Fair-skinned people are more prone to it than darker-skinned individuals. Typically, it shows up as thickened red areas covered with silvery scales, usually on the scalp, elbows, knees, back and buttocks. It can also affect fingernails and toenails. The patches usually flare up periodically and symmetrically on both sides of the body.

Psoriasis is considered mild if it affects less than 5 per cent of the body surface, moderate if it involves 5-30 per cent of the skin, and severe if it covers more than 30 per cent.

About one in 10 of those with psoriasis will develop psoriatic arthritis, characterised by painful swelling and stiffness in the joints as well as the usual skin plaques.

What causes it? Many doctors believe that psoriasis is caused by an accumulation of dead skin cells on the skin surface, the result of skin-cell replication at a rate 1000 times faster than normal. This cell growth is too rapid to allow skin cells to be shed normally. Their accumulation results in the silvery scales typical of psoriasis.

But, as some psoriatics improve while taking prescription drugs that interfere with the immune system, it may be that the too-rapid cell formation reflects immune dysfunction.

Indeed, psoriasis is most likely an autoimmune disorder, probably the result of a T cell-mediated immune response (Arch Dermatol, 1999; 135: 1104-10).

The question is, what triggers the immune response? The tendency to psoriasis is inherited, but a number of other factors appear to cause or contribute. In women, the most common trigger is smoking (BMJ, 1994; 308: 428-9); in men, it is alcohol. But other triggers include incomplete protein digestion, bowel toxaemia, impaired liver function, excess consumption of animal fats, nutritional factors and stress.

Psoriasis can also flare up after bacterial infections such as strep throat, skin injuries and vaccinations as well as with the use of medicines such as lithium, beta-blockers and antimalarials, the heart drug quinidine, systemic steroids (oral or injected) and the non-steroidal anti-inflammatory indomethacin.

What doctors tell you There are three categories of treatment. Medicated creams and ointments (including corticosteroids, vitamin D or synthetic vitamin A called ‘retinoids’) are the mainstay of treatment for mild-to-moderate psoriasis. Over-the-counter coaltar-containing products and hydrocortisone may also work.

Stronger immunosuppressants such as methotrexate and cyclosporin A are generally only prescribed for the most severe cases. Phototherapy (PUVA) using ultraviolet A and psoralen (a photosensitising agent) is usually reserved for individuals who do not respond well to other therapies.

Methotrexate should not be taken by pregnant women or by those who wish to become pregnant because of their adverse effects on the unborn child. Steroids can cause serious side-effects in all users. Cyclosporin A is toxic to the kidneys; methotrexate is toxic to the liver and bone marrow. Retinoids are associated with birth defects, and PUVA is known to cause cancer and may lead to light sensitivity. High blood pressure, anaemia and immune-system suppression that can lead to infection have also been documented with conventional antipsoriasis treatments.

Pat Thomas