Author: Dr Amanda Oakley, Dermatologist, Hamilton, New Zealand, 2011.
Melanoma of the nail unit is usually a variant of acral lentiginous melanoma (melanoma arising on the palms of the hands and soles of the feet). Other types of melanoma rarely arising under the nails are nodular melanoma and desmoplastic melanoma.
Melanoma of the nail unit usually affects either a thumb nail or great toenail but any finger or toenail may be involved. The term includes:
Melanoma of the nail unit is rare, accounting for only about 1% melanoma in white-skinned individuals. It arises in people of all races, whatever their skin colour. Although no more common in dark skin than fair skin, it is the most common type of melanoma diagnosed in deeply pigmented individuals. It is most diagnosed between the age of 40 and 70.
It is not thought to be due to sun exposure. Trauma may be a factor, accounting for the greater incidence in the great toe and thumb.
Subungual melanoma often starts as a pigment band visible the length of the nail plate (melanonychia). Over weeks to months, the pigment band:
However, in up to half of all cases subungual melanoma is amelanotic (not pigmented). Ungual melanoma can form a nodule under the nail plate, lifting it up (onycholysis). It may sometimes look like a wart (verrucous). It is usually painless, but an advanced tumour invading underlying bone may cause severe pain.
Subungual melanoma may be suspected clinically because of a wide (> 3 mm) new or changing pigment band in a single nail. Dermatoscopic examination may reveal more details showing pigmented lines of varying colour, width and spacing. These lines tend to lose their usual tendency to run parallel to each other along the length of the nail. Ungual melanoma forms a non-pigmented lump under the nail plate, eventually resulting in its destruction.
The diagnosis of melanoma is confirmed by biopsy of the nail matrix and nail bed. It can be a difficult diagnosis requiring examination by expert dermatopathologists. The pathologist should report whether the melanoma is in-situ or invasive. The description of invasive melanoma should include its thickness in millimetres and what level of tissue has been invaded.
If the clinical diagnosis is subungual haematoma (a purple mark under the nail due to bleeding or bruising), the nail may be observed for a few weeks. Normal-appearing nail should then be seen growing behind the mark. Dermoscopy is helpful, as haematoma does not conform to the band-like pattern of subungual melanoma. However, it must be remembered that melanoma may bleed.
The melanoma must be removed surgically. This requires removal of the entire nail apparatus. Sometimes the end of the finger or toe is amputated.
Some patients may be offered sentinel node biopsy to determine whether the melanoma has spread to local lymph nodes.
The main factor associated with risk of spread of melanoma (metastasis) and death is the thickness of the melanoma at the time of complete excision of the primary tumour. Delay in diagnosis is common with subungual melanoma, particularly when it affects the toe, and some of these tumours have already spread at the time of diagnosis.
The 5-year survival rate ranges widely from 16% to 87%, depending on the series, with 2 larger series in the 51% to 55% range2.
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