38.2 Fungal Skin Infections: Tinea Corporis, Cruris, Pedis & Onychomycosis

Biology & Pathogenesis of Dermatophytoses

Dermatophytoses (commonly designated as "tinea" infections followed by the anatomical site) represent superficial fungal infections caused by a specialized group of filamentous fungi called dermatophytes. Dermatophytes are taxonomically classified into three primary genera: Trichophyton (e.g., T. rubrum, T. mentagrophytes, T. tonsurans), Microsporum (e.g., M. canis), and Epidermophyton (e.g., E. floccosum).

Pathophysiologic Mechanisms

• Keratinotropism: Dermatophytes are obligate keratinophilic fungi. They possess specialized extracellular proteases (keratinases) that digest keratin, allowing the organisms to adhere to, invade, and proliferate within non-viable, keratinized host structures: the stratum corneum of the epidermis, hair shafts and follicles, and nail plates.
• Depth of Invasion: In immunocompetent hosts, dermatophytes are restricted strictly to fully cornified tissue. They are incapable of penetrating viable deep epidermal layers or the dermis because they are inhibited by host non-specific serum inhibitory factors (transferrin) and cannot survive at core human body temperature (≥37°C).
• Transmission: Dermatophytes are transmitted through direct contact with infected humans (anthropophilic species, e.g., T. rubrum, T. tonsurans), animals (zoophilic species, e.g., M. canis from cats and dogs), or contaminated soil (geophilic species). Anthropophilic infections tend to cause chronic, low-grade, minimally inflammatory disease, whereas zoophilic and geophilic species trigger intense cell-mediated hypersensitivity reactions in human hosts.

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Diagnostic Techniques in Cutaneous Mycology

Accurate diagnosis of fungal skin disease relies on bedside direct microscopy and confirmatory laboratory testing. Presumptive visual diagnosis without diagnostic confirmation leads to frequent misdiagnosis, inappropriate corticosteroid prescription, and prolonged clinical morbidity.

1. Potassium Hydroxide (KOH) Direct Microscopy

• Sampling Technique: The leading, active, erythematous, scaly border of the cutaneous lesion contains the highest density of viable hyphae. Using a sterile #15 scalpel blade held at a 45-degree angle, the clinician gently scrapes epidermal scales from the advancing margin onto a clean glass microscope slide.
• Preparation: A drop of 10% to 20% potassium hydroxide (KOH) solution is applied, and a coverslip is placed over the specimen. The slide is gently warmed over an alcohol flame (without boiling) or allowed to stand at room temperature for 10 to 15 minutes. KOH acts as a powerful keratolytic agent, dissolving host keratin, lipids, and cellular debris while leaving the fungal chitinous cell walls intact.
• Microscopic Findings: Under low power (10x) and confirmatory high power (40x), dermatophyte infection is confirmed by the visualization of branching, septate hyphae of uniform caliber (width) traversing squamous cells, often accompanied by chains of spherical arthrospores.
• Artifact Pitfalls: Clinicians must distinguish true fungal hyphae from the "mosaic fungus" artifact (an optical illusion formed by lipid droplets and cholesterol crystals aligning along the undulating cell borders of keratinocytes) and cotton fibers (which lack uniform caliber, have irregular branching, and lack true internal septations).

2. Wood's Lamp Examination

• A Wood's lamp emits long-wave ultraviolet radiation (peak wavelength ~365 nm) in a completely darkened room.
• Dermatophytes: Hairs infected with certain Microsporum species (Microsporum canis, M. audouinii) emit a striking, bright blue-green fluorescence due to pteridine metabolites deposited in the hair cortex. Important Pearl: The vast majority of tinea capitis cases in North America today are caused by Trichophyton tonsurans, which does NOT fluoresce under a Wood's lamp!
• Other Cutaneous Organisms:
  • Corynebacterium minutissimum (erythrasma): Emits an intense, pathognomonic coral-red fluorescence due to coproporphyrin III production;
  • Pseudomonas aeruginosa: Emits a bright yellow-green fluorescence due to pyoverdine synthesis;
  • Malassezia (tinea versicolor): Emits a faint yellow-gold or copper-orange fluorescence.

