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Chapter IV
Irregular Astigmatism
Management
Lucero Pedro-Aguilar, Alejandro Navas,
and Enrique Graue-Hernandez
Department of Cornea and Refractive Surgery,
Institute of Ophthalmology “Conde de Valenciana”,
México City, Distrito Federal,
Mexico
Abstract
The term irregular astigmatism is often applied to a distorted corneal
topography, whereby two major meridians may not even be easily
identified; in simple terms, it can be defined as an “unrefractable” eye.
As a consequence, visual function can be severely affected.
Conventional treatments are often insufficient and/or unsatisfactory.
Spectacles may lead to intolerable aniseikonia, and contact lens fitting is
usually challenging. Conservative approaches do not always yield
maximal visual results leaving the patient unsatisfied. Numerous surgical
procedures have been developed trying to improve astigmatism, including
relaxing incisions, thermal and conductive keratoplasty, wedge

E-mail: [email protected].
116
L. Pedro-Aguilar, A. Navas, and E. Graue-Hernandez
resections, photorefractive procedures, phakic and intraocular lens
implantation, among others.
Each procedure needs to be evaluated within the patient’s context
and chosen accordingly. The purpose of this chapter is to review the most
commonly used techniques for the correction of irregular astigmatism.
Introduction
The term comes from the Greek α- (a-) meaning "without" and στίγμα
(stigma), "a mark/spot”.
An optical system with astigmatism is one where rays that propagate in
two perpendicular planes have different foci. If an optical system with
astigmatism is used to form an image of a cross, the vertical and horizontal
lines will be in sharp focus at two different distances. The cornea of the
normal eye has a uniform curvature, with resulting equal refracting power over
its entire surface.
In the eye, astigmatism results from the refraction of light through an
aspheric or toric cornea or an abnormal crystalline lens. Light rays refracted by
this cornea are not focused to a single point, and retinal images from objects
both distant and near are blurred and may appear broadened or elongated. The
main differences between regular and irregular astigmatism are summarized in
Table 1.
Table 1. Regular versus irregular astigmatism
Regular astigmatism is correctable using a cylindrical spectacle lens.
Irregular astigmatism: refraction in different meridians conforms to no
geometrical plan and the refracted rays have no planes of symmetry, not
correctable by a sphero-cylindrical lens, “unrefractable”.
The further distinction of irregular astigmatism includes regularly or
irregularly irregular astigmatism and relates to the presence of pattern
recognition on corneal topography (see Figure 1).
Irregular Astigmatism Management
117
Astigmatism can be originated from any of the surfaces of the eye, thus it
can corneal (or keratometric), lenticular, and retinal, but in most instances it is
originated from the anterior surface of the cornea.
Astigmatism (more than 0.5 diopters) is a commonly encountered
refractive error, accounting for about 13 percent of the refractive errors of the
human eye. It is commonly encountered with prevalence rates up to 30% or
higher depending on the age or ethnic groups. Studies using videokeratoscopy
have found that approximately 40% of normal corneas with a toric refractive
error as possessing primary irregular astigmatism. [1]
In regular astigmatism, which is the more common form, the cornea
would resemble a football or rugby ball with the two principal meridians
having different curvature radii. The two principal meridians separated by 90
degrees; the best-corrected visual acuity (BCVA) is at least 20/20 and, in the
case of corneal astigmatism, corneal topography displays a symmetrical bowtie pattern. The meridian of greatest and least power, the so-called principal
meridians, are always located at meridian 90 degrees apart.
Irregular astigmatism (IA) is defined as astigmatism where the principal
meridians are not 90 degrees apart. Sir William S. Duke-Elder defined IA as
follows: “refraction in different meridians conforms to no geometrical plan
and the refracted rays have no planes of symmetry”. [2] It may be defined as
an astigmatic state not correctable by a sphero-cylindrical lens thus the term
“unrefractable”. Irregular astigmatism can be either “regularly irregular” or
“irregularly irregular”. In regularly irregular astigmatism, two principal
meridians are either asymmetrical or not 90 degrees apart and characterized by
either unequal slopes along a single meridian which generates the classical
“asymmetric bow-tie” patterns corneal topography maps or hemimeridians of
equal slope but not aligned with each other (the “angled bow-tie” or nonorthogonal astigmatism). Irregularly irregular astigmatism does not have
identifiable principal meridians (Figure 1). It is commonly present in diseases
that alter corneal shape such as keratoconus and other corneal ecstasies,
anterior stromal scarring from various corneal conditions basement membrane
and stromal dystrophies, post-traumatic corneal distortion and iatrogenic
procedures such as lamellar and penetrating keratoplasty, corneal based
refractive procedures (e.g. radial keratotomy). In any case, the magnitude and
the axis of astigmatism vary from point to point across the entrance pupil and
corneal topography cannot demonstrate a recognizable pattern.
