This affects joints mainly in the hands and feet but sometimes also in the back and legs, causing inflammation, swelling, pain and stiffness. There is a large genetic component to psoriasis, with more than one third of patients having a positive family history. If both parents are affected, a child has a 50-60 per cent chance of developing the disease. However, the disease cannot be entirely explained by genetics. The genetics of psoriasis is complex.
Research indicates that it is abnormalities in two or three genes that usually cause psoriasis.
A trigger, usually environmental, is also needed to set off the disease. Trauma to the skin is a common trigger and can include grazes, mechanical friction, and thermal or sun burns. Infection is another trigger, with streptococcal sore throat often precipitating the onset of childhood psoriasis. Stress can exacerbate psoriasis, as can heavy drinking and smoking. Cold weather and the dryness caused by central heating often worsens the condition.