Acquired longitudinal melanonychia after puberty in a whiteskinned individual requires urgent biopsy

Approximately 2-3% of melanomas in whites, and 15-20% in blacks are located in the nail unit. However, malignant melanoma is rare in black people; thus the number of nail melanomas does not significantly differ between these population groups. Most white patients have a fair complexion, light hair, and blue or hazel eyes. There is no sex predominance, although some reports show variable female or male predominance. The mean age at onset is 55-60 years. Most tumours are found in the thumbs or great toes.

Melanoma of the nail region is often asymptomatic. Many patients only notice a pigmented lesion after trauma to the area; only approximately two-thirds seek medical advice because of the appearance of the lesion; pain or discomfort is rare, and nail deformity, spontaneous ulceration, sudden change in colour, bleeding or tumour mass breaking through the nail are even more infrequent. It is useful to remember that a pigmented subungual lesion is more likely to be malignant than benign. If the melanoma is pigmented it may show one or more of the following characteristics:

1 A spot appearing in the matrix, nail bed or plate. This may vary in colour from brown to black; it may be homogeneous or irregular, and is seldom painful.

2 A longitudinal brown to black band of variable width running through the whole visible nail.

3 Less frequently, Hutchinson's sign—periungual extension of brown-black pigmentation from LM onto the proximal and lateral nail folds—is an important indicator of subungual melanoma (but note the reservations discussed below).

Current experience has demonstrated that Hutchinson's sign, while valuable, is not an infallible predictor of melanoma, for the following reasons:

• Periungual pigmentation is present in a variety of benign disorders and, under these circumstances, may lead to overdiagnosis of subungual melanoma.

• Periungual hyperpigmentation occurs in at least one non-melanoma skin cancer: Bowen's disease of the nail unit.

• Hyperpigmentation of the nail bed and matrix may reflect through the 'transparent' nail folds, simulating Hutchinson's sign ('pseudo-Hutchinson's sign'). Each of the above may incorrectly suggest a diagnosis of subungual melanoma. Table 5.6 lists disorders in which pseudo-Hutchinson's sign occurs.

Total reliance on the (apparent) presence or absence of periungual pigmentation may lead to over- or underdiagnosis of subungual melanoma. All relevant clinical and historical information, including the presence or absence of periungual pigmentation, must be carefully evaluated in a patient suspected of having subungual melanoma. Ultimately, the diagnosis of subungual melanoma is made histologically. Hutchinson's sign is a single, important clue to this diagnosis. The nail plate may also become thickened or fissured and permanently shed.

Approximately 25% of melanomas are amelanotic (pigmentation not an obvious or prominent sign; Figure 5.36) and may mimic pyogenic granuloma, granulation tissue or ingrowing nail. The risk of misdiagnosis is particularly high in these cases.

Malignant melanoma must be considered in the differential diagnosis (see Table 5.3) in all cases of inexplicable chronic paronychia, whether painful or not, in torpid granulomatous ulceration of the proximal nail fold and in pseudoverrucous keratotic lesions of the nail bed and lateral nail groove. Subungual melanoma may also simulate mycobacterial infections, mycotic onychodystrophy, recalcitrant paronychia and ingrowing nail. Subungual haematoma is not rare and may present

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