Dermatology · Fungal Infections
The facts most likely to be tested
The most common causative pathogen for tinea pedis is Trichophyton rubrum.
The interdigital type is the most common clinical presentation, characterized by erythema, scaling, and maceration between the toes.
The moccasin-type (chronic hyperkeratotic) presentation involves diffuse hyperkeratosis and fine silvery scaling on the soles and lateral aspects of the feet.
Diagnosis is confirmed via potassium hydroxide (KOH) preparation showing septate hyphae.
First-line treatment for localized tinea pedis is topical antifungal therapy with terbinafine or imidazole agents.
The two-feet-one-hand syndrome is a classic clinical association where tinea pedis is accompanied by tinea manuum on the dominant hand.
Oral antifungal therapy, such as oral terbinafine, is reserved for refractory cases or extensive hyperkeratotic disease.
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A 34-year-old male presents to the clinic complaining of persistent itching on his feet for the past three months. On physical examination, there is diffuse hyperkeratosis and fine silvery scaling involving the plantar surfaces of both feet, extending to the lateral borders. The interdigital spaces are spared. He reports that he has tried over-the-counter hydrocortisone cream without improvement. A KOH preparation of the skin scrapings reveals septate hyphae.
What is the most appropriate next step in management?
Topical terbinafine
The patient presents with the classic moccasin-type tinea pedis, confirmed by KOH prep; topical antifungals are the first-line treatment for this presentation.
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Etiology / Epidemiology
Common in damp, communal environments (gyms, pools). Caused by dermatophytes, primarily Trichophyton rubrum.
Clinical Manifestations
Presents as interdigital maceration or moccasin distribution hyperkeratosis. Often associated with tinea unguium.
Diagnosis
KOH preparation showing septate hyphae is the gold standard. Fungal culture is reserved for diagnostic uncertainty.
Treatment
Topical terbinafine is first-line. Oral terbinafine requires baseline LFT monitoring.
Prognosis
High recurrence rate; secondary bacterial infection (cellulitis) is the primary complication.
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Epidemiology & Etiology
Prevalence is highest in adolescents and young adults due to frequent exposure to public locker rooms. The primary pathogen is Trichophyton rubrum, which thrives in warm, moist environments. Transmission occurs via direct contact or fomites like contaminated towels or flooring.
Pertinent Anatomy
The infection typically targets the stratum corneum of the feet. The interdigital spaces are the most common site due to trapped moisture and lack of airflow. The plantar surface is often involved in the chronic, dry, hyperkeratotic form.
Pathophysiology
Dermatophytes produce keratinases that digest keratin, allowing the fungus to colonize the superficial skin layers. The host immune response triggers inflammation, leading to pruritus and scaling. Chronic infection can lead to nail bed invasion (onychomycosis), which serves as a persistent reservoir for reinfection.
Clinical Manifestations
The interdigital type presents with fissuring and maceration between the 4th and 5th toes. The chronic hyperkeratotic type exhibits a moccasin distribution of fine, silvery-white scale. Red flags include warmth, erythema, and purulence, suggesting secondary bacterial cellulitis.
Diagnosis
The KOH preparation is the gold standard diagnostic test, revealing branching, septate hyphae. A skin scraping should be taken from the active, leading edge of the lesion. If the KOH is negative but clinical suspicion remains high, a fungal culture or periodic acid-Schiff (PAS) stain can be utilized.
Treatment
First-line therapy is topical terbinafine applied twice daily for 1-4 weeks. For hyperkeratotic or recalcitrant cases, oral terbinafine is indicated. Hepatotoxicity is a major concern with oral antifungals, necessitating baseline and periodic LFT monitoring.
Prognosis
While generally benign, the condition is prone to frequent recurrence. Patients must maintain foot hygiene and use antifungal powders to prevent relapse. The most significant complication is the development of cellulitis or erysipelas, particularly in diabetic patients.
Differential Diagnosis
Contact dermatitis: usually bilateral and associated with a specific allergen exposure
Psoriasis: presents with thick, silvery scales and often involves other body surfaces
Erythrasma: fluoresces coral-red under Wood's lamp
Dyshidrotic eczema: characterized by deep-seated, pruritic vesicles on palms and soles
Candida intertrigo: typically involves the base of the web space with satellite lesions