I have a new black longitudinal streak on a single nail as an adult—could it be subungual melanoma and how should I manage it?

New Black Longitudinal Streak on a Single Nail in an Adult

A new pigmented longitudinal streak on a single nail in an adult requires urgent specialist referral to dermatology or a multidisciplinary skin cancer team, as this is a warning sign for possible subungual melanoma and should never be biopsied or removed in primary care. 1

Immediate Action Required

Do not attempt removal or biopsy in primary care settings. 1 The UK guidelines explicitly state that lesions suspicious for melanoma require specialist evaluation because:

• Clinicopathological correlation is vital for diagnostic accuracy 1
• Diagnostic surgery requires specialist training in nail apparatus biopsy 1
• The nail must be sufficiently removed to expose underlying lesions and the nail matrix must be adequately sampled by a practitioner skilled in this technique 2

Red Flags Strongly Suggesting Subungual Melanoma

Your presentation warrants urgent evaluation if any of these features are present:

• New onset in adulthood (especially after age 50) 3, 4
• Single digit involvement rather than multiple nails 3, 5
• Bandwidth >3 mm 3, 4
• Irregular borders or blurry lateral edges 3, 5
• Pigment variegation (multiple shades of brown to black) 3, 6
• Proximal widening of the band (triangle shape) 3
• Hutchinson's sign: pigment spreading onto the cuticle or proximal/lateral nail folds 3, 4, 7
• Rapid growth or darkening of the streak 3, 6
• Associated nail dystrophy or nail plate changes 3
• Bleeding or nodule formation 2

Why This Matters: The Melanoma Risk

Subungual melanoma represents 2% of melanomas in Western populations but carries a poor prognosis due to delayed diagnosis. 7 Key concerns:

• Up to 33% of subungual melanomas are amelanotic (non-pigmented), making diagnosis even more challenging 7
• Non-specific features often lead to misdiagnosis as benign conditions like fungal infection or trauma 2
• The thumb and great toe are most commonly affected 5
• Metastasis rate for invasive carcinoma is approximately 2% 2

Diagnostic Pitfalls to Avoid

The most common error is delayed diagnosis due to initial misdiagnosis as a benign lesion or hesitation to perform biopsy. 2 Other critical mistakes include:

• Assuming fungal infection without considering melanoma 2
• Mistaking it for subungual hematoma from trauma 4
• Performing inadequate biopsy that doesn't sample the nail matrix properly 1, 2
• Attempting shave or punch biopsy, which leads to sampling error and incorrect staging 1

Differential Diagnosis Considerations

While melanoma must be excluded first, other causes of longitudinal melanonychia include:

Benign causes (more common in children and multiple nails):

• Ethnic melanonychia in dark-skinned individuals 3
• Nail matrix nevus or lentigo 3
• Trauma-related pigmentation 3, 4
• Medications causing melanocytic activation 3
• Pregnancy 3

Non-melanin pigmentation:

• Bacterial infection (Pseudomonas) causing greenish-gray discoloration, typically at lateral nail edge 4
• Subungual hematoma (usually oval, not a neat streak, often on medial great toe) 4

Specialist Evaluation Process

When referred, the specialist will:

1. Photograph the lesion before any intervention 1
2. Perform dermoscopy of the nail plate and free edge to assess specific patterns 5
3. Remove the nail plate sufficiently to expose the underlying lesion 1, 2
4. Perform excisional biopsy with 2 mm clinical margin including the entire tumor and a cuff of fat 1
5. Orient the excision to facilitate possible subsequent wide local excision 1

Management Algorithm for Subungual Melanoma (If Confirmed)

The traditional approach involves amputation, though recent evidence supports more conservative options: 1

• Distal phalanx amputation at the distal interphalangeal joint (fingers) or metatarsophalangeal joint (toes) has been standard 1
• Digit preservation with partial distal phalanx resection shows no difference in recurrence-free survival compared to proximal amputation 1
• Functional results of nail bed excision with skin grafting are generally suboptimal with persistent sensitivity and pain issues 1

Bottom line: Refer urgently to dermatology or a skin cancer multidisciplinary team for proper evaluation. Do not wait, do not biopsy yourself, and do not dismiss this as benign without specialist assessment. 1