Tinea Unguium

01Pathophysiology

Tinea unguium is a dermatophyte infection of the nail unit and represents the most common cause of abnormal-appearing nails. The term "onychomycosis" is broader and includes infections by Candida species and non-dermatophyte molds, but exam vignettes predominantly test dermatophyte infections, so "tinea unguium" and "onychomycosis" are often used interchangeably in that context.

The infection begins when dermatophytes, most often Trichophyton rubrum, invade the nail apparatus. These organisms produce keratinases, enzymes that digest keratin, the structural protein of the nail plate. This enzymatic degradation is the central mechanism behind the clinical findings. As the fungus digests the nail plate from below, the nail becomes thickened, opaque, and dystrophic. Accumulation of keratinous debris beneath the nail plate produces subungual hyperkeratosis, the hallmark physical finding. As the bond between the nail plate and the nail bed is disrupted, onycholysis (separation of the nail from the bed) develops.

The route of fungal entry determines the clinical subtype. In the most common pattern (distal-lateral subungual), fungi enter through the hyponychium (the junction of the distal nail bed and fingertip skin) and spread proximally along the nail bed. In proximal subungual onychomycosis, the fungus enters through the proximal nail fold, and this pattern is strongly associated with immunosuppression, particularly HIV/AIDS. This association is a frequently tested point: a young patient with proximal white subungual onychomycosis should prompt consideration of underlying immunodeficiency.

The reason tinea unguium commonly coexists with tinea pedis is that the foot skin serves as a reservoir. Dermatophytes colonize the interdigital spaces and plantar skin first, then spread to the adjacent nail unit. This "two feet, one hand" pattern (bilateral tinea pedis with unilateral tinea manuum and onychomycosis) is a classic exam presentation.

02Classification and Clinical Manifestation

The clinical subtypes matter for the exam because the pattern of nail involvement narrows the differential. DLSO is so common that it is the "default" presentation in vignettes. PSO appearing in a young patient without trauma history is essentially a prompt to evaluate for HIV. WSO is the most superficial form and responds best to topical therapy alone.

03Diagnostic Workup

KOH (potassium hydroxide) preparation is the best initial diagnostic step. Subungual debris is collected from the most proximal area of involvement (where active infection is occurring, not the distal crumbling edge) and dissolved in 10 to 20% KOH solution. The KOH dissolves keratinocytes but preserves fungal cell walls, allowing visualization of septate, branching hyphae under light microscopy. The sensitivity of KOH is moderate (approximately 60 to 80%), meaning a negative KOH does not rule out the diagnosis.

Fungal culture on Sabouraud dextrose agar is considered the most accurate confirmatory test. It identifies the causative organism to the species level, which can guide therapy (dermatophytes respond to terbinafine, while Candida species may require azole therapy). The major drawback is the slow growth rate: cultures may require 3 to 6 weeks for a result, and the false-negative rate is notable (up to 30%).

PAS (Periodic Acid-Schiff) staining of nail plate clippings sent for histopathology is the most sensitive single test for detecting fungal elements in the nail. It is particularly useful when KOH and culture are both negative but clinical suspicion remains high. This is a commonly tested point: when a vignette describes a classic presentation but both KOH and culture return negative, the next step is PAS staining of the nail clipping.

A critical exam principle is that oral antifungal therapy should not be started without laboratory confirmation of fungal infection. Many nail dystrophies mimic onychomycosis (psoriasis, lichen planus, trauma), and systemic antifungals carry hepatotoxicity risk. Confirm first, then treat.

04Management and Treatment

Oral terbinafine is the first-line treatment for dermatophyte onychomycosis and the answer the exam expects unless the vignette introduces a contraindication. It is fungicidal (not merely fungistatic), has the highest mycological and clinical cure rates, and has fewer drug interactions than the azoles. The standard regimen is 250 mg once daily for 6 weeks (fingernails) or 12 weeks (toenails). Baseline and periodic liver function tests are recommended because terbinafine is hepatically metabolized, and rare cases of hepatotoxicity have been reported.

Oral itraconazole is the alternative when terbinafine is contraindicated or when the pathogen is Candida or a non-dermatophyte mold (which do not respond well to terbinafine). Itraconazole can be given continuously or as pulse therapy (200 mg twice daily for 1 week per month, repeated for 3 to 4 cycles). The pulse dosing approach exploits itraconazole's property of persisting in the nail for months after discontinuation. The main concern is its potent CYP3A4 inhibition, leading to dangerous interactions with statins, benzodiazepines, calcium channel blockers, and many other drugs. Itraconazole is also contraindicated in heart failure due to negative inotropic effects.

Topical therapy alone is reserved for patients with mild disease (less than 50% of the nail plate affected, no matrix involvement) or those who cannot tolerate systemic agents. Topical agents have significantly lower cure rates because the nail plate acts as a barrier to drug penetration. Efinaconazole 10% solution and tavaborole 5% solution are newer topical agents with better nail penetration than ciclopirox.

The concept of "next best step" in management logic:

1. Patient presents with dystrophic nail: Confirm the diagnosis (KOH, culture, or PAS stain) before any treatment.

2. Diagnosis confirmed as dermatophyte: Oral terbinafine is first-line.

3. Contraindication to terbinafine (liver disease, drug interaction): Switch to oral itraconazole.

4. Patient refuses systemic therapy or disease is mild and limited: Topical efinaconazole or ciclopirox.

5. At the end of treatment, the nail may still appear abnormal because the damaged nail must grow out completely. A "cured" nail takes 12 to 18 months to appear fully normal for toenails.

05Differential Diagnosis and Distractors

The most tested distractor is nail psoriasis. Both conditions cause onycholysis and subungual debris, and they can coexist. The vignette will differentiate them by including nail pitting, the oil-drop sign, or concurrent psoriatic skin/joint findings for psoriasis, or positive KOH/culture for tinea unguium.

06Traps and High-Yield Pearls

The single most common way students lose points on tinea unguium questions is by jumping to treatment without confirming the diagnosis. The exam tests the principle that nail dystrophy has many mimics, and initiating oral terbinafine based on appearance alone is premature and potentially harmful. When the vignette shows a dystrophic nail, the first correct answer is almost always a diagnostic step (KOH, culture, or nail biopsy/PAS stain), not a prescription.

A second major trap involves proximal subungual onychomycosis in a young, otherwise healthy patient. This pattern is uncommon without immunosuppression, and the vignette is testing whether you recognize it as a clue to screen for HIV. Students who focus only on the nail and choose antifungal therapy miss the broader clinical picture.

Third, remember that a negative KOH does not exclude the diagnosis. If clinical suspicion is strong and KOH is negative, the next step is fungal culture or PAS stain, not abandonment of the diagnosis. Conversely, a positive KOH in a patient with concurrent psoriasis means both conditions may coexist, and both must be addressed.

Finally, be aware of the treatment duration trap. Terbinafine is given for 12 weeks for toenails, but the nail will not look normal at the end of therapy. The drug persists in the nail plate for months, and the damaged nail must grow out over 12 to 18 months. A question stem describing a patient who "completed therapy but the nail still looks abnormal" is not asking you to extend or change treatment; it is testing your understanding that clinical improvement lags behind mycological cure.

The core competency tested across all tinea unguium questions is the ability to confirm a clinical suspicion with the appropriate diagnostic test before committing to systemic therapy, and to recognize atypical presentations (particularly PSO) as red flags for underlying disease.