What are the possible causes and treatments for a discolored single toe?

Discolored Single Toe: Causes and Treatment

Immediate Diagnostic Priority

A discolored single toe most likely represents either onychomycosis (fungal nail infection), bacterial infection (Green Nail Syndrome from Pseudomonas), or a non-infectious cause such as trauma, psoriasis, or lichen planus—and you must obtain laboratory confirmation before initiating systemic antifungal therapy, as 50% of dystrophic nails are non-fungal despite similar appearance. 1

Diagnostic Algorithm

Step 1: Visual Inspection and Texture Assessment

• Examine the nail texture carefully: Soft, friable texture suggests fungal infection, while hard, brittle texture indicates non-infectious causes 2
• Green discoloration specifically indicates Pseudomonas infection (Green Nail Syndrome) 2
• Thickening with yellow-brown discoloration and onycholysis suggests Distal and Lateral Subungual Onychomycosis (DLSO), the most common fungal pattern 3
• White crumbling lesions on the nail surface indicate Superficial White Onychomycosis (SWO), particularly in children 3

Step 2: Mandatory Laboratory Confirmation

Never diagnose fungal infection based on appearance alone—always obtain mycological confirmation before starting systemic therapy. 1

• Perform direct microscopy with potassium hydroxide (KOH) preparation from the discolored, dystrophic portions of the nail 1
• Send specimen for fungal culture on Sabouraud's glucose agar 1
• Consider calcofluor white staining to enhance visualization of fungal elements 1
• Rule out non-fungal causes including psoriasis, trauma, lichen planus, and yellow nail syndrome 3

Step 3: Examine for Associated Findings

• Check for paronychia (nail fold swelling/erythema), which suggests Candida infection, particularly with occupational moisture exposure 2
• Examine family members for onychomycosis and tinea pedis, as household transmission is common 1
• Look for concomitant infections such as tinea capitis and tinea pedis 1

Treatment Based on Confirmed Etiology

For Confirmed Fungal Infection (Dermatophyte Onychomycosis)

Terbinafine is the preferred first-line treatment for dermatophyte onychomycosis with superior efficacy over other agents. 1

Terbinafine Dosing (Weight-Based):

• <20 kg: 62.5 mg daily 1
• 20-40 kg: 125 mg daily 1
• >40 kg: 250 mg daily 1, 4

Treatment Duration:

• Fingernails: 6 weeks 1
• Toenails: 12 weeks 1, 4
• Expected mycological cure rate: 70% for toenails, 79% for fingernails 4

Alternative First-Line: Itraconazole Pulse Therapy

• Dosing: 5 mg/kg/day for 1 week per month 1
• Duration: 2 pulses (2 months) for fingernails, 3 pulses (3 months) for toenails 1
• Particularly useful when Candida species are suspected 1
• Clinical cure rates: 94-100% 1

Monitoring Requirements:

• Baseline liver function tests and complete blood count before starting terbinafine 1
• Monitor liver function tests for patients on itraconazole, particularly with pre-existing abnormalities or continuous therapy >1 month 1
• Patients must immediately report persistent nausea, anorexia, fatigue, vomiting, right upper abdominal pain, jaundice, dark urine, or pale stools 4

For Bacterial Infection (Green Nail Syndrome)

• Keep the affected area dry 1, 2
• Apply topical povidone iodine 2% twice daily 1, 2

For Candida Onychomycosis

• Azoles are preferred when onychomycosis is caused by Candida species 1
• Address underlying factors such as chronic moisture exposure 3

Preventive Measures

• Apply regular antifungal powder inside shoes 1
• Periodic terbinafine solution spraying into shoes 1
• Daily topical emollients to cuticles and periungual tissues 1, 2
• Avoid excessive moisture exposure and wear gloves while cleaning or working with chemicals 2

Critical Pitfalls to Avoid

• Do not assume fungal infection without laboratory confirmation—the nail surface in non-infectious conditions typically doesn't become soft and friable as in fungal infection 2
• Do not overlook examination of family members for fungal infections, as household transmission is common 1
• Do not ignore occupational counseling—patients with chronic moisture exposure need guidance on protective measures 2
• Warn patients about photosensitivity with terbinafine and advise minimizing exposure to natural and artificial sunlight 4
• Discontinue treatment immediately if patients develop hives, mouth sores, blistering and peeling of skin, or difficulty swallowing/breathing 4

Expected Timeline

• Mean time to overall success: approximately 10 months for toenails, 4 months for fingernails 4
• Optimal clinical effect is seen months after mycological cure due to time required for outgrowth of healthy nail 4
• Clinical relapse rate: approximately 15% at least six months after achieving clinical cure 4