3. Fungal Culture & Histopathology

• Fungal Culture: Inoculation of scraped scales, hair, or nail fragments onto Sabouraud dextrose agar containing chloramphenicol and cycloheximide. Culture requires 2 to 4 weeks of incubation to identify specific fungal species and determine antifungal susceptibilities.
• Periodic Acid-Schiff (PAS) Staining: The gold-standard histopathologic stain for nail plate clippings and skin biopsies. PAS oxidizes 1,2-glycol groups in fungal wall polysaccharides into aldehydes, reacting with Schiff reagent to stain fungal hyphae a vibrant, unmistakable magenta/pink.

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Superficial Dermatophytic Syndromes

                     SUPERFICIAL DERMATOPHYTIC INFECTIONS

  Syndrome            Primary Pathogens     Hallmark Clinical Manifestations
  ─────────────────────────────────────────────────────────────────────────────
  Tinea Corporis      T. rubrum,            Annular erythematous plaque, raised
  ("Ringworm")        T. mentagrophytes,    scaly advancing border, central clearing;
                      M. canis              centrifugal expansion.
  ─────────────────────────────────────────────────────────────────────────────
  Tinea Cruris        T. rubrum,            Pruritic erythematous rash in groin folds
  ("Jock Itch")       E. floccosum          and inner thighs; ADVANCING SCALY BORDER;
                                            CHARACTERISTICALLY SPARES SCROTUM & PENIS.
  ─────────────────────────────────────────────────────────────────────────────
  Tinea Pedis         T. rubrum,            Interdigital maceration & fissuring (4th/5th);
  ("Athlete's Foot")  T. mentagrophytes     Moccasin distribution (scaling on soles/sides);
                                            Vesiculobullous inflammatory flares.
  ─────────────────────────────────────────────────────────────────────────────
  Tinea Capitis       T. tonsurans (>90%),  Patchy alopecia, scaly scalp, "black dot"
                      M. canis              hairs, occipital LAD; Kerion (boggy mass);
                                            MANDATORY SYSTEMIC ORAL THERAPY!
  ─────────────────────────────────────────────────────────────────────────────
  Onychomycosis       T. rubrum,            Subungual hyperkeratosis, yellow-brown
  (Tinea Unguium)     T. mentagrophytes     discoloration, onycholysis, brittle nail plate;
                                            MANDATORY PRE-TREATMENT CONFIRMATION!

1. Tinea Corporis ("Ringworm")

• Clinical Presentation: Begins as a pruritic, circular erythematous macule or papule that spreads centrifugally. As the central area clears, it evolves into the classic annular ("ring-shaped") plaque featuring a raised, active, erythematous, vesicular or scaly advancing border and a relatively clear, hypopigmented or normal central zone.
• Topical Therapy: First-line for localized tinea corporis:
  • Allylamines: Topical Terbinafine 1% cream applied once or twice daily for 1 to 2 weeks. Allylamines inhibit squalene epoxidase, blocking ergosterol synthesis and accumulating toxic squalene; they are fungicidal and associated with lower relapse rates and shorter treatment durations than azoles.
  • Imidazoles / Azoles: Topical Clotrimazole 1%, Ketoconazole 2%, or Miconazole 2% cream applied twice daily for 2 to 4 weeks. Azoles inhibit 14-alpha-demethylase; they are fungistatic and require application for at least 1 to 2 weeks after visible clinical resolution to eradicate latent fungal elements.
• Systemic Therapy: Indicated for extensive body surface area involvement, immunosuppressed hosts, deep follicular penetration (Majocchi granuloma), or failure of topical therapy: Oral Terbinafine 250 mg PO daily for 2 to 4 weeks, or Oral Itraconazole 200 mg PO daily for 1 to 2 weeks.
• Tinea Incognito Warning: Application of topical corticosteroids (alone or in combination products such as betamethasone dipropionate-clotrimazole [Lotrisone]) blunts the local host inflammatory response. This temporarily masks symptoms while allowing fungal hyphae to proliferate unchecked into deeper follicular structures. The rash loses its classic annular raised scaly border, becoming an ill-defined, violaceous, diffuse plaque known as tinea incognito, which is notoriously difficult to diagnose and eradicate.

