Definition. A chronic, dry, inflammatory disease of the skin characterized by variously sized, reddish, sharply defined patches covered with abundant, silvery-white, adherent scales.
Symptoms. Psoriasis is one of the common diseases, constituting three to four per cent. of all dermatological cases, but this is owing more to its persistency and tendency to recur than to the number of actual new cases. It shows its systemic origin by occurring symmetrically, and its probable connection with some internal derangement by beginning on the surfaces where the circulation is less active; namely, the extensor surfaces. Without prodromal signs of any kind psoriasis makes it appearance in pin-head-sized reddish flat or acuminated papules which, within a few hours, become capped with a whitish scale (psoriasis punctata). The spots gradually enlarge peripherally until, when they are about one-quarter of an inch across, with thickened scales, they look like drops of mortar (psoriasis guttata). When they reach the size of familiar coins, they are known as psoriasis nummularis. When they continue to spread, two or more patches may coalesce, forming an irregular shaped lesion, or resolution may occur in the center of a patch, leaving a ring-like patch (psoriasis annulata). If two or more of these lesions join, as the points of contact melt away, one or two lines are left (psoriasis gyrata). While these forms are transitional, and are slow in their evolution, and while it is possible that they may be arrested at any stage, it is common to find more than a single form existing at the same time. The union of patches may go on until a wide extent of surface is involved (psoriasis diffusa). Rarely it may be so widely distributed that it has been called psoriasis universalis, or if there is induration of the skin, fissures may develop, and adherent scales form, then it has been termed psoriasis inveterata. When there is a tendency to the heaping of scales, it has been termed psoriasis rupioides or psoriasis ostreacea, and rarely when papillary hypertrophy is noted, psoriasis verrucosa.
The scaling, while a pronounced feature of psoriasis, varies in different cases, in different spots and at different times. Though they are always adherent, they can readily be removed and minute bleeding points appear at the point of removal. The eruption is dry from beginning to end, whether of short or long duration. Untreated, the disease tends to continue although spontaneous remissions are likely to occur, especially in the summer time. Some cases go through a lifetime without a complete disappearance of the eruption, at the same time without any disturbance of the general health. Other may show temporary disturbance of nutrition or some constitutional defect. There may be more or less persistent pigmentation, especially on the legs after the eruption has disappeared. This is more apt to follow treatment with full doses of arsenic.
Many cases of psoriasis will show lesions only of the scalp and extensor surfaces of the extremities, particularly the elbows and knees, but they may be found anywhere even on the palms and soles. It never develops, however, in the last named locations without first appearing elsewhere on he surface of the skin, and, indeed, Crocker has asserted that the majority of cases of so-called palmar or plantar psoriasis are of syphilitic origin, or else are eczema. While psoriasis never attacks the mucous membranes, it may extend to the the glans penis. On the scrotum, it will lead to considerable thickening and fissuring of the skin, and occasionally a thin exudation. The nails may be affected but present nothing characteristic. There may be slight depression or the nail only, or any degree of hypertrophy, but the presence of the disease elsewhere must be relied upon to determine its nature. While psoriasis often occurs in the scalp, and may remain there for months or years before becoming generalized, it rarely interferes with growth of the hair. A red line or strip may be seen as it advances to the hair border, resembling the corona of eczema or seborrheic dermatitis, but it is never moist.
Psoriasis is a disease of all ages and conditions of life. It begins primarily in early life, my own experience being that two-thirds of the cases first appear between the ages of ten and twenty-five, while the cases first appear between the ages of ten and twenty-five, while Bulkley gives fifty-five per cent. as occurring between ten and twenty-five years of life and a few others after the fifth year. In this connection of I have recently seen one appearing before the fourth month and another initial appearance in a man seventy-nine years of age. While eczema and syphilis usually have no effect on an already present psoriasis, the acute fevers, especially the exanthemata, may cause it to disappear, only to return again with the restoration of normal health. On the other hand, scrofula, gout, parturition and lactation tend to aggravate the disease. Pruritus, which is never present in simple cases, may result from pus infection or from eczematous, ecthymatous, or seborrheic complications. As has already been noted, persistent pigmentation may attend or follow psoriasis, and very rarely superficial scarring has been noted as a sequence.
Etiology and Pathology. Modern views as to the nature and origin of tuberculosis and leprosy would suggest a possible parasitic etiology, especially in cases occurring in the same family, quite as reasonably as the theory of heredity. But, despite the fact that from ten to twenty-five per cent. of all cases give some variety of hereditary influence, no specific microorganism has yet been found, and such evidence as has been presented to establish a specific microbe is not convincing. It is barely possible that the cutaneous manifestations are produced by a parasite, but there is not question that the underlying causes are internal and constitutional. I believe that the theory of the nervous origin of psoriasis presents many elements of truth and explains some of the vagaries of the disease. It has been claimed that syphilis, rheumatism, gout, struma and tuberculosis stand in etiological relation to psoriasis, but scientific proof is wanting, although I have noted the presence of a pronounced uric acid diathesis in nearly fifty per cent. of the case of long standing, inveterate psoriasis. Rare instances have been noted where psoriasis followed vaccination, tattooing and other excoriations of the skin. Further internal use of biborate of soda and injections of tuberculin have produced psoriasis. It has been commonly asserted that psoriasis is a disease of the strong and well, but my personal experience does not support this view because, in a large percentage of over eleven hundred cases observed, the average one did not present normal assimilation or sufficient elimination. Often defective hepatic function, constipation, hyperacid urine, intestinal fermentation and renal irritation of some sort may be noted; further, a great number of these patients prospered on a properly regulated diet. Whatever the dynamic origin of psoriasis may be, its development seems uninfluenced to any extent by age, sex, class or occupation. The general run of cases are, however, worse in winter.
Without going into the plausible theories that this disease is either due to an undiscovered parasite planted on susceptible soil or has a neuropathic basis, it is fairly well agreed that psoriasis begins as a non inflammatory hyperplasis and multiplication of the cells of the mucous layer of the epidermis, producing downward growth of the interpapillary processes and apparent elongation of the papillae, followed by dilatation of the blood vessels of the papillary layer of the corium, with serous exudation and cell infiltration around the vessels. The silvery white color of the scales is caused by the presence of air between the cells forming the scales. There is imperfect keratinization of the horny layer of the epidermis, perhaps due to the rapidity of cell growth.
Diagnosis. It is only in the atypical cases that doubt may arise, and then it may be confounded with one of the following diseases:
Squamous eczema will generally give some history of moisture, its patches have an ill-defined border, its scales are thickest in the center, darker and less abundant than those of psoriasis, it is located in the flexures of the joints or on the flexor surfaces, and invariably itches. Both psoriasis and eczema may affect the nails, but eczema often attacks all, psoriasis only one or more, never all at once.
Seborrhea of the scalp is generally more extensive than psoriasis. If the latter extends beyond the hairline, its characteristic appearance, and other lesions on its favourite locations may usually be found. Its scales are not dirty and fatty as are those of seborrhea.
Seborrheic dermatitis of the non-hairy parts may closely resemble psoriasis, but the scales are greasy, tend to form crusts, and the affected skin is generally a yellowish red. It seldom occurs in the favorite locations of psoriasis, and a history of a primary seborrhea on a hairy surface, and its evolution, clearly establishes its nature.