LAUSR

DEAR EDITOR, We describe three cases of solar urticaria (SU) developing in patients with a diagnosis of erythropoietic protoporphyria (EPPs). We suggest that this may imply that the endothelium the location of a chromophore in SU. Patient 1 was a 42-year-old woman. She had lifelong EPP characterized by 3-day-long bouts of burning pain and swelling on the back of her hands and on her face, induced by 10 min of exposure to spring or summer sun, and an increased red-cell-free protoporphyrin IX concentration [36 lmol L 1 (normal range < 0 2 lmol L )]. She described a different type of photosensitive reaction that she also had since childhood. After a few minutes of bright sun exposure, the patient develops an itchy erythematous eruption. This itchy eruption affects the arms and legs but does not affect the hands and face. It fully resolves within 2 h. Unlike the painful ‘EPP’ reaction, this itchy eruption is almost fully controlled by oral antihistamine medications (cetirizine, fexofenadine and ranitidine). Patient 2 was a 16-year-old male patient with EPP that was characterized by a lifelong history of bouts of burning pain and swelling on the face, back of his hands and the top of his feet, which was triggered by 30 min of bright sun exposure. These painful bouts last 2–3 days. His increased red-cell-free protoporphyrin IX concentration [26 8 lmol L 1 (normal range < 0 2 lmol L )] confirmed the diagnosis of EPP. He experienced a different photosensitivity reaction from the age of 11 years. Exposure to the sun for a duration of 5 min causes an itchy red eruption on the back of the neck, thighs and lower legs, which does not affect the face, hands and feet. This itchy rash resolves within 15–20 min. These responses are in keeping with a diagnosis of SU and did not respond to oral antihistamine therapy. Patient 3 was a 33-year-old man who had ‘classical’ EPP. From the age of 3 years he had experienced photosensitivity characterized by bouts of burning pain with swelling on the back of the hands and on the face, which was triggered by 3–4 h of exposure to U.K. spring or summer sun. Each bout lasts around 3 days. His increased red-cell-free protoporphyrin IX concentration [41 4 lmol L 1 (normal range < 0 2 lmol L )] confirmed the diagnosis of EPP. From the age of 14 years he has experienced a different photosensitive reaction. Around 5 min of sun exposure induces an itchy red eruption on the tops of the feet, the shoulders and the forearms, but does not affect the dorsum of the hands, the neck or the face. This itchy eruption resolves within 15 min once he is out of the sun. These symptoms did not respond to antihistamine therapy. We carried out monochromator phototesting in these patients. In all three, the monochromatic irradiation induced weal and flare responses to long wavelength ultraviolet A and visible light within 5 min. The response resolved within 20 min in patient 1 and within 30 min in patient 3 (the time to resolution was not recorded in patient 2). The action spectrum was similar in the three patients; 380–500 nm in patient 1, 360–600 nm in patient 2 and 360–600 nm in patient 3, with the most severe reaction (visually assessed by estimation of the diameter and thickness of the weal and flare response) occurring at 400 nm in all of the patients. These three patients have been diagnosed with biochemically proven EPP, with typical bouts of painful photosensitivity affecting the dorsal extremities and face, which persist for a few days. The diagnosis of SU in these patients is based on clinical characteristics and the typical weal and flare reactions of SU on monochromator testing. In two of the three patients, the SU started several years after childhood onset of painful EPP photosensitivity, and the eruption was controlled by antihistamines in one patient. The diagnosis of SU in these patients is based on these clinical characteristics. Although the monochromator responses are typical of SU, these results must be treated with caution because phototest responses in EPP have never been characterized in detail. SU and EPP are rare. In the U.K., the prevalence of SU is estimated to be 3 1 cases per 100 000 individuals, and the prevalence of EPP in the U.K. is 2 54 cases per 100 000 individuals. SU has been reported in one patient with porphyria cutanea tarda. The coexistence of these two rare photodermatoses in these patients may not be coincidental. In SU, it has been postulated that ultraviolet radiation (UVR) and/or visible light are absorbed by a chromophore in the skin to create a photoallergen that causes an urticarial reaction via IgE-mediated mast cell degranulation. One study of six patients provided evidence that the chromophore in visible-light-allergic SU (400–500 nm) has a molecular mass below 45 kDa, and the chromophore for UVR-allergic SU has a molecular mass of around 1000 kDa. In EPP, painful photosensitivity is thought to result from the activation of protoporphyrin IX to an excited triplet state by Soret wavelength light (408 nm), resulting in the