Differential Diagnosis for Rash Made Worse with Heat/Sun
Single Most Likely Diagnosis
- Polymorphic Light Eruption (PMLE): This is the most common photodermatosis, characterized by a rash that occurs after exposure to sunlight or UV radiation. It typically appears as an itchy, bumpy, or blistering rash on sun-exposed areas.
Other Likely Diagnoses
- Solar Urticaria: A rare condition where hives or itchy wheals appear on sun-exposed skin, usually within minutes of sun exposure.
- Phototoxicity: A non-immunologic reaction to certain medications (like doxycycline or sulfonamides) or substances (like St. John's Wort) that occurs when the skin is exposed to sunlight, leading to a sunburn-like reaction.
- Porphyria Cutanea Tarda: A disorder that results from a deficiency of the enzyme uroporphyrinogen decarboxylase, leading to blistering skin lesions on sun-exposed areas due to the accumulation of porphyrins.
Do Not Miss Diagnoses
- Lupus Erythematosus: An autoimmune disease that can cause a photosensitive rash, among other systemic symptoms. Missing this diagnosis could lead to delayed treatment and worsening of the disease.
- Pellagra: A condition caused by niacin deficiency, characterized by the "three Ds": diarrhea, dermatitis, and dementia. The dermatitis can be exacerbated by sun exposure.
- Xeroderma Pigmentosum: A rare genetic disorder where the body is unable to repair DNA damage caused by UV light, leading to an increased risk of skin cancers and premature aging.
Rare Diagnoses
- Chronic Actinic Dermatitis: A rare condition characterized by a persistent, eczematous dermatitis on sun-exposed areas, often seen in older men.
- Hydroa Vacciniforme: A rare photodermatosis of childhood, characterized by a vesiculopapular eruption on sun-exposed areas, often accompanied by scarring.
- Erythropoietic Protoporphyria: A rare genetic disorder that leads to an accumulation of protoporphyrin in the skin, causing pain, swelling, and blistering upon sun exposure.