Corneal ectasia remains one of the most insidious complications after laser in situ keratomileusis (LASIK). Since the first reports by Seiler and colleagues in 1998, fewer than 150 cases have been reported in the ophthalmic literature, although this number is likely an underrepresentation of the actual incidence. Thus, while rare, post-LASIK ectasia can have dramatic consequences and in some instances require corneal transplantation for visual rehabilitation.
Due to not only the visual and medical but also the medico-legal ramifications of post-LASIK ectasia, seven members of the American Academy of Ophthalmology, International Society of Refractive Surgery, and the American Society of Cataract and Refractive Surgery have rendered a consensus opinion paper to summarize current knowledge on the subject.
Eliminating this complication altogether may prove difficult. Increased understanding of the alterations in corneal biomechanics after LASIK, improved recognition of risk factors for post-LASIK ectasia, and improved management strategies, however, can considerably reduce both the incidence and the severity of this potentially significant complication.
Incidence and etiology
Corneal ectasia after LASIK is a progressive corneal steepening, usually inferiorly, with an increase in myopia and astigmatism, loss of uncorrected visual acuity, and often loss of best-corrected visual acuity that can present days to years after LASIK. The actual incidence remains undetermined, and no good data support firm predictions; previous estimates, however, have ranged from 0.04% to 0.2% to 0.6%. Among members of the International Society of Refractive Surgery (ISRS) of the American Academy of Ophthalmology (AAO) responding to the practice patterns survey in 2004, more than 50% had at least one case of ectasia develop in their practice. Approximately 50% of cases present within the first 12 months, but late onset ectasia can also occur.
Corneal refractive surgery by definition alters the effective shape, thickness, curvature, and tensile strength of the cornea. Chang and Stulting found that intraocular pressure reductions after LASIK were directly correlated with increasing refractive correction, with an independent effect of flap creation, signifying that the lamellar flap does not contribute to the tensile strength of the post-LASIK cornea. Andreassen and colleagues found the elastic modulus of the keratoconic cornea to be 1.6–2.5 (average 2.1) times less than that of a normal cornea. Post-LASIK ectasia may mimic this altered corneal elastic modulus.
The specific mechanisms resulting in extreme corneal shape alterations manifesting as post-LASIK ectasia remain undetermined, although complex biomechanical modeling that takes into account factors such as corneal ‘plasticity’ and ‘viscoelasticity’ and corneal parameters such as Young's modulus, Poisson's ratio, and curvature radius, among others, may provide insight in the future.
Risk factors
LASIK inevitably reduces the tensile strength of the cornea. Among the first four reported post-LASIK ectasia cases, all had greater than 10 diopters of myopia preoperatively and less than 250 µm residual stromal bed thickness (RSB) postoperatively, and two patients had forme fruste keratoconus. Extrapolating from Andreassen's data, Seiler postulated that reduction in the load-bearing portion of a normal cornea from 525 to 250 µm (a factor of 2.1) might simulate the elasticity of a cornea with keratoconus, and predispose to developing ectasia. This value also coincides with Barraquer's early recommendation that a RSB of at least 250–300 µm be maintained to prevent corneal ectasia after myopic keratomileusis. Pallikaris and colleagues also reported a large series of patients with myopia greater than 10 diopters that developed post-LASIK ectasia with variable RSB.
A population of post-LASIK ectasia patients was compared with a general post-LASIK control population and a post-LASIK control population with high myopia preoperatively (greater than -8 diopters). This study confirmed earlier suspicions and found a significant difference between control populations and post-LASIK ectasia patients, with ectasia patients having higher myopia preoperatively and low RSB postoperatively – usually less than 225 µm. Additionally, 88% of ectasia cases had forme fruste keratoconus preoperatively as defined by Rabinowitz and McDonnell, compared with 2–4% of control cases. Additionally, multiple postoperative LASIK enhancements have also been correlated with postoperative ectasia. These findings were further supported by subsequent reports on the characteristics of post-LASIK ectasia. Binder provides comprehensive analysis of features of reported ectasia cases through 2003.
High myopia
It follows logically that patients with high myopia will have lower residual stromal beds after appropriate tissue ablation than their counterparts with less myopia; thus, it remains possible that high myopia simply predisposes to a thinner RSB rather than independently increasing the risk for developing ectasia. Some of the patients with post-LASIK ectasia reported by Pallikaris and colleagues, however, had theoretically adequate RSB (more than 250 µm) despite treatments for high myopia.