2. Tinea Cruris ("Jock Itch")

• Clinical Presentation: Intensely pruritic, sharply demarcated, symmetrical erythematous rash located in the intertriginous groin creases and extending onto the upper inner thighs. Features an elevated, active, scaling advancing margin.
• The Crucial Board Differentiating Feature: Tinea cruris characteristically SPARES the scrotum and penis! In contrast:
  • Cutaneous Candidiasis (Intertrigo): Overwhelmingly involves the scrotum, presenting with bright beefy-red erythema, maceration, and pathognomonic peripheral satellite pustules;
  • Erythrasma: Presents as a uniform, reddish-brown, finely wrinkled patch in the groin that lacks an active raised border and exhibits brilliant coral-red fluorescence under Wood's lamp.
• Treatment: Topical terbinafine 1% or topical azoles applied twice daily for 2 to 4 weeks. Patients must keep the inguinal folds dry, wear loose-fitting cotton boxers, and concurrently treat coexisting tinea pedis to prevent reinfection.

3. Tinea Pedis ("Athlete's Foot")

• Epidemiology: The single most common dermatophytosis worldwide. Fungal transmission occurs via walking barefoot on contaminated floors in communal showers, locker rooms, and pool decks.
• Clinical Patterns:
  1. Interdigital Tinea Pedis: Most frequent pattern. Features maceration, erythema, peeling, and painful fissuring in the toe web spaces, particularly between the fourth and fifth toes.
  2. Moccasin Type (Chronic Hyperkeratotic): Features chronic, diffuse, fine, silvery-white scaling and bilateral hyperkeratotic thickening distributed over the soles, heels, and lateral borders of both feet in a "moccasin" distribution. Commonly manifests as the classic "two feet, one hand syndrome" (chronic tinea pedis involving both feet accompanied by unilateral tinea manuum on the dominant hand).
  3. Vesiculobullous (Inflammatory): Acute eruption of intensely pruritic, painful vesicles or bullae on the plantar instep, often triggered by warmth.
• Clinical Significance: Interdigital tinea pedis creates microscopic disruptions in the cutaneous barrier, serving as the primary anatomical portal of bacterial entry for recurrent lower-extremity non-purulent cellulitis and erysipelas!
• Management: Interdigital disease responds to topical terbinafine 1% cream BID for 1 to 2 weeks. Moccasin-type tinea pedis frequently fails topical monotherapy due to the thick hyperkeratotic stratum corneum and requires Oral Terbinafine 250 mg daily for 2 to 4 weeks combined with topical keratolytic agents (salicylic acid 6% or urea 20-40% cream) to thin the cornified layer.

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Tinea Capitis & Kerion

Tinea capitis is a dermatophyte infection of scalp hair follicles and hair shafts, occurring almost exclusively in prepubertal children (peak incidence ages 3 to 9 years, with highest prevalence in African American children).

Pathophysiology & Hair Shaft Invasion

• Causative Organisms: In the United States and North America, >90% of cases are caused by Trichophyton tonsurans (anthropophilic, spread person-to-person via shared combs, brushes, pillows, hats). Zoophilic cases are caused by Microsporum canis (transmitted from kittens or puppies).
• Patterns of Hair Invasion:
  • Endothrix: Fungal arthroconidia proliferate entirely within the interior of the hair shaft, destroying the internal keratin matrix. The weakened hair breaks off flush at the level of the follicular scalp orifice, producing the pathognomonic physical finding of "black dot" tinea capitis (classic for T. tonsurans);
  • Ectothrix: Arthroconidia form a sheath around the external surface of the hair shaft, breaking hairs a few millimeters above the scalp (characteristic of M. canis).

Clinical Presentation

• Presents as one or multiple patches of scaling, pruritic alopecia studded with broken hair stubs ("black dots").
• Prominent, tender posterior cervical and suboccipital lymphadenopathy is an exceptionally sensitive clinical hallmark distinguishing tinea capitis from non-infectious causes of pediatric hair loss (e.g., alopecia areata, trichotillomania).
• Kerion: A severe, aggressive, cell-mediated hypersensitivity reaction to the dermatophyte. Presents as a markedly elevated, boggy, fluctuant, purulent, inflammatory granulomatous tumefaction studded with open pustules, draining sinus tracts, and thick crusts. Critical Board Pearl: A kerion is frequently misdiagnosed as a bacterial scalp abscess or carbuncle, leading to inappropriate surgical incision! Incision is strictly contraindicated and ineffective because the process is hypersensitivity-driven rather than a bacterial pocket. If untreated, a kerion causes permanent cicatricial (scarring) alopecia.