Main causes of corneal irregular astigmatism are diverse, including
keratoconus (KC), pellucid degeneration (PD), keratoglobus (KG), corneal
surgery (keratoplasty, photorefractive keratectomy (PRK), laser in situ
L. Pedro-Aguilar, A. Navas, and E. Graue-Hernandez
118
60°
90°
0°
60°
12
9mm
5
7
+2
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+4
N
-4
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°
-2
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270°
0
+2
0°
15
5
3
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50.3
24
7
-2
T
-4
+4
T
-4
3
52.9 53.0
5
7
51.8
51.3
50.6
0°
3
-4
0
0°
54.3
50.8
24
0°
-2
33
52.1
3
0°
-2
51.2
33
15
0°
0°
55.1
41.0
48.7 45.4
21
33
3
0°
°
00
270°
0
5
21
0°
24
-2
7
54.9
49.2
42.4
45.3
51.8
0°
0
42.5
°
7
+2
30
5
48.2
0°
46.4
42.3
180°
42.8 42.2
49.1
-4
+4
0°
43.2 3
49.2
T
60°
°
7
5 42.9 3 42.8
180°
42.6
21
-4
+2
90°
0°
12
46.2
0°
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-2
9mm
30
°
47.6
48.0
0
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46.2
30
+2
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9mm
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+4
15
keratomileusis (LASIK), radial keratotomy (RK), arcuate keratotomy), and
scars secondary to infections or burns. Lenticonus may cause astigmatism due
to abnormality of the lens. Clinical manifestations of irregular astigmatism
could be blurred vision, glare, visual disturbances and ghost images. Through
retinoscopy we can observe typical patterns including “scissoring reflex” and
jumbled or uninterpretable reflexes. By keratometry, irregular mires are
presented. The most common topographic values assessed in irregular
astigmatism shown in Table 2.
0°
30
270°
0
+2
N
+4
Figure 1. Topographic patterns of irregular astigmatism: Regularly irregular: A) the
“asymmetric bow-tie” and B) the “angled bow-tie” or nonorthogonal astigmatism. C)
Irregularly irregular, topography cannot demonstrate a recognizable pattern.
Table 2. Most common values assessed in corneal topography
(statistical indices)
Simulated keratometry (SimK) provides the power and location of the steepest and
flattest meridian analogous to values obtained by a keratometer.
Surface asymmetry index (SAI) is a centrally weighted summation of differences in
corneal power between corresponding points 180°apart on 128 equally spaced
meridians crosing all the mires. Increases as the corneal shape becomes more
asymmetrical.
The surface regularity index (SRI) reflects local power fluctuations along 256
equally spaced meridians. Correlates with localized surface regularity.
Shape Factor (SF) is the measurement of corneal asphericity.
Corneal Irregularity Measurement (CIM) represents the irregularity of the corneal
surface. Higher the value of CIM predicts more irregularity.
Mean Toric Keratometry (MTK) use elevation data to compare the toric reference to
the actual cornea.
Irregular Astigmatism Management
119
In the past decades, the available treatment options have increased
dramatically, however, the debate continues regarding to the systematic
approach necessary to achieve an optimal surface of the eye. Treatment of
irregular astigmatism is directed toward the rearrange the orientation of the
astigmatism by concurrently regularizing and rotating the axes of astigmatism
in both hemidivisions so that they are aligned with each other and closer to the
vertical or horizontal meridian with a more favorable position. [3] Treatment
options have increased in recent years, particularly with the use of combined
procedures, however, the discussion continues regarding to the systematic
approach necessary to achieve an optimal surface for the eye. Some examples
can be seen in Figure 2.
