The UV + riboflavin crosslinking procedure triggers the development of mild stromal haze in rabbit corneas, but it is not because of myofibroblast generation,a s it is for late haze after PRK.
"Riboflavin/UV crosslinking is a potentially effective procedure for treating patients with ectasia and keratoconus," said Dr. Wilson, staff refractive and corneal surgeon and director of Corneal Research at the Cole Eye Institute at Cleveland Clinic, Cleveland. "Several studies have suggested that the procedure can halt progression of those disorders, but not much is known about the biology involved."
With that in mind, Dr. Wilson and his colleagues designed a study to evaluate the cellular effects of riboflavin/UV crosslinking in rabbit eyes, which exhibit corneal wound healing that is similar to that in humans following surface ablation but are more prone to developing haze.
Riboflavin, UV, and combined therapy
Using the TUNEL assay, Dr. Wilson said, early cell apoptosis was apparent 24 hours after treatment. The numbers of dead cells in the anterior stroma were similar among the three treatment groups.
"There was a significant increased number of apoptotic cells after treatment with the combination procedure compared with the other two groups," he said. (Salomão MQ, Chaurasia SS, Sinha-Roy A, Ambròsio, R Jr., Espòsito A, Sepulveda R, Agrawal V, Wilson SE. Corneal wound healing after ultraviolet-A/riboflavin collagen cross-linking: A rabbit study. J Ref Surg. 2011;27:401-407.)
The degree of inflammatory cell infiltration at 24 hours was similar among the three groups. The degree of inflammation did not seem to be augmented when UV treatment was combined with riboflavin treatment, Dr. Wilson said.
Slight stromal opacity was apparent in all rabbit corneas at the 1-month evaluation, according to Dr. Wilson. The haze was minimal in the UV-alone and riboflavin-alone groups.
Staining for alpha-smooth muscle actin, the marker for myofibroblasts, showed that none of the three groups had myofibroblast cells.
These cells are associated with especially severe haze after PRK. When PRK is performed in rabbit eyes, there is heavy myofibroblast generation, he noted.
"Riboflavin/UV-A crosslinking does cause increased cell death through apoptosis compared with epithelial scraping alone, but the inflammatory response is similar among the three groups," Dr. Wilson said, "Surprisingly, very few smooth muscle actin-positive myofibroblasts were generated, presumably because the basement membrane is not damaged as it is in some high-correction PRK procedures.
"The haze that is present after a normal treatment is, therefore, much more transient, likely because corneal fibroblasts are generated that do not persist nearly as long in the cornea and are likely to resolve over just a few months," he concluded, noting that rare patients with severe persistent haze after UV/riboflavin crosslinking may have myofibroblast generation.