Pigmented lesions of the nail unit: clinical and histopathologic features.

Probably the most common reason to perform biopsy of the nail unit is for the evaluation of irregular pigmentation, especially longitudinal melanonychia or pigmented bands. When narrow and solitary, these are usually the product of melanocytic activation/hypermelanosis, lentigines, or melanocytic nevi. Multiple pigmented bands are generally a benign finding, the result of melanocytic activation, as seen in racial pigmentation in darker-skinned patients, for example. In the context of an irregular, broad, heterogeneous or "streaky" band, the chief concern is the exclusion of subungual melanoma. Before assessing the histologic features of any such entities, it is important to understand the normal nail anatomy and melanocytic density of nail unit epithelium, as well as the type of specimen submitted, and whether it is adequate to undertake a proper histologic evaluation. The criteria for diagnosis and prognosis of melanoma of the nail unit are still evolving, and a variety of factors must be weighed in the balance to make a correct diagnosis. The importance of the clinical context cannot be overemphasized. There are also nonmelanocytic conditions to be considered that may produce worrisome nail discoloration, such as subungual hemorrhage, squamous cell carcinoma, and pigmented onychomycosis.