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PHOTOREFRACTIVE KERATECTOMY -1- 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 PHOTOREFRACTIVE KERATECTOMY -2- 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 -3- PHOTOREFRACTIVE KERATECTOMY -4- 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 PHOTOREFRACTIVE KERATECTOMY -5- 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. PHOTOREFRACTIVE KERATECTOMY -6- 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 PHOTOREFRACTIVE KERATECTOMY -7- 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 PHOTOREFRACTIVE KERATECTOMY -8- 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 PHOTOREFRACTIVE KERATECTOMY -9- 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 PHOTOREFRACTIVE KERATECTOMY -10- 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 PHOTOREFRACTIVE KERATECTOMY -11- 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. PHOTOREFRACTIVE KERATECTOMY -12- 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 PHOTOREFRACTIVE KERATECTOMY -13- 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 PHOTOREFRACTIVE KERATECTOMY -14- 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 PHOTOREFRACTIVE KERATECTOMY -15- 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 PHOTOREFRACTIVE KERATECTOMY -16- 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. PHOTOREFRACTIVE KERATECTOMY -17- 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 PHOTOREFRACTIVE KERATECTOMY -18- 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 PHOTOREFRACTIVE KERATECTOMY -19- 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 -20- 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 -21- 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 PHOTOREFRACTIVE KERATECTOMY -22- 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 PHOTOREFRACTIVE KERATECTOMY -23- 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 PHOTOREFRACTIVE KERATECTOMY -24- 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 PHOTOREFRACTIVE KERATECTOMY -25- 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%, PHOTOREFRACTIVE KERATECTOMY -27- 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 PHOTOREFRACTIVE KERATECTOMY -28- 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 PHOTOREFRACTIVE KERATECTOMY -29- 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 -30- 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 -31- 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 -32- 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 PHOTOREFRACTIVE KERATECTOMY -33- 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 -34- 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 -36- 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 -37- 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 PHOTOREFRACTIVE KERATECTOMY -38- 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 -39- 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 -40- 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 -41- 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 -42- 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 -43- 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 -44- 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 -45- 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 PHOTOREFRACTIVE KERATECTOMY -46- 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 -47- 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 -48- 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 -49- 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. 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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. 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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. 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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