**4. Histology**

Nail matrix biopsy is still essential for SUM diagnosis. Normal nail matrix has between 4 and 14 melanocytes per mm (mean 6.86 cells/mm per mm stretch of nail matrix epithelium) [7].

The presence of nests without atypia is distinctive of nevi, especially in a child with a well-demarcated, uniformly pigmented, single, longitudinal band [8].

The histologic distinction between a benign subungual pigmented macule (lentigo or lentigo-like hyperpigmentation) and an early lesion of SUM can be difficult.

This benign lentigos may histologically only show an increase in melanin deposition in keratinocytes, melanocytes, and/or macrophages without proliferation of melanocytes (melanocytic activation). However, these benign lesions may show proliferation of melanocytes as well. The mean density of melanocytes in lentigos is around 15.3 cells per 1-mm-stretch nail matrix. There is no confluence of melanocytes. Cytologic atypia has to be absent or mild. There is no inflammation associated. Pagetoid spread may be present but only focally.

SUM in situ shows a much greater proliferation of melanocytes (mean 58.9 cells per 1 mm of stretched nail matrix) that ranges from 39 to 136 melanocytes per 1 mm of stretched nail matrix. There is at least focal confluence of cells with various grades of cytologic atypia: nuclear enlargement, hyperchromatism, irregular nuclear contours, and prominent nucleolus. Dendrites are thicker and larger. Pagetoid spread is found in almost all lesions of SUM, and inflammation in the

epithelial stromal interface is frequent [6, 7]. In some cases of SUM with lentiginous growth of single atypical melanocytes, immunohistochemical stains with MELAN-A and HMB-45 may ease the diagnosis.

Invasive SUM has denser proliferation of atypical melanocytes arranged in aggregates and sheaths and may lead to nail dystrophy, nail destruction, and ulceration.

It can be difficult to measure Clark level and Breslow thickness, because the distinction of the onychodermis is not always clear and the underlying phalanx is separated by only a thin dermal collagen layer [6].
