What are the causes and treatments of dark lines in fingernails?

Dark Lines in Fingernails: Causes and Management

Immediate Diagnostic Priority

Dark lines in fingernails require urgent evaluation to rule out melanoma, which presents as longitudinal brown-black lines in approximately 70% of cases and represents the most serious cause requiring immediate biopsy. 1, 2

Differential Diagnosis by Color Pattern

Brown-Black Lines (Longitudinal Melanonychia)

Melanonychia is caused by either melanocyte activation or proliferation in the nail matrix, and distinguishing between these mechanisms is critical for determining malignancy risk. 1, 2

• Malignant causes: Subungual melanoma typically presents as a single longitudinal brown-black band in adults, often with irregular borders, color variegation, and extension onto the periungual skin (Hutchinson sign) 3, 4, 2
• Benign causes: Include racial pigmentation (common in darker-skinned individuals), trauma, medications, inflammatory conditions (lichen planus, psoriasis), and fungal infections 3, 4
• Dermoscopy is essential for triaging lesions—regular parallel lines suggest benign activation, while irregular patterns suggest proliferation requiring biopsy 1, 2

Green-Black Discoloration

• Green Nail Syndrome from Pseudomonas aeruginosa or Proteus species infection causes green-black discoloration 5, 6
• Treatment requires keeping the area dry and applying topical povidone-iodine 2% twice daily 5, 6

Yellow-Brown to Black Lines

• Fungal infection (onychomycosis) causes thickening, yellow-brown to black discoloration, and friable texture 5, 7
• Trichophyton rubrum accounts for 64% of cases, with distal and lateral subungual onychomycosis being the most common pattern 5, 7
• Critical pitfall: 50% of dystrophic nails are non-fungal despite similar appearance—always obtain mycological confirmation with KOH preparation and fungal culture before initiating treatment 5, 7

Diagnostic Algorithm

Step 1: Determine if Single or Multiple Nails Affected

• Single nail with longitudinal brown-black band in an adult: High suspicion for melanoma—proceed directly to dermatology referral for dermoscopy and likely biopsy 2
• Multiple nails affected: Consider systemic causes (medications, inflammatory conditions) or infectious etiologies 3, 4

Step 2: Assess Associated Features

• Hutchinson sign (pigment extending onto periungual skin): Highly concerning for melanoma—urgent biopsy required 4
• Nail thickening, onycholysis, friable texture: Suggests fungal infection—obtain KOH preparation and fungal culture 5, 7
• Nail fold inflammation, pain, purulent discharge: Suggests bacterial infection (Pseudomonas)—culture and treat accordingly 5, 6

Step 3: Laboratory Confirmation

• For suspected melanoma: Dermoscopy followed by nail matrix biopsy with histopathologic examination is the gold standard 2
• For suspected fungal infection: Direct microscopy with KOH preparation and fungal culture on Sabouraud's glucose agar are essential 5
• Calcofluor white staining enhances visualization of fungal elements 5

Treatment Based on Confirmed Etiology

Melanoma or Suspicious Lesions

Any longitudinal melanonychia in a single digit in an adult without clear benign cause requires biopsy—malignant lesions must be excised. 4, 2

Confirmed Fungal Infection

Terbinafine is the preferred first-line systemic treatment for dermatophyte onychomycosis, with superior efficacy over other agents. 5

• Terbinafine dosing: 250 mg daily for 6 weeks (fingernails) or 12-16 weeks (toenails) 8
• Baseline liver function tests and complete blood count are recommended 8
• Alternative: Itraconazole 200 mg daily for 12 weeks continuously, or pulse therapy 400 mg daily for 1 week per month (2 pulses for fingernails, 3 for toenails) 8
• For Candida species: Azoles (itraconazole or fluconazole) are preferred 5

Bacterial Infection (Green Nail Syndrome)

• Keep affected area dry and apply topical povidone-iodine 2% twice daily 5, 6
• Avoid prolonged water immersion and wear protective gloves during wet work 6

Critical Pitfalls to Avoid

• Never assume fungal infection based on appearance alone—50% of dystrophic nails are non-fungal, and initiating antifungal therapy without mycological confirmation leads to treatment failure and delayed diagnosis of other conditions 5, 7
• Never dismiss a single longitudinal brown-black band in an adult as benign without dermoscopy or biopsy—melanoma can present subtly and early detection is critical for survival 2
• Never overlook examination of family members when fungal infection is confirmed, as household transmission is common 5
• Never neglect occupational counseling for patients with chronic moisture exposure—protective measures like wearing gloves are essential for preventing recurrence 6

Preventive Measures

• Apply daily topical emollients to periungual folds, nail matrix, and nail plate 6
• Use protective nail lacquers to limit water loss 6
• Avoid trauma, nail biting, use of nails as tools, prolonged water soaking, and exposure to harsh chemicals 6
• For confirmed fungal infections, apply regular antifungal powder inside shoes and periodic terbinafine solution spraying into shoes 5