Beyond Tinea: Other Skin Conditions Detectable with a Wood's Lamp

I. Introduction: Wood's Lamp Beyond Ringworm

For many, the mention of a Wood's lamp conjures an image of a doctor in a darkened room, shining a mysterious ultraviolet light on a suspicious rash to confirm a case of ringworm (tinea). Indeed, the characteristic bright green or blue-green fluorescence of certain dermatophyte infections under Wood's light is a classic diagnostic sign. However, to pigeonhole this venerable diagnostic tool as merely a "tinea detector" is to overlook its remarkable versatility in clinical dermatology. A Wood's lamp emits long-wave ultraviolet A (UVA) light, typically around 365 nanometers. When this light interacts with various chemicals, pigments, and organisms in or on the skin, it can induce fluorescence—the emission of visible light of a different color. This simple, non-invasive, and rapid bedside examination provides a unique window into skin pathology that is invisible to the naked eye. Its utility extends far beyond fungal infections, offering valuable clues for diagnosing and managing a spectrum of bacterial infections, pigmentary disorders, metabolic diseases, and genetic conditions. In the modern era, the traditional Wood's lamp has even found a digital counterpart in the form of the smartphone dermatoscope. These attachments, while primarily used for magnified dermoscopic imaging, often incorporate specific wavelength LEDs, including UVA, bringing the principles of Wood's lamp examination into a more portable and potentially quantifiable format. This article will explore the diverse landscape of skin conditions where a Wood's lamp, or its modern adaptations, proves to be an indispensable diagnostic ally, moving well beyond its traditional association with tinea.

II. Bacterial Infections

While fungi fluoresce under Wood's light, certain bacteria possess their own photogenic properties, producing distinct and diagnostically helpful colors. This makes the Wood's lamp a quick screening tool for specific cutaneous bacterial infections.

A. Erythrasma (coral-red fluorescence)

Erythrasma is a superficial, mild skin infection caused by the bacterium Corynebacterium minutissimum. It commonly presents as well-defined, reddish-brown, slightly scaly patches in intertriginous areas like the axillae, groin, and between the toes. To the naked eye, it can be easily mistaken for a fungal infection (tinea cruris or corporis), eczema, or psoriasis. This is where the Wood's lamp becomes a rapid and definitive point-of-care test. When the lamp is shone on an area of erythrasma in a completely dark room, the affected skin emits a characteristic bright coral-red or pink fluorescence. This striking fluorescence is due to porphyrins, particularly coproporphyrin III, produced by the bacteria. The test is highly specific, with studies indicating a sensitivity and specificity of around 100% for the diagnosis when performed correctly. In a clinical setting in Hong Kong, where humid climates predispose individuals to intertriginous infections, a Wood's lamp examination can instantly differentiate erythrasma from other common causes of groin or axillary rash, preventing unnecessary antifungal treatment. It is crucial to note that the patient should not have washed the area recently, as soap and water can remove the bacterial porphyrins and lead to a false-negative result. The immediate visual confirmation provided by the Wood's lamp allows for targeted therapy with topical clindamycin or erythromycin, or oral antibiotics for more extensive cases.

B. Pseudomonas infections (green fluorescence)

Pseudomonas aeruginosa is a notorious opportunistic pathogen that can cause various skin infections, especially in burn wounds, chronic ulcers (e.g., diabetic foot ulcers), and in the macerated web spaces of the toes (green nail syndrome). This bacterium produces several pigments, most notably pyocyanin and pyoverdine. Under Wood's lamp illumination, these pigments can cause the wound exudate, pus, or even the nail plate itself to fluoresce with a distinctive yellow-green or bright green hue. This fluorescence is a valuable clinical sign, alerting the physician to a likely Pseudomonas colonization or infection. For instance, in the management of chronic venous leg ulcers in Hong Kong's aging population, a routine Wood's lamp check of a non-healing, malodorous wound revealing green fluorescence would strongly suggest a superimposed Pseudomonas infection, guiding appropriate wound culture and the initiation of targeted antimicrobials like topical acetic acid or silver dressings. It is a rapid, adjunctive tool that complements, but does not replace, standard microbiological culture. The advent of the smartphone dermatoscope with UVA capabilities offers potential for teledermatology applications, allowing community nurses or patients to capture and share images of fluorescent wounds for remote specialist assessment.

