Melanonychia: Causes, When to Worry, and Diagnosis

Melanonychia — brown or black pigmentation within the nail plate — is one of the most anxiety-provoking nail findings because subungual melanoma is in the differential. However, the majority of melanonychia cases are benign. The clinical challenge is accurately stratifying risk to avoid missing the rare malignant case while not over-biopsying the common benign ones.

Melanonychia Causes: Benign vs Malignant Risk Stratification

Cause	Type	Typical Appearance	Risk Level
Ethnic/racial melanonychia	Benign — melanocyte activation	Multiple nails; homogeneous tan-brown band; stable for years	Very low — common in darker skin phototypes (Fitzpatrick IV-VI)
Traumatic melanonychia	Benign — post-hemorrhagic or melanocyte activation	Often dark brown-black; may move distally as nail grows	Low — subungual hematoma confirms with dermoscopy (red/purple globules)
Drug-induced (hydroxyurea, chemotherapy, antimalarials)	Benign — melanocyte stimulation	Multiple nails; diffuse pigmentation; clears after drug cessation	Low — medication history confirms
Nevus (nail matrix melanocytic nevus)	Benign but monitor	Single nail; homogeneous brown band; present since childhood	Low-moderate — biopsy if changing or atypical
Laugier-Hunziker syndrome	Benign systemic	Multiple nails; pigmented lip and oral mucosa also present	Low — characteristic clinical pattern
Subungual melanoma	Malignant	Hutchinson sign; irregular width/color; rapid change; single nail; nail destruction	High — biopsy mandatory

ABCDEF Melanonychia Rule for Melanoma Risk Assessment

Letter	Feature	High-Risk Finding
A	Age and race	Over 50 years old; Asian, African, or Native American (higher prevalence); any age if changing rapidly
B	Band characteristics	Brown-black color; blurred borders; width over 3 mm; rapid width increase
C	Change	Rapid change in color, width, or pattern; failure of treated nail to improve
D	Digit involved	Thumb, hallux, or index finger (most common melanoma sites); single digit
E	Extension (Hutchinson sign)	Pigmentation extending from nail onto the proximal or lateral nail fold skin
F	Family/personal history	Dysplastic nevus syndrome; personal history of melanoma

Hutchinson sign — periungual pigmentation extending from the nail onto the surrounding skin — is the single most important clinical feature for melanoma risk. It is pathognomonic enough that its presence mandates urgent nail biopsy regardless of other features. Pseudo-Hutchinson sign (benign periungual pigmentation visible through the transparent cuticle without actually extending onto skin) can mimic it and is distinguished by dermoscopy.

At Balance Foot & Ankle in Howell and Bloomfield Township, we evaluate pigmented nail lesions with dermoscopy and perform nail biopsy when subungual melanoma cannot be excluded. Call (810) 206-1402.