Histopathology of Salzmann Nodular Corneal Degeneration

PURPOSE:To define the histopathology of Salzmann nodular degeneration and suggest potential mechanisms involved in its pathogenesis. METHODS:Archived corneal biopsy specimens from 5 patients with Salzmann nodular degeneration were evaluated by chemical and immunohistochemical staining to describe the structure of the Salzmann nodules and phenotypes of nodule epithelium and stromal cells. RESULTS:Each Salzmann nodule appeared as a hypercellular mound of extracellular matrix located between a thinned corneal epithelium and a fragmented Bowman layer. Stromal cells within each nodule stained positively for vimentin, consistent with a fibroblast phenotype, whereas the epithelial cells overlying each nodule were positive for matrix metalloproteinase-2 and negative for matrix metalloproteinase-9. CONCLUSIONS:The observed epithelial expression of matrix metalloproteinase-2 overlying Salzmann nodules is consistent with chronic epithelial wounding in the disorder but does not identify a cause-effect relationship. Salzmann nodules might develop because of enzymatic disruption of the Bowman layer, anterior migration and proliferation of keratocytes, and secondary deposition of extracellular matrix. Alternatively, desiccation secondary to the elevation of the nodule might induce increased epithelial metalloproteinase expression.