Mandatory Systemic Therapy for Tinea Capitis

[!IMPORTANT] THE CARDINAL RULE OF TINEA CAPITIS: TOPICAL THERAPY CANNOT CURE SCALP RINGWORM! Topical antifungal creams, lotions, and shampoos cannot penetrate down into the hair follicle bulb or enter the interior of the hair shaft. Therefore, SYSTEMIC ORAL ANTIFUNGAL THERAPY IS MANDATORY for the treatment of all tinea capitis cases! Topical antifungal shampoos (selenium sulfide 2.5% or ketoconazole 2%) are used strictly as adjunctive therapy (applied twice weekly) to decrease viable spore shedding, reducing secondary transmission to family members and classmates.

First-Line Oral Regimens for Tinea Capitis

1. Oral Griseofulvin:
   • Dosing: Microsize formulation at 20 to 25 mg/kg/day PO (or ultramicrosize formulation at 10 to 15 mg/kg/day) once daily or divided BID for 6 to 8 weeks.
   • Administration Rule: Griseofulvin is highly lipophilic and has poor aqueous absorption. It MUST be ingested with a high-fat meal or beverage (whole milk, peanut butter, ice cream, yogurt) to ensure adequate gastrointestinal absorption and therapeutic tissue concentrations.
   • Clinical Indication: Historically the gold-standard agent; remains the drug of choice for Microsporum canis infections.
   • Adverse Effects: Gastrointestinal distress, headache, drug eruptions, photosensitivity, and rare hepatotoxicity. Avoid in patients with porphyria or severe hepatic disease.
2. Oral Terbinafine:
   • Dosing: Weight-based once-daily dosing for 4 to 6 weeks:
     • Children <20 kg: 62.5 mg PO once daily;
     • Children 20 to 40 kg: 125 mg PO once daily;
     • Children >40 kg: 250 mg PO once daily.
   • Clinical Indication: The preferred first-line agent for Trichophyton tonsurans due to its shorter treatment duration (4-6 weeks vs. 6-8 weeks for griseofulvin) and superior mycological cure rates.
3. Management of Kerion:
   • Initiate systemic oral antifungal therapy (terbinafine or griseofulvin) immediately.
   • Add an adjunctive course of Oral Prednisone (1 mg/kg/day, maximum 40 mg daily) for 10 to 14 days to rapidly suppress the destructive cell-mediated hypersensitivity reaction, relieving severe pain and preventing permanent cicatricial scarring alopecia.

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Onychomycosis (Tinea Unguium)

Onychomycosis is a fungal infection of the nail unit (nail bed, nail plate, and nail matrix), accounting for roughly 50% of all nail disorders in adults. Its prevalence increases with advancing age, diabetes mellitus, peripheral vascular disease, and immunosuppression.

Etiology & Clinical Subtypes

• Over 85% to 90% of cases are caused by dermatophytes, overwhelmingly Trichophyton rubrum, followed by Trichophyton mentagrophytes. Non-dermatophyte molds (Fusarium, Acremonium) and Candida species account for the remainder.
• Distal Lateral Subungual Onychomycosis (DLSO): The most frequent clinical pattern. Dermatophytes invade the hyponychium and distal nail bed, spreading proximally. Clinical features include:
  • Subungual hyperkeratosis (accumulation of thick, cheesy keratinaceous debris under the nail plate);
  • Onycholysis (separation of the nail plate from the underlying pink nail bed);
  • Yellowish-brown, chalky discoloration of the nail plate;
  • Brittleness, crumbling, and progressive nail plate dystrophy.

