What are fungal nails (onychomycosis)?

Fungal Nails (Onychomycosis)

Fungal nails, or onychomycosis, is a chronic fungal infection of the nail bed and plate that causes thickening, discoloration, brittleness, and separation of the nail, most commonly affecting toenails and caused by dermatophytes, particularly Trichophyton rubrum. 1

Clinical Presentation and Patterns

Onychomycosis manifests in five distinct clinical patterns, each with characteristic features 1:

Distal and Lateral Subungual Onychomycosis (DLSO)

• Most common presentation of dermatophyte nail infection 1
• Toenails affected more frequently than fingernails 1
• Fungus invades through distal or lateral nail margins 1
• Nail becomes thickened and discolored with varying degrees of onycholysis (separation of nail plate from nail bed) 1
• Nail plate initially remains intact 1

Superficial White Onychomycosis (SWO)

• Infection begins at superficial nail plate layer and spreads deeper 1
• Crumbling white lesions appear on nail surface, particularly toenails 1
• Gradually spreads until entire nail plate involved 1
• Most commonly seen in children, usually caused by T. interdigitale 1
• Deep penetrating forms do not respond well to topical therapy 1

Proximal Subungual Onychomycosis (PSO)

• Least common presentation in general population but common in persons with AIDS and considered a useful marker of HIV infection 1
• Infection originates in proximal nail fold or beneath proximal nail plate 1
• Distal nail portion remains normal until late in disease course 1
• T. rubrum is usual cause 1
• In AIDS patients, infection spreads rapidly producing gross white discoloration without obvious thickening 1

Total Dystrophic Onychomycosis (TDO)

• Advanced stage where nail plate is almost completely destroyed 2
• Represents end-stage of untreated onychomycosis 1

Secondary Candidal Onychomycosis

• Occurs in other nail diseases, most notably psoriasis 1
• Usually begins in proximal nail plate with nail fold infection (paronychia) present 1
• Most common causative organisms are T. soudanense and T. violaceum 1

Clinical Impact and Complications

Onychomycosis is not merely cosmetic—it significantly impacts quality of life and can cause serious medical complications, particularly in vulnerable populations. 1

Functional and Quality of Life Impact

• Approximately 50% of patients experience pain or discomfort 1
• About 30% have difficulty wearing footwear 1
• Difficulty walking, emotional embarrassment, and work-related difficulties are most commonly reported 1
• Severe cases negatively influence patients' sex lives 1
• Socks and stockings frequently damaged due to friction with sharp, dystrophic nails 1

Medical Complications

• Infected nails serve as reservoir of fungi with potential for spread to feet, hands, and groin 1
• Fungal diseases are contagious and may spread to other family members if untreated 1
• In diabetic patients, onychomycosis disrupts skin integrity, providing entry point for bacteria leading to foot ulcers, osteomyelitis, cellulitis, and gangrene 1
• Huge financial implications of neglecting onychomycosis in diabetic patients 1
• Sensitizing fungal/dermatophytic antigens in nail plate may predispose to asthma/respiratory sensitization, atopic dermatitis, urticaria, and erythema nodosum 1

Differential Diagnosis

Many noninfectious conditions mimic onychomycosis, but the nail surface typically does not become soft and friable as in fungal infection. 1

Key Distinguishing Features

• Bacterial infection (particularly Pseudomonas aeruginosa) causes green or black discoloration 1
• Candidal onychomycosis typically shows lesser degree of subungual hyperkeratosis and absence of toenail involvement 1
• Psoriasis produces nail changes but without soft, friable texture 1
• Other mimics include chronic trauma, onycholysis, onychogryphosis, subungual malignant melanoma, lichen planus, Darier disease, and yellow nail syndrome 1

Diagnostic Confirmation

• Laboratory confirmation is essential before initiating treatment because therapy is lengthy and can cause adverse effects 1, 3
• Direct microscopy with potassium hydroxide (KOH) preparation and fungal culture are gold standard 1, 4, 3
• Calcofluor white staining enhances visualization of fungal elements, increasing sensitivity over KOH alone 1
• Real-time PCR assays detect dermatophytes in <2 days with higher detection rates than culture, but may detect nonpathogenic or dead fungus 1
• Specimens should be taken from discolored, dystrophic, or brittle nail parts, cutting through entire thickness including crumbly material 1

Treatment Considerations

Treatment duration is lengthy, typically 6-12 months, with optimal clinical effect seen months after mycological cure due to time required for healthy nail outgrowth. 2, 5

Oral Therapy

• Oral terbinafine is preferred over topical therapy due to better effectiveness and shorter treatment duration 3
• Terbinafine: 250 mg once daily for 6 weeks (fingernails) or 12 weeks (toenails) 5
• Mycological cure rates of 70% for toenails, 79% for fingernails 5
• Clinical relapse rate approximately 15% at one year after completing therapy 5
• Liver function tests (ALT and AST) advised before starting terbinafine 5
• Monitor for drug-drug interactions with tricyclic antidepressants, SSRIs, atypical antipsychotics, beta blockers, or tamoxifen 3

Topical Therapy

• Less effective than oral agents but useful for mild to moderate disease with fewer adverse effects 3
• Options include ciclopirox 8%, efinaconazole 10%, and tavaborole 5% 3
• Deep penetrating forms of SWO do not respond well to topical therapy 1

Adjunctive Measures

• Nail trimming and debridement used concurrently with pharmacologic therapy improve treatment response 3
• Preventive measures such as avoiding walking barefoot in public places and disinfecting shoes and socks thought to reduce 25% relapse rate 3