Toenail Discoloration: Causes, Diagnosis, and Management
Immediate Diagnostic Priority
Do not initiate treatment based on clinical appearance alone—mycological confirmation is mandatory before starting antifungal therapy, as approximately 50% of all dystrophic nails are non-fungal despite appearing clinically identical to fungal infections. 1
Differential Diagnosis by Clinical Pattern
Fungal Infection (Onychomycosis) – Most Common Cause
Clinical features:
- Distal and lateral subungual onychomycosis (DLSO) is the most common pattern, presenting with nail plate thickening, yellow-brown discoloration, subungual hyperkeratosis, onycholysis, and a soft, friable texture starting distally/laterally 1
- Toenails are affected in 80% of onychomycosis cases 1
- Often associated with concurrent tinea pedis 1
- Trichophyton rubrum causes approximately 64% of cases 1, 2
Bacterial Infection (Green Nail Syndrome)
Clinical features:
- Green or black discoloration indicates Pseudomonas aeruginosa or Proteus species infection 3, 2
- Unlike fungal infection, the nail surface typically doesn't become soft and friable 3
- May coexist with fungal infection 4
Inflammatory Dermatoses
Psoriasis:
- Nail pitting, oil drop sign, and subungual hyperkeratosis 1
- Usually involves multiple nails with associated cutaneous plaques 4
Lichen planus:
- Thinning of nail plate, subungual hyperkeratosis, onycholysis, dorsal pterygium, and longitudinal ridging 1, 4
- Occurs in approximately 10% of cases with nail involvement 1
Trauma
Clinical features:
- Examination of nail bed after clipping shows intact longitudinal epidermal ridges stretching to lunula (normal nail bed) 1
- Hard, brittle texture rather than soft and friable 4
Diagnostic Algorithm
Step 1: Specimen Collection (Critical for Accuracy)
Proper technique:
- Collect specimens from discolored, dystrophic, or brittle parts of the nail, cutting through the entire thickness including crumbly material 1
- For DLSO, scrape material from beneath the nail plate and from the nail bed itself 4
- Collect subungual debris from the most proximal part of the infection using a dental scraper 4
- Submit as much material as possible due to relative paucity of fungal elements 4
Step 2: Laboratory Confirmation (Choose One or More)
First-line testing:
- Potassium hydroxide (KOH) preparation with microscopy—first-line screening test 1, 4
- Calcofluor white staining—significantly increases sensitivity compared to KOH alone 1
- Fungal culture on Sabouraud's glucose agar—identifies specific pathogen species, incubated at 28°C for at least 3 weeks 1, 4
Enhanced sensitivity options:
- Histopathological examination with periodic acid-Schiff (PAS) staining—more sensitive than direct microscopy or culture in recent studies 1, 5
- Real-time PCR assays—turnaround time less than 2 days with significantly increased detection rate compared to culture (though may detect dead fungus) 1
Step 3: Rule Out Non-Fungal Mimics
Examine for:
- Paronychia (nail fold swelling/erythema)—suggests Candida infection, particularly with occupational moisture exposure 4
- Nail pitting or onycholysis involving majority of nails—suggests psoriasis 4
- Soft, friable texture with thickening—strongly suggests dermatophyte infection 4
- Green or black discoloration—indicates bacterial infection 4
Treatment Based on Confirmed Etiology
Confirmed Fungal Infection (Dermatophyte)
Oral therapy (preferred):
- Terbinafine is first-line: 250 mg daily for 12 weeks for toenails (88-76% cure rate) 3, 6, 5
- Itraconazole pulse therapy: 200 mg twice daily for 1 week per month × 3 pulses (3 months total) for toenails (63% cure rate) 5
- Fluconazole (second-line): Weekly dosing for 18-26 weeks for toenails 3
Topical therapy (for mild to moderate disease affecting <50% of nail plate):
- Efinaconazole 10% or tavaborole 5% or ciclopirox 8% 6
- Less effective than oral agents but fewer adverse effects and drug interactions 6
Adjunctive measures:
- Nail trimming and debridement used concurrently with pharmacologic therapy improve treatment response 6
- Daily urea-based keratolytic cream to reduce nail thickness 4
Confirmed Candida Infection
Oral therapy:
- Azoles are preferred when onychomycosis is caused by Candida species 3
- Itraconazole or fluconazole 3, 2
Adjunctive measures:
- Keep affected area dry and avoid prolonged water exposure 4
- Use protective gloves during wet work or when handling chemicals 4
Bacterial Infection (Pseudomonas)
Treatment:
- Keep affected area dry 3, 4
- Apply topical povidone iodine 2% twice daily 3
- Topical antiseptic agents (octenidine) or topical antibiotics (nadifloxacin, gentamicin) 2
- Oral ciprofloxacin for severe Pseudomonas infections 2
Inflammatory Dermatoses (Psoriasis/Lichen Planus)
For <3 nails affected:
- Intralesional triamcinolone acetonide 5-10 mg/cc for nail matrix involvement 4
- Topical steroids with or without vitamin D analogs for nail bed involvement 4
Critical Pitfalls to Avoid
Most common cause of treatment failure:
- Incorrect diagnosis based solely on clinical grounds without laboratory confirmation 4
- Inadequate specimen collection from superficial nail plate rather than proximal subungual debris contributes to false negatives 4
Do not overlook:
- Examination of family members for fungal infections, as household transmission is common 3
- Screening for systemic conditions (thyroid function, complete blood count) in cases of brittle nails 4
- Baseline liver function tests and complete blood count for patients on terbinafine 3
Suspect non-dermatophyte moulds when:
- Previous antifungal treatment has failed repeatedly 4
- Direct microscopy is positive but no dermatophyte isolated 4
- No associated skin infection is present 4
Prevention and Recurrence Reduction
Preventive measures (25% relapse rate):