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, infection, ethnic pigmentation) and malignant melanoma, with treatment ranging from conservative observation to urgent surgical excision depending on the underlying etiology.

Diagnostic Approach: Identifying the Cause

The color and pattern of nail discoloration provides critical diagnostic clues:

Infectious Causes

Fungal infection presents with thickening, discoloration, and friable texture of the nail plate 1. Black marks can appear in the proximal and lateral portions of the nail in Candida paronychia, often accompanied by painful, swollen periungual skin 1. However, do not diagnose fungal infection based on appearance alone—50% of dystrophic nails are non-fungal despite similar clinical presentation 2.

• Direct microscopy with potassium hydroxide (KOH) preparation and fungal culture on Sabouraud's glucose agar are essential for confirming fungal infection 2
• Calcofluor white staining enhances visualization of fungal elements 2

Bacterial infection (Green Nail Syndrome) from Pseudomonas aeruginosa or Proteus species produces greenish or grayish-black discoloration, typically confined to the lateral nail edge 2, 3.

Traumatic Causes

Subungual hematoma results from single heavy trauma or repeated microtrauma and appears as an oval-shaped discoloration that does not form a neat longitudinal streak 3. This is commonly found on the medial aspect of the great toe from repetitive minor trauma 3.

Melanocytic Causes (Most Critical to Identify)

Longitudinal melanonychia appears as a tan, brown, or black longitudinal streak running from the proximal nail fold to the distal nail plate 4. This occurs due to increased melanocyte activity or melanocytic hyperplasia in the nail matrix 4, 5.

Benign causes include:

• Ethnic melanonychia in dark-skinned individuals 4
• Pregnancy, chronic local trauma, medications 4
• Nail matrix melanocytic nevus or nail lentigo 4

Malignant melanoma warning signs (Hutchinson sign and associated features):

• Abrupt onset after middle age 4
• Rapid growth or darkening of the melanonychia band 4
• Bandwidth >3 mm with proximal widening 4
• Blurry lateral borders with pigment variegation 4
• Periungual spread of pigmentation onto the adjacent cuticle, proximal or lateral nail folds 4, 3
• Associated nail plate dystrophy or bleeding mass 3
• Single digit involvement rather than multiple digits 4

Treatment Based on Etiology

For Confirmed Fungal Infection

In children, pulse itraconazole (5 mg/kg/day for 1 week every month) for 2 months for fingernails or 3 months for toenails achieves 94-100% clinical cure rates 2.

Alternatively, daily terbinafine for 6 weeks (fingernails) or 12 weeks (toenails) provides 88% cure rate 2:

• 62.5 mg/day if weight <20 kg
• 125 mg/day for 20-40 kg
• 250 mg/day if weight >40 kg 2

Azoles are preferred when onychomycosis is caused by Candida species 2.

For limited disease (<80% nail plate involvement, no lunula involvement), topical amorolfine 5% lacquer applied once or twice weekly for 6-12 months achieves approximately 50% effectiveness 1.

For Bacterial Infection

Keep the affected area dry and apply topical povidone iodine 2% twice daily 2, 6.

For Subungual Hematoma

Conservative observation is appropriate as the discoloration will grow out with the nail 3.

For Melanocytic Lesions

Benign melanonychia in children warrants a wait-and-see approach with prolonged follow-up for early detection of malignant changes 4, 7.

Suspected melanoma requires urgent biopsy and excision 3, 7. Treatment should be as conservative as possible to preserve the digit tip; once completely removed, amputations have not been shown to prolong disease-free survival time 3.

Critical Pitfalls to Avoid

• Never assume fungal infection without laboratory confirmation—50% of dystrophic nails are non-fungal 2
• Do not overlook examination of family members for fungal infections, as household transmission is common 2
• Do not dismiss melanonychia in adults over 40 years old—subungual melanomas carry a poor prognosis due to average diagnostic delays of 2 years 8
• Splinter hemorrhages are not a primary feature of onychomycosis; their presence should prompt consideration of other diagnoses including vascular insufficiency, psoriasis, or drug-induced causes 9