Melanonychia of the Big Toe: Differential Diagnosis and Specialist Referral
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, 2
Differential Diagnosis
Malignant Causes (Highest Priority)
- Subungual melanoma: The most critical diagnosis to exclude, particularly in adults with new or changing pigmentation 1, 2
- Mean time from initial symptom to correct diagnosis is approximately 2 years, contributing to poor outcomes 2
- Melanoma arises in the nail matrix and requires matrix sampling for diagnosis 1
Infectious Causes
Onychomycosis (fungal infection): The most common cause of nail discoloration, accounting for 54.54% of melanonychia cases in one dermoscopy study 3
Bacterial infection: Pseudomonas aeruginosa causes green or black discoloration 1
Nondermatophyte mould infection: Accounts for approximately 5% of onychomycosis cases in the UK 1
Benign Melanocytic Causes
- Melanocytic activation or hyperplasia: Can produce longitudinal melanonychia 4, 5
- Junctional nevus: Diagnosed in 12.12% of melanonychia cases in dermoscopy studies 3
Traumatic/Mechanical Causes
- Chronic trauma: Repetitive trauma to the big toe can cause nail discoloration and dystrophy 1
- Subungual hemorrhage: Blood under the nail can mimic melanonychia 5
Other Inflammatory Conditions
- Psoriasis: Can cause nail dystrophy with discoloration 1
- Lichen planus: Approximately 10% of affected patients have abnormal nails with potential pigmentation changes 1
Clinical Features Suggesting Malignancy (Red Flags)
Urgent biopsy is indicated when melanonychia demonstrates: 1
- New pigmented line appearing in an adult nail
- Irregular borders or color variation (three or more colors)
- Progressive change in size, shape, or color
- Extension of pigmentation onto the periungual skin (Hutchinson's sign)
- Associated nail plate damage or ulceration
- Width greater than 3mm or involving multiple digits asymmetrically
Recommended Specialist and Workup
Primary Specialist Referral
Refer urgently to a dermatologist with expertise in nail apparatus biopsy. 1
- Suspicious nail lesions should NOT be removed in primary care 1
- The complexity of nail anatomy and the fact that melanoma arises in the nail matrix requires a practitioner skilled in nail biopsy techniques 1
- Diagnostic accuracy depends on proper sampling technique and clinicopathological correlation 1
Diagnostic Workup by Specialist
Clinical evaluation should include: 1
- Complete skin examination to assess for other melanocytic lesions
- Assessment of risk factors (personal/family history of melanoma, immunosuppression)
- Dermoscopy of the nail lesion to evaluate pigmentation patterns 1, 3
- Clinical photography for documentation 1
Laboratory/Pathological workup: 1
- For suspected melanoma: Nail matrix biopsy with sufficient nail plate removal to expose the underlying lesion 1
- For suspected fungal infection: Direct microscopy with potassium hydroxide and fungal culture on Sabouraud's glucose agar 1
- Bacterial culture if infection is suspected 1
Alternative Specialist Consideration
Podiatrist referral may be appropriate for preventive nail care or if fungal infection is strongly suspected, but dermatology remains the primary referral for diagnostic evaluation of pigmented lesions 1
Critical Pitfalls to Avoid
- 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—the 2-year average diagnostic delay for subungual melanoma is already unacceptably long 2
- Do not assume fungal infection without mycological confirmation, as this can mask melanoma 1, 2
- Do not treat empirically with antifungals before establishing a diagnosis, particularly in single-nail involvement without associated skin infection 1