The plaques may be solid with sharply demarcated borders and may vary from about the size of a coin to extensive lesions resembling a chart or map. The latter are aptly called geographic plaques and may cover the back or chest. Annular plaques with raised, scaling borders and central clearings may develop and assume serpiginous outlines. Another characteristic feature of psoriasis is symmetric distribution of lesions. In some patients, no more than one or two lesions may be present at a time, although widespread involvement with significant cosmetic disability is common.
Nail involvement is an important diagnostic feature.
Lesions may also occur on the palms and soles and are often seen on the glans penis and vulva. Patients who have psoriatic erythroderma are systemically ill and may exhibit disturbance of body temperature regulation. Although psoriasis may first appear as an exfoli-ative erythroderma, this state may also be induced by withdrawal of systemic corticosteroids, the use of synthetic antimalarial drugs, and harsh topical therapy. Not all patients with exfoliative erythroderma have underlying psoriasis. Other conditions, such as atopic eczema, lymphomas, and drug eruptions, however, can also present with exfoliative erythroderma and therefore must be included in the differential diagnosis.