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Melanonychia is nail pigmentation from melanin deposition. When it occurs in bands, it is called melanonychia longitudina (ML) or melanonychia striata, and it can be formed of melanotic macules, lentigines, nevi, or malignant melanoma (MM). These authors sought histologic characteristics that distinguish benign subungual ML from malignant junctional melanomas. They compared density of melanocytes, the presence of multinucleated cells, pagetoid spread, cytologic atypia, inflammation, and the distribution of melanin pigment in biopsy samples and excisions from 15 benign MLs, 20 subungual MMs (10 in situ and 10 invasive), and 10 controls with non-ML nail conditions.
Most of the patients were women (mean ages: 46 in the benign ML group; 60 in the in-situ MM group; and 65 in the invasive MM group). Thumbs and great toes were the most common sites. In the control group, the density of melanocytes was low (4–9 melanocytes per mm), and the melanocytes were present at the basal and suprabasal levels. In the benign ML group, the mean melanocytic concentration was 15.3 cells per mm; none of these lesions were confluent or multinucleated. Focal pagetoid spread occurred in about half the ML lesions. Atypia was mild or nonexistent. In the MM group, the mean melanocytic concentrations in the in situ and invasive MM groups were 58.9 per mm and 102.0 per mm, respectively. Confluence was apparent in all MM lesions. Pagetoid spread was also seen in all lesions, but florid pagetoid spread increased from 20% in in situ MM to 70% in invasive lesions. Most MM lesions were moderately atypical.
Amin B et al. Histologic distinction between subungual lentigo and melanoma. Am J Surg Pathol 2008 Jun; 32:835.
Comment
Nail pigmentation can represent keratinocytic or melanocytic proliferations or infection. Certain clinical signs and dermoscopic findings point to the lesion type, but histology of the matrix and nail bed is critical for diagnosis. Nearly two thirds of single MLs in adults are just increased melanin pigment without melanocyte proliferation. However, most longitudinal melanonychias in children are nevi, of which a reported 30% are benign subungual melanonychia. These findings highlight the importance of melanocytic density as one of the earliest and most reliable clues to potentially aggressive lesions. Severe atypia is unusual in these lesions, and the implication of pagetoid spread needs to be cautiously weighed, as it can occur in normal melanocytic distribution.