Which histologic findings are characteristic of melanonychia?

Updated: Sep 10, 2018
  • Author: Chris G Adigun, MD, FAAD; Chief Editor: William D James, MD  more...
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Answer

The histopathologic findings vary based on the etiology of melanonychia.

Nail matrix nevi can have an unusual appearance histologically, and they are considered to be “special site nevi,” a subset of acral pigmented lesions. The majority of nail matrix nevi are junctional, although compound nail matrix nevi have been reported. Histologically, nail matrix nevi can be highly cellular; can have hyperchromatic, large cells that do not form discrete nests; and can possess prominent, abundant, and uneven cytoplasmic dendrites. [38]

One study evaluated 20 subungual melanomas and 15 benign subungual melanotic macules to determine the histopathologic features that can help to differentiate these 2 entities. A statistically significant difference in the mean melanocyte count (number of melanocytes per 1-mm stretch of subungual dermoepithelial junction) was noted for invasive melanoma (mean 102; range 51-212) compared with subungual melanotic lentigines (mean, 15.3; range 5-31). Other histopathologic features seen in subungual melanoma compared with subungual melanotic macules included a confluence of melanocytes, multinucleation of melanocytes, florid pagetoid spread of melanocytes, inflammation at the epithelial stromal interface, and moderate and/or severe cytologic atypia of melanocytes. [39]

A Fontana-Masson stain highlights melanin and thus may be useful in the determination of location of pigment within the nail matrix epithelium. Of note, 20-30% of subungual melanomas may be amelanotic, [24, 40] and immunostains for melanocytes, such as S-100, Melan-A, HMB-45, and MITF, may provide important diagnostic information.


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