What causes psoriasis?

No one really knows for certain what causes psoriasis. But there are two factors that may contribute to the development of the condition, namely (1) genetic predisposition and (2) triggers that make psoriasis appear.

GENETIC PREDISPOSITION

Psoriasis often, but not always, runs in families. However, the inherit­ance of psoriasis is not predictable because the abnormalities that cause it are carried on several different genes. There is no single gene test to predict with certainty whether you’ll get psoriasis or not. If you have several members of your family who have psoriasis, you are more likely to have psoriasis than someone who has no affected relatives – but it is by no means certain that you’ll ever get it.
Similarly, some people develop psoriasis out of the blue, and have no relatives they can think of who’ve ever had the condition. In fact, only around one-third of people with psoriasis can think of any family members with the condition. In this sense, the inheritance of psoriasis is like that of eye colour. If you have two parents with brown eyes, you and your siblings are likely to have brown eyes too, but this is not definitely the case. You may have hazel or blue eyes because – as with psoriasis – eye colour is carried on several genes and has an unpredictable pattern of inheritance. However, the fact that genetics is involved means that psoriasis is more likely to develop in individuals who have many affected close family members.
The most powerful genetic evidence arises from twin studies. If a non-identical twin has psoriasis, then the other twin has a 20 per cent chance of developing psor­iasis. If the twins are identical, however, the risk rises to 73 per cent. Non-identical twins are genetically similar to ordinary brothers and sisters, whereas iden­tical twins are identical because they literally have the same genes. Perhaps it is surprising that in identical twins it is only a 73 per cent and not a 100 per cent risk – this indicates that there are other, probably environmental, factors involved.
It seems that what happens is that you inherit a tendency to psoriasis. The condition develops only if you are exposed to specific triggers in the environment, such as viral illnesses, certain drugs or stress.
If your child has psoriasis, you may want to know whether any other children in your family are likely to develop the condition too. If a child has developed psoriasis, and if both parents have psoriasis, the chances of another sibling developing the condition is 50 per cent (the incidence of psoriasis in the general population is only about two per cent). If, however, only one parent has psoriasis, the chance of a sibling developing the condition is 16.4 per cent. If a child with psoriasis has no parents with psoriasis, then the risk of the sibling being affected falls to 7.8 per cent.
If you have a sibling who has psoriasis, you will be wondering how likely you are to develop psoriasis yourself. The answer depends on at what age your sib­ling developed the condition for the first time. If your brother or sister first developed psoriasis before they were 15, your risk of devel­oping it yourself is three times higher than if your brother or sister developed it for the first time when they were over 30.
Recent molecular genetic studies have shown that the tendency to have psoriasis is linked to certain genes found at sites on the long arms of two specific chromosomes (thread-like structures carrying several genes): chromosome 17 and chromosome 4. However, this has applied only in families where there are a large number of members with psoriasis. These findings have not been confirmed in the general population. Therefore, a genetic test for the condition is still a long way off.

IMMUNOLOGICAL FACTORS

Until relatively recently, the mechanisms of producing psoriasis were thought to depend entirely on the epidermis in psoriatic skin making new cells much more rapidly than in normal skin. The associated inflammation and changes in blood vessels were largely ignored.
Intensive study of the inflammatory cells in the skin in psoriatic plaques has shown mainly T lymphocytes (a category of white blood cells) of a type known as CD4 positive. These cells produce cytokines (soluble proteins which help regulate the immune system) which stimulate keratinocytes to proliferate. As the psoriasis lesion subsides, either spontaneously or as a result of treatment, then the CD4­ positive cells disappear. The case for the central role of T lymphocytes in stimulating this epidermal proliferation is now overwhelming.
These findings are supported by the fact that ciclosporin, which is very effective in treating psoriasis, has its effect entirely on the white cells of the immune system and has no effect directly on the multiplication of the epidermal cells. Moreover immune system proteins which inactivate CD4 cells inhibit the multiplication of epidermal cells and lead to a resolution of the psoriatic plaque.
Genetic factors are crucial and triggers such as streptococcal infection are also important in the modulation and control of these events.

TRIGGERS THAT CAUSE PSORIASIS

If you have a genetic predisposition to develop psoriasis, certain triggers may activate the condition. However, in the vast majority of people who develop psoriasis, the relevant triggers cannot be identified.

Streptococcal infection

Streptococcal infection, which is a common cause of sore throats and tonsillitis, is a clear trigger in some people, particularly in children and young adults. The type of psoriasis that is most typically induced is guttate psoriasis, the rash appearing about ten days after the streptococcal tonsillitis. In patients with the more common plaque psoriasis, it is rare for streptococcal infection to be the trigger.

Hormonal factors

There is a peak onset of psoriasis at the menarche (when the periods start) and the menopause (when the periods stop). Psoriasis tends to improve in pregnancy and worsen after delivery. However, this is by no means invariable and there are no clear-cut hormonal triggers.

‘Trauma’ to the skin

If the skin is injured, psoriasis may develop. This is known as the Köbner reaction – for more relevant section.

Sun exposure

Ultraviolet light helps the vast majority of people with psoriasis. However, a few people (less than five per cent) notice that even small amounts of sunlight – at levels that do not cause sunburn – aggravate their condition. Some people will have psoriasis only on areas of their skin that are often exposed to the sun, such as their face, hands and forearms. If a person is sunburnt, there is a risk that the psoriasis will flare up because of the Köbner reaction.

Drugs

Some drugs worsen already existing psoriasis. This applies particularly to lithium, which is commonly used in the management of people with manic depression. If you have been taking corticosteroid drugs by mouth for other conditions, you may find that your psoriasis flares up when the corticosteroids are withdrawn. The same reaction may occur if you have been using potent topical steroid creams and oint­ments on your skin – if you sud­denly stop using topical steroids, a nasty and often pustular flare-up of your psoriasis can result.

Psychological factors

It is always difficult to prove a relationship of the mind to psor­iasis, and indeed to most skin disorders. Undoubtedly, in some people, psychological factors are important and they notice that when they are under stress their psoriasis worsens. On the other hand, it is stressful to have an unpleasant skin disease, so it is hard to sort out the cause and effect. In individual patients, stress can be a clear aggravating factor, but, for the majority, it is not a strong trigger of psoriasis.