Mandatory Pre-Treatment Mycological Confirmation

               ONCHOMYCOSIS: MANDATORY CONFIRMATION PROTOCOL

   Suspected Onychomycosis (Thickened, Dystrophic, Discolored Nail Plate)
                                     │
                                     ▼
               MANDATORY PRE-TREATMENT CONFIRMATION TESTING
             • Up to 50% of dystrophic nails are NOT fungal!
               (Differential: Psoriasis, Lichen Planus, Trauma)
             • Oral Terbinafine requires 6-12 weeks of treatment
             • Risk of drug-induced liver injury & CYP450 drug interactions
                                     │
                                     ▼
        SELECT DIAGNOSTIC MODALITY (Order at least one before treating):
        [1] PAS Histology of Nail Clipping (Highest sensitivity: 80-90%)
        [2] KOH Direct Microscopy of subungual scraping (Rapid bedside)
        [3] Fungal Culture on Sabouraud agar (Identifies specific species)
                                     │
                        Is Fungal Etiology Confirmed?
                                ╱         ╲
                             NO             YES
                            ╱                 ╲
                           ▼                   ▼
           DO NOT PRESCRIBE ORAL TERBINAFINE!  CONFIRMED ONYCHOMYCOSIS:
           • Re-evaluate differential:         • Baseline LFTs (ALT/AST)
             Nail psoriasis (pitting, oil drop)• Oral Terbinafine 250 mg daily
             Lichen planus, repetitive trauma  • Fingernails: 6 weeks
           • Repeat biopsy / nail clipping     • Toenails: 12 weeks

[!CAUTION] CRITICAL BOARD & SAFETY RULE: NEVER TREAT UNCONFIRMED ONYCHOMYCOSIS! Up to 50% of thickened, discolored, dystrophic nails are NOT caused by fungal infections! Common non-fungal etiologies include nail psoriasis (characterized by nail pitting, salmon/oil-drop patches, and onycholysis), lichen planus (longitudinal ridging and pterygium), chronic microtrauma (from poorly fitting footwear), chronic eczema, and peripheral arterial disease. Because systemic antifungal therapy requires prolonged administration (6 to 12 weeks) and carries significant risks of drug-induced liver injury (severe hepatotoxicity), adverse taste disturbances, and potent CYP450 drug interactions, CLINICIANS MUST DEFIRMATIVELY CONFIRM A FUNGAL INFECTION (via PAS stain, KOH, or fungal culture) BEFORE PRESCRIBING SYSTEMIC ANTIFUNGAL THERAPY!

Evidence-Based Treatment Regimens for Onychomycosis

1. Oral Terbinafine (First-Line Systemic Gold Standard):
   • Dosing: 250 mg PO once daily for 6 weeks (fingernails) or 12 weeks (toenails).
   • Efficacy: Highest mycological cure rate (~70% to 75%) and clinical cure rate (~40% to 50%).
   • Laboratory Monitoring: Check baseline liver function tests (serum ALT and AST) prior to treatment initiation. In patients with pre-existing hepatic disease or prolonged therapy, repeat transaminases midway through treatment. Counsel patients to immediately discontinue the drug and report symptoms of hepatitis: persistent nausea, fatigue, right upper quadrant abdominal pain, dark urine, or jaundice.
   • Contraindications: Active or chronic liver disease, severe renal insufficiency (creatinine clearance <50 mL/min).
   • Drug Interactions: Terbinafine is a potent inhibitor of the cytochrome P450 CYP2D6 isoenzyme; co-administration increases serum levels of tricyclic antidepressants, beta-blockers, and SSRIs.
2. Oral Itraconazole (Alternative Systemic Agent):
   • Pulse Dosing: 200 mg PO BID for 1 week per month; 2 pulses for fingernails, 3 pulses for toenails.
   • Precautions: Potent inhibitor of CYP3A4 (numerous drug-drug interactions with statins, direct oral anticoagulants, calcium channel blockers). Boxed warning: Negative inotropic effect—strictly contraindicated in patients with congestive heart failure or left ventricular dysfunction.
3. Topical Antifungal Therapies:
   • Agents include Ciclopirox 8% nail lacquer applied daily for 48 weeks, Efinaconazole 10% solution daily for 48 weeks, and Tavaborole 5% solution daily for 48 weeks.
   • Limitations: Modest efficacy (complete cure rates only 8% to 18%) due to poor penetration through the dense keratin nail plate.
   • Indications: Reserved for mild-to-moderate superficial distal disease involving <50% of the nail plate without lunula/matrix involvement, or when oral antifungals are contraindicated.

• Patient Expectation Counseling: Even after completing the full 12-week course of oral terbinafine, the toenail will not look cosmetically normal immediately. Because human toenails grow at a rate of only 1 to 1.5 mm per month, a fully normal, clear nail plate requires 9 to 18 months to completely grow out.