Erythrasma

Erythrasma is a common, chronic bacterial infection caused by Corynebacterium minutissimum. It is often mistaken for a fungal infection (like Tinea Cruris) due to its location in skin folds.

• Onset and Location: Patients report a slowly enlarging, reddish-brown rash in intertriginous (moist) areas—most commonly the groin, armpits (axillae), and the spaces between the toes (toe webs).

• Symptoms: Often asymptomatic, though some patients report mild pruritus (itching) or a burning sensation, especially when aggravated by heat, friction, or sweating.

• Medical History: The risk is significantly higher in patients with Diabetes Mellitus, obesity, hyperhidrosis (excessive sweating), or those living in warm, humid climates. It is also more prevalent in patients with suppressed immune systems.

Diagnosis is primarily clinical but can be confirmed instantly with a bedside test.

1. Skin Appearance:

   • Morphology: Well-demarcated, dry, reddish-brown patches.

   • Texture: The skin may appear slightly wrinkled or "crenelated" with fine scales. Unlike fungal infections, there is typically no central clearing or active "border" of pustules.

   • Toe Webs: It often presents as maceration (soggy, white skin) between the 4th and 5th toes, which can coexist with fungal athlete's foot.

2. The "Gold Standard" Test:

   • Wood’s Lamp Examination: When exposed to ultraviolet light (Wood's lamp), the patch glows a brilliant coral-red fluorescence. This is caused by porphyrins produced by the Corynebacterium.

Crucial Note for Darker Skin Tones

• Color Presentation: In darker skin, the "reddish" hue is often absent. The patches typically appear deep brown or black, making it look very similar to Acanthosis Nigricans or post-inflammatory hyperpigmentation.

• Differential Diagnosis: * Acanthosis Nigricans: Also common in diabetic/obese patients in the same areas, but the skin is "velvety" and thickened, and it will not fluoresce under a Wood's lamp.

  • Tinea Cruris: Usually has a raised, scaly leading edge and lacks the uniform brown "sheet" appearance of erythrasma.

• Under-diagnosis: Because the coral-red glow is so distinctive, a Wood’s lamp is an essential tool for avoiding misdiagnosis in patients with high baseline skin pigment.

1. Topical Antibiotics:

   • Clindamycin or Erythromycin creams applied twice daily for 7–14 days. These are effective first-line treatments.

2. Oral Antibiotics:

   • Erythromycin (250 mg orally four times daily for 7 days) is an option for extensive infections. It’s effective against Corynebacterium minutissimum, the bacteria causing erythrasma.

3. General Care and Prevention:

   • Encourage keeping affected areas dry, regular washing, and avoiding tight clothing. Recurrence is common, so maintaining hygiene in moist areas is crucial.

Risk Factors, Prognosis, and Complications
Prognosis is generally favorable with treatment, though recurrences are frequent if risk factors persist. Rarely, untreated erythrasma can spread, leading to extensive skin involvement.

Darker Skin Tones – Specific Treatment Risks:

• Pigment Persistence: Even after the Corynebacterium is eradicated (confirmed by a negative Wood's lamp test), the brown hyperpigmentation may remain for weeks or months. Patients should be counseled that "the color is the last thing to go" and doesn't necessarily mean the infection is still active.

• Irritation: Topical treatments can sometimes cause localized dryness. Ensure the patient isn't over-scrubbing the area, as friction can worsen the dark appearance (lichenification).

• DermNet NZ – Erythrasma: https://dermnetnz.org/topics/erythrasma

• StatPearls – Erythrasma (2024 update): https://www.ncbi.nlm.nih.gov/books/NBK544334/

• AOCD – Erythrasma Patient Education: https://www.aocd.org/page/Erythrasma