Diagnostic Workup for Melanonychia of the Big Toe to Rule Out Subungual Melanoma
The nail plate must be sufficiently removed to expose the underlying lesion, and the nail matrix must be sampled by a practitioner skilled in nail apparatus biopsy, with either excisional or incisional biopsy performed depending on lesion size. 1
Clinical Evaluation
Before proceeding to biopsy, assess the following high-risk features that increase suspicion for subungual melanoma:
- Age and ethnicity: Peak incidence in 5th-7th decades; higher risk in African Americans, Asians, and Native Americans where subungual melanoma accounts for up to one-third of all melanomas 2
- Band characteristics: Brown to black pigmentation with breadth ≥3 mm and variegated borders 2
- Hutchinson's sign: Extension of pigment onto the proximal and/or lateral nail fold 2
- Change or lack of change: Progressive change in the nail band or lack of improvement despite adequate treatment 2
- Personal/family history: History of dysplastic nevus syndrome or melanoma 2
Document the site, size (maximum diameter), presence of ulceration, bleeding, or nodular features, and photograph the lesion before biopsy. 1
Biopsy Technique
Because melanoma arises in the nail matrix and due to the complexity of nail anatomy, suspicious nail lesions must be evaluated and sampled by a practitioner skilled in biopsy of the nail apparatus. 1
The biopsy approach depends on clinical presentation:
- Remove the nail plate sufficiently to expose the underlying lesion and adequately sample the nail matrix 1
- Excisional biopsy is preferred when feasible, encompassing the entire lesion 1
- Incisional biopsy is acceptable for larger lesions, but must include adequate sampling of the nail matrix 1
- Avoid shave or punch biopsies as these techniques are inadequate for nail unit lesions and may miss the diagnosis 1
Critical Pitfalls to Avoid
Do not perform superficial or partial biopsies that fail to sample the nail matrix, as this is where subungual melanoma originates. 1 The diagnosis is frequently delayed because of initial misdiagnosis as benign lesions or inadequate biopsy technique. 3 Median time from symptom onset to diagnosis can be 15-20 months, with lesions on the toes particularly prone to delayed diagnosis and more advanced disease at presentation. 3
Biopsies can be misinterpreted histologically—if the lesion persists after initial biopsy showing benign findings, maintain a low threshold for repeat biopsy. 1 Only 5 of 34 patients in one series presented with classic longitudinal melanonychia, emphasizing that absence of this feature does not exclude melanoma. 3
Histopathologic Features to Request
Ensure the pathologist experienced in melanocytic lesions evaluates the specimen and reports: 1
- Breslow thickness (measured to nearest 0.1 mm)
- Presence or absence of ulceration
- Mitotic rate (number per mm²)
- Margins of excision (peripheral and deep)
- Clark level
- Presence of pagetoid spread, cytologic atypia, and tumor-infiltrating lymphocytes 4
Staging Considerations After Diagnosis
If melanoma is confirmed, do not perform routine imaging (CT, PET/CT, MRI) for stage I-II disease unless investigating specific signs or symptoms. 5, 6 For stage IIB-IIC, chest radiograph is optional. 5, 6
For clinically positive regional lymph nodes, confirm with fine-needle aspiration or open biopsy before proceeding with definitive treatment. 5, 6 Subungual melanomas often present at advanced stages (79% Clark level IV-V, median Breslow thickness 3.2 mm), with 32% having stage III disease at diagnosis. 4