Diagnosis and Treatment of Onychomycosis

Importance of Laboratory Confirmation

The British Association of Dermatologists recommends that treatment should not be instituted on clinical grounds alone, and mycological confirmation is mandatory before initiating antifungal therapy, as approximately 50% of all dystrophic nails are non-fungal in origin despite appearing clinically identical to fungal infections 1, 2
The British Association of Dermatologists explicitly states that treatment should not be prescribed without mycological confirmation because long treatment duration is required, treatment is not always successful, and multiple non-fungal mimics exist 1, 2

The British Association of Dermatologists recommends obtaining mycological confirmation through one or more of the following methods: potassium hydroxide (KOH) preparation with microscopy, calcofluor white staining, fungal culture on Sabouraud's glucose agar, histopathological examination with periodic acid-Schiff (PAS) staining, and real-time PCR assays 3, 4
Potassium hydroxide (KOH) preparation with microscopy should be performed by collecting specimens from discolored, dystrophic, or brittle parts of the nail, cutting through the entire thickness including crumbly material 4
Calcofluor white staining significantly increases sensitivity compared to KOH alone by enhancing visualization of fungal elements 3, 4
Histopathological examination with periodic acid-Schiff (PAS) staining is more sensitive than direct microscopy or culture in recent studies 4
Real-time PCR assays have a turnaround time of less than 2 days and significantly increased detection rate compared to culture, though may detect dead fungus 4

Distal and lateral subungual onychomycosis (DLSO) is the most common pattern, with subungual hyperkeratosis, onycholysis, and nail plate thickening starting distally/laterally, and is often associated with tinea pedis 1, 3
Toenails are more affected than fingernails, with 80% of onychomycosis cases involving toenails 1, 2
Lichen planus should be suspected in approximately 10% of cases with nail involvement, characterized by thinning of nail plate, subungual hyperkeratosis, onycholysis, and dorsal pterygium 3, 4
Psoriasis should be suspected in cases with nail pitting, oil drop sign, and subungual hyperkeratosis 4
Trauma should be suspected in cases with examination of nail bed after clipping showing intact longitudinal epidermal ridges stretching to lunula (normal nail bed) 4
Proximal subungual onychomycosis (PSO) should be suspected in cases with underlying conditions such as HIV, immunosuppression, peripheral vascular disease, or diabetes 1, 2

The British Journal of Dermatology describes onychomycosis as characterized by nail plate thickening, yellow-brown discoloration, and varying degrees of onycholysis, with distal and lateral subungual onychomycosis (DLSO) being the most common clinical pattern 5
Nail plate thickening, also known as subungual hyperkeratosis, occurs due to fungal invasion of the nail bed, resulting in an abnormally thick nail 5
Onycholysis, or separation of the nail plate from the underlying nail bed, typically starts distally and progresses proximally 5
The nail becomes brittle and crumbly in advanced cases, with a friable texture 6

DLSO accounts for the majority of fungal nail infections in adults, where the fungus invades through the distal or lateral nail margins, according to the British Journal of Dermatology 5, 7
Toenails are more commonly affected than fingernails in this pattern 5
Trichophyton rubrum is the causative organism in the majority of cases, approximately 64% of some series 8
The American Academy of Dermatology recommends mycological confirmation before initiating treatment, which can be achieved through potassium hydroxide (KOH) preparation and fungal culture on Sabouraud's glucose agar 6, 8
Calcofluor white staining enhances visualization of fungal elements, aiding in diagnosis 8

Approximately 50% of dystrophic nails are non-fungal, despite similar clinical presentation, highlighting the importance of mycological confirmation 6, 8
Alternative diagnoses include psoriasis, lichen planus, trauma, and yellow nail syndrome, which must be considered before initiating treatment 8
Associated findings, such as onycholysis, subungual debris, or nail bed inflammation, should be documented, as they are important for treatment selection and outcome assessment 5, 8