Deep Purple Under Wood's Lamp: Diagnosis and Treatment
A deep purple discoloration under Wood's lamp examination is most likely erythrasma, which should be treated with topical or oral erythromycin or clindamycin.
Diagnosis
When examining skin under Wood's lamp (ultraviolet light), different conditions produce characteristic fluorescence patterns:
- Deep purple/coral-red fluorescence: Diagnostic of erythrasma, a superficial bacterial infection caused by Corynebacterium minutissimum
- Other fluorescence patterns (for differential diagnosis):
- Yellow-green: Fungal infections (tinea)
- Blue-white: Certain bacterial infections
- Pink: Some Pseudomonas infections
Treatment Options
First-line Treatment
- Topical antibiotics:
- Erythromycin 2% solution applied twice daily for 2 weeks
- Clindamycin 1% solution applied twice daily for 2 weeks
For Extensive Disease or Treatment Failures
- Oral antibiotics:
- Erythromycin 250mg four times daily for 14 days
- Clarithromycin 500mg twice daily for 14 days
- Tetracycline 500mg twice daily for 14 days
Adjunctive Measures
- Antiseptic washes containing chlorhexidine
- Keep affected areas dry
- Avoid tight-fitting clothing over affected areas
- Treat any underlying conditions (diabetes, obesity)
Management Algorithm
- Confirm diagnosis with Wood's lamp examination (coral-red/deep purple fluorescence)
- Assess extent of infection:
- Limited/localized: Use topical therapy
- Extensive/recurrent: Consider oral therapy
- Evaluate for predisposing factors:
- Diabetes
- Obesity
- Hyperhidrosis
- Immunosuppression
- Initiate appropriate treatment based on extent and severity
- Follow-up after 2-3 weeks to confirm resolution
Clinical Pearls and Pitfalls
- Erythrasma commonly affects intertriginous areas (groin, axillae, interdigital spaces)
- The condition may be mistaken for tinea or other dermatophyte infections
- Wood's lamp examination is a simple, non-invasive diagnostic tool that can immediately differentiate erythrasma from other conditions
- Recurrent cases may require investigation for underlying metabolic disorders
- Failure to respond to appropriate therapy should prompt consideration of alternative diagnoses
While the British Association of Dermatologists guidelines discuss various phototherapy treatments including PUVA (Psoralen plus UVA) for numerous dermatological conditions, they do not specifically address the management of erythrasma with deep purple fluorescence under Wood's lamp. The diagnosis and treatment recommendations provided are based on standard dermatological practice for this specific condition.