorefractive keratectomy. J Cataract Refract Surg 27:404-10,
2001.
246. Nagy ZZ, Hiscott P, Seitz B, et al: Ultraviolet-B enhances corneal stromal response to 193nm excimer laser treatment. Ophthalmology 104:375-80, 1997.
247. Pineda R, Talamo JH: Late onset of haze associated with viral keratoconjunctivitis
following photorefractive keratectomy. J Refract Surg 14:147-51, 1998.
248. Campos M, Takahashi R, Tanaka H, et al: Inflammation-related scarring after
photorefractive keratectomy. Cornea 17:607-10, 1998.
249. Vinciguerra P, Torres I, Camesasca FI: Applications of confocal microscopy in refractive
surgery. J Refract Surg 18:S378-81, 2002.
250. Tabbara KF, El-Sheikh HF, Sharara NA, et al: Corneal haze among blue eyes and brown
eyes after photorefractive keratectomy. Ophthalmology 106:2210-5, 1999.
251. Aras C, Ozdamar A, Aktunc R, et al: The effects of topical steroids on refractive outcome
and corneal haze, thickness, and curvature after photorefractive keratectomy with a 6.0-mm
ablation diameter. Ophthalmic Surg Lasers 29:621-7, 1998.
252. Corbett MC, O'Brart DP, Marshall J: Do topical corticosteroids have a role following
excimer laser photorefractive keratectomy? J Refract Surg 11:380-7, 1995.
253. Bilgihan K, Ozdek S, Ozogul C, et al: Topical vitamin E and hydrocortisone acetate
treatment after photorefractive keratectomy. Eye 14 ( Pt 2):231-7, 2000.
254. Park WC, Tseng SC: Modulation of acute inflammation and keratocyte death by suturing,
blood, and amniotic membrane in PRK. Invest Ophthalmol Vis Sci 41:2906-14, 2000.
PHOTOREFRACTIVE KERATECTOMY
-75-
255. Levy Y, Zadok D, Glovinsky Y, et al: Tono-Pen versus Goldmann tonometry after excimer
laser photorefractive keratectomy. J Cataract Refract Surg 25:486-91, 1999.
256. Chatterjee A, Shah S, Bessant DA, et al: Results of excimer laser retreatment of residual
myopia after previous photorefractive keratectomy. Ophthalmology 104:1321-6, 1997.
257. Gartry DS, Larkin DF, Hill AR, et al: Retreatment for significant regression after excimer
laser photorefractive keratectomy. A prospective, randomized, masked trial. Ophthalmology
105:131-41, 1998.
258. Pop M, Aras M: Photorefractive keratectomy retreatments for regression. One-year followup. Ophthalmology 103:1979-84, 1996.
259. Schipper I, Suppelt C, Gebbers JO: Mitomycin C reduces scar formation after excimer laser
(193 nm) photorefractive keratectomy in rabbits. Eye 11 ( Pt 5):649-55, 1997.
260. Maldonado MJ: Intraoperative MMC after excimer laser surgery for myopia.
Ophthalmology 109:826; discussion -8, 2002.
261. Comaish IF, Domniz YY, Lawless MA, et al: Laser in situ keratomileusis for residual
myopia after photorefractive keratectomy. J Cataract Refract Surg 28:775-81, 2002.
262. Choi YS, Kim JY, Wee WR, et al: Effect of the application of human amniotic membrane
on rabbit corneal wound healing after excimer laser photorefractive keratectomy. Cornea 17:38995, 1998.
263. Rasheed K, Rabinowitz YS: Superficial lamellar keratectomy using an automated
microkeratome to excise corneal scarring caused by photorefractive keratectomy. J Cataract
Refract Surg 25:1184-7, 1999.
PHOTOREFRACTIVE KERATECTOMY
-76-
264. Thom SB, Myers JS, Rapuano CJ, et al: Effect of topical anti-transforming growth factorbeta on corneal stromal haze after photorefractive keratectomy in rabbits. J Cataract Refract Surg
23:1324-30, 1997.
