You are here:

Home Information Psoriasis


What causes psoriasis?

The skin is a complex organ made up of several layers of different types of cell. Those in the outer layer - the epidermis – skin cells change gradually as they move towards the surface where they are continually shed. This process normally takes between 3 and 4 weeks. In psoriasis, the rate of turnover is dramatically increased within the affected skin, so that the process takes as little as 3 or 4 days.  The reasons for this are still not fully understood.

Some people are clearly more likely to develop psoriasis than others, particularly if there is someone else in their family who has psoriasis: in other words, for some individuals, it appears to be a genetic or hereditary disease (see below). However the trigger for psoriasis is often an outside event, such as a throat infection, stress or an injury to the skin.

Nevertheless, in most patients who develop psoriasis, or who have a recurrence, no obvious cause can be detected. Usually, sunlight improves psoriasis, though occasionally it makes it worse (especially if the skin gets burned). A high alcohol intake and smoking can worsen psoriasis too, as can medicines used for other conditions - such as lithium, some tablets used to treat malaria, and rarely other drugs.  There is no apparent relationship between diet and psoriasis.



Is psoriasis hereditary?

Yes – but the way it is inherited is complex and not yet fully understood.  Many genes are involved, and even if the right combination of genes has been inherited psoriasis may not appear. When psoriasis comes on later in life the genetic contribution is less important.  Other features of the inheritance of psoriasis are:

There is more likely to be a family history of psoriasis in people who get it when they are young than in those who develop it when they are old.

A child with one parent with psoriasis has roughly a 1 in 4 or chance of developing psoriasis too.

If one of a pair of twins has psoriasis, the other twin has a 70% chance of having it too if the twins are identical, but only a 20% chance if the twins are not identical.

What are the symptoms of psoriasis?

Psoriasis can itch, and painful splits may form within it.

Some 5 to 10% of those who have psoriasis also have stiff painful joints, which can be due to an associated ‘psoriatic arthropathy’. The joints most commonly affected are those at the ends of the fingers and toes.

The main problem with psoriasis for many people lies in the way it looks, and the way it attracts comments from others. This can affect their quality of life.



What does psoriasis look like?

Patches of psoriasis (often known as plaques) are red but covered with silvery white scales. They can take up a variety of shapes and sizes, and have well defined edges from the surrounding skin.  Some come up where the skin has been damaged: this is known as the Köbner phenomenon.  Patches triggered by a cut or a scratch in this way tend to be long and thin.  In the scalp, the scales heap up so that the underlying redness is hard to see.  In contrast, in body folds such as the armpits, the red well-defined areas are easy to see but are seldom scaly.

The severity of psoriasis varies from time to time, and from person to person.  When it is mild, there may be only one or two plaques: when it is more severe there may be large numbers.

The plaques can take up a variety of patterns on the skin:

The most common pattern is ‘chronic stable plaque psoriasis’.  The persistent plaques tend to appear symmetrically, most often on the knees, elbows, trunk and scalp, though any area can be involved.

‘Guttate psoriasis’ is another variant.  It is seen most often in children and is sometimes triggered by a sore throat.  The patches of guttate psoriasis are usually small (often less than 1 cm across) but numerous.

In ‘unstable psoriasis’, the plaques of psoriasis lose their clear-cut sharp edges, enlarge, and sometimes join up.  New ones may appear too.  Occasionally the skin becomes red all over – a condition known as erythrodermic psoriasis.  Unstable psoriasis must be treated with bland preparations as stronger ones can make it worse.

There are two main types of ‘pustular psoriasis’.  The first and most common involves only the palms and soles, where the red areas are studded with a mixture of new yellow pus spots and older brown dried up pus spots.  This type is slow to clear and often responds poorly to treatment.  A different, unrelated, and more widespread (generalised) type of pustular psoriasis can affect any part of the skin and is more severe.


Changes in the nails can often be seen too, if looked for carefully. They appear in up to a half of people with psoriasis.  The most striking ones are:

Irregular pitting (a pin prick effect) of the surface of the nail.

Circular areas of discolouration under the nail.

The nail may separate away from the underlying nail bed, and sometimes becomes too thick.

How will psoriasis be diagnosed?

Psoriasis is usually easy to recognise and a biopsy is seldom needed.

If a sore throat has triggered an attack of psoriasis, your doctor may take a swab from your throat to see if bacteria known as beta-haemolytic streptococci are present.  If they are, a course of an antibiotic may help.


If you are suffering from painful joints, your doctor may want to take a blood test to rule out rheumatoid arthritis. If you have several red or tender joints you may need to be assessed by a rheumatologist.




Can psoriasis be cured?

No.  However, treatment to control the signs and symptoms is usually effective. The skin becomes less scaly and may then look completely normal. However, even if your psoriasis disappears after treatment, there is a tendency for it to return. This may not happen for many years, but can do so within a few weeks.




All information from The British Skin Foundation. The Charity for Skin Disease Research