Evidence that anterior episcleral nerve sheath tumors arise from the Axenfeld nerve loop.

Comment. Although perineurial cell proliferation may be suspected histologically with routine hematoxylineosin staining, a definite identification needs the demonstration of perineurial cell features using immunohistochemical studies. The morphologic criteria include spindle cells with curved or wavy thin nuclei and thin, elongated cytoplasmic processes, arranged in lamellae, and a storiform growth pattern forming loose whorls and bundles. By immunohistochemistry, perineurial cells stain positive for EMA, GLUT-1, and claudin-1; may stain positive for protein gene product 9.5 and CD34; and stain negative for S-100 protein, CD57, and neurofilaments. Perineuriomas are benign soft-tissue neoplasms, but atypical and malignant examples have been reported. According to Hornick and Fletcher, cases with atypical pleomorphic cytologic results that show abrupt transition from typical cytomorphologic findings and storiform architecture to markedly hypercellular areas with a fascicular growth pattern and diffuse infiltration of adjacent skeletal muscle have been classified as atypical perineuriomas. Hornick and Fletcher also state that the mitotic count is variable. These histological features have no clinical significance and may be comparable to the degenerative changes observed in ancient schwannoma. In benign and atypical forms of perineurioma, surgical resection with free margins is curative; only 5% of cases have shown recurrence, and there are no reported cases with metastases. The differential diagnosis of soft-tissue perineurioma includes cellular schwannoma (S-100 protein ; CD34 /−; EMA −), low-grade fibromyxoid sarcoma of Evans (S-100 protein −; EMA /−; CD34 −), solitary fibrous tumor (CD34 ; S-100 protein −; EMA −), and benign fibrous histiocytoma or low-grade malignant fibrous histiocytoma (CD34 −; S-100 protein −; EMA −; CD68 ). Immunohistochemical studies will help separate these 4 entities. To our knowledge, this is the first case of atypical perineurioma arising in the bulbar conjunctiva reported in the English-language literature.