Diagnosis of Fungal Infections

Initial Approach - Spotting Spores Sooner

• Clinical Clues:
  • Tinea (Ringworm): Annular, erythematous lesions with central clearing & active, raised borders. Common sites: corporis, cruris, pedis, capitis.
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  • Cutaneous Candidiasis: Erythematous plaques, satellite pustules/papules. Favors intertriginous areas (axillae, groin, inframammary).
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  • Pityriasis (Tinea) Versicolor: Well-demarcated, hypo/hyperpigmented macules with fine scales. Typically on trunk, upper arms. *
• Wood's Lamp Examination (365 nm UV light):
  • Indications: Suspected tinea capitis, pityriasis versicolor, erythrasma.
  • Fluorescence Patterns:
    • Microsporum canis, M. audouinii: Blue-green (📌 M for Microsporum, M for Moon-like glow).
    • Pityriasis versicolor (Malassezia furfur): Yellowish-gold or coppery-orange.
    • Erythrasma (Corynebacterium minutissimum): Coral-red (DDx for tinea cruris).
    • Trichophyton schoenleinii: Dull blue (favus).

⭐ Microsporum species (e.g., M. canis, M. audouinii) are ectothrix hair invaders and typically show a characteristic blue-green fluorescence under Wood's lamp. This is a key diagnostic feature, especially in tinea capitis evaluation. * * *

Microscopy - Fungi Under the Lens

• Specimen Collection: Key to accurate diagnosis.

  • Skin: Scrape from the active, erythematous, scaling edge of the lesion using a scalpel blade.
  • Nails: Collect nail clippings (especially from the distal, crumbling part) and subungual debris.
  • Hair (Tinea Capitis): Pluck hairs with roots; look for broken hairs or those fluorescing under Wood's lamp (for some species).

• $KOH$ Mount (Potassium Hydroxide Preparation): Rapid, cost-effective primary test.

  • Reagent: 10-20% $KOH$ solution; sometimes with Dimethyl Sulfoxide (DMSO) to accelerate keratin dissolution.
  • Procedure: Place specimen on a glass slide, add a drop of $KOH$, apply coverslip. Gentle heating may be used to speed clearing.
  • 📌 Mnemonic: KOH helps See The Fungus.

• Microscopic Findings: Visualize fungal elements.

  • Dermatophytes (e.g., Trichophyton, Microsporum): Branching, septate hyphae; arthroconidia may be present.
  • Candida species: Budding yeast cells and pseudohyphae (chains of elongated budding cells).
  • Malassezia furfur (Pityriasis versicolor): Characteristic 'spaghetti and meatballs' appearance - clusters of yeast cells and short, curved hyphal elements.

• Stains (Optional Enhancement):

  • Parker Quink ink or Chicago Sky Blue can be added to $KOH$ to improve contrast and visualization of fungal walls.

⭐ The 'spaghetti and meatballs' appearance (clusters of yeast cells and short, curved hyphal elements) on a KOH mount is pathognomonic for Pityriasis (Tinea) versicolor caused by Malassezia species.

Culture & Confirmation - Growing & Naming Culprits

• Key Culture Media:
  • Sabouraud Dextrose Agar (SDA): Gold standard, often with Chloramphenicol (antibacterial) & Cycloheximide (anti-saprophytic).
  • Dermatophyte Test Medium (DTM): Screening; alkaline byproducts turn phenol red indicator from yellow → red.
• Incubation: 25-30°C for up to 4 weeks.
• Identification:
  • Gross Colony Morphology: Color, texture, obverse/reverse pigments.
  • Microscopic Morphology (Speciation): Macroconidia & microconidia (e.g., Trichophyton, Microsporum, Epidermophyton).
• Histopathology (Deeper/Atypical Infections):
  • Biopsy stains: Periodic Acid-Schiff (PAS), Gomori Methenamine Silver (GMS) to see fungal elements.

⭐ Dermatophyte Test Medium (DTM) changes from yellow to red, typically within 7-14 days, due to alkaline metabolites produced by dermatophytes acting on the phenol red pH indicator.

High‑Yield Points - ⚡ Biggest Takeaways

• KOH microscopy is the key initial test for superficial fungal infections.
• Wood's lamp helps identify specific fungi like Microsporum (shows green fluorescence).
• Fungal culture (e.g., SDA) is confirmatory and identifies species, but is slow.
• Skin biopsy with PAS stain is valuable for deep or atypical infections.
• Dermatoscopy can show specific signs like "comma hairs" in tinea capitis.
• Clinical presentation (e.g., annular lesions, active borders) is crucial for suspicion.