Phakomatous Choristoma of the Eyelid: Immunohistochemical and Electron Microscopic Observations

Background: A 13-month-old Hispanic boy underwent excision of a congenital inferonasal orbital mass arising from the right lower lid. Results of histopathologic examination of the tumor showed a phakomatous choristoma of the eyelid. An immunohistochemical and electron microscopic study of this rare, benign, congenital tumor of lenticular anlage was performed. Methods: Immunohistochemistry was performed on 4-μm thick sections from paraffin-embedded tissue. Electron microscopy was performed on thin sections stained with uranyl acetate and lead citrate. Findings: The cuboidal epithelial cells that comprise this choristoma showed strongly positive cytoplasmic staining with S-100 protein and vimentin and focally positive staining with a keratin cocktail (AE₁/AE₃). Electron microscopy showed the presence of numerous 10-nm whorled cytoplasmic microfilaments within degenerating epithelial cells. Conclusion: The immunoreactivity of this tumor to keratin and vimentin are newly described in this detailed clinicopathologic report and, together with its S-100 positivity, support the proposal that this tumor is of lenticular anlage. The authors hypothesize that the intracytoplasmic 10-nm intermediate filaments observed with electron microscopic examination within the epithelial cells that comprise this choristoma represent vimentin as detected by immunohistochemistry.