What skin conditions can a Woods lamp help diagnose?

A Woods lamp is a valuable tool in dermatology for diagnosing various skin conditions. Here are the skin conditions it can help diagnose:

### Infections
- **Fungal Infections**:
  - **Tinea Capitis**: Caused by *Microsporum* species, which fluoresce blue-green under a Woods lamp.
  - **Flower Spot Tinea**: Fluoresces yellow-green.
  - **Tinea Versicolor**: Activity lesions fluoresce yellow-white or copper-orange.
- **Bacterial Infections**:
  - **Erythrasma**: Caused by *Corynebacterium minutissimum*, which fluoresces coral-red.
  - **Acne**: *Propionibacterium acnes* produces porphyrins that fluoresce orange-red.
  - **Pseudomonas Infections**: *Pseudomonas aeruginosa* produces pyocyanin, which fluoresces green under a Woods lamp.

### Pigmentation Disorders
- **Vitiligo**: Depigmented areas appear bright white or blue-white due to the absence of melanin.
- **Melasma**: Hyperpigmented areas may fluoresce yellow-brown.
- **Pigmentation Changes**:
  - **Pigment Reduction and Loss**: For individuals with fair skin, it is difficult to distinguish. In such cases, the fluorescence induced by the Woods lamp originates only from the dermal collagen. Due to the sudden interruption of emitted visible light at the lesion site, pigment-reduced or depigmented lesions appear as bright blue-white patches under the Woods lamp.
  - **Pigment Deposition**: When Woods lamp light illuminates the skin, shorter-wavelength ultraviolet (UVB and UVA) is easily scattered by the stratum corneum and epidermis, while longer-wavelength visible light (400–800 nm) penetrates deeper into the dermis. Melanin has a strong ability to absorb both UV and visible light. When Woods lamp light shines on melanin-rich epidermis, most of it is absorbed, whereas on adjacent melanin-poor skin, it is scattered and reflected. This creates a distinct boundary at the junction. Woods lamp primarily reveals epidermal pigmentation but is less effective for dermal pigmentation, as the fluorescence from collagen fibers in the dermis reduces the reflected fluorescence.

### Porphyria
- **Porphyria Cutanea Tarda**: Urine samples show bright pink-orange fluorescence under Woods lamp.
- **Congenital Porphyria**: Teeth, urine, and bone marrow may exhibit red fluorescence.
- **Erythropoietic Protoporphyria**: Red blood cells show transient fluorescence under fluorescence microscopy, while urine does not fluoresce.

### Other Conditions
- **Scabies**: Burrows created by scabies mites may appear as thin, dark lines under UV light.
- **Head Lice**: Can be identified through fluorescence.
- **Monitoring Local Drug Efficiency**: Some drugs show special fluorescence under Woods lamp, such as yellow-green fluorescence of mefloquine hydrochloride and yellow fluorescence of tetracycline. Sometimes, to understand the patient's topical drug use, fluorescent agents can be added to the medication.
- **Photochemotherapy**: Woods lamp can be used for photochemotherapy of psoriasis and vitiligo.
- **Skin Tumors**: Woods lamp can be used for the diagnosis of skin tumors.

Woods lamp examination is painless and non-invasive, providing immediate results. However, it should be used in conjunction with other diagnostic methods for accurate diagnosis. Proper interpretation requires training and clinical knowledge.

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