Contact Lenses
In patients with irregular astigmatism, optical correction with spectacles
has limited success, as this may not provide satisfactory visual function to the
patient. Therefore, in attempt to improve the anterior refracting surface,
contact lens fitting is and should be the first line of treatment to visually
rehabilitate this kind of patients. They are also useful diagnostic tools to assess
the visual potential and BCVA and thus may assist in the decision of the other
treatment modalities. Corneal topography can and should be obtained in
patients with irregular astigmatism. The data from the anterior corneal surface
can help in fitting the cornea with the contact lens.
Currently many lens designs are available. Soft multicurve lens sets
(progressively flatter) with parameters designed for keratoconic corneas are
often used as they provide comfort and relatively good visual outcomes,
however, when the irregularity is significant a rigid anterior surface is needed
to correct the refractive error moderate astigmatism. Piggyback fit is a method
fitting a rigid gas permeable lens over a soft lens, to be tried in the
management of patients with difficulties in adapting. [4, 5]
Fluid-ventilated and gas-permeable scleral lens have proved to be a
valuable therapeutic tool in the management of irregular astigmatism resulting
from several corneal and ocular surface conditions, who had not responded
satisfactorily to conventional methods of optical correction and is an
alternative to penetrating keratoplasty for patients with corneal ectasia who are
contact lens intolerant. [6-8]
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L. Pedro-Aguilar, A. Navas, and E. Graue-Hernandez
The Prosthetic Replacement of Ocular Surface Ecosystem (PROSE), is a
gas permeable scleral lens that vaults the cornea and submerges the entire
corneal surface in a pool of oxygenated tears, serve as a protective measure in
thin and protruding corneas in addition to providing improved vision. [9, 10]
Figure 2. Examples of surgical and nonsurgical therapeutic options available for the
management of irregular astigmatism. (A) Fluorogram of contact lens fitting in a patient
with keratoconus, (B) Compressive sutures and conductive keratoplasty in a case of high
astigmatism post-Radial Keratotomy, (C) Arcuate keratotomy for irregular astigmatism
after cataract surgery, (D) Intrastromal ring segments, (E) Pseudophakic toric intraocular
lens in keratoconus, (F) Combination of intrastromal ring segments and toric implantable
collamer lenses, (G) Wedge resection in a patient with history of pellucid marginal
degeneration, and (H) Lamellar keratoplasty in a case of ectasia.
Irregular Astigmatism Management
121
Previous studies have reported improved visual functions and decreased
higher-order aberrations in subjects with irregular cornea fitted with PROSE
device. [11] Hence, PROSE fitting in corneas with steeper curvature such as
ectatic disorders has to be carefully monitored while dispensing the PROSE
device because reduced vault over a period may cause trauma to the cornea
leading to corneal scarring. [12]
Contact lenses fitting is possible in patients with mild, moderate and
severe irregular astigmatism and even in cases with postoperative astigmatism,
provides good comfort, improves visual acuity, and therefore, may postpone
the need for surgery. [13]
Photorefractive Procedures
The goal of modern refractive surgery is to achieve emmetropia and to
decrease higher order aberrations. Even a mild residual spherical or cylindrical
errors may be considered failure, especially when compared to older refractive
surgery procedures.
Ablation-related complications following refractive surgery such as
decentered ablations, small optical zones, or irregular ablations can produce
irregularities in the optical system. [14] Topographically customized ablations
have been suggested for the treatment of such corneal irregularities. It may be
necessary to perform a treatment that does not change the manifest refraction
but that improves vision by “regularizing” the corneal surface technique
named topography smoothing.
