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Editor,—Several cases of iatrogenic keratectasia have been reported after laser in situ keratomileusis (LASIK).1-4 Until now, there has been no report of keratectasia after phototherapeutic keratectomy (PTK), presumably because PTK employs less laser ablation of the corneal stroma than LASIK. Previous studies demonstrated that thickness of the residual stromal bed is critical in the development of this complication.134
We describe a case of progressive corneal ectasia following PTK for the treatment of band keratopathy. Even though the residual corneal thickness was greater than 500 μm, the patient developed steepening of the cornea, irregular astigmatism, and progressive myopia.
A 76 year old woman was referred to Miyata Eye Hospital for the treatment of bilateral band keratopathy. She had no apparent systemic or ocular disorders related to the development of band keratopathy, such as uveitis, long term use of miotics, hypercalcaemia, chronic renal disease, tuberculosis, or connective tissue diseases. The best spectacle corrected visual acuity (BSCVA) was 6/20 in the right eye with a refraction of cyl −1.5 dioptres (D) and 8/20 in the left eye with +1.5 D cyl +1.0 D. The central corneal thickness measured with the ultrasound pachymeter (UP-2000, Nidek Co, Ltd, Aichi, Japan) was 541 μm and 540 μm in the right and left eyes, respectively. PTK was performed on the right eye with Star excimer laser system version 2.50 (Visx, Inc, Santa Clara, CA, USA). Using the transepithelial technique, 200 pulses were applied to ablate 48 μm of the cornea. The treatment zone was 6 mm in diameter with a 0.7 mm transition zone. A soft contact lens was worn for 3 days following the procedure. The re-epithelialisation was complete within the first postoperative week. One month after surgery, BSCVA was 20/20 with −3.5 D and corneal thickness was 517 μm. By the third month after surgery, BSCVA deteriorated to 10/20 with a spectacle lens of −10.0 D. At 6 months after surgery, BSCVA was 10/20 with −10.0 D and the central corneal thickness was 513 μm. The colour coded maps of the videokeratography (TMS-2, Computed Anatomy Inc, New York, NY, USA) obtained serially after surgery showed progressive keratectasia in the central area (Fig1). The scanning slit corneal topography (Orbscan, Orbtek, Inc, Salt Lake City, UT, USA) taken 6 months after surgery revealed a marked elevation of the posterior surface in the central area, indicating anterior protrusion of the central cornea (Fig 2). Since then, the anterior and posterior topographies did not show apparent progression during the observation period up to 1 year after PTK.
This is the first documentation of iatrogenic keratectasia after PTK. In LASIK, the minimum thickness of the residual stromal bed to avoid corneal ectasia has been claimed to be 250–300 μm,134 approximately corresponding to the postoperative total corneal thickness of 400–450 μm. The postoperative corneal thickness in the current patient was greater than 500 μm. Moreover, it has been postulated that the risk of keratectasia following surface excimer laser surgery might be lower than that following LASIK because of the relatively thicker effective stress bearing corneal stroma after surgery.45Nevertheless, this patient demonstrated keratectasia as evidenced by steepening of the cornea, irregular astigmatism, and progressive myopia. It seems that the histopathological changes due to band keratopathy had already compromised the tensile strength of the cornea6 and the laser ablation further weakened the tissue to the degree that progressive ectasia ensued. The age of patient might have played a part. It was suggested that the safety limit of residual corneal thickness in normal eyes may not directly apply to diseased corneas.
None of the authors has a proprietary interest in any material or method mentioned here.
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