Adult‐onset actinic prurigo in Thailand

Background: Actinic prurigo (AP) is one of the rare idiopathic photodermatosis. It is said to be a familial disease and is usually seen in certain specific geographical areas. The adult‐onset type of AP is reported less frequently in the Asian population and has never been reported in Thailand.Methods: The study population comprised 30 patients. Demographic data were collected. Photo‐tests and photo provocation tests for UVA, UVB and visible light were carried out on non‐exposed skin. The other investigations included antinuclear antibody, anti‐HIV antibody and urine porphyrin level. Histopathology studies were also carried out.Results: There were 18 males and 12 females. The mean age of onset was 36.86 years. The duration of disease was from 1 month to 20 years. Forearms (27 patients) were the most frequently affected site. Other screening tests showed negative results. Five patients had abnormal MED to UVA and one patient had abnormal MED to UVA as well as UVB. Photo provocation tests showed positive responses to both UVA and UVB in 12 cases (40%), a positive response to UVA in 11 cases (37%), a positive response to UVB alone in four cases (13.3%) and a normal response in three patients (10%). None of the patients had a positive response to visible light. Skin biopsies were performed on nodular lesions in 23 cases. Histopathology from these 23 cases showed hyperkeratosis ortho‐ or parakeratosis and acanthosis in 20 of the 23 cases.Conclusions: Adult‐onset AP in our country may have different geographic and racial distribution from previous reports or may be the tropical variant as described by Tham et al. It may not be an uncommon disease in our country, if there is increased awareness of this disease. Only 16.6% of patients had reduced MED. Photo provocation tests were positive in 90% of cases. Most of the positive wavelengths were UVA or both UVA and UVB. Therefore, photo provocation tests should be performed in cases suspected of AP. The prognosis for AP is not good, despite combinations of treatment. The disorder may run a chronic course. This may be because of our sunny climate and the sun‐exposed occupations of patients.