Solar urticaria in Adult
Solar urticaria presents with classic urticarial wheals, erythema, and/or edema limited to sun-exposed areas of the body within minutes of sun exposure. Lesions typically resolve within 1-2 hours after exposure. Regularly sun-exposed areas, such as the face and dorsal hands, may be less sensitive due to the hardening phenomenon. Fixed solar urticaria is a subtype in which lesions occur exclusively in the same localized area of the skin.
Most patients experience accompanying pruritus or a burning sensation, and rarely pain has been described. Systemic symptoms are uncommon but may occur after prolonged exposure of a large body surface area to inciting radiation; symptoms may include headache, nausea, dizziness, wheezing, dyspnea, and syncope. Rarely, severe attacks are associated with anaphylactic shock.
Solar urticaria is caused by immunoglobulin E (IgE)-mediated mast cell degranulation. However, the exact mechanism triggering the degranulation is not known. It is thought to be an IgE-mediated response to a photoinduced allergen. Chlorpromazine, tar, benoxaprofen, and repirinast are known causes of drug-induced solar urticaria.
Solar urticaria typically follows a chronic course. The mean age of onset is 35 years, with less than 4% of patients presenting before age 5. Spontaneous resolution may occur in 15% of patients after 5 years and approximately 25% of patients after 10 years of symptoms. Occasionally, repeat exposure may produce a hardening effect with decreased severity or frequency of symptoms.
L56.3 – Solar urticaria
10347006 – Solar urticaria
- Other forms of urticaria, especially heat-induced urticaria (see physical urticaria). Careful history taking should help differentiate idiopathic from inducible urticaria. The use of a water filter in front of the light source during phototesting helps to rule out heat-induced urticaria.
- Polymorphous light eruption occurs within hours, not minutes, of sun exposure, and lesions last for days, not minutes or hours, as in solar urticaria. Whereas solar urticaria may occur on the face, polymorphic light eruption usually spares the face.
- Porphyrias also have lesions on sun-exposed skin. Erythropoietic protoporphyria (EPP) would be clinically most similar to solar urticaria in children. EPP can be differentiated in that it manifests during early childhood; it is typically painful rather than pruritic; petechial, purpuric, hemorrhagic crusted, and/or scarring lesions may be present; and it may have increased protoporphyrin in plasma and red blood cells. Patients with EPP typically have a positive family history, as the disorder is inherited in an autosomal dominant fashion. Porphyria cutanea tarda, also in the differential diagnosis for adults, manifests with blisters and erythema on sun-exposed skin, with scarring and elevated uroporphyrinogen in the urine.
- Systemic lupus erythematosus should be excluded in all patients with photoinduced skin eruptions. Subacute cutaneous lupus erythematosus spares the midface. Both acute and subacute lupus erythematosus spare the knuckles and may be scaly. Lupus erythematosus tumidus may have urticarial plaques, but lesions last longer than hours.
- Brachioradial pruritus presents with localized pruritus of the extensor forearm, upper arm, or neck without an associated rash. Sunlight is a common reported trigger. Like solar urticaria, symptoms present commonly in middle-aged females. Burning and stinging may accompany pruritus, and symptoms are often relieved with an ice pack.
- Photoallergic reactions are type 4 hypersensitivity reactions that present with eczematous dermatitis in sun-exposed areas of the body. Symptoms present 1-2 days after sun exposure, in contrast to solar urticaria. See drug-induced photoallergic reaction.
- Phototoxic drug reactions occur after ingestion of a photosensitizing agent. Like solar urticaria, the reaction occurs within minutes to hours of sun exposure. However, presentation resembles a sunburn instead of urticaria. Since the reaction is not immune mediated, it will resolve with removal of the offending drug.