Lasers producing more stromal surface irregularity are associated with higher incidence of severe haze, and haze at lower levels of correction.
“In these situations, there is a mechanical impediment to the basement membrane healing,” he said. “Poorly maintained excimer lasers are more likely to produce such irregularities.”
Haze can occur after infection as well, he said, citing a pseudomonas rabbit model in which defects in the epithelial basement membrane led to the development of fibrosis.
The epithelial basement membrane slowly regenerates in many corneas with severe haze, causing localized myofibrobast apoptosis and reabsorption of abnormal extracelluar matrix by the repopulating keratocytes. The keratocytes form lacunae which coalesce in a clear cornea.
Mytomycin C can prevent haze after PRK by inhibiting proliferation of myofibroblast precursors.
Haze developing despite retreatment--“breakthrough haze”--is less likely to resolve over time, probably due to a prolonged decrease in normal anterior keratocyte density or altered keratocyte function.
Breakthrough haze is more common in humans with a 0.01% concentration than with a 0.02% concentration of mytomycin C.
In most eyes, transient early mild haze following PRK is caused by corneal fibroblasts, not myofibroblasts. In rabbits, haze can be associated with fibroblasts in normal riboflavin-UV cross-linking and with RK or AK incisions.
A future gene therapy may transiently express components--such as nidogen-1 and nidogen-2, perlecan, or laminin alpha-3-- which would be provided by keratocytes to regenerate the basement membrane, he predicted.