What is the role of a Wood (Wood's lamp) lamp in diagnosing skin diseases?

Role of Wood's Lamp in Diagnosing Skin Diseases

Wood's lamp is a valuable diagnostic tool in dermatology that aids in the diagnosis of various skin conditions by emitting long-wave ultraviolet A (UVA) light that reveals characteristics not visible under normal lighting conditions.

Basic Principles and Mechanism

• Wood's lamp emits long-wavelength ultraviolet A (UVA) light (340-450 nm, with peak at 365 nm) that causes certain substances to fluoresce 1
• The device was originally invented by physicist Robert Williams Wood for ultraviolet photography before being adopted for dermatologic applications 2
• The lamp works by projecting UVA light onto the skin which then reflects visible light that can be interpreted by clinicians 3

Key Diagnostic Applications

Pigmentary Disorders

• Vitiligo assessment: Wood's lamp delineates areas of pigment loss, especially in lighter skin types (I and II)
• Wood's lamp can reveal actively depigmenting areas that appear larger under UV illumination than with visible light 4
• Helps determine depth of melanin pigmentation: contrast in epidermal pigmentation increases while contrast in dermal pigmentation decreases under Wood's lamp illumination 5

Fungal Infections

• Tinea capitis: Reveals characteristic fluorescence when caused by Microsporum canis
• Tinea versicolor: Shows yellow-green or golden fluorescence
• Erythrasma: Displays coral-red fluorescence due to porphyrin production by Corynebacterium minutissimum 1
• Onychomycosis: Helps define clear margins for debridement and distinguish between fungal infection and medication staining 6

Bacterial Infections

• Pseudomonas infections: Produce characteristic blue-green fluorescence 1

Photodynamic Diagnosis (PDD)

• In photodynamic therapy (PDT), Wood's lamp can be used to check surface fluorescence after application of photosensitizing agents like 5-aminolaevulinic acid (ALA) or methyl aminolevulinate (MAL)
• This confirms protoporphyrin IX generation at least in the superficial part of the lesion 4
• The simplest method involves illumination of a porphyrin-enriched tumor by Wood's lamp, revealing a brick-red fluorescence 4

Porphyrias

• Detects characteristic fluorescence in various types of porphyria 1

Clinical Utility and Recommendations

• Wood's lamp is recommended for the diagnosis of vitiligo and demonstration of disease extent and activity, particularly in skin types I and II 4
• It can be useful for monitoring response to therapy in conditions like vitiligo 4
• The British Association of Dermatologists recommends Wood's lamp for photodynamic diagnosis, though notes it is a relatively crude technique compared to more advanced imaging methods 4

Advantages and Limitations

Advantages

• Non-invasive diagnostic tool
• Cost-effective and widely available
• Requires minimal training to operate
• Provides immediate results
• Safe for use in all patient populations

Limitations

• Limited to superficial skin assessment
• Results can be subjective and require clinical experience for interpretation
• Environmental factors (ambient lighting) can affect visualization
• Some conditions may have similar fluorescence patterns, requiring additional diagnostic tests

Best Practices for Use

1. Perform examination in a completely darkened room
2. Allow eyes to adapt to darkness for 1-2 minutes before examination
3. Hold the lamp 4-5 inches (10-12 cm) from the skin surface
4. Systematically examine all affected and relevant areas
5. Compare findings with normal skin appearance
6. Document findings for monitoring disease progression or treatment response

Wood's lamp remains a valuable tool in dermatological practice due to its simplicity, cost-effectiveness, and ability to improve diagnostic precision for numerous skin conditions, despite the development of more advanced diagnostic technologies.