Published reports to date have found topography-guided treatment in
general to be effective, successfully improving uncorrected visual acuity
(UCVA), BCVA in the setting of previous refractive surgery or even
improving the corneal shape after corneal transplantation. However,
customized ablations including both, topography and wavefront-guided
treatments have reported lower effectiveness in reducing irregular astigmatism
secondary to scars, postoperative cataract wound gape (incisional), and postkeratotomy irregularities. [15]
It is important to consider that most cornea based refractive procedures
may increase higher order aberrations while attempting to neutralize high
cylinder. Also, the unpredictability of the refractive outcome appears to be the
major disadvantage of current laser systems. Complications associated with
photoablative surgery are also an important drawback. For example,
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L. Pedro-Aguilar, A. Navas, and E. Graue-Hernandez
photorefractive keratectomy performed over corneal grafts may cause
significant haze and associated irregularities and regression; [16] on the other
hand, performing LASIK in such cases increases the chance of wound
dehiscence as a result of high vacuum pressure with the microkeratome. Also
flap-related complications are of concern, as these corneas tend to be either too
steep or too flat to be safely severed with a microkeratome blade or even with
femtosecond lasers.
Laser based surgery in keratoconus is debatable, specifically increasing
the risk of disease progression. However, recent evidence suggest that limited
ablation performed to improve corneal shape may help in stabilizing the
cornea and improve visual function thus giving the ablation. Crosslinking may
be used in conjunction to improve corneal biomechanics. [17, 18]
Corneal Tuck
The basis of the corneal tuck is suture-induced tissue compression. In this
procedure an incision is made across the flat meridian, concentric with the
limbus and down to Descemet´s membrane and then it is closed with
interrupted sutures. However, sutures eventually degrade and loose their
tensile strength thereby loosing the effect. Busin and colleagues performed a
excision of a part of the diseased tissue and reinforcement of the residual
ectatic periphery by means of corneal tuck for the treatment of extremely
advanced pellucid marginal corneal degeneration, resulting in a marked
reduction of the preoperative irregular astigmatism, thus allowing a BCVA of
20/50 or better. [19] Due to the unpredictability of this technique, it is reserved
almost exclusively for those patients in which conventional corneal
transplantation has to extend from limbus to limbus to excise the ectatic cornea
thus dramatically increasing the risk of rejection. Improvement in contact lens
technology and the introduction of intracorneal rings into the refractive
armamentarium have limited the use of corneal tuck, however it may still be of
use in patients with limited peripheral thinning (Terrien´s or pellucid marginal
degeneration) or in graft patients with ectatic peripheral host cornea. Although
theoretically simple, this technique may be extremely difficult to perform and
may have deleterious effects on visual function.
Irregular Astigmatism Management
123
Conductive Keratoplasty
Conductive keratoplasty (CK) uses a 450-micron-sized probe that releases
a controlled high-frequency radio wave (350 kHz) to predetermined locations
on the peripheral cornea. This causes shrinkage of the collagen fibers with a
resultant change in corneal curvature. [20] Several studies have shown CK to
be a safe and effective treatment of low-to-moderate irregular hyperopic
astigmatism after refractive surgery. These studies reported that most of the
correction loss occurred during the first postoperative year, with a minimal
regression. [21-23] The few published reports of complications include iron
ring lines, keratitis and corneal perforation. [24-26] Conductive keratoplasty
may be a minimally invasive option for patients with irregular hyperopic
astigmatism, however larger series and longer follow-up periods may be
needed to assess this especific indications.
Semilunar and Wedge Resections
Both were used in last decades to correct astigmatism. Lans used deep,
double wedge resections, unsutured or sutured, to modified the corneal
toricity. In addition, Wiener used elliptical peeling and crescentic resections
adjacent to the limbus in patients with keratoconus. [27] The semilunar
resection developed by Professor Jose I. Barraquer consisting of a crescentric
piece of tissue with perpendicular walls is resected in full thickness from the
limbal area, often under a conjunctival flap. [28]
Corneal wedge resections were developed by Richard Troutman. [29] In
this procedure, a very deep crescentric tissue with various central widths of up
to 1.5 mm is resected from the area of the graft across the flat corneal
meridian; the length of the resection is approximately 70 to 90º in arc length.
The two curvilinear incisions can be made sequentially with a single blade or
simultaneously with a doubled-bladed diamond knife and the wound is
sutured. In general, the amount of correction obtained is related to the central
width of the resection, although the results correlate only mildly and may be
somewhat unpredictable. [30, 31]
Crescentic lamellar keratoplasty acts tectonically, simply reinforcing the
ectatic area of the cornea, usually with poor visual and refractive outcomes.
