144 OPHTHALMOLOGY SECRETS IN COLOR
38.What is the role of drugs in refractive surgery?
The first issue is pain, which is important in all treatment modalities but most significant for PRK. After PRK, increased levels of prostaglandin E-2 have been found, which sensitize the pain response of nerves. Topical nonsteroidal anti-inflammatory drugs (NSAIDs) such as ketorolac and diclofenac sodium have been shown to decrease pain by reducing prostaglandin E-2 levels. However, these agents also increase white blood cell response in the cornea and should be used concomitantly with a topical steroid. One study found increased sterile corneal infiltrates when topical NSAIDs were used alone.
Another issue is corneal haze after PRK. The cornea undergoes a wound-healing response to the excimer laser ablation. Activated keratocytes lay down new collagen and proteoglycan matrix (the haze). This is first apparent at 1 month postoperatively, peaks at 3 months, and then decreases as remodeling ensues. Several experimental and retrospective studies have shown that topical steroids reduce corneal haze after PRK. However, a prospective, double-masked study revealed no benefit from topical steroids versus placebo. Still, in a subgroup of patients steroids may be beneficial, and they are typically used postoperatively.
Topical steroids also have been studied in the modulation of corneal curvature. Despite controversy in the literature, topical steroids apparently help to prevent regression of myopic effect after PRK. In fact, cessation of steroids has been associated with myopic regression, which may be reversed on reinstituting therapy in certain patients.
Mitomycin C (MMC) is a cell-cycle-nonspecific alkylating agent that targets rapidly dividing cells. MMC is being used by topical application during PRK for people with moderate to high myopia. The goal is to reduce the proliferation of keratocytes and fibroblasts, thus reducing the haze seen after moderate to deep ablations. MMC has shown great promise in refractive surgery and is felt to be safe for use on the cornea; however, the adverse reactions to MMC when used on the conjunctiva or
sclera can be quite severe. Reported complications of MMC include corneal and scleral melts, cataract formation, and corneal edema.
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