Low residual stromal bed thickness
Factors contributing to low postoperative RSB in addition to high myopia include excessive flap thickness and deeper than expected stromal ablations. There can be significant variability in the measurement of corneal thickness, flap thickness, and ablation depth measurements. While most of the microkeratome plate markings overestimate average actual flap thickness, flap thickness can vary widely and excessively thick flaps still occur. Additionally, previous studies have found that actual ablation depth is usually greater than estimated ablation depth. Among surveyed members of the ISRS/AAO, only 31% currently routinely measure flap or RSB intraoperatively.
Multiple enhancements
Multiple enhancements have also been correlated with ectasia; it remains difficult, however, to determine whether multiple enhancements are a contributing cause of ectasia in ways other than simple residual stromal bed reduction, and whether multiple enhancements stimulate ectasia by RSB reduction or whether they are simply performed more frequently in these cases in response to the progressive myopic shift associated with ectasia. Previous studies have demonstrated significant inconsistencies between RSB estimations by various direct and indirect calculations and highlight the value of directly measuring RSB via intraoperative pachymetry measurements.
Topographic abnormalities
Most authors recognize preoperative topographic abnormalities as uniquely indicative of increased risk for post-LASIK ectasia; the definition of ‘abnormal’, however, remains a source of great debate. Forme fruste keratoconus as defined by the Rabinowitz criteria is a risk factor for post-LASIK ectasia. Pellucid marginal corneal degeneration suspects are also at increased risk. Rao and colleagues reported that preoperative posterior float elevations greater than 40 µm may signify increased ectasia risk. Based on extensive review of the literature, the members of the AAO/ISRS/American Society of Cataract and Refractive Surgery (ASCRS) joint committee now recommend avoiding LASIK in patients with asymmetric inferior corneal steepening or asymmetric bowtie patterns with skewed steep radial axes above and below the horizontal meridian.
Ambrosio and colleagues found that approximately 1% of refractive surgery candidates were excluded due to abnormal topographies, including keratoconus, keratoconus suspect, and pellucid marginal corneal degeneration. The risk profile for more subtle topographic abnormalities, including asymmetric bowtie and mild inferior steepening patterns, however, remains unclear. Varsanno and colleagues and Kanpolat and colleagues both independently found that up to one-third of patients presenting for refractive surgery do so with an asymmetric bowtie topographic pattern.
Ectasia without risk factors
Although most reported cases have at least one of the aforementioned risk factors, there are reports of post-LASIK ectasia developing in patients without risk factors, and of patients developing ectasia after surface ablation.
Risk factors summary
At this time, currently recognized risk factors for the development of post-LASIK ectasia include keratoconus, high myopia, low RSB (probably less than 225–250 µm) resulting from excessive ablation relative to preoperative corneal thickness for high myopia or thicker than expected lamellar flap creation, and preoperative topographic abnormalities, including forme fruste keratoconus and pellucid marginal corneal degeneration.
As discussed eloquently by the AAO/ISRS/ASCRS joint committee, however, no single risk factor or defined combination of risk factors stands alone as an absolute predictor of post-LASIK ectasia occurrence. Ectasia can certainly also occur in patients without any recognized risk factors.
Management
Post-LASIK ectasia management includes first and foremost prevention of its occurrence, which requires recognition of the problem, identifying and refining known risk factors, and utilizing alternative treatment strategies in high-risk patients. When ectasia occurs, prior to performing corneal transplantation, a variety of treatments are available currently, including intraocular pressure reduction, rigid gas permeable contact lenses, and intracorneal ring segments.
Hiatt and colleagues recently reported a case of early ectasia reversed by intraocular pressure (IOP) reduction. The authors postulate that there exists a ‘window of opportunity’ very early in the development of ectasia when IOP lowering may be beneficial, whereas they report no success in reversing long-standing post-LASIK ectasia with IOP management. The full effect of IOP management on post-LASIK ectasia remains to be determined.
When ectasia becomes manifest, rigid gas permeable contact lenses are usually necessary and frequently sufficient for visual rehabilitation. Fitting strategies for post-LASIK ectasia are often similar to those for keratoconic eyes. Various specific lens styles can be used, including standard aspheric, multicurve, or reverse-geometry lenses either alone or in combination with high oxygen-transmissible soft lenses in a piggyback lens system to improve comfort. The specific fitting parameters should be customized to each case, as post-LASIK ectatic corneas may present quite dissimilarly.
