The Wood’s lamp is a long-standing diagnostic tool utilized in dermatology to help identify various skin, hair, and scalp conditions. This device emits long-wave ultraviolet A (UVA) light, commonly known as black light, which is invisible to the human eye. Illuminating the skin with this specific wavelength reveals characteristics and substances unseen under normal lighting. This simple, non-invasive examination aids clinicians in pinpointing disorders of pigmentation, infection, and certain metabolic diseases.
How the Wood’s Lamp Works
The underlying principle of the Wood’s lamp examination is fluorescence, a physical phenomenon where certain molecules absorb light energy at one wavelength and then quickly re-emit it at a longer, visible wavelength. The UVA light excites specific substances, such as metabolites, pigments, or compounds produced by microorganisms residing on the skin. When these substances return to their normal energy state, they release the absorbed energy as a visible glow.
The resulting color of the fluorescence depends entirely on the chemical composition of the excited substance, providing immediate diagnostic clues. For instance, some bacterial infections may glow bright green, while certain fungal infections might appear yellow-green. The examination must be performed in a completely dark environment to maximize contrast and ensure the subtle fluorescence is not missed.
Porphyria Cutanea Tarda
The specific condition most often associated with a purple, pink-purple, or coral-red fluorescence under the Wood’s lamp is Porphyria Cutanea Tarda (PCT). This metabolic disorder stems from a deficiency in an enzyme necessary for the body’s heme production pathway. The resulting accumulation of specific compounds, known as porphyrins, is responsible for the unique fluorescent property.
The clinical symptoms of PCT primarily manifest on sun-exposed areas, such as the hands and forearms. Patients experience extreme skin fragility and develop tense, fluid-filled blisters (bullae) after minimal trauma or sun exposure, which heal slowly, often leaving behind small, white cysts (milia), scarring, and hyperpigmentation. The pink-red fluorescence is not typically visible on intact skin but is most strikingly observed when examining blister fluid, urine, or feces.
The Role of Porphyrins in Fluorescence
The fluorescence observed in Porphyria Cutanea Tarda is a direct result of the chemical structure of the accumulated porphyrin molecules. In PCT, the defective enzyme, uroporphyrinogen decarboxylase, causes an excess of uroporphyrin and heptacarboxyl porphyrin to build up in the tissues; these compounds possess a molecular structure that makes them highly photoactive.
When the porphyrin molecules absorb the UVA light, they become energized and immediately release this absorbed energy by emitting light in the longer, visible wavelength range, specifically around 600 nanometers. This emission corresponds to the orange-red or pink-red part of the visible spectrum, which appears purple or pink-purple in the dark examination room, making the Wood’s lamp a valuable initial screening test for detecting these light-absorbing molecules.