Prevalence of interdigital psoriasis of the feet (“psoriasis alba”) in mild, moderate, and severe psoriasis

Interdigital psoriasis of the feet ("psoriasis alba") is a rare form of inverse psoriasis. We conducted a cross-sectional study of the prevalence of interdigital psoriasis in mild, moderate, and severe psoriasis, compared to atopic dermatitis and normal controls. Data were collected during 2010-2013 from 232 psoriatic patients, 190 patients with atopic dermatitis, and 202 normal controls. The psoriatic and atopic dermatitis patients were from the dermatology department and outpatient clinic of the Hadassah-Hebrew University Medical Center in Jerusalem, Israel. The normal controls were healthy workers and volunteers from Hadassah Hospital who were not aware of any dermatological disease and had never consulted a general practitioner or dermatologist for skin problems of the feet. Our study revealed a prevalence of 2.6% of interdigital psoriasis of the feet in psoriatic patients, especially in men, and none in atopic dermatitis and normal controls. Three of the six affected patients with interdigital psoriasis of the feet complained of itching, both feet were involved in four patients, while two presented with additional palmoplantar psoriasis. The hematoxylin and eosin histopathological findings were in line with those found in inverse psoriasis. Dermatologists should be aware of this entity and treat it correctly. The diagnosis should be considered in psoriatic patients presenting with whitish plaque or patches in the toe-webs, in whom the fungal test is negative and are not responding to antimycotic treatment.