What causes brown fingernails and how are they treated?

Brown Fingernails: Causes and Treatment

Brown fingernail discoloration (melanonychia) requires immediate evaluation to distinguish between benign causes like medication effects, fungal infection, or trauma versus potentially malignant lesions, with treatment directed at the underlying etiology.

Primary Causes to Consider

Medication-Induced Melanonychia

• Chemotherapeutic agents, particularly taxanes (docetaxel, paclitaxel), are the most common medication cause of brown nail discoloration, occurring in 34.9-43.7% of patients receiving these drugs 1
• Other anticancer agents including capecitabine, etoposide, cytarabine, cyclophosphamide, and doxorubicin can also cause melanonychia 2
• The discoloration results from melanin deposition in the nail plate and typically affects multiple nails 1

Fungal Infection (Onychomycosis)

• Fungal nail infections commonly cause brown discoloration along with nail thickening, friability, and dystrophy 3
• Diagnosis requires laboratory confirmation with KOH preparation, fungal culture, or nail biopsy before initiating treatment 4
• Bacterial superinfection with Pseudomonas produces characteristic green-brown discoloration rather than pure brown 5

Subungual Hematoma

• Blood collection under the nail from trauma creates brown-black discoloration that is typically oval-shaped rather than forming a neat longitudinal streak 6
• May result from single heavy trauma or repeated microtrauma (common on medial aspect of great toe) 6
• Unlike melanonychia, hematomas move distally with nail growth 7

Melanocytic Activation

• Longitudinal brown streaks result from melanin-producing melanocytes in the nail matrix 7, 6
• Malignant melanoma must be excluded, particularly if the streak is wider than 5mm, shows irregular borders, involves nail dystrophy, or demonstrates periungual pigmentation (Hutchinson's sign) 6
• Benign melanonychia can occur with systemic conditions including hyperthyroidism, liver disease, and chronic kidney disease 8, 9

Diagnostic Approach

Key Clinical Features to Assess

• Number of nails affected: Multiple nails suggest medication effect or systemic disease; single nail raises concern for melanoma or localized infection 3, 6
• Pattern of discoloration: Longitudinal streaks suggest melanocytic origin; diffuse brown suggests fungal infection or hematoma 7, 6
• Width and borders: Streaks >5mm with irregular borders warrant biopsy for melanoma exclusion 6
• Associated findings: Nail thickening/friability (fungal), nail dystrophy/bleeding mass (melanoma), or periungual pigmentation (Hutchinson's sign) 6

Laboratory Testing

• For suspected fungal infection: Obtain KOH preparation, fungal culture, or nail biopsy before treatment 4
• For suspected melanoma: Nail matrix biopsy with histopathology is mandatory 7, 6
• Consider systemic workup (renal function, liver function, thyroid studies) if multiple nails affected without clear cause 9, 8

Treatment Strategies

Medication-Induced Melanonychia

• Daily application of topical emollients to periungual folds, matrix, and nail plate 1
• Protective nail lacquers to limit water loss from the nail plate 1
• Frozen gloves (10-30°C for 90 minutes during taxane infusion) significantly reduce nail changes 1
• Avoid nail trauma, excessive water exposure, and harsh chemicals 1
• Discoloration typically resolves months after drug discontinuation as healthy nail grows out 1

Confirmed Onychomycosis

• Terbinafine 250mg orally once daily: 6 weeks for fingernails, 12 weeks for toenails 4
• Mycological cure rates: 79% for fingernails, 70% for toenails 4
• Optimal clinical effect seen months after treatment completion due to time required for healthy nail outgrowth 4
• Monitor liver transaminases (ALT/AST) before initiating treatment 4
• Clinical relapse rate approximately 15% at one year post-treatment 4

Malignant Lesions

• Complete surgical excision is mandatory for confirmed melanoma 7, 6
• Conservative excision to preserve digit function; amputation does not improve disease-free survival once melanoma completely removed 6

Benign Melanonychia

• Wait-and-see approach with serial photography to monitor for changes 7
• Patient education about warning signs requiring re-evaluation 7

Critical Pitfalls to Avoid

• Never assume brown discoloration is benign without proper evaluation, especially in single-nail involvement with irregular features 6
• Do not treat presumed fungal infection without laboratory confirmation, as this delays diagnosis of potentially serious conditions 4
• Recognize that color intensity and patient age alone cannot distinguish benign from malignant melanonychia, though subungual melanoma is rare in children 6
• Bacterial infection (Pseudomonas) produces green-brown, not pure brown discoloration, and is typically confined to lateral nail edges 6, 5