Ectasia After LASIK
Corneal ectasia is one of the most devastating complications after Laser In situ Keratomileusis (LASIK) and other refractive surgical procedures. 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, posterior corneal elevation, with or without central and paracentral corneal thinning, and topographic evidence of asymmetric inferior corneal steepening after LASIK procedure.Ectatic changes can occur as early as 1 week or can be delayed for years after LASIK.The actual incidence of ectasia is undetermined, although the incidence rate of 0.04% to 0.2% to 0.6% has been reported.
The Ectasia Risk Score System designed by Randleman and colleagues, is a controversial screening tool carefully developed by an evidence-based review of a large series of LASIK ectasia cases. The Ectasia Risk Score System scale may help to identify high-risk patients preoperatively. It is controversial in its utility because diagnostic methods of screening for preoperative ectasia have changed dramatically over the past 10 years, and now include possibly more sensitive techniques of corneal 'tomography' (Pentacam, Orbscan, Gallei) which can measure posterior corneal curvature rather than placido-disc based 'topography' which only measure anterior corneal curvatures. In the Randleman et al 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 more 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.
2. Low residual stromal bed (RSB) thickness
RSB thickness is especially important after LASIK because both stress-strain analysis and tensile strength analysis indicate greater strength in the anterior 40% relative to posterior 60% of stroma and LASIK reduces anterior corneal structural integrity.
Ectasia increases reciprocally relative to RSB thickness and a RSB of < 300 microns has been correlated with increased risk of ectasia.
3. Young age
Younger age may be a significant risk factor for ectasia in patients without other risk factors. One hypothesis is 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
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 as
|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.
One significant criticism of the Risk Score System is that any individual less than 22 years old is automatically classified as at least 'Moderate Risk', despite the low incidence of ectasia in this age group.
6. Percent tissue altered (PTA)
PTA is an additional significant risk factor for post-LASIK ectasia and may be considered in addition to residual stromal bed when determining the safety of an excimer laser treatment. PTA is equal to the flap thickness (FT) plus the ablation depth (AD) divided by the pre-operative thinnest central corneal thickness (CCT). PTA = (FT + AD)/CCT]. Some studies have indicated that PTA is one of the most predictive factors of the risk for corneal ectasia, even in eyes with a normal pre-operative corneal topography. A PTA of 40% is an indicator of a higher risk for ectasia.
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.
Transmission electon microscopy (TEM) showed thinning of the collagen lamellae and loss of lamellar number in the RSB and decreased interfibril distance.
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.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.
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 the next options. Custom wavefront-guided soft contact lenses have been used in keratoconus with similar visual acuity compared to RGP and decreased aberrations.
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. Different wound location, size, symmetry and number of segments that are used depend on surgeon and patient.
4. Combination treatments
5. Penetrating keratoplasty (PKP)
- Ectasia Risk in Topography
- http://www.aao.org (American Academy of Ophthalmology)
- http://www.nkcf.org (NationalKeratoconusFoundation)
- https://nei.nih.gov/health/cornea (National Eye Institute)
- https://clinicaltrials.gov (Clinical trials for corneal diseases)
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