Ectasia After LASIK
Corneal ectasia is one of the most devastating complications after Laser In situ Keratomileusis (LASIK). Post-LASIK ectasia is considered in patients who developed increasing myopia, with or without increasing astigmatism, loss of uncorrected visual acuity, often loss of best-corrected visual acuity, with keratometric steepening, with or without central and paracentral corneal thinning, and topographic evidence of asymmetric inferior corneal steepening after LASIK procedure.1 Ectatic changes can occur as early as 1 week2 or can be delayed up to several years after LASIK.3,4 The actual incidence of ectasia is undetermined, although the incidence rate of 0.04%5 to 0.2%6 to 0.6%7 has been reported.
The Ectasia Risk Score System designed by Randleman and colleagues8, is the most legitimate screening tool for identifying high-risk patients for post-LASIK ectasia preoperatively. In this score system, the most common risk factors, in order of significance include:
1. Abnormal preoperative topography
Abnormal topography compromises of keratoconus, pellucid marginal corneal degeneration, or forme fruste keratoconus with an I-S value of 1.4 or more9 and is the most significant factor with highest relative risk.
The axial map placido disc-based topography pattern classification using in this score system consists of:
includes round, oval, or symmetric bowtie patterns.
includes the following asymmetric patterns:
a. Asymmetric bowtie
i. Asymmetric steepening in any direction less than 1.0 diopter (D)
ii. No skewed radial axis
b. Inferior steep/skewed radial axis
i. Signiﬁcant skewed radial axis with or without inferior steepening
ii. One diopter or more of inferior steepening in some areas but an I-S value of less than 1.4.9,10
includes keratoconus, pellucid marginal corneal degeneration, or forme fruste keratoconus with an I-S value of 1.4 or more.9
2. Low residual stromal bed (RSB) thickness
RSB thickness is especially important after LASIK because both stress-strain analysis11 and tensile strength analysis12 indicate greater strength in the anterior 40% relative to posterior 60% of stroma and LASIK reduces anterior corneal structural integrity.
It is clear that cutoff of 250 microns does not absolutely discriminate development of ectasia, however the risk of ectasia increases reciprocally relative to RSB thickness.
3. Young age
Younger age is a significant risk factor for ectasia in patients without other risk factors. One might hypothesize that some of these individuals would have developed delayed onset forme fruste or keratoconus even without LASIK procedure.
4. Low preoperative corneal thickness
Corneal thickness, degree of myopia and RSB are related and RSB thickness is the most significant predictor of ectasia among them.
5. High myopia
Despite the early reported cases of ectasia for extreme myopia (more than 12 D), post-LASIK ectasia has been reported in numerous patients with low myopia13 and even hyperopia.14
The Ectasia Risk Score System is a cumulative score system. Risk categories based on points are:
0-2 points=low risk, 3 points=moderate risk, 4 points=high risk.
The score system may be summarized as8:
Abnormal topography, RSB <240 microns, corneal thickness less than 450 microns and Manifest refraction spherical equivalent (MRSE)> -14 D: each 4 points
Inferior steepening pattern or skewed radial axis in topography, RSB between 240 to 259 microns, age between 18 to 21 years, corneal thickness between 451 to 480 microns, MRSE between -12 to -14 D: each 3 points
RSB between 260 and 279 microns, age between 22 to 25 years, corneal thickness between 481 to 510 microns and MRSE between -10 to -12 D: each 2 points
Asymmetric bowtie pattern in topography, RSB between 280 to 290 microns, age between 26 to 29 years, MRSE between -8 to -10 D: each one point
Normal pattern or symmetric bowtie, RSB more than 300 microns, age more than 30 years, corneal thickness more than 510 microns, MRSE less than -8 D: each 0 point.
Other risk factors include eye rubbing, family history of keratoconus, refractive instability, BCVA less than 20/20 preoperatively, and male gender and should be considered especially in borderline cases.
Histopathology and Immunohistochemical characteristics
Light microscopy and hematoxylin-eosin staining of post-LASIK ectasia corneas demonstrated RSB thinning, hypocellular stromal scar, larger than normal artifacteous interlamellar cleft in RSB and fewer areas of Bowman’s layer disruption than keratoconus.15
Transmission electon microscopy (TEM) showed thinning of the collagen lamellae and loss of lamellar number in the RSB and decreased interfibril distance.15
Immunohistochemical evaluation of post-LASIK ectasia revealed abnormal epithelial basement membrane (EBM) structure similar to keratoconus and bullous keratopathy and increase in certain proteinases indicating lysis and remodeling of EBM.16
Confocal microscopic analysis of post-LASIK ectasia showed unevenly distributed highly reflective collagen scars with reduced keratocyte density and background transparency at the anterior stroma compared to normal post-LASIK eyes.17
1. Contact lens
Various types of contact lenses such as rigid gas permeable (RGP), custom wavefront-guided soft contact lenses, hybrid lenses and tandem soft contact lens-rigid gas permeable lenses can be used for visual rehabilitation. If patient has intolerance to RGPs, then soft contact lenses in tandem use, hybrid contact lenses (SynergEyes, Synerg Eyes Inc., Carlsbad, CA, USA) and scleral lenses are next options. Custom wavefront-guided soft contact lenses have been used in keratoconus with similar visual acuity compared to RGP and decreased aberrations18,19 may also be relevant to post-LASIK ectasia.
2. Intracorneal ring segments (ICRS)
ICRSs including Intacs (Addition Technology Inc. Des Plaines, Illinois, USA) and KeraRings (Ferrara Ophthalmics, Belo Horizonte, Brazil) may improve visual function in ecstatic corneas. Intracorneal rings are placed symmetrically or asymmetrically on the steep meridian or about the cone.20,21 Different wound location, size, symmetry and number of segments that are used depend on surgeon and patient.
It has been well documented that cross-linking can stop the progression of ectasia.22,23 Most of the cross-linking effect is occurs in the anterior 200 microns of cornea that is weakened by flap creation.
4. Combination treatments
The most potential options include combining ICRSs with CXL 24 or CXL with customized excimer laser ablation.25,26
5. Penetrating keratoplasty (PKP)
PK is the last resort for visual rehabilitation in patients with post-LASIK ectasia.27
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