What causes a black fingernail and how is it treated?

Black Fingernail: Causes and Treatment

A black fingernail requires immediate differentiation between benign causes (trauma, fungal/bacterial infection, melanocytic activation) and malignant melanoma, with the diagnostic approach prioritizing dermoscopy and laboratory confirmation before initiating treatment.

Diagnostic Approach: Determine the Pigment Source

Exogenous (External) Causes

• Bacterial infection (Pseudomonas aeruginosa) causes green-to-black discoloration and requires keeping the area dry with topical povidone iodine 2% twice daily 1
• Fungal infection (onychomycosis) presents with thickening, black/white/green marks, friable texture, and nail plate opacity—but diagnosis must be confirmed via KOH preparation and fungal culture on Sabouraud's glucose agar, as 50% of dystrophic nails are non-fungal despite similar appearance 2, 1
• Trauma/blood under the nail appears as dark discoloration and typically has a clear history of injury 3

Endogenous (Melanin-Based) Causes

• Melanocytic activation (benign) occurs in dark-skinned individuals, pregnancy, chronic trauma, medications, or systemic conditions—presenting as longitudinal brown-black bands 4, 5
• Melanocytic hyperplasia includes benign nevi/lentigo versus nail unit melanoma, which is the critical diagnosis not to miss 5, 6

Red Flags for Malignant Melanoma

Suspect nail unit melanoma if ANY of the following are present:

• Abrupt onset after middle age with rapid growth or darkening 4
• Bandwidth >3 mm with blurry lateral borders and pigment variegation 4
• Proximal widening of the band (triangle shape) 4
• Hutchinson sign: periungual pigment spread onto cuticle or nail folds 4
• Single digit involvement (rather than multiple) with associated nail dystrophy 4
• Personal or family history of melanoma 4

Diagnostic Tools

Initial Evaluation

• Dermoscopy should be used routinely to differentiate melanin from non-melanin pigment and to assess for malignancy patterns before considering biopsy 5, 7
• Laboratory confirmation for suspected infection: KOH preparation, fungal culture, and calcofluor white staining enhance visualization of fungal elements 1
• Examine family members for fungal infections, as household transmission is common 1

When to Biopsy

• Nail matrix tangential excisional biopsy is the gold standard for suspicious melanonychia to diagnose or rule out melanoma 5
• Longitudinal excision is recommended when invasive melanoma is highly suspected 5
• In children, benign melanonychia may mimic adult melanoma clinically, so a conservative wait-and-see approach with close follow-up is recommended rather than immediate biopsy 5

Treatment Based on Confirmed Etiology

Bacterial Infection (Black/Green Nail)

• Keep affected area dry and apply topical povidone iodine 2% twice daily 1, 8

Confirmed Fungal Infection (Onychomycosis)

Systemic therapy is required for nail plate involvement:

• Pulse itraconazole (5 mg/kg/day for 1 week every month): 2 months for fingernails, 3 months for toenails, with 94-100% clinical cure rates 1
• Daily terbinafine: 6 weeks for fingernails, 12 weeks for toenails, with 88% cure rate 1, 9
  • Dosing: 62.5 mg/day if <20 kg, 125 mg/day for 20-40 kg, 250 mg/day if >40 kg 1
  • FDA warning: Obtain baseline liver function tests before prescribing; discontinue immediately if persistent nausea, vomiting, right upper abdominal pain, jaundice, dark urine, or pale stools develop 9
• Azoles are preferred when Candida species are confirmed 1
• Topical amorolfine 5% lacquer may be used for superficial white onychomycosis or early distal involvement affecting <80% of nail plate without lunula involvement, applied once weekly for 6-12 months 2

Benign Melanonychia

• Wait-and-see approach with prolonged follow-up for early detection of malignant changes 3, 4
• Especially appropriate in children and dark-skinned individuals with multiple digit involvement 4, 5

Malignant Melanoma

• Surgical excision is mandatory 3

Common Pitfalls to Avoid

• Never diagnose fungal infection based on appearance alone—50% of dystrophic nails are non-fungal, requiring laboratory confirmation 1
• Never assume benign melanonychia in adults with single-digit involvement—maintain high suspicion for melanoma and use dermoscopy 4, 5
• Never overlook family examination when fungal infection is suspected, as household transmission is common 1
• Never ignore Hutchinson sign (periungual pigment spread)—this mandates immediate biopsy for melanoma 4
• In Candida paronychia, bacterial superinfection is common, making it difficult to determine which organism causes nail damage—consider both 2