Photosensitivity Disorders
Photosensitivity represents an abnormal cutaneous response to ultraviolet (UV) or visible light radiation. It is broadly categorized into idiopathic, metabolic, and drug/chemical-induced etiologies.
Mechanisms of Action
| Category | Clinical Examples |
|---|---|
| Idiopathic | Polymorphous Light Eruption (PMLE), Solar Urticaria. |
| Metabolic | Porphyria Cutanea Tarda (PCT). |
| Exogenous | Drug-induced (Photoallergic/Phototoxic reactions). |
Distinguishing Phototoxic vs. Photoallergic
- Phototoxic (Common): Non-immune mediated; resembles a severe sunburn. Dose-dependent; occurs in anyone given enough drug + UV. (Examples: Tetracyclines, Amiodarone, Psoralens).
- Photoallergic (Rare): Immune-mediated (Type IV); resembles contact dermatitis. Not dose-dependent; occurs only in sensitized individuals. (Examples: Fragrances, Sulfonamides, NSAIDs).
High-Yield Exam Pearl:
- PMLE (Polymorphous Light Eruption): The “hardening effect” is characteristic—repeated exposure to sunlight leads to tolerance later in the season.
- Porphyria Cutanea Tarda (PCT): Look for blisters and increased skin fragility on the **dorsum of hands**, often associated with liver disease or alcohol use.
- Sparing: Note that lesions usually appear on sun-exposed areas (face, V-neck, dorsum of hands), but spare “shadow” areas like under the chin or behind the ears.