[Nerve sheath myxoma of the hyponychium]

Nerve sheath myxoma is a rare benign tumour of the extremities that was long confounded with neurothekeoma. Herein, we describe a rare case of interest because of its site on the hyponychium.

A 31-year-old woman presented with a painless distal tumour on the right ring finger that had been present for 3 to 4 years. It consisted of a firm, round nodule under the nail and spreading to the fingertip. Complete excision was carried out after cutting away the distal nail plate. Histological examination revealed a myxoid tumour comprising very clearly delineated lobules containing pale fusiform cells with small nuclear inclusions. These cells expressed S100 protein but no CD34 or epithelial membrane antigen (EMA). Complete excision was performed and a full recovery was made.

This type of tumour is characteristic of nerve sheath myxoma, and is almost certainly of Schwannian origin, although distinct from Schwannoma. It is rare, occurs after the age of 35 years and is preferentially located in the extremities of the limbs. There has only been one other description of its occurrence under the fingernail, in which it was described as neurothekeoma. However, neurothekeoma is entirely different, being more cellular, with no expression of protein S100, and marked by the NKIC3 antibody; it occurs in children or young adults, and is frequently found on the face. These two tumours were confused for some time, but today they must be completely distinguished from one another. These myxomas must be completely excised because of the risk of relapse. Finally, they should be distinguished from other myxoid tumours of the digits, certain of which can be malignant.