III. Pigmentary Disorders

The interaction of UVA light with skin pigment (melanin) forms the basis for the Wood's lamp's utility in evaluating disorders of pigmentation. Melanin is a natural chromophore that absorbs UVA light. Differences in melanin concentration and depth dramatically alter how skin appears under the lamp, providing critical diagnostic and therapeutic insights.

A. Vitiligo (accentuation of depigmented areas)

Vitiligo is an autoimmune condition characterized by the destruction of melanocytes, leading to well-defined, milky-white patches of complete depigmentation. Under a Wood's lamp, these depigmented areas become starkly accentuated, appearing bright, chalky, or bluish-white. This is because the complete absence of melanin allows the UVA light to reflect off the underlying dermal collagen, creating a sharp contrast with the surrounding normally pigmented skin, which absorbs the UVA light and appears darker or dull. This accentuation is particularly useful in several scenarios: 1) Diagnosing early or subtle vitiligo in individuals with very fair skin (Fitzpatrick phototypes I-II), where the contrast may not be obvious in normal light. 2) Differentiating vitiligo from other hypopigmented conditions like post-inflammatory hypopigmentation or pityriasis alba. In these latter conditions, some melanin remains, so the areas do not show the brilliant, sharp-bordered fluorescence of true vitiligo but rather a dull, off-white or partial enhancement. 3) Mapping the extent of vitiligo, especially on sun-exposed areas where tanning can mask the borders. The Wood's lamp helps in accurately assessing the total body surface area involved, which is important for prognosis and treatment planning, such as for phototherapy. In Hong Kong, where cultural perceptions of skin conditions can significantly impact quality of life, an accurate and immediate diagnosis facilitated by a simple Wood's lamp examination is the first crucial step in patient counseling and management.

B. Melasma (visualization of pigment distribution)

Melasma presents a more complex picture. It is a common acquired hyperpigmentation disorder, often hormonally driven, presenting as symmetric, brown or gray-brown patches on the face. A key to effective treatment lies in determining the depth of the excess pigment: is it primarily in the epidermis (superficial), dermis (deep), or mixed? A Wood's lamp examination (often called a "black light examination") is a traditional, non-invasive method to help make this distinction. In a dark room, epidermal melanin absorbs UVA light, making epidermal melasma appear more pronounced and with sharper borders under the lamp. In contrast, dermal melanin is less affected by UVA; therefore, dermal melasma often shows little to no enhancement and may even become less visible. Mixed melasma will show partial enhancement. This information guides therapy: epidermal pigmentation typically responds better to topical agents (hydroquinone, retinoids, azelaic acid) and certain laser treatments, while dermal melasma is notoriously more resistant. While the Wood's lamp assessment for melasma has limitations (especially in darker skin tones where even normal skin absorbs much UVA), it remains a valuable first-line clinical tool. Modern tools like reflectance confocal microscopy provide more precise depth analysis, but the accessibility and simplicity of the Wood's lamp ensure its continued relevance in everyday practice, including in dermatology clinics across Hong Kong.

IV. Other Skin Conditions

The diagnostic reach of the Wood's lamp extends into the realms of metabolic disorders and genetic syndromes, where its findings can be pathognomonic or highly suggestive.

A. Porphyria cutanea tarda

Porphyria Cutanea Tarda (PCT) is the most common type of porphyria, a disorder of heme biosynthesis. It manifests clinically with fragile skin, blistering, milia, and hyperpigmentation on sun-exposed areas like the hands and face. A hallmark laboratory feature is the excretion of excessive porphyrins in the urine. When a fresh urine sample from a patient with active PCT is examined under a Wood's lamp, it often exhibits a striking pink-orange or coral-red fluorescence. This is due to the high concentration of uroporphyrin and coproporphyrin. This simple bedside test can provide an immediate, strong clue to the diagnosis before quantitative urine porphyrin analysis is available. The fluorescence may also be observed in the teeth (erythrodontia) in some congenital porphyrias. It's a dramatic demonstration of how a metabolic byproduct can be visually detected, prompting further investigation into liver function, iron studies, and potential triggers like hepatitis C, estrogen use, or alcohol consumption—all relevant considerations in a diverse patient population like Hong Kong's.

