Psoriasis is a common condition affecting the skin. It causes red, scaly patches. In addition it can affect the joints, nails and eyes. Psoriasis can cause as little as a single dimple on one of your finger or toe nails, or affect as much as the majority of your skin surface, your joints, and your eyes. 2% of people (1 in 50) have psoriasis to some degree.

The most commonly affected areas are the back of the elbows and the front of the knees. It often affects the scalp, and can affect any part of the body. The standard appearance is of red areas where the skin is thickened and crusty, often with silvery flakes which come off easily. This appears as patches, which are known as plaques.

Types of psoriasis

• Plaque psoriasis. The patches most commonly seen are called plaques. They especially affect the back of the elbows and the front of the knees and the back.
• Guttate psoriasis is many small patches of psoriasis, all over the body, and often happens after a throat infection.
• Flexural psoriasis causes red, shiny areas in skin folds, for example under breasts, between buttocks etc.
• Pustular psoriasis. Smaller, circular patches, filled with pus, appear on the palms of the hands and soles of the feet. This can sometimes cause a fever and may need treatment with an antibiotic.
• Scalp psoriasis. Scaling and flakes of the scalp, often particularly affecting the hair margins.
• A serious, but rare, complication of psoriasis is erythroderma, where large areas of the skin become hot, red and dry. This is one of the few emergencies involving skin conditions. If you suffer from this your doctor will admit you to hospital.

Sometimes parts of the body other than the skin can be affected:

• The joints can be affected by a form of arthritis (Psoriatic arthropathic). This can affect any joint, but often it is only one joint that becomes inflamed at a time.
• One or more of your finger or toe nails may develop little pits as on a thimble, or may become generally more opaque and thickened (nail dystrophy).
• The eyes may become inflamed (uveitis).

Though the rash is sometimes quite obvious, it is not infectious and cannot be caught by contact.

Psoriasis runs in some families, but that is not to say that everyone in a family will get it. It can start at any stage in life, but most develop their first symptoms between 11 and 45 years old. Often it starts at puberty.

The cause is unknown but, as well as a genetic link, a number of things seem to trigger a first attack:

• Often a patch will start where the skin has been scratched or injured
• A throat infection
• Certain medicines or drugs

The skin, in the patches that are affected, replaces itself at a much quicker rate than the rest of the skin normally does. Normal skin replaces itself by pushing up new skin cells from below over a period of 28 days, but in psoriasis this takes as little as 4 days.

As with all diseases, and problems of the skin in particular, stress can aggravate psoriasis.

Usually your doctor will make the diagnosis from the appearance of the rash. If you have inflamed joints your doctor may want to arrange some blood tests. Rarely, in cases of doubt, a sample of flakes scraped from the skin, or a small sample of skin (a biopsy), will be sent to the laboratory.

There is no cure for psoriasis, but many people have long periods when it does not trouble them. Sometimes it gets better on its own, but most people need some treatment.

Treatments include:

• Moisturizing creams and ointments. Used to moisturize dry skin, and also as a substitute for soap when washing the skin.
• Oils for the bath. Some of these contain tar or antiseptics, which can add other benefits in addition to the moisturizing effect.
• The mainstay of treatment has for years been creams, ointments, lotions and shampoos based on tar. These help cut down scaling of the skin and also have an anti-inflammatory effect. The main worry is their smell!
• Preparations to be applied, based on Vitamin D have been found to be very effective, and are probably becoming the first choice with patients and doctors alike.
• Applications based on salicylic acid (which was originally developed from willow bark, and is related to aspirin) are helpful at removing thick layers of over-grown skin and scales.
• Sun shine has been known, for years, to help. A development of that, especially as dermatologists are always very suspicious of the sun, is the controlled use of ultraviolet radiation often given with a medication (a psoralen) to prime the skin. (PUVA, Psoralens with long wave ultraviolet radiation.)
• Stronger medications – prescribed only by a dermatologist, and carefully monitored – are occasionally used and can be very helpful, for example methotrexate.
• Mild steroid creams and ointments, used for short periods, for psoriasis affecting the face or body folds. (Stronger preparations and steroids by mouth are sometimes used, but this should be under specialist supervision.)