incidence of prominent corneal nerves in multiple endocrine neoplasia type 2a we studied the increased visibility of corneal nerves inside an 8mm diameter central corneal area in 14 patients with multiple endocrine neoplasia type 2a one patient with multiple endocrine neoplasia type 2b five patients with nonhereditary medullary thyroid carcinoma ten patients with anterior keratoconus and ten normal subjects we used a grading system grade 0 through grade 4 for nerve visibility based on slitlamp biomicroscopic examination and photographic documentation all 20 normal eyes showed either grade 0 or grade 1 which indicated no pathologic thickening of corneal nerves sixteen of the 28 eyes 57 with multiple endocrine neoplasia type 2a however were evaluated as grade 2 or higher which indicated thickened corneal nerves the incidence of high nerve visibility in eyes with multiple endocrine neoplasia type 2a was significantly greater compared to normal eyes p less than 0001 anterior keratoconus p less than 0001 and nonhereditary medullary thyroid carcinoma p 0012 furthermore eight of the 28 eyes 29 with multiple endocrine neoplasia type 2a showed markedly prominent corneal nerves grade 3 and 4 a prominence similar to those seen in eyes with multiple endocrine neoplasia type 2b there was no definite relationship among prominent nerve age of the patient and occurrence of pheochromocytoma these findings suggest that over half of all patients with multiple endocrine neoplasia type 2a show corneal nerves pathologically thickened to different degrees