Diagnosing Skin Conditions with Wood's Lamp

A Wood's lamp, also known as a black light, is a diagnostic tool that emits ultraviolet (UV) light to identify various skin conditions using a Wood's lamp examination. Dermatologists use this simple, non-invasive technique to detect fluorescence patterns that are invisible under normal lighting. By observing skin conditions examined under black light, clinicians can differentiate between infections, pigmentary disorders, and other cutaneous abnormalities. This article explores the science behind Wood's lamp, common diagnostic findings, and how it aids in dermatological practice.

How Does a Wood's Lamp Work?

The Wood's lamp produces long-wave UVA light (365 nm) that causes certain substances in the skin to emit visible fluorescence. When examining skin conditions under UV light, the lamp is held 4–5 inches away from the skin in a dark room. Normal healthy skin appears blue-white or violet, while abnormal areas may glow in distinct colors depending on the underlying condition. The fluorescence is due to the presence of porphyrins, melanin, or other chromophores that absorb UV light and re-emit it at longer wavelengths.

For instance, conditions detectable with a Wood's lamp such as tinea capitis (scalp ringworm) produce a bright green fluorescence, while erythrasma caused by Corynebacterium minutissimum shows coral-red fluorescence. The lamp is also invaluable for detecting pigmentary disorders like vitiligo, which appears as well-defined white patches under UV light due to the absence of melanin. Understanding the fluorescence patterns is key to accurate diagnosis.

Common Skin Conditions Identified with Wood's Lamp

A wide range of skin conditions evaluated with Wood's lamp can be diagnosed by their characteristic fluorescence. Below is a list of the most common findings:

• Fungal Infections: Microsporum species (e.g., tinea capitis) fluoresce bright green; Trichophyton species usually do not fluoresce.
• Bacterial Infections: Erythrasma (coral-red), Pseudomonas (green fluorescence), Propionibacterium acnes (orange-red follicular fluorescence).
• Pigmentary Disorders: Vitiligo (white, well-demarcated), hypopigmentation from scars or post-inflammatory changes (faint white).
• Metabolic & Other Conditions: Porphyria cutanea tarda (pink-red urine fluorescence), melasma (accentuated hyperpigmentation).

Each condition has a specific color pattern, making the Wood's lamp a valuable screening tool. For example, dermatological conditions under black light like vitiligo are often mistaken for tinea versicolor, but the latter shows a golden-yellow fluorescence due to Malassezia furfur. Thus, the lamp helps avoid misdiagnosis and guides appropriate treatment.

Procedure and Interpretation

Performing a Wood's lamp examination is straightforward. After explaining the procedure to the patient, darken the room and turn on the lamp. Hold it 10–15 cm (4–6 inches) from the skin surface and scan the area of interest. Record the color, intensity, and pattern of any fluorescence. Normal skin appears blue-white due to the reflection of UV light from collagen; hyperkeratotic areas may appear more white. Note that cutaneous conditions under UV light may be subtle, so careful observation is necessary.

Interpretation requires knowledge of common fluorescence patterns. For example, a coral-red fluorescence strongly suggests erythrasma, while a green fluorescence indicates certain dermatophyte infections. In vitiligo, the affected skin is pure white with sharp borders, contrasting with normal skin. The lamp can also reveal subclinical lesions, helping in early treatment.

Limitations and Considerations

While the Wood's lamp is a useful diagnostic aid, it has limitations. Not all infections fluoresce; for example, tinea pedis (athlete's foot) often shows no fluorescence. Additionally, false positives can occur from topical products, fibers, or cosmetics that fluoresce. The lamp should not replace microbiological cultures or biopsy when needed. However, as a quick, painless screening tool, it remains a staple in dermatology for evaluating Wood's lamp-visible conditions.

In summary, the Wood's lamp is an essential instrument for diagnosing skin conditions with Wood's lamp examination. From fungal infections to pigmentary disorders, its ability to reveal hidden fluorescence makes it a valuable first step in clinical evaluation. By understanding the patterns of skin conditions under black light, healthcare providers can improve diagnostic accuracy and patient outcomes.

For more detailed information on specific skin conditions under UV light, consult a dermatologist or refer to clinical guidelines. The Wood's lamp remains a simple yet powerful tool in modern dermatology.