265. Gillies MC, Garrett SK, Shina SM, et al: Topical interferon alpha 2b for corneal haze after
excimer laser photorefractive keratectomy. The Melbourne Excimer Laser Group. J Cataract
Refract Surg 22:891-900, 1996.
266. Chang JH, Kook MC, Lee JH, et al: Effects of synthetic inhibitor of metalloproteinase and
cyclosporin A on corneal haze after excimer laser photorefractive keratectomy in rabbits. Exp
Eye Res 66:389-96, 1998.
267. Winkler von Mohrenfels C, Reischl U, Lohmann CP: Corneal haze after photorefractive
keratectomy for myopia: role of collagen IV mRNA typing as a predictor of haze. J Cataract
Refract Surg 28:1446-51, 2002.
268. Azar DT, Ang RT, Lee JB, et al: Laser subepithelial keratomileusis: electron microscopy
and visual outcomes of flap photorefractive keratectomy. Curr Opin Ophthalmol 12:323-8, 2001.
269. Lee JB, Choe CM, Kim HS, et al: Comparison of TGF-beta1 in tears following laser
subepithelial keratomileusis and photorefractive keratectomy. J Refract Surg 18:130-4, 2002.
270. Rosa N, Capasso L, Romano A: A new method of calculating intraocular lens power after
photorefractive keratectomy. J Refract Surg 18:720-4, 2002.
271. Kim JH, Lee DH, Joo CK: Measuring corneal power for intraocular lens power calculation
after refractive surgery. Comparison of methods. J Cataract Refract Surg 28:1932-8, 2002.
272. Seitz B, Langenbucher A: Intraocular lens power calculation in eyes after corneal refractive
surgery. J Refract Surg 16:349-61, 2000.
PHOTOREFRACTIVE KERATECTOMY
-77-
273. Ghaith AA, Daniel J, Stulting RD, et al: Contrast sensitivity and glare disability after radial
keratotomy and photorefractive keratectomy. Arch Ophthalmol 116:12-8, 1998.
274. Niesen UM, Businger U, Schipper I: Disability glare after excimer laser photorefractive
keratectomy for myopia. J Refract Surg 12:S267-8, 1996.
275. Niesen U, Businger U, Hartmann P, et al: Glare sensitivity and visual acuity after excimer
laser photorefractive keratectomy for myopia. Br J Ophthalmol 81:136-40, 1997.
276. Hersh PS, Steinert RF, Brint SF: Photorefractive keratectomy versus laser in situ
keratomileusis: comparison of optical side effects. Summit PRK-LASIK Study Group.
Ophthalmology 107:925-33, 2000.
277. Verdon W, Bullimore M, Maloney RK: Visual performance after photorefractive
keratectomy. A prospective study. Arch Ophthalmol 114:1465-72, 1996.
278. Hodkin MJ, Lemos MM, McDonald MB, et al: Near vision contrast sensitivity after
photorefractive keratectomy. J Cataract Refract Surg 23:192-5, 1997.
279. Hadden OB, Ring CP, Morris AT, et al: Visual, refractive, and subjective outcomes after
photorefractive keratectomy for myopia of 6 to 10 diopters using the Nidek laser. J Cataract
Refract Surg 25:936-42, 1999.
280. Hjortdal JO, Olsen H, Ehlers N: Prospective randomized study of corneal aberrations 1 year
after radial keratotomy or photorefractive keratectomy. J Refract Surg 18:23-9, 2002.
281. Martinez CE, Applegate RA, Klyce SD, et al: Effect of pupillary dilation on corneal optical
aberrations after photorefractive keratectomy. Arch Ophthalmol 116:1053-62, 1998.
282. Maeda N: Evaluation of optical quality of corneas using corneal topographers. Cornea
21:S75-8, 2002.
