History
An accurate history is important for the diagnosis of solar urticaria because of the transient nature of the eruption. Patients often have no obvious lesions.
Patients may report pruritus, erythema, and wheal formation of varying degrees after a short period (< 30 min) of sun exposure. As with most other photodermatoses, skin lesions in solar urticaria may occur on any exposed area, even if skin was covered with thin clothing.
The face and the dorsal aspect of the hands, which are chronically exposed to the sun, are less severely affected than other parts of the body, perhaps owing to acclimatization and "hardening."
Mucosal involvement (eg, tongue and/or lip swelling) has been reported. Headache, nausea, vomiting, bronchospasm, and syncope have been reported as well but are considered rare.
Upon cessation of sun exposure, the rash begins to disappear within several minutes to a few hours and rarely lasts beyond 24 hours. Rapid disappearance of the rash upon cessation of further sun exposure is essential to the diagnosis of solar urticaria.
Ascertain the following aspects of the patient’s history to exclude other differential diagnoses:
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Oral medication intake (eg, chlorpromazine), which may cause a similar photo-induced reaction
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Currently used topical agents (eg, sunscreen, fragrance), which can cause photocontact dermatitis
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Family history of photosensitivity, as may occur in some porphyrias
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Medical history regarding other body systems in order to detect other underlying causes of photosensitivity, such as connective-tissue disorders
Physical Examination
In most cases of solar urticaria, physical examination findings will be normal. During an acute episode of solar urticaria, vital signs are usually unaffected; however, systemic symptoms accompanying the cutaneous eruption have been reported. In rare cases, cardiac and respiratory rates increase and blood pressure decreases. Wheezing may be heard upon auscultation of the chest when bronchospasm is present.
Examination of the skin during an acute solar urticaria episode may reveal lesions in the form of erythematous macules to distinct wheals. Although the lesions’ morphology may be no different than that of lesions found in acute urticaria secondary to other causes, the eruption will follow a photodistribution modified by the type of clothing worn by the affected individual at the time of exposure.
Lesions may also be present in areas covered with thin clothing, depending on the causative light wavelength and sheerness of the fabric. Mucosal areas, such as the tongue and lips, may be swollen or edematous.
The reaction leaves no residual skin changes. Consequently, examination of the skin after an acute eruption reveals no evidence of the condition.
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Solar urticaria. Courtesy of DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/reactions/solar-urticaria.jpg).
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Solar urticaria. Courtesy of DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/reactions/solar-urticaria1.jpg).