B. Tuberous sclerosis (ash-leaf spots)

Tuberous sclerosis complex (TSC) is a multisystem genetic disorder that can affect the skin, brain, kidneys, and other organs. One of the earliest and most common cutaneous signs is the presence of hypomelanotic macules, often called "ash-leaf spots" due to their shape. These are areas of reduced pigmentation that can be subtle, especially in infants and individuals with fair skin. A Wood's lamp examination is an essential part of the dermatological assessment for suspected TSC. Under the UVA light, these hypopigmented patches are greatly enhanced, becoming clearly visible as well-defined, off-white or pale areas. This allows for the identification of even very small or faint spots that might be missed in room light. The detection of three or more hypomelanotic macules (of typical shape) is one of the major diagnostic criteria for TSC. Early diagnosis is critical for initiating surveillance for associated complications like seizures, intellectual disability, and renal tumors. Therefore, a simple, non-invasive Wood's lamp screening in a pediatric or dermatology clinic can be the pivotal step that triggers a comprehensive diagnostic workup for this serious condition.

V. Proper Technique and Interpretation

The diagnostic value of a Wood's lamp is entirely dependent on proper technique and a nuanced understanding of its limitations. It is a supportive tool, not a standalone diagnostic machine.

A. Best practices for using Wood's lamp for various conditions

To obtain reliable results, a standardized approach is necessary. The examination should always be conducted in a completely dark room, allowing several minutes for both the examiner's and the patient's eyes to adapt to the darkness. The lamp should be warmed up for about a minute to achieve stable UVA output. The lamp head should be held 4 to 5 inches (10-13 cm) from the skin surface. It is vital to consider potential confounding factors:

• Recent washing or topical products: Soaps, makeup, moisturizers, deodorants, and topical medications (especially those containing petrolatum or certain antibiotics) can fluoresce themselves, causing false-positive readings. The skin should ideally be clean and free of products.
• Ambient light: Any external light source will wash out the subtle fluorescence.
• Skin type: Interpretation in darker skin tones (Fitzpatrick V-VI) can be more challenging as the baseline skin absorbs more UVA, potentially masking subtle fluorescence. Experience is key.
• Condition-specific protocols: For erythrasma, do not clean the area. For pigment assessment, ensure the patient has not recently tanned.

The rise of the smartphone dermatoscope with UVA filters offers new possibilities for documentation and telemedicine. However, the intensity and wavelength purity of these consumer-grade devices can vary, and they should not yet be considered a direct replacement for a medical-grade Wood's lamp in critical diagnostic decisions without proper validation.

B. Limitations and the need for differential diagnosis

While evocative, Wood's lamp findings are rarely 100% pathognomonic and must be integrated into the full clinical context. A positive fluorescence must be interpreted cautiously. For example, a coral-red glow is highly suggestive of erythrasma, but other Corynebacterium species or even some fabrics or topical residues can produce similar colors. The classic green fluorescence of tinea capitis caused by Microsporum species is a valuable sign, but not all ringworm fluoresces (e.g., Trichophyton species usually do not). Furthermore, the absence of fluorescence does not rule out a condition. A non-fluorescing scalp scaling could still be tinea caused by a Trichophyton species, requiring microscopy and culture for confirmation—the gold standard for fungal diagnosis. Similarly, the tinea woods lamp examination is negative in onychomycosis (nail fungus). The lamp's role in pigment disorders is qualitative, not quantitative. Therefore, the Wood's lamp is best viewed as a powerful, immediate triage and guiding tool. It can point the clinician in the right direction, suggest the next diagnostic step (e.g., "this fluoresces red, let's prescribe an antibacterial cream and see if it clears"), or help monitor treatment response. However, for definitive diagnosis, especially for infections, it should be combined with other methods like potassium hydroxide (KOH) preparation, fungal/bacterial culture, skin biopsy, or advanced imaging. This holistic approach, leveraging both simple tools like the Wood's lamp and modern diagnostics, embodies the art and science of clinical dermatology.