What is the cause of and treatment for dark toenails in a 15-year-old male football player?

Dark Toenails in a 15-Year-Old Football Player

In a 15-year-old football player with dark toenails, the most likely diagnosis is onychomycosis (fungal nail infection), which occurs 15 times more frequently in athletes compared to non-athletes due to repetitive trauma, increased sweating, and occlusive footwear, and requires mycological confirmation before initiating treatment with oral terbinafine. 1

Differential Diagnosis

The dark discoloration in this athletic adolescent most likely represents one of three conditions:

Most Likely: Onychomycosis (Fungal Infection)

• Athletes have a 15-fold higher prevalence of onychomycosis compared to non-athletes, with football specifically listed as a high-risk sport due to sudden starting/stopping movements that cause repetitive nail trauma 1
• The most common causative organism in children is T. rubrum (64% of pediatric cases), followed by T. tonsurans 1, 2
• Clinical features include nail thickening, yellow-orange discoloration, subungual hyperkeratosis, friable texture, and onycholysis 2, 3
• Concomitant tinea pedis occurs in 25% of pediatric onychomycosis cases 1, 4

Alternative: Subungual Hematoma (Trauma)

• Results from single heavy trauma or repeated microtrauma common in football players 5
• Usually oval-shaped but does not form a neat longitudinal streak 5
• More commonly found on the medial aspect of the great toe 5

Less Likely: Bacterial Infection (Green Nail Syndrome)

• Caused by Pseudomonas aeruginosa or Proteus species 2, 5
• Presents with greenish or grayish hue, not the typical dark brown/black coloration 2, 5
• Usually confined to lateral nail edge 5

Diagnostic Approach

Do not diagnose fungal infection based on appearance alone—50% of dystrophic nails in children are non-fungal despite similar clinical presentation. 2, 4

Essential Diagnostic Steps:

1. Obtain mycological confirmation before treatment using KOH preparation with direct microscopy and fungal culture on Sabouraud's glucose agar 2, 4
2. Calcofluor white staining enhances visualization of fungal elements 2
3. Examine for concomitant infections: Check for tinea pedis (feet), tinea capitis (scalp), and other body sites 2, 4
4. Screen family members: Examine parents and siblings for onychomycosis and tinea pedis, as household transmission is the most common route 4

Clinical Features Supporting Fungal Diagnosis:

• Subungual hyperkeratosis (thickening under the nail) 3
• White-yellow-orange subungual scales 3
• Absence of salmon-pink coloration 3
• Nail thickening and onycholysis 3
• Distal and lateral subungual pattern (most common in children) 1, 2

Treatment Algorithm

If Mycological Confirmation Shows Dermatophyte Infection:

First-Line: Oral Terbinafine 2, 4, 6

• Preferred systemic treatment with superior efficacy over other agents 2, 4
• Weight-based dosing:
  • <20 kg: 62.5 mg daily 2, 4, 6
  • 20-40 kg: 125 mg daily 2, 4, 6

  • 40 kg: 250 mg daily 2, 4, 6

• Duration: 12 weeks for toenails (6 weeks for fingernails if involved) 2, 6
• Clinical cure rate: 88% 2
• Baseline liver function tests and CBC recommended; monitor LFTs if continuous therapy >1 month 2, 4

Alternative First-Line: Itraconazole Pulse Therapy 2

• Equally effective, particularly useful if Candida species suspected 2
• Dosing: 5 mg/kg/day for 1 week per month 2
• Duration: 3 pulses (3 months total) for toenails 2
• Clinical cure rate: 94-100% 2

Adjunctive Measures:

• Topical terbinafine 1% cream twice daily for 1 week if concomitant tinea pedis present 4
• Nail debridement improves treatment response when used with pharmacologic therapy 7
• Daily topical emollients to cuticles and periungual tissues 2, 8

Prevention Strategies:

• Apply antifungal powder (miconazole, clotrimazole, or tolnaftate) inside shoes daily 2, 4
• Periodic terbinafine solution spraying into shoes 2
• Wear protective footwear in locker rooms and communal bathing facilities 4
• Keep affected area dry 2
• Cover active foot lesions with socks before wearing underwear to prevent groin spread 4

If Bacterial Infection Confirmed:

• Keep area dry and apply topical povidone iodine 2% twice daily 2

Follow-Up and Monitoring

• Optimal clinical effect seen months after treatment completion due to time required for healthy nail outgrowth 6
• Monitor for at least 48 weeks from treatment start to identify potential relapse 4
• Reassess clinical response at 4-6 weeks after initiating therapy 4
• Children achieve higher cure rates than adults but require extended monitoring 4
• Clinical relapse rate approximately 15% for patients evaluated at least 6 months after achieving clinical cure 6

Critical Pitfalls to Avoid

• Never assume fungal infection without laboratory confirmation—50% of dystrophic nails are non-fungal 2, 4
• Do not overlook family member examination—household transmission is common 2
• Do not ignore concomitant tinea pedis—present in 25% of pediatric onychomycosis cases and requires concurrent treatment 1, 4
• Warn patients to immediately report persistent nausea, anorexia, fatigue, vomiting, right upper abdominal pain, jaundice, dark urine, or pale stools (hepatotoxicity signs) 6
• Advise reporting taste/smell disturbances or depressive symptoms (terbinafine side effects requiring discontinuation) 6