Bilateral keratectasia after unilateral laser in situ keratomileusis

In their report of bilateral keratectasia after unilateral laser in situ keratomileusis (LASIK), Wang and coauthors1 highlight an important etiology of post-LASIK keratectasia, ie, preexisting ectatic corneal disorder. Their patient had against-the-rule astigmatism with normal keratometric and central pachymetry values in both eyes. The preoperative parameters made the patient an ideal candidate for laser refractive surgery. However, if the corneal topography maps are examined carefully, certain abnormalities seem more apparent in light of the post-LASIK keratectasia. The topography map of the right eye initially shows an asymmetric bow-tie pattern, with skewing of the inferior mire temporally (Figure 1). At a later date, this shows the formation of the loop cylinder, in which the superior and inferior mires skew temporally and meet each other (Figure 3). The left eye also shows a vertical corridor of flattening in the preoperative topography (Figure 2). The posttreatment map shows inferior steepening with characteristic marked flattening of the central cornea along the vertical meridian (Figure 4). The topographic features are suggestive of pellucid marginal corneal degeneration (PMCD) in both eyes. We have reported progressive keratectasia after LASIK in eyes with PMCD.2 Wang and coauthors correctly highlight the presence of an asymmetric topography pattern initially that was not considered abnormal until later. The creation of a corneal flap, followed by a minimal ablation of 14 μm, resulted in weakening of the anterior lamellae and rapid progression of the subclinical corneal PMCD. We would like to emphasize the importance of interpreting corneal topography maps carefully to detect subclinical corneal ectatic disorders before LASIK is performed. An asymmetric bow-tie pattern with skewing of the mires should be considered suspicious. Serial topography maps should be performed at least 6 months apart to rule out progressive changes. Abnormal patterns with inferior steepening can also occur with contact lens wear. These changes disappear after discontinuation of contact lens wear. It is important to ensure that the topographic changes are contact lens related and not from another source. Regional pachymetry is also important in detecting subclinical corneal ectatic disorders. It is important not to rely on central corneal thickness values only, as these may be normal in patients with PMCD. In conclusion, PMCD is a rare ectatic corneal disorder that can be associated with progressive keratectasia after LASIK. Careful interpretation of the preoperative corneal topography is essential to arouse suspicion of subclinical PMCD. Serial topography should be performed before LASIK in these cases. Rajesh Fogla DNB, FRCSED Prema Padmanabhan MS Chennai, India