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Consultation section

refractive surgical problem

Davidorf, Jonathan M MDa

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Journal of Cataract & Refractive Surgery: January 2001 - Volume 27 - Issue 1 - p 13-15
doi: 10.1016/S0886-3350(00)00824-5
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This patient had LASIK for moderate compound myopic astigmatism. Three years later, he is more myopic and has decreased BCVA. The patient's decreased best spectacle-corrected visual acuity (BSCVA) peoperatively (20/30), his progressive myopia postoperatively, and the appearance of the postoperative topography maps raise red flags that the patient may have had forme fruste keratoconus from the outset. Additional preoperative data, including topography, pachymetry, refractive history, and comments on the retinoscopy reflex, would be helpful in making this diagnosis. It is possible that the patient was not a good LASIK candidate.

We can try to deduce the preoperative pachymetry measurements from the postoperative corneal thickness maps. In the right eye, the thinnest area is 420 μm and located inferotemporally. By including the tear film in its measurement, the Orbscan would be expected to overestimate corneal thickness. An estimated ablation depth of 55 μm in the right eye (6.0 mm optical zone, −4.50 diopter [D] preoperative spherical equivalent refraction) would correspond to a preoperative minimum corneal thickness of 420 + 55 = 475 μm. This is quite thin, particularly if we acknowledge that the estimate is generous. Also, the area of thinning on the topography is peripheral to the area of maximum ablation depth, meaning that the minimum corneal thickness may have been close to 420 μm preoperatively. If any epithelial hyperplasia occurred postoperatively, the preoperative corneal thickness must have been even lower.

More worrisome is the inferior location of the thin area. This could be explained by (1) an inferiorly decentered ablation, causing increased tissue removal in that location, or (2) the manifestation of a naturally occurring thin cornea in a patient with keratoconus. Given the appearance of the anterior curvature maps, the chance that the ablation was decentered is low. Similar calculations to estimate preoperative corneal thickness may be performed for the left eye.

The patient's refraction 3 years after LASIK demonstrates a substantially higher level of myopia and astigmatism than preoperatively. A manifest refraction is almost meaningless when the BSCVA is 20/100. Assuming the refraction was remotely accurate and (important) was confirmed with retinoscopy, there are a few possible etiologies of the increased myopia:

1. A regression of this magnitude (actually a progression of myopia) is most likely a result of an ectatic process. The elevation of the posterior corneas (Figure 2), which corresponds to the thinned areas of the corneas, lends credence to this theory.

2. The regression could represent an aggressive healing response, which would be more common in a patient who presents with corneal haze.

3. Presuming the patient had forme fruste keratoconus from the outset, the progressive myopia may simply represent the natural history of the disease and would have occurred regardless of LASIK.

4. Nuclear sclerosis with a myopic shift must also be ruled out as a cause for the refractive changes.

Intraoperatively, the patient developed epithelial defects, which were probably relatively large as they took 2 weeks to heal. The patient may have had underlying anterior basement membrane dystrophy, predisposing him to this complication. There is a hint of epithelial nests in Figure 1, B, which would support this diagnosis. It is also possible that this patient, with forme fruste keratoconus, was at increased risk of experiencing a thin/irregular flap and an epithelial defect. I suspect that the substantial haze response is a result of a partial buttonhole. The violation of Bowman's layer that occurs with a partial buttonhole, especially when laser ablation proceeds, increases the risk of haze in the flap itself. The haze is described as such, and forme fruste keratoconus predisposes to such flaps. The type of reticular fibrosis seen in Figure 1, B can present most intensely in areas of flap striae (also more common with thin flaps).

Although attempting to piece together the important historical events is interesting, the important fact is that the patient, now 3 years after elective laser surgery, has a substantial reduction in BSCVA, which is modestly improved with RGP lenses, to which he is becoming increasingly intolerant. The RGP lenses correct irregular astigmatism that is manifest on the anterior corneal surface, but not that occurring in the flap (striae) or at the level of the posterior cornea (ectasia). Both factors can lead to further reduction in BSCVA and best RGP-corrected visual acuity. The flap fibrosis, worse in the right eye than in the left, has caused additional reduction in BCVA.

The most troublesome problem is the ectasia, which is amenable only to lamellar or penetrating keratoplasty (PKP). Serial refractions, ultrasonic pachymetry measurements, and topographies will help confirm the diagnosis. If ectasia is ruled out or refraction and vision stabilize, the haze may be treated by performing phototherapeutic keratectomy (PTK)/photorefractive keratectomy (PRK) through the flap or by removing and discarding the scarred flap. Both options, however, cause further corneal thinning and increase the risk of ectasia. If the patient truly cannot tolerate RGP lenses, proceeding with lamellar keratoplasty or PKP, followed by LASIK once the sutures have been removed, is the best hope for attaining reasonable and stable spectacle-corrected visual acuity.

© 2001 by Lippincott Williams & Wilkins, Inc.