Diagnosis and Management of Melanonychia

Introduction to Melanonychia

The American Academy of Dermatology recommends that melanonychia of the big toe requires urgent evaluation by a dermatologist skilled in nail apparatus biopsy, as subungual melanoma is a critical diagnosis that cannot be missed and has a mean diagnostic delay of 2 years with only 43% 10-year survival 1

Subungual melanoma is the most critical diagnosis to exclude, particularly in adults with new or changing pigmentation, and arises in the nail matrix, requiring matrix sampling for diagnosis 1
Onychomycosis, a fungal infection, is a common cause of nail discoloration, with dermatophyte infection causing thickening and discoloration, and the big toenail being the most frequently affected nail 2
Bacterial infection, such as Pseudomonas aeruginosa, can cause green or black discoloration 2
Nondermatophyte mould infection accounts for approximately 5% of onychomycosis cases in the UK 4
Chronic trauma to the big toe can cause nail discoloration and dystrophy 2
Psoriasis can cause nail dystrophy with discoloration 2
Lichen planus can cause abnormal nails with potential pigmentation changes in approximately 10% of affected patients 2

Urgent biopsy is indicated when melanonychia demonstrates new pigmented line appearing in an adult nail, irregular borders or color variation, progressive change in size, shape, or color, extension of pigmentation onto the periungual skin, associated nail plate damage or ulceration, or width greater than 3mm or involving multiple digits asymmetrically 1, 5, 6

Refer urgently to a dermatologist with expertise in nail apparatus biopsy, as suspicious nail lesions should not be removed in primary care, and diagnostic accuracy depends on proper sampling technique and clinicopathological correlation 1, 5
Clinical evaluation should include complete skin examination, assessment of risk factors, dermoscopy of the nail lesion, and clinical photography for documentation 1, 5
Laboratory/pathological workup may include nail matrix biopsy, direct microscopy with potassium hydroxide, fungal culture, and bacterial culture 1, 2

Never perform a superficial shave biopsy of suspected melanoma, as this may underestimate depth and stage 1
Do not delay referral for "watchful waiting" in adults with new or changing melanonychia, and do not assume fungal infection without mycological confirmation 1, 2
Do not treat empirically with antifungals before establishing a diagnosis, particularly in single-nail involvement without associated skin infection 4