Longitudinal melanonychia of the thumb

Subungual acral lentiginous melanoma in situ, treated by digit-sparing nail-unit excision

Learning points

• The ABCDEF criteria for subungual melanoma: Age 50–70, African / Asian / Native; Band: brown-black, ≥ 3 mm width, irregular borders; Change or lack of Change to treatment; Digit involved (thumb > great toe > index); Extension of pigment to nail folds (Hutchinson's sign); Family / personal history of melanoma or dysplastic naevus.
• Hutchinson's sign (true periungual pigmentation) is the most specific clinical sign of subungual melanoma; pseudo-Hutchinson (pigment visible through the transparent cuticle but not in the nail-fold skin) is benign.
• Nail dermoscopy: disrupted parallel lines (irregular colour, width, spacing) suggest melanoma; uniform parallel lines suggest benign melanocytic activation. A triangular band wider proximally is concerning.
• Nail-matrix biopsy is required for definitive diagnosis — tangential matrix shave or longitudinal punch biopsy by an experienced operator. Avoid superficial nail-plate biopsy alone.
• Subungual melanoma is histologically usually acral lentiginous melanoma; staging follows AJCC 8 and is driven by Breslow thickness.
• Digit-sparing nail-unit excision can be considered for subungual melanoma in situ in specialist hands. Invasive subungual melanoma should be planned through the melanoma MDT, with oncological clearance taking priority over reconstruction; amputation remains appropriate where bone, joint or functionally critical deep structures are involved.
• Acral lentiginous melanoma is proportionally commoner in Fitzpatrick IV–VI skin — have a low threshold for biopsy in skin of colour.