One of the most common skin disorders, psoriasis comes in various forms and needs specific treatments, says Dr Lesley Hickin

What is psoriasis?
Most skin disorders do not pose a serious threat to your health, but can affect the quality of your life and require long-term treatment. Psoriasis is a genetically determined inflammatory condition of the skin, with unknown cause, and is common in Western countries, Australia, South America and parts of Africa and Asia. It is one of the most common of the inflammatory skin disorders and is important because of its persistent or recurrent nature and its severity in a number of those affected.

Who does it affect?
Psoriasis affects one to two per cent of the population and can begin at any age from very young to very old. The commonest time for it to appear is in the 15-20 age group, but it also appears for the first time frequently in people over 70. In general, the earlier it appears the more severe the disease is likely to be in terms of frequency and severity of attacks. There are several different types of the disease, affecting people at various ages, and there are some drugs that can provoke an attack in a susceptible person, including antidepressants, beta blockers and drugs used to prevent malaria.

What causes psoriasis?
You cannot 'catch' psoriasis from someone who has it, but it often runs in families. The way it is inherited is not clear-cut. If one parent is affected there is a 30 per cent chance that a child will develop the condition, with two affected parents the risk is 60 per cent. With identical twins (the 'gold standard' for working out inheritance of a disease) the chances of both twins having psoriasis is 70 per cent. The current theory about the cause of psoriasis is that something triggers the immune system in an affected person and the control of cell division at the skin surface is altered.

What does it look like?
Each individual lesion (called a plaque) has a well-defined border and is raised above the skin surface. Plaques vary enormously in size and shape. They often begin as discs but as adjacent plaques merge become more irregular. The affected skin is red, and often has large silvery scales stuck to the surface.

What is happening to the skin?
In areas of skin affected by psoriasis new skin cells are produced at a much faster rate than dead cells are shed and the excess cells accumulate to form a thick scale. Underlying the surface scale is an area with increased numbers of small blood vessels.

Do I have psoriasis or could it be something else?
Other skin conditions can look similar to psoriasis, particularly if it is not the classic type. These include certain forms of eczema and ringworm, with other rarer skin diseases also being a possibility.

Types of psoriasis

1. Plaque psoriasis
This is the most common form of psoriasis and is a lifelong disorder that may develop at any age. Any part of the outer surface of the body can be affected but especially the outer borders of the limbs, the knees, elbows, lower back and scalp. Plaques can also arise in areas of previously damaged skin like scar tissue. The plaques can be itchy and sore, but the symptom that bothers people most is the continual shedding of the skin scales, particularly from the scalp, which is embarrassing and can cause severe psychological upset.

Sometimes the areas affected are the inner areas of the skin, so-called reverse psoriasis, when the armpits, groins and the skin between the buttocks or under the breasts is inflamed. There is no characteristic scale at these sites because it is rubbed off by friction, and so it can be harder to distinguish it from other skin disorders. Psoriasis in these areas can also become infected by bacteria or fungi and be very sore and uncomfortable.

The nails are also commonly involved, with small pits scattered across the nails, separation of the nail from the nail bed, and scale accumulating under the nail plate. Rarely, the nails only are affected, with no skin changes at all.

2. Guttate psoriasis
This form most commonly affects children and adolescents and frequently follows a throat infection or other viral illness. The attack lasts three to six months. Many plaques of psoriasis appear all over the body, mainly very small (less than one centimetre in diameter). About half of those affected can go on to develop one of the other forms of psoriasis later in life.

3. Pustular psoriasis
There are two types of this form of psoriasis. The more common is the type localised to the palms and soles, which consists of small blisters filled with pus, red tender inflamed surrounding areas of skin and some areas of thickened scaly cracked skin. Occasionally this type can affect the whole body and this is a serious and life-threatening condition needing hospital admission.

4. Joint disease
Around five per cent of people with psoriasis develop a typical form of arthritis. This may take the form of inflammation (pain, redness and swelling) of the small joints of the hands, feet, wrists and ankles. The other common type of arthritis associated with psoriasis is a more severe inflammation affecting and deforming various joints.

Treatment

1. Topical (surface) treatments
There are several types of creams and ointments that are useful and many of the preparations are available as bath additives, scalp applications or shampoos. The problem that people with psoriasis have is the time it takes to apply the treatment and the fact that it may take up to six weeks to know whether an individual treatment will work. No wonder so many give up on the treatments. The more old-fashioned treatments, although they were effective, were smelly and messy, but fortunately newer drugs have been developed to overcome these problems.