PHOTOREFRACTIVE KERATECTOMY
-78-
283. Lee JB, Kim JS, Choe C, et al: Comparison of two procedures: photorefractive keratectomy
versus laser in situ keratomileusis for low to moderate myopia. Jpn J Ophthalmol 45:487-91,
2001.
284. Holladay JT, Dudeja DR, Chang J: Functional vision and corneal changes after laser in situ
keratomileusis determined by contrast sensitivity, glare testing, and corneal topography. J
Cataract Refract Surg 25:663-9, 1999.
285. Eggink FA, Houdijn Beekhuis W, Trokel SL, et al: Enlargement of the photorefractive
keratectomy optical zone. J Cataract Refract Surg 22:1159-64, 1996.
286. Lafond G: Treatment of halos after photorefractive keratectomy. J Refract Surg 13:83-8,
1997.
287. Chen S, Wang IJ: Effect of tear film stability on fluctuation of vision after photorefractive
keratectomy. J Refract Surg 15:668-72, 1999.
288. Matsui H, Kumano Y, Zushi I, et al: Corneal sensation after correction of myopia by
photorefractive keratectomy and laser in situ keratomileusis. J Cataract Refract Surg 27:370-3,
2001.
289. Hong JW, Kim HM: The changes of tear break up time after myopic excimer laser
photorefractive keratectomy. Korean J Ophthalmol 11:89-93, 1997.
290. Seiler T, McDonnell PJ: Excimer laser photorefractive keratectomy. Surv Ophthalmol
40:89-118, 1995.
291. Vetrugno M, Maino A, Quaranta GM, et al: A randomized, comparative open-label study
on the efficacy of latanoprost and timolol in steroid induced ocular hypertension after
photorefractive keratectomy. Eur J Ophthalmol 10:205-11, 2000.
PHOTOREFRACTIVE KERATECTOMY
-79-
292. Morales J, Good D: Permanent glaucomatous visual loss after photorefractive keratectomy.
J Cataract Refract Surg 24:715-8, 1998.
293. Abbasoglu OE, Bowman RW, Cavanagh HD, et al: Reliability of intraocular pressure
measurements after myopic excimer photorefractive keratectomy. Ophthalmology 105:2193-6,
1998.
294. Faucher A, Gregoire J, Blondeau P: Accuracy of Goldmann tonometry after refractive
surgery. J Cataract Refract Surg 23:832-8, 1997.
295. Rosa N, Cennamo G, Breve MA, et al: Goldmann applanation tonometry after myopic
photorefractive keratectomy. Acta Ophthalmol Scand 76:550-4, 1998.
296. Schipper I, Senn P, Oyo-Szerenyi K, et al: Central and peripheral pressure measurements
with the Goldmann tonometer and Tono-Pen after photorefractive keratectomy for myopia. J
Cataract Refract Surg 26:929-33, 2000.
297. Loewenstein A, Goldstein M, Lazar M: Retinal pathology occurring after excimer laser
surgery or phakic intraocular lens implantation: evaluation of possible relationship. Surv
Ophthalmol 47:125-35, 2002.
298. Geerling G, Neppert B, Wirbelauer C, et al: Relative mydriasis after photorefractive
keratectomy. J Refract Surg 16:69-74, 2000.
