Longitudinal melanonychia (LM) is a pigmented band extending from the matrix to the distal edge of a nail. It is caused by increased production of melanin within the matrix, and integration into the nail plate. The origin of this production is usually benign, due to activation, hyperplasia or proliferation of melanocytes normally present in the matrix. In some cases, however, LM is the manifestation of a subungual melanoma, the diagnosis of which must be made early.

Biographical, clinical and dermoscopic criteria make it possible to suspect melanoma and decide whether to perform biopsy. None of these criteria, however, are specific and definitive diagnosis requires pathologic examination of a matrix biopsy.

The biopsy technique should enable reliable histological study while limiting the risk of secondary nail dystrophy. Initial resection should ideally involve the entire lesion. Complete elevation of the nail plate enables the lesion to be precisely located. Lesions up to 3 mm can be removed by longitudinal resection biopsies without significant sequelae. In more extensive lesions, incision or tangential (“shave”) biopsy can be performed without impairing prognosis. In clinical presentations strongly suggestive of melanoma, immediate complete resection of the entire nail unit may be proposed.