Bimatoprost-induced periocular skin hyperpigmentation: histopathological study.

Objective: To investigate light microscopic and ultrastructural changes in bimatoprost-induced skin hyperpigmentation.

Methods: Eyelid biopsy specimens from bimatoprost-treated patients and matched controls were examined by light microscopy and transmission electron microscopy. Using an image analyzer, melanin granules were counted on Fontana-Masson-stained sections, and melanosomes were counted on electron micrographs. Immunohistochemical analysis was performed with antibodies against S100 and CD3. Positively labeled cells were counted.

Results: By light microscopy, a marked increase in the number of melanin granules was noted in the bimatoprost-treated specimens. Electron microscopy demonstrated dermal melanocytes with prominent rough endoplasmic reticulum and abundant normal-sized melanosomes in different stages of maturation as compared with control specimens. Furthermore, the keratinocytes of the bimatoprost-treated specimens showed abundant mature melanosomes when compared with controls. Also of note, atypical melanocytes were absent in both specimens. The S100-positive melanocytes were comparable in bimatoprost-treated and control specimens. Few CD3- and CD68-positive cells in the bimatoprost-treated specimens were noted in both groups.

Conclusion: Bimatoprost-induced periocular hyperpigmentation is caused by increased melanogenesis. There was no evidence of melanocyte proliferation or prostaglandin-induced inflammation in the specimens that were examined.