PHOTOREFRACTIVE KERATECTOMY
-80-
PHOTOREFRACTIVE KERATECTOMY
-81-
Myopic PRK
Laser & Surgeon
VISX STAR
Jackson et al
Sphere range
Cyl range (D)
-1.00 to -6.00
+0.25 to +5.00
Mean
Pre-op (D)
-3.61
-7.47
VISX STAR S2
Stevens et al
Meditec MEL70 G
-6.12 to -12.00
+0.25 to +4.00
-0.50 to -5.90
0 to -3.50
-1.50 to -6.00
-6.10 to -9.00
-
31
12 months
25.8
-
74.2
61
83.8
-
3.2
-
-9.10 to -14.00
-
0
11.1
22.2
44.4
66.6
-
22.2
-
-1.00 to -10.50
0 to -3.00
-1.00 to -15.50
0 to -4.00
-1.50 to -6.25
-
9
12 months
36
12 months
54
12 months
93
12 months
414
12 months
175
12 months
83.3
100
100
-
-
-
-
-
78
-
98
77
98
14.9
0
7.4
70
-
99
75
93
-
1
-
72
-
98.1
76.4
94.4
-
1.8
-
97.4
73.9
95
-
2.5
-
-3.50
-
# of eyes
Follow-up
253
24 months
%
20/20
74
%
20/25
86
%
%
20/40  0.5 D
96
87
%
 1.00 D
99
1 line loss
BSCVA %
29
2 line loss
BSCVA %
1
%
Re-operation
3.6
97
24 months
198
12 months
90
12 months
55
78
96
73
93
31
3
5.6
82
-
97
58
86
8.6
0.5
5.6
77.7
-
95.5
73.3
98.8
-
1.1
-
Nagy
Nidek EC-5000
Amoils
LaserSight LSX
Stojanovic et al
Autonomous
Pallikaris et al
Autonomous
McDonald et al
-1.00 to -5.99
PRK
-1.00 to -5.99
PARK
-4.72
-
61.7
PHOTOREFRACTIVE KERATECTOMY
-82-
Hyperopic PRK
Laser & Surgeon
VISX STAR
Jackson et al
VISX STAR
Stevens et al
Chiron Technolas
217-C
Pacella et al
Summit
O’Brart et al
Aesculap-Meditec
MEL60
Nagy et al
Sphere range
Cyl range (D)
+1 to +4
+4 to +6
Mean
Pre-op (D)
+2.36
+5.17
+1 to +4
+0.25 to +3.00
+1.75 to +5.00
0 to -2.50
+1.00 to +7.75
0 to +0.50
+2.58
+1.75 to +7.50
+4.54
 +3.50
+2.88
 +3.75
+5.64
+2.90
+4.82
# of eyes
Follow-up
82
24 months
19
24 months
14
24 months
27
12 months
28
18 months
%
20/20
65
%
20/25
84
%
%
20/40  0.5 D
98
88
%
 1.00 D
95
21
42
84
63
84
50
64
100
93
70
-
93
46
-
43
24 months
482
12 months
318
12 months
62
12 months
-
1 line loss
BSCVA %
26
2 line loss
BSCVA %
0
%
Re-operation
3.5
11
5
6.8
100
36
0
0
65
88
30
0
12
100
57
92.8
-
0
-
-
-
-
68
-
-
-
75.7
-
88.4
74.4
84.8
15
2.1
-
34.2
-
47.5
22.3
46.8
32.1
19.1
88
-
96
82
93
3.2
1.6
-
Aesculap-Meditec
MEL70 G-Scan
+1.00 to +3.50
(cyl <1.00)
+2.48
Nagy et al
+1.00 to +3.50
(cyl  1.00)
+1.81
44
12 months
77
-
81
68
81
4.6
9
-
+3.75 to +7.50
(cyl < 1.00)
+4.88
56
12 months
78
-
85
76
89
7.1
5.4
-
> +3.50
(cyl  1.00)
+3.56
(+2.64)
38
12 months
60
-
71
42
63
7.9
15.8
-
PHOTOREFRACTIVE KERATECTOMY
-83-
Wavefront
Laser & Surgeon
Asclepion-Meditec
MEL70 G-Scan
Sphere range
Cyl range (D)
-1.50 to -6.50
Mean
Pre-op (D)
-4.02
# of eyes
Follow-up
150
6 months
%
