Tumors of the nail unit. A review. Part I: acquired localized longitudinal melanonychia and erythronychia.

The article aims to be a guide to the interpretation of tumors specific to the nail, that is, tumors presenting peculiar histological features linked specifically to the nail unit. Therefore, the classical epithelial, fibroepithelial, and fibrous skin tumors occurring in the nail region are not analyzed. The interpretation of nail biopsies requires the identification and integration of the 2 main clinical modes of presentation of nail tumors, the acquired localized (monodactylous) longitudinal (ALL) band pattern, and the "masked" nail tumor. The ALL band pattern often allows the recognition of a nail tumor in its early phase of progression, with a limited differential diagnosis. The masked nail tumor mimics an inflammatory nail process, as a clinically misleading reactive benign lesion, which delays diagnosis with the subsequent development of partial nail loss and a locally destructive evolution. ALL band pattern appears as a longitudinal band starting at the matrix and extending to the tip of the nail plate. The band is usually single, rarely bifid. This clinical pattern can divided into 2 presentations. The generic term of ALL maculonychia could be proposed to define the macular aspect of the colored band of the nail plate. It encompasses 3 syndromes: longitudinal melanonychia, longitudinal erythronychia, and longitudinal leukonychia. ALL pachyonychia is a rare presentation. Pachyonychia indicates a localized thickening of the nail plate specific to the matrical nail tumor. In this group, there is differentiation toward cells of the nail matrix. The prototype tumor is the onychomatricoma, which present classically with a yellow (xantholeukonychia) band pattern. Recently, a new clinical band pattern has been described as longitudinal pachymelanonychia with 2 etiologies: pigmented onychomatricoma and onychocytic matricoma. The first part of this review delineate, in the first section, the distinctive microanatomical features of the nail unit and the second is dedicated to the most important pitfalls in pathological diagnosis of nail tumors because of nail surgery techniques. In the third section, the histopathology of ALL melanonychia and ALL erythronychia is discussed in a detailed description.