Thick Discolored Nail Beds: Diagnosis and Treatment
Most Likely Diagnosis
The most likely diagnosis is onychomycosis (fungal nail infection), specifically Distal and Lateral Subungual Onychomycosis (DLSO), which accounts for the majority of cases and presents with nail thickening, yellow-brown discoloration, and separation of the nail plate from the bed. 1, 2
However, you must obtain laboratory confirmation before starting any treatment—approximately 50% of dystrophic nails are non-fungal despite appearing clinically identical to fungal infections. 1, 2
Diagnostic Approach
Mandatory Laboratory Testing
Do not prescribe treatment based on clinical appearance alone. 1 The British Association of Dermatologists explicitly mandates mycological confirmation because:
- Treatment duration is lengthy (6-12 months) 3
- Treatment is not always successful 1
- Multiple non-fungal mimics exist 1
Recommended Diagnostic Methods (in order of preference):
Potassium hydroxide (KOH) preparation with microscopy - Collect specimens from discolored, dystrophic, or brittle parts of the nail, cutting through the entire thickness including crumbly material 1, 4
Calcofluor white staining - Significantly increases sensitivity compared to KOH alone by enhancing visualization of fungal elements 1
Histopathological examination with periodic acid-Schiff (PAS) staining - More sensitive than direct microscopy or culture in recent studies 1
Real-time PCR assays - Turnaround time less than 2 days with significantly increased detection rate compared to culture, though may detect dead fungus 1
Fungal culture on Sabouraud's glucose agar - Identifies specific organism (Trichophyton rubrum causes ~64% of cases) 1, 4
Alternative Diagnoses to Rule Out
Before confirming onychomycosis, consider these non-fungal causes:
- Psoriasis - Look for nail pitting, oil drop sign, and subungual hyperkeratosis 1, 3
- Lichen planus - Characterized by nail plate thinning, subungual hyperkeratosis, onycholysis, and dorsal pterygium (occurs in ~10% of cases with nail involvement) 1
- Trauma - Examine nail bed after clipping; intact longitudinal epidermal ridges stretching to lunula indicate normal nail bed (trauma rather than infection) 1, 2
- Bacterial infection (Green Nail Syndrome) - Green or black discoloration from Pseudomonas aeruginosa 2
Treatment Algorithm
If Dermatophyte Infection is Confirmed:
First-line therapy: Oral terbinafine 250 mg daily 2, 4
- Fingernails: 6 weeks 2, 4
- Toenails: 12 weeks 2, 4
- Mycological cure rates: 70% for toenails, 79% for fingernails 4
- Effective treatment rates: 59% for toenails, 75% for fingernails 4
Important monitoring: Measure serum transaminases (ALT and AST) before starting treatment 4. Monitor for drug-drug interactions if patient takes tricyclic antidepressants, SSRIs, atypical antipsychotics, beta blockers, or tamoxifen 5
If Candida Species are Confirmed:
Use azole antifungals (itraconazole or fluconazole) instead of terbinafine 2, as azoles are preferred for Candida onychomycosis 2
Address underlying factors such as wet occupations or chronic paronychia 2, 3
Adjunctive Measures to Increase Cure Rates:
- Concomitant nail debridement 2, 5, 6
- Apply regular antifungal powder inside shoes 2
- Periodic terbinafine solution spraying of shoes 2
- Minimize exposure to natural and artificial sunlight (photosensitivity reactions reported) 4
Topical Therapy (Less Effective Alternative)
Consider topical therapy only for mild to moderate disease limited to distal nail plate 5, 7:
Topical agents have fewer adverse effects and drug interactions but lower cure rates and much longer treatment duration 5, 8
Critical Red Flags
Proximal subungual pattern is uncommon except in immunocompromised patients and may indicate HIV infection—this warrants systemic disease evaluation 2, 3. Also suspect this pattern in patients with peripheral vascular disease or diabetes 1
Expected Timeline and Follow-up
- Mean time to overall success: ~10 months for toenails, ~4 months for fingernails 4
- Optimal clinical effect: Seen months after mycological cure due to time required for outgrowth of healthy nail 4
- Clinical relapse rate: Approximately 15% at one year after completing therapy 4
- Overall recurrence rate: 10-50% due to reinfection or lack of mycological cure 9, 6
Patient Education for Prevention
- Avoid walking barefoot in public places 5, 8
- Disinfect shoes and socks 5, 8
- Prophylactic application of topical antifungals may help prevent recurrence 8
Common Pitfalls to Avoid
- Starting treatment without laboratory confirmation - This is the most critical error, as 50% of dystrophic nails are non-fungal 1, 2
- Early termination of treatment - Full course is essential even if nail appears improved 9
- Ignoring associated tinea pedis - DLSO is nearly always accompanied by tinea pedis on surrounding skin, which must also be treated 2
- Not monitoring liver function - Hepatotoxicity is a known adverse effect of oral antifungals 4, 5