Nail apparatus melanoma initially diagnosed as nail matrix blue nevus: a case report with dermatoscopy and dermatopathology

We present a case of nail apparatus melanoma in a 50-year-old woman presenting as new and changing longitudinal melanonychia of the right thumb. Very heavy melanin pigmentation involving both the epidermis and dermis interfered with dermatopathological assessment, which initially leads to a diagnosis of nail matrix blue nevus. After consultation with a specialist multidisciplinary clinic the diagnosis was revised to invasive melanoma, a diagnosis consistent with the clinical and dermatoscopic assessment.

We present a case of nail apparatus melanoma in a 50-year-old woman presenting as new and changing longitudinal melanonychia of the right thumb. Very heavy melanin pigmentation involving both the epidermis and dermis interfered with dermatopathological assessment, which initially leads to a diagnosis of nail matrix blue nevus. After consultation with a specialist multidisciplinary clinic the diagnosis was revised to invasive melanoma, a diagnosis consistent with the clinical and dermatoscopic assessment.  mitoses or lymphovascular or peri-neural invasion observed.
The patient agreed to definitive treatment by distal phalanx amputation.

Conclusions
Nail apparatus melanoma is uncommon but has a relatively high mortality [1]. There is a reported female preponderance with location on the thumb having the highest prevalence (41%) [2]. The proportion of nail apparatus melanoma which are pigmented has been reported as 71.7% [3] with the median Breslow thickness of nail apparatus melanoma being reported as 0.8 mm with up to 18% having spindle shaped cells [4]. All reported nail apparatus melanomas have apparently arisen de novo, there being no reports of any pre-existing associated nevus [4]. The diagnosis can be challenging both clinically and dermatopathologically [5], although this is not expected in a mature invasive melanoma. A history of new and changing longitudinal melanonychia at mature age is a clue to malignancy as is the specific dermatoscopic clue to nail apparatus melanoma of longitudinal melanonychia (brown, black, grey or blue) with lines parallel varying in width, interval and color [6]. In the present case, there was one broad band of blue color and two narrow bands of the same color.
Lines varying in width, but not interval and color, raised the possibility of a benign etiology, but this appearance could also be explained by the very heavy density of pigmentation. In the present case, the index of suspicion of the treating dermatologist was high and the patient was referred promptly for nail matrix biopsy. Subsequent dermatopathological assessment, including H&E, Melan A and Ki-67 staining, was hindered by dense melanin deposition. Bleaching of histological sections was employed in response to this, but it is known that this process can damage the tissue, compromising interpretation [7]. As a result of the dense melanin deposition and equivocal dermal melanocyte cytology, an initial dermatopathological diagnosis of blue nevus was rendered.
This diagnosis of blue nevus was questioned primarily because of a history of progressive evolution clinically and also because, with only two exceptions in the literature, nail apparatus blue nevus does not exhibit epidermal pigmentation or longitudinal melanonychia. A pigmented melanocytic proliferation restricted to the dermis of the nail matrix is not expected to transfer pigment to the developing nail plate, and