Instead, wedge resection offers the advantage of removing the ectatic tissue
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L. Pedro-Aguilar, A. Navas, and E. Graue-Hernandez
and preserving the recipient cornea, restructured in a shape compatible with
useful vision.
As in corneal tuck, these procedures are often limited to peripheral
thinning disorders. Careful case selection is key to success. During the
procedure is often recommended to tighten the sutures so that, the astigmatism
is inverted, to allow for suture removal at the slit-lamp and as well generates a
reserve for regression as the suture tensile strength decreases.
Radial Keratotomy
Keratotomies of various types have been proposed for the correction of
astigmatism, these include radial, arcuate, transversal and combinations of all.
However it is generally accepted that all these techniques are limited to the
correction of low degrees of astigmatism.
No standard method has been established to perform arcuate keratotomy
(AK), and even with mechanical devices, such as the Hanna keratome, results
showed a low reliability. [32-34] Femtosecond laser (FSL) is a step forward in
the evolution of corneal surgery, aiming to reduce the deviation of depth and
size of corneal incisions to a minimum. The use of this technology has several
advantages, including improved accuracy and safety, associated with enhanced
reproducibility. [35-37] The Alpins method for vector analysis that using
refractive, keratometric, and astigmatism data has shown a decrease of 2.56 D
for laser AK and 4 D for mechanized AK. [33] Previous reports have variable
results, Hjortdal and Ehlers found a 54% reduction in refractive cylinder with
paired AKs in 21 post-keratoplasty eyes, and Hovding found a 49% reduction
with transverse keratotomies in 12 post-keratoplasty eyes. [38, 39]
Limbal relaxing incisions (LRIs) are used to treat low-to-moderate degrees
of astigmatism and are either performed at the time of cataract extraction or as
an independent procedure. Planning incisions precisely on axis is not critical
since it produces lesser effect than corneal incisions and significant over
corrections are rare.
The forgiving nature of LRIs is due to placement and length of the
incision. LRI produces lesser effect than corneal relaxing incision, thus precise
alignment of the axis is not as critical. LRIs can be useful in patients with
astigmatism outside the range of the toric lenses; in patients with astigmatic
eyes that cannot support a toric lens in the capsular bag; in patients with
asymmetric and nonorthogonal astigmatic axes, in which each incision can be
Irregular Astigmatism Management
125
placed on each arm of the steep axis. [40, 41] Carvalho et al. have shown that
LRI´s performed during phacoemulsification surgery are a safe, effective, and
stable procedure to reduce pre-existing corneal astigmatism. [42]
Author like Belmont et al. described 48.9% of reduction in postoperative
refractive astigmatism using wedge resection and relaxing incisions after PK.
[43]
Complications associated with these techniques include transient glare,
fluctuating visual acuity, undercorrection, overcorrection, scarring,
perforation, wound leak, and infection. [44]
Intrastromal Corneal Ring Segments
The concept of intrastromal corneal rings was introduced in 1978, were
first implanted in humans in 1991 to correct myopia. [45, 46]
ICR´s are space ocupying elements that inserted in the corneal stroma help
flatten the steep meridians while the coupling effect steepens the flatter.
Multiple arc lengths and thickness and optical zone diameters are available
commercially, however none have proven to be superior. The aim of ICRS
surgery is to induce a geometric change in the central corneal curvature, thus,
reducing the refractive error and the mean keratometry and improving the
visual acuity. An improvement in the optical quality of the cornea and a
reduction in optical aberrations can also be expected.
ICRS have been used for other indications other than primary KC. The
results of implant insertion on corneal topography, refraction, and visual acuity
in post-LASIK ectasia were similar to those reported for KC. The evidence for
this indication, however, only involved case reports and small case series.
ICRS were also successfully used to treat recurrent KC after corneal transplant
but this was based on only a small case reports. [47]
The channels for the insertion of the segments can be created
mechanically or with a femtosecond laser. The most common complications
associated with the mechanical dissection are: epithelial defects on the
insertion site, anterior and posterior perforations, inadequate depth placement
of the ring, extrusion, infectious keratitis, stromal thinning, stromal edema,
intraepithelial growth in the tunnel, corneal melting and tunnel vascularization.
