Psoriasis, which affects people of all ages in every country, is one of the most common chronic dermatoses in industrialized countries. It is estimated to affect about 2% of the population in the United States and northern Europe. Considerable local variations in its prevalence have been variously attributed to genetic, climatic, nutritional and ecological factors. Various biological events may trigger its expression, including streptococcal or viral infections, an emotional crisis or pregnancy. Other cases appear to be related to the administration of certain drugs, including lithium, chloroquine and β-adrenoreceptor antagonists.

Psoriasis vulgaris, the most common form of the disease, is characterized by erythematous, scaly plaques of varying size. These usually occur on the scalp and the extensor surfaces of the limbs, but the trunk may also be involved. Pitting, thickening and yellowish coloration of the nails are frequently observed. When lesions are widespread a fine desquamative scaling, known as erythrodermic psoriasis, may occur over the entire surface. About 5-10% of patients develop peripheral inflammatory arthritis of the hands and feet, sometimes in association with ankylosing spondylitis. Psoriasiform lesions, together with severe seborrhoeic dermatitis and severe Reiter syndrome, are often seen in patients with AIDS.

Guttate psoriasis occurs mainly in children. It is often triggered by a streptococcal infection, but may also occur during an acute exacerbation of chronic psoriasis vulgaris. It is characterized by the sudden appearance of smaller but widespread lesions. The condition sometimes resolves spontaneously, but usually transforms into psoriasis vulgaris.

Management

Many different approaches to treatment are used, each of which has advantages and disadvantages. However, none of the methods currently available has been shown to be effective in preventing relapse.

Topical treatment

Localized psoriasis vulgaris can frequently be cleared, sometimes for many months, by daily applications of dithranol ointment for 2-4 weeks. A low strength (0.1%) ointment should be applied initially. Higher strengths may be used subsequently, provided that they do not produce irritation or burning. A “short contact” method in which each application is rinsed off within 10-20 minutes causes little, if any, irritation or staining of normal skin. This is particularly useful for outpatient management, although there is a risk of severe conjunctivitis if the dithranol accidentally enters the eye. Dithranol should be used only under the direction of a physician trained in its use.

Crude coal tar is also effective in the treatment of psoriasis. The odour, staining and irritant properties are reduced when refined products are used in cream-based preparations. Some preparations also contain salicylic acid as a keratolytic. Good results are often obtained when daily applications or tar baths are combined with ultraviolet irradiation or exposure to sunlight.

Emollients containing low concentrations (1-2%) of salicylic acid are a useful adjunct to treatment, particularly where there is thick scaling.

Topical corticosteroids are widely used for short-term treatment of mild to moderate psoriasis. However, when extensive areas of the body surface are treated, such as in erythrodermic psoriasis, they can induce systemic adrenal suppression. In addition, relapse frequently occurs after withdrawal, and this may result in a more unstable form of the disease.

Recently it has been shown that topically applied calcipotriol, a vitamin D₃ analogue that promotes the differentiation of epidermal keratinocytes, is effective. The drug appears to be well accepted by patients, but is very expensive.

Systemic treatment

Patients with guttate psoriasis induced by streptococcal infection require treatment with oral antibiotics. Several options exist for the systemic treatment of patients with psoriasis vulgaris unresponsive to topical therapy. These involve the use of drugs such as antimetabolites, immunosuppressants, ciclosporin, oral retinoids or methoxsalen (in conjunction with exposure to ultraviolet irradiation (PUVA therapy)) which are expensive, have potentially serious side-effects and necessitate close monitoring of patients. These forms of therapy should be given only by dermatologists in a secondary- or tertiary-level health facility. It is also advisable, when treatment is prolonged, to change the regimen from time to time to reduce the risk of cumulative toxicity.

Systemic corticosteroids should not be used because of the risk of severe exacerbations on withdrawal.