20/20
80
%
20/30
100
+2.90
40
6 months
70
-
%
%
20/40  0.5 D
100
98
%
 1.00 D
100
1 line loss
BSCVA %
0.7
2 line loss
BSCVA %
0
%
Re-operation
-
0 to -2.50
Nagy et al
Asclepion-Meditec
MEL70 G-Scan
Nagy et al
+1.50 to +4.00
90
85
100
17.5
12.5
-
PHOTOREFRACTIVE KERATECTOMY
-84-
PRK vs LASIK for Myopia
Laser & Surgeon
Nidek EC-5000
Automatic
Microkeratome
(Chiron ophthalmic)
Sphere range
Cyl range (D)
PRK
-1.00 to -3.00
Mean
Pre-op (D)
# of eyes
Follow-up
%
20/20
%
20/25
-3.28
75
12 months
53
-
91.5
69
87
36
-
82
63
72
-
95
58.5
-
-3.25 to -6.00
%
%
20/40  0.5 D
%
 1.00 D
1 line loss
BSCVA %
2 line loss
BSCVA %
%
Re-operation
6
8
-
87
7
7.5
-
68
86
-
-
-
97.5
70
88
-
-
-
Fernadez et al
LASIK
-1.00 to -3.00
-3.86
133
12 months
-3.25 to -6.00
PRK
-1.50 to -6.00
-4.54
45
6 months
77.8
-
-
62.2
-
-
-
-
LASIK
-1.50 to -6.00
-4.82
45
6 months
62.2
-
-
53.4
-
-
-
-
Nidek EC-5000
Chiron microkeratome
PRK
-0.50 to -3.00
-2.10
209
6 months
69
84
98
87
96
5.1
0
3.6
Tole et al
-3.10 to -6.00
-4.10
61
78
90
78
95
6.3
1.8
1.6
LASIK
-0.50 to -3.00
-2.26
89
96
100
94
100
2.7
0
-
-3.10 to -6.00
-4.61
77
84
95
73
90
8.8
1
-
?
Lee et al
139
6 months
PHOTOREFRACTIVE KERATECTOMY
Chiron Keracor 116
PRK
-1.25 to -6.00
-
307
12 months
72
-
-
61
83
-
-
-
LASIK
-1.25 to -6.00
-
103
12 months
83
-
-
71
89
-
-
-
-5.55
82
12 months
85.4
-
97.6
82.9
93.9
1.2
0
9.3
-5.55
77
12 months
83.1
-
100
77.9
98.7
1.3
0
2.8
-3.23
24
12 months
62.5
-
100
83.3
100
4
0
-
-3.44
24
12 months
79.2
-
100
87.5
100
4
0
-
-4.70
30
12 months
36
-
96
67
87
-
6
10
-4.80
30
12 months
61
-
100
73
90
-
6
-
Wang et al
Nidek EC-5000
Pop et al
Multiple lasers
El-Danasoury et al
Summit Omnimed I
Chiron Automated
-85-
PRK
-1.00 to -9.50
0 to -3.75
LASIK
-1.00 to -9.50
0 to -4.50
PRK
-2.00 to -5.50
0 to -2.25
LASIK
-2.00 to -5.50
0 to -1.50
PRK
-2.30 to -8.10
0 to +1.50
El-Maghraby et al
LASIK
-2.30 to -8.10
0 to +1.75
PHOTOREFRACTIVE KERATECTOMY
-86-
PRK vs LASIK for Hyperopia
Laser & Surgeon
VISX STAR S2
Sphere range
Cyl range (D)
PRK
Mean
Pre-op (D)
+2.25
# of eyes
Follow-up
22
12 months
%
20/20
47.1
%
20/25
-
+1.81
26
12 months
54.5
-
%
%
20/40  0.5 D
100
83.3
%
 1.00 D
-
1 line loss
BSCVA %
-
2 line loss
BSCVA %
9
%
Re-operation
-
El-Agha et al
LASIK
100
61.5
-
-
7
-
PHOTOREFRACTIVE KERATECTOMY
Peripheral Haze post Hyperopic PRK
Hyperopic Nodule
-87-
PHOTOREFRACTIVE KERATECTOMY
-88-
Pre-op and Post-op MMC application
Pre-op
PRK post Refractive Surgery (LASIK)
showing remaining striae after smoothing
Post-op