A rapidly growing pigmented nail streak resulting in diffuse melanosis of the nail. A possible sign of subungual melanoma in situ.

Subungual melanomas are one of the most common types of malignant melanoma among the Japanese population. Although most pigmented nail streaks are benign and remain unchanged in their color and shape for a long time, rarely are they precursor lesions of subungual melanomas i.e., a rapid growing pigmented nail streak resulting in diffuse melanosis of the nail is thought to be an early stage of subungual melanoma in situ. We found four patients with these changes: three of these patients were children. The lesions occurred on the right index finger, right thumb, left middle finger, and right great toe, respectively. A slightly haphazard combination of colors ranging from dark brown to black, the important characteristic of subungual melanoma in situ, was observed in two cases. In the remaining two cases, although the haphazard combination of colors was not distinctive, many fine, dark longitudinal lines were seen within diffuse, light-brownish pigmentation. Serial histologic examination of the excised tissue specimens showed great proliferation of vacuolated melanocytes with variable nuclear atypicality along the entire basal layer in all cases. These histologic changes were compatible with those of atypical melanocytic hyperplasia or intraepidermal melanoma (in situ melanoma), which is an early lesion of subungual melanoma. An adult case is thought to be a definite example of a subungual melanoma in situ. We also made the diagnosis of melanoma in situ in the remaining three cases of children with rapidly growing pigmented nail streaks because their histopathologic features were distinguishable from those of the adult case. However, there remains some hesitation about this because invasive subungual melanoma is rare in children.