Other Tests
All patients who are surgical candidates for photorefractive keratectomy (PRK) should undergo preoperative screening with videokeratography (corneal topography) and corneal pachymetry. A high percentage of candidates for keratorefractive surgery have clinical or subclinical keratoconus. Preoperative corneal topography must be radially symmetric and free of irregular astigmatism; however, topographic-guided treatment of irregular astigmatism can be performed and is gaining in popularity. Patients suspected of having keratoconus are detected most easily with videokeratography using a 1.5-D interval scale by looking for a local area of corneal steepening.
Pseudokeratoconus is a form of localized corneal steepening caused by an artifact. The most common cause of pseudokeratoconus is found in contact lens wearers who have worn decentered contact lenses.
Contact lens warpage is a phenomenon resulting in destabilization of a patient's refraction. This effect may last for several weeks after contact lens use has been discontinued.
Quantitative descriptors of corneal optical performance, such as the Surface Regularity Index (SRI), can be helpful to quantify the smoothness of the ablation postoperatively.
Histologic Findings
The following histologic excimer-induced changes in corneal morphology have been reported:
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Epithelium: Initial reepithelialization occurs within the first 3-5 days. Over the following 6-18 months, the epithelium thickens primarily at the deepest part of the ablation site. No clear explanation exists for this phenomenon of epithelial hyperplasia. It is most likely secondary to the epithelium "smoothing out" the ocular surface in conjunction with the mechanical smoothing from the eyelids with each blink. [5] Morphologic and immunohistochemical studies demonstrate normal epithelial attachment complexes as evidenced by the presence of type III collagen (anchoring fibrils), beta 4-integrin (epithelial hemidesmosomes), and type IV collagen (basement membrane).
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Stroma: Within the first 24 hours after excimer PRK, stromal wound healing begins, as inflammatory cells invade the corneal stroma from the tear film. For at least 3 weeks following PRK, activated fibrocytes repopulate the treated area. These cells are responsible for the formation of new collagen and proteoglycan matrix.
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No significant changes have been found in the corneal endothelium or the Descemet membrane following excimer PRK as long as 250 μm of stroma remain.
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Human eye anatomy.
