What is the cause and appropriate treatment for white lines appearing on the fingernails, potentially indicating a nutritional deficiency or systemic disease?

White Lines on Fingernails: Diagnostic and Management Approach

White lines on fingernails (leukonychia) require systematic evaluation to distinguish between benign causes, fungal infection, and serious systemic disease—laboratory confirmation is mandatory before initiating treatment, as 50% of nail dystrophies are non-fungal despite similar appearance. 1

Immediate Diagnostic Classification

White nail discoloration must be categorized anatomically to guide workup:

• True leukonychia originates from the nail plate itself and moves distally with nail growth 2
• Apparent leukonychia arises from nail bed abnormalities and does not move with growth (e.g., Terry's nails, Muehrcke's lines) 2
• Pseudoleukonychia affects only the nail surface and can be scraped off 2

Critical Differential Diagnosis

Fungal Infection (Superficial White Onychomycosis)

• Superficial white onychomycosis presents as white (not cream) discoloration with noticeably flaky nail surface, affecting the nail plate rather than the nail bed 3
• Most commonly caused by T. mentagrophytes, and onycholysis is NOT a common feature 3
• Never diagnose fungal infection based on appearance alone—direct microscopy with KOH preparation and fungal culture on Sabouraud's agar are essential, as 50% of nail dystrophy cases are non-fungal 1
• Calcofluor white staining enhances visualization of fungal elements 1

Systemic Disease Indicators

• Transverse white bands (Mees' lines) can indicate previous severe systemic illness, heavy metal toxicity, or chemotherapy exposure 2, 4
• Transverse leukonychia has been documented following acute systemic infections including tuberculosis and bacterial empyema 4
• Muehrcke's lines (paired transverse white bands that do not move with nail growth) warrant albumin level checking and workup if low, suggesting hypoalbuminemia from liver or kidney disease 5
• Terry's nails (proximal white discoloration with distal pink band) may indicate liver cirrhosis, congestive heart failure, or diabetes 6

Other Important Causes

• Trauma and manicure-related injury are common benign causes 2
• Psoriasis, lichen planus, and yellow nail syndrome can mimic infectious causes 1, 7
• Chemotherapy (particularly taxanes) causes melanonychia in 43.7% of patients, though this presents as dark rather than white discoloration 8

Diagnostic Algorithm

Step 1: Determine if white discoloration moves with nail growth

• If moves distally → true leukonychia (nail plate origin)
• If stationary → apparent leukonychia (nail bed origin, suggests systemic disease)
• If superficial and scrapable → pseudoleukonychia 2

Step 2: Obtain detailed history

• Recent severe illness or hospitalization 4
• Medication history including chemotherapy 8
• Occupational water exposure (for candidal infection) 3
• Examine family members for fungal infections 1

Step 3: Laboratory confirmation before treatment

• Scrape surface of nail plate for KOH preparation and fungal culture if superficial white appearance 3, 1
• Check albumin level if transverse bands present 5
• Consider liver and kidney function tests if total/partial leukonychia present 6

Treatment Based on Confirmed Etiology

For Confirmed Superficial White Onychomycosis

First-line systemic therapy:

• Terbinafine (preferred): Weight-based dosing for 6 weeks for fingernails 1
  • <20 kg: 62.5 mg daily
  • 20-40 kg: 125 mg daily

  • 40 kg: 250 mg daily

• Itraconazole pulse therapy: 5 mg/kg/day for 1 week per month for 2 pulses (2 months total) for fingernails, with 94-100% clinical cure rates 1

Monitoring requirements:

• Baseline liver function tests and complete blood count for terbinafine 1
• Monitor liver function for itraconazole, particularly if continuous therapy >1 month 1

For Systemic Disease-Related Leukonychia

• Address underlying condition (liver disease, kidney disease, hypoalbuminemia) 5, 6
• No antifungal treatment indicated 2

Critical Pitfalls to Avoid

• Never assume fungal infection without laboratory confirmation—50% of dystrophic nails are non-fungal despite similar appearance 1
• Do not overlook examination of family members for fungal infections, as household transmission is common 1
• Do not initiate long-term antifungal therapy on clinical grounds alone—treatment requires 6-12 months for complete nail regrowth, making empiric treatment costly and potentially harmful 3, 7
• Consider immunosuppression if proximal subungual pattern present, as this may indicate HIV infection or other immunocompromised states 3