The use of the femtosecond laser reduces the risk of complications in the
creation of the tunnels, however it has been reported that the main
126
L. Pedro-Aguilar, A. Navas, and E. Graue-Hernandez
complication with this technique is the incomplete formation of the tunnel (up
to 2.7% of the cases), among those cited previously for the manual technique.
Improvements in visual acuity and refractive error following insertion of
ICRS, however, were not noted for all patients. Although improvements were
not found to vary across age groups there were differences across stages of
disease. Several reports suggested that improvements in visual acuity and
refractive outcomes may not be as large or predictable in more advanced
stages of KC. [48]
Corneal collagen cross-linking (CXL) used in conjunction with ICRS
implantation has shown promising results in improving visual acuity and
slowing the progression of keratoconus. ICRS, although they flatten the
corneal surface, do not prevent the progression of keratoconus. Several longterm follow up studies have demonstrated that the beneficial effects of ICRS
regress over time. [49] CXL has the potential to slow or stop the progression
of keratoconus by strengthening and stabilizing the collagen in the corneal
stroma via rearrangement of corneal lamellae. The combination of the two
treatments is potentially synergistic. [50]
Toric Intraocular Lenses
Corneal astigmatism is usually treated at the corneal plane. Nevertheless,
some cases cannot be treated in such a manner, because for example large
amount of cylinder or ocular surface disease. In such cases, correction of
astigmatism with an intraocular lens may offer better and more stable results.
The selection criteria, intraocular lens (IOLs) calculation issues, and surgical
techniques for IOL implantation, clinical outcomes including uncorrected
visual acuity, residual refractive astigmatism, and spectacle dependency,
should be discussed with patients to generate realistic expectations.
Emmetropia was not an achievable target for most of the study eyes because of
low toric cylindrical powers, even with the expanded range.
The safety, efficacy and predictability indexes of all of the studies have
demonstrated to be very suitable in cases in which the patient has been
selected appropriately, in particular, adequately identifying progression and
refractability. Multiple studies with different brands of toric lenses have been
employed for the treatment of irregular astigmatism and for the management
of residual ametropia following penetrating keratoplasty. [51-55] This
technique allows for the appropriate ametropia correction in cases of non-
Irregular Astigmatism Management
127
progressive keratoconus, furthermore may be used with additional treatments,
for example, combined intraocular lens treatment (piggyback) for the
management of residual ametropia in post-keratoplasty patients.
The misalignment is the most frequent complication of toric IOLs,
however corneal decompensation; graft failure rates, retinal detachment and
macular edema have been associated with cataract surgery. Another drawback
of this technique is the potential for miscalculation of the lens power. Irregular
corneas frequently display not only asymmetrical or irregular cylinder power,
but the relationship between the anterior and posterior corneal curvature are
lost, thus the IOL formulas are not as precise when compared to unoperated
eyes. This raises the potential for a refractive surprise.
However, we have shown that toric IOLs implantation in patients with
irregular keratoconus (regularly irregular) may be an effective therapeutic
option in the optical rehabilitation of patients with stable and nonprogressive
keratoconus. [52, 56, 57]
Phakic IOLs are based on the patient’s spectacle refraction. The benefits
of phakic IOLs are reversibility and preservation of accommodation. The
selection criteria for the placement of a phakic lens include a stable refraction
or ectatic disorder, correctable refractive error due to the types of phakic
lenses available, endothelial count greater than 2500 cells/mm2, anterior
chamber depth > 2.8 mm and absence of uveal pathology or glaucoma.
[17, 58]
Patients undergoing posterior chamber phakic IOLs should be counseled
about the risk of cataract and the potential need for further surgical
intervention. [59] Furthermore there may be more long-term risks unique to
patients with phakic IOLs, such as continued endothelial cell loss and cataract
formation.
Bioptics
The bioptics approach is a sequential combination of different refractive
techniques to treat large and complex refractive errors. The use of this
approach in the management of refractive errors secondary to keratectasia has
been reported previously, combining phakic intraocular lens implantation with
intracorneal ring segments or refractive lens exchange combined with
keratorefractive surgery.
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L. Pedro-Aguilar, A. Navas, and E. Graue-Hernandez
Performed bioptics procedures with sequential intracorneal ring and IOL
implantation allow us to improve the corneal characteristics and visual acuity
in eyes with corneal ectasia.
