Melanonychia and Subungual Melanoma
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Direct answer
Melanonychia is brown or black pigmentation of the nail plate, most often a longitudinal band caused by melanocytes in the nail matrix. Most cases are benign, including naevi and racial pigmentation, but subungual melanoma must be excluded when a band is broad, irregular or extends onto the nail fold. The Hutchinson sign, pigment spreading onto the proximal or lateral nail fold, is the most specific clinical warning.
What you must remember
- Longitudinal melanonychia: a pigmented band running from cuticle to free edge, the commonest form of nail pigmentation that needs assessment.
- Benign causes: a melanocytic naevus of the matrix, racial pigmentation in darker-skinned people often affecting several nails, zidovudine and other drugs, and traumatic or fungal causes.
- Hutchinson sign: pigment extension onto the proximal or lateral nail fold, which is highly suspicious for melanoma.
- Band width: bands wider than 3 mm are suspicious, particularly when they are irregular, blurred or variably coloured.
- Subungual melanoma: most often affects the thumb and great toe in middle-aged adults, and diagnosis is often delayed.
- Biopsy: a longitudinal nail matrix biopsy settles the diagnosis when features are worrying.
- Staging: once confirmed, subungual melanoma is staged like cutaneous melanoma using Breslow thickness and nodal assessment.
Stepwise assessment of a pigmented nail band
Pigmented bands are common, so assessment should separate benign pigmentation from melanoma using a standard sequence. Each step narrows the differential before any invasive test.
- Document onset and course: note when the band started, whether it is widening, and whether other nails are affected.
- Measure the band: a band wider than 3 mm, with blurred edges or varied colour, raises concern.
- Look for the Hutchinson sign: pigment on the proximal or lateral nail fold is the strongest warning.
- Use dermoscopy: parallel pigment lines suggest a benign cause, while irregular streaks and wide, blotchy bands favour melanoma.
- Biopsy the matrix: a longitudinal matrix biopsy is taken when the features are worrying or the band is changing.
- Stage and refer: a confirmed melanoma needs excision, sentinel node assessment and specialist referral.
Steps 2 and 3 are the quickest bedside checks, and a single positive feature is enough reason to move on to dermoscopy and biopsy.
Where melanonychia questions lead candidates astray
Exam stems often describe a single dark band and then test the warning sign.
- Trap: reassuring a patient because the band is in a dark-skinned person. Correct: racial melanonychia is common, but a changing or wide band still needs assessment, since melanoma also occurs in darker skin.
- Trap: calling a thumb band benign because it is narrow and stable. Correct: width, colour variegation and the Hutchinson sign matter more than the nail site.
- Trap: taking a nail plate clipping as the biopsy. Correct: the diagnostic sample comes from the matrix, usually by a longitudinal approach.
- Trap: attributing a dark band to trauma without asking. Correct: a subungual haematoma can mimic melanonychia but grows out with the nail, so persistence needs biopsy.
Frequently asked questions
What is the Hutchinson sign?
The Hutchinson sign is pigmentation extending from the nail onto the proximal or lateral nail fold, and it is highly suspicious for subungual melanoma. Melanonychia without periungual spread is usually benign, but the sign alone warrants biopsy.
Which nails are most often affected by subungual melanoma?
Subungual melanoma most often affects the thumb and great toe, and it is usually diagnosed late because the lesion lies beneath the nail. Any new, widening or changing band in these nails deserves early referral.
Is racial melanonychia normal in Indian patients?
Longitudinal pigmented bands are common in darker-skinned people, often affecting several nails, and are usually benign. Even so, a single widening or changing band requires assessment, because subungual melanoma can occur in darker skin as well.
Which drugs cause melanonychia?
Zidovudine is a well-known drug cause of melanonychia, and some cytotoxic agents have also been implicated. Drug-induced pigmentation is usually multiple and diffuse, which helps distinguish it from a solitary band.
What band width is considered suspicious for melanoma?
A band wider than 3 mm is traditionally considered suspicious, especially when it is irregular, varies in colour or is accompanied by nail dystrophy. Width is a warning feature, and biopsy decides the diagnosis.