Recently, intracorneal ring segments (Intacs; Addition Technology Inc., Sunnyvale, California, USA) have been approved for use in keratoconus, and some promising results have been reported when used off-label for post-LASIK ectasia. Techniques reported have varied in terms of wound location and the size, symmetry, and number of Intacs placed. Some surgeons have placed symmetric thickness Intacs segments in the same manner as for low myopic correction, with the wound in the 180° meridian, while other surgeons have used both symmetric and asymmetric segments, with a thicker segment inferiorly, with standard wound placement. Alio and colleagues reported success with placement of symmetric thickness segments with the wound location in the steep corneal meridian. Pokroy and colleagues reported success with single Intacs segment implantation placed inferiorly. While the early results for Intacs appear promising, few longitudinal follow-up data exist; thus the long-term utility of this procedure for post-LASIK ectasia also remains to be determined.
Future directions: recognition and prevention
When potential at-risk patients present for surgery, especially those with high myopia, thinner corneas, or subtle topographic abnormalities, novel screening approaches and surgical options other than LASIK, including surface ablation and phakic intraocular lens implantation, should be explored.
Utilizing alternative treatment strategies for at-risk patients
Once a LASIK flap has been created, collagen fibers anterior to the flap contribute little if anything to the overall tensile strength of the cornea. The posterior stroma also appears more vulnerable to biomechanical weakening than the anterior stroma. For these reasons, surface ablation techniques including photorefractive keratectomy, laser subepithelial keratomileusis, and epi-LASIK, may be more suitable for borderline candidates, especially those with thinner corneas or mild topographic irregularities.
Phakic intraocular lens implantation avoids altering a potentially unstable cornea for highly myopic at-risk patients, and these lenses have recently shown promising results in keratoconic eyes and may also be applicable to some of the at-risk patients presenting for LASIK.
Utilizing new technology to identify abnormal corneas
Some studies suggest that patients developing ectasia without risk factors may be younger or have surgery prior to manifesting latent topographic abnormalities, as evidenced by a case in which bilateral corneal ectasia developed after unilateral LASIK. This suggests that current screening approaches rely on relatively late indicators of reduced corneal integrity, namely topographic evidence of corneal weakness. Newer techniques, including corneal interferometry, corneal hysteresis measurements, and dynamic corneal imaging may allow identification of at-risk patients with normal topographies but reduced biomechanical integrity preoperatively.
Avoiding retreatment in corneas with low residual stromal bed thickness
As corneal thickness measurements taken months after initial LASIK usually overestimate RSB thickness at the time of LASIK retreatment, accurate assessment of actual RSB is critical to avoid excessive ablation of the posterior stroma. This can be avoided by utilizing intraoperative pachymetry measurements prior to laser ablation, or by utilizing confocal microscopy prior to enhancement, as the confocal microscope can accurately measure RSB thickness without ever lifting the flap.
Future directions: management and reversal
In addition to the aforementioned treatment strategies, corneal collagen cross-linking may improve the course of post-LASIK ectasia. Using riboflavin as a photosensitizer followed by ultraviolet-A exposure, Wollensak and colleagues found that collagen cross-linking halted the progression of ectasia in keratoconus patients and in many cases reversed the process, as evidenced by a reduction in corneal steepening and refractive error. Future work will examine the safety and efficacy of this approach for post-LASIK ectasia patients.
Conclusion
Current evidence suggests that, although cases of post-LASIK ectasia will likely continue to infrequently occur, there should not be an impending ‘ectasia epidemic’. In fact, as methods for identifying at-risk patients improve and alternative treatment strategies are employed, the incidence should significantly decrease. This decrease will be facilitated by improved preoperative patient evaluation utilizing advanced topographic analysis and corneal elasticity measurements. Measurement of intraoperative pachymetry should become routine to avoid treatment after inadvertent thick flap creation. Alternative surgical options, including surface ablation and phakic intraocular lenses, should be considered for at-risk patients. When ectasia initially occurs, effective treatment strategies including intraocular pressure reduction and collagen cross-linking may be able to reverse ectasia. When ectasia becomes manifest, rigid gas permeable contact lenses and intracorneal ring segments may both be utilized as effective first-line treatments for visual rehabilitation. Thus, with time, the need for corneal transplantation for post-LASIK ectasia should significantly diminish.
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