# 1911 Encyclopædia Britannica/Psoriasis

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​PSORIASIS, a skin affection characterized by the occurrence of flat dry patches of varying size covered with silvery white scales. Next to eczema and ringworm it is one of the most commonly found skin diseases. It occurs frequently during infancy and early adult life, and rarely begins after the age of fifty. Though a parasitic origin has been suggested, no bacteriological factor has yet been found, and it has been demonstrated that psoriasis may follow on nervous shock, gout, mental emotion and insufficient nourishment. It may also follow an attack of scarlet fever or erysipelas. The site of the disease may be determined by an abrasion or other injury of the skin, or even an irritation caused by friction of the clothing. The favourite starting point of the lesion is either the elbows or the fronts of the knees. It is nearly always symmetrical in its distribution, and spreads over the trunk and the extensor surfaces of the limbs, in contrast to eczema, which selects the flexor surfaces. The hairy scalp may also be affected. The eruption generally first shows itself as one or more papules, at first red and spreading, and later white from the formation of scales and red at the spreading margin, where it is surrounded by a hyperaemic zone. On removing the scales is seen a smooth hyperaemic zone dotted with red spots. The patches spread centrifugally and may remain stationary for a long time or coalesce with other patches and cover large areas of skin. In some cases involution of the central portion accompanies the spreading of the patch, and large concentric rings are formed. The lesions may persist for years, or spontaneously disappear, leaving behind a slight brown stain. The symptoms are usually slight and there is little or no irritation or itching, and no pain except in a form which is associated with osteo-arthritis. The disease, though of noted chronicity, is subject to sudden exacerbations, and may reappear at intervals after it has completely disappeared. It has little or no effect upon the general health. Several forms have been described, viz. the simple uncomplicated, the nervous, the osteo-arthritic, and the seborrhoeic. Varieties have also been named according to the character of the patches, such as psoriasis punctata, guttata, circinata or nummularis, or when large areas are involved and the skin is harsh, dry and cracked, it is known as psoriasis inveterate. The pathological changes taking place in the skin have been described as an inflammation of the papillae and corium, with a down-growth of the stratum mucosum between the papillae and an increase of the horny layer (keratosis). This latter, however, has been said to be due to the formation in it of tiny dry abscesses. The silvery appearance of the scales is due to the inclusion of air globules within them. The treatment is hygienic, constitutional and local. The clothing must be regulated so as to prevent undue perspiration or irritation or chafing of the skin. The most effective local application is chrysarobin used as an ointment. A bath of hot water and soap should first be given, or an alkaline bath, in order to remove all the scales; the ointment is then applied, but must be used over a small area at a time, as it is apt to set up dermatitis. Tarry applications, such as unguentum picis liquidae, creosote ointment or liquor carbonis detergens, are also useful and radio-therapy has caused a rapid removal of the lesions, but neither it nor the ointment has prevented subsequent recurrence. In chronic cases the sulphur-water baths of Harrogate, Aix-les-Bains and Aachen have been successful. The internal administration of small doses of vinum antimoniale, in acute cases, or of arsenic (in gradually increasing doses of the liquor arsenicalis) in chronic cases, is undoubtedly beneficial.