Psoriasis (from the Greek word psora, which signifies a cutaneous eruption, supposed by some to be the itch) is now employed to signify a disease characterised by slight elevations of the surface of the skin covered with whitish scales. The eruption begins in small rounded spots, which may remain small, or may enlarge indefinitely, the centre becoming more normal while the inflamed margin continues to extend. The spots are covered by white silvery scales, not easily detached from the skin, which, however, when they are removed, is seen to be red and dry. The parts most often affected are the fronts of the knees and backs of the elbows; whatever other parts may be attacked, these are rarely free from the eruption, and the distribution is always nearly the same on the two sides of the body. Itching is often absent altogether, and very seldom severe. The disease may occur at any age, but usually first manifests itself in youth, rarely before the age of six. It is extremely apt to recur: it is rare for a person to suffer from it only once.
Numerous causes have been assigned for the disease; it has been attributed to scrofula, gout, and many other constitutional states; and doubtless it may be associated with them. But, with the exception of heredity, no cause has been satisfactorily shown to lead to its development. It frequently occurs in persons otherwise in perfect health, and, except in very severe cases, does not interfere in any way with their employments.
If left to itself, the disease generally tends to persist indefinitely. But in the great majority of cases it is very amenable to treatment, both local and constitutional. Locally, ointments containing tar, resorcin, pyrogallic acid, &c. are most in use; internally, arsenic is far the most valuable remedy. Where it has failed, iodide of potassium in large doses, liquor potassæ, and carbolic acid have sometimes succeeded. But some cases resist the most varied and persevering efforts for their cure; and nothing has yet been discovered which will prevent the tendency to recurrence of the disease.