Cakir et al. performed sequential intracorneal Keraring segments and
Artisan/Artiflex iris-fixated IOL implantation yielded satisfactory visual and
refractive results and could be considered as an alternative to penetrating
keratoplasty in patients with different corneal ectatic conditions with high
myopic refractive errors, they suggest performing intracorneal ring
implantation first, which corrects the refractive myopia and astigmatism in a
limited manner and are often unpredictable in different stages of the ectatic
disease, it is important to wait until the corneal edema disappears and the
refractive error stabilizes before proceeding to the next step of the bioptics
procedure. [60]
Few studies have reported the combined effect of the three procedures; AlTuwairqi et al. reported placement of ICRS followed by combined
topography-guided PRK/CXL appears to be safe and an effective option in
select patients with keratoconus. [61]
Keratoplasty
If contact lens fitting is unsuccessful or the above treatments cannot be
performed a penetrating or lamellar keratoplasty may be performed to restore
corneal anatomy.
The penetrating transplant was considered the treatment of choice for
keratoconus for many decades; [62] nevertheless, one of the principal
disadvantages has to do with the risk of immunological rejection which can
occur in up to 20% of the patients with good prognosis, such as the case of
keratoconus. [63, 64]
This technique continues to be the treatment of choice when there are
endothelial scars (secondary scars to hydrops) or low receptor endothelial cell
count. The advantages of the lamellar techniques over the penetrating
keratoplasty are that these techniques have lower recuperative time periods,
earlier management of astigmatism and sutures and lower incidence of postoperative glaucoma and graft rejection. [65-67] The most important limitations
of the lamelar techniques continue to be the irregular borders of the corneal
surface dissection that are obtained through manual technique. [68, 69]
Irregular Astigmatism Management
129
Recommendations
The first step to managing corneal astigmatism is to accurately assess the
magnitude and axis of astigmatism, also the stability, especially in ectatic
diseases.
Total refractive astigmatism is best identified by the manifest or the
cycloplegic refraction. The total refractive astigmatism is composed of both
corneal astigmatism and lenticular astigmatism. It is important to differentiate
these two components because the lenticular component will be replacing in
the crystalline lens.
Regularly irregular astigmatism
Myopic
Photoablative
procedures
(topo or wavefrontguided PRK, LASIK)
Hyperopic
IOLs *
ICRS
IOLs*
CK
Compresive
sutures
PRK: photorefractive keratectomy; ICRS: intrastromal corneal rings segments; IOLs:
intraocular lenses.* If the patient < 50 years: toric phakic lens, if > 50 years:
posterior chamber intraocular lens.
Irregularly irregular astigmatism
Myopic (e.g. post-PK)
Topo or wavefrontguided PRK,
LASIK)
Semilunar,
Wedge
Resections
PK
Combined
procedures:
ICRS +IOLs
ICRS+PRK/LASIK
Hyperopic (e.g. post-RK)
PK
Compresive
sutures
PK: penetrating keratoplasty; RK: radial keratotomy; PRK: photorefractive
keratectomy ; ICRS: intrastromal corneal rings segments; IOLs: intraocular
lenses.* If the patient < 50 years: toric phakic lens, if > 50 years: posterior
chamber intraocular lens.
Figure 3. Proposed algorithm for management in irregular astigmatism based on spherical
refractive error and topographic pattern.
130
L. Pedro-Aguilar, A. Navas, and E. Graue-Hernandez
Topographic analysis of the corneal surface, wavefront analysis of ocular
refractive aberrations, and vector planning to enable the appropriate balance in
emphasis between these two diagnostic modalities.
In recent times, a spate of combinations of procedures, such as collagen
crosslinking, intracorneal rings, ICLs, and photorefractive keratectomy have
been reported, these procedure reduces residual manifest refraction cylinder to
predictably low levels with corresponding improvement in UDVA and CDVA.
In Figure 3 we suggest a management algorithm for patients with irregular
astigmatism, based on the spherical refractive and topographic pattern.
By examining the success of each surgical technique, the refractive
surgeon may be able to make an informed decision on its indications and
limitations, based on the specific patient’s characteristics.
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