Comorbidities associated with psoriasis

There are a number of diseases which are associated with psoriasis; some of which have been recognised for a long time and others which have emerged from more recent research.

The well-recognised comorbidities include psoriatic arthritis (PSA) (which is discussed in the next section), psychological/psychiatric disorders and inflammatory bowel disease, particularly Crohn’s disease. Research has confirmed that the associations between psoriasis and these conditions are well established. Thus up to 30% of those with psoriasis have joint pain (Osborn and Wilke, 2004). The impact on quality of life for those with psoriasis is comparable to those who have cancer, arthritis and depression (Rapp et al., 1999) and patients with inflammatory bowel disease are up to seven times more likely than the normal population to develop psoriasis (Christophers, 2007).

There also appears to be an increased risk of malignancy in those with psoriasis; however, whilst this may be associated with underlying immunological changes in the skin, it is also likely that some treatments (notably PUVA and high-dose methotrexate) lead to an increased risk of malignancy (Gulliver, 2008).

Whilst not strictly speaking comorbidities, there does seem to be a link between psoriasis and certain lifestyle choices, namely smoking and drinking. For example, a study carried out in Italy found that there was a negative effect on the severity of psoriasis (particularly in women) in those who smoked (Fortes et al., 2005). A particularly strong association has been shown between smoking and palmoplantar psoriasis with smokers five times more likely to suffer from the disease than non-smokers (Naldi et al., 2005). The likelihood of alcohol abuse is increased in patients with psoriasis with a German study showing that increased alcohol consumption was seen twice as frequently in patients with moderate to severe psoriasis than in hospital-based controls. It is not clear if psoriasis is a risk factor for tendency to smoking or excessive drinking, but both the behaviours will increase the risk of mortality (Sommer et al., 2006). The direction of the association between psoriasis and drinking and smoking is not clear. It is still not certain whether smoking and drinking are risk factors for developing psoriasis or whether they are behaviours that people develop in order to help them cope with having psoriasis.

More recently research has turned its attention to making connections between diseases which are known to have a chronic inflammatory association and particularly those that are mediated by proinflammatory T-helper type 1 cytokines (as psoriasis is). In a review article, Gulliver (2008) summarises what he calls the emerging comorbidities: obesity, dyslipidaemia, hypertension and glucose intolerance (otherwise known as metabolic syndrome) all show increased prevalence in people with psoriasis. For obesity and diabetes, at least, it seems there is a greater prevalence in people with severe disease than those with mild disease.

Because of the greater prevalence of cardiovascular risk factors in those with psoriasis, it has been suggested that psoriasis might be an independent risk factor in itself for cardiovascular events including myocardial infarction. In other words just the very fact of having psoriasis makes an individual at higher risk of having cardiovascular events. The evidence here is mixed. However, what does seem clearer is that patients with psoriasis appear to have an increased risk of coronary artery calcification (which predisposes to atherosclerosis) and psoriasis is an independent risk factor for coronary artery calcification. It might therefore be expected that those with psoriasis would have a lower life expectancy than those without, here again the evidence is not wholly clear; however, a study in 2007 indicated that for those with severe disease there was a significantly increased risk of mortality(Gelfand et al., 2007). When comparing those with severe psoriasis to those without psoriasis, the figures from the study indicated that men died 3.5 years earlier and women 4.4 years earlier than the normal population.

Further research is needed in this field to clarify the relationships between psoriasis and the aforementioned comorbidities. The processes which lead to low-grade persistent inflammation as is seen in psoriasis are common to these conditions. Thus obesity is associated with low grade chronic inflammation mediated by levels of circulating TNF-α, IL-2, IL-6 and C-reactive protein. Metabolic syndrome and atherosclerosis show similarities to psoriasis in that Th 1 cytokines drive all three processes.

As there is increased understanding of the importance of the inflammatory process, treatments will increasingly look to tackle the causes of the inflammatory cascades within the skin. The biological therapies which are discussed later in this section will be key in this process. When discussing these associations with patients it is important not to create unwarranted anxiety. Patients need to know that there is a theoretical link between psoriasis and cardiovascular diseases but the evidence is still being collected and analysed. It seems that the links are most important for those with severe disease. Thus ‘switching off’ the severe inflammatory response quickly may be important not only to reduce the level of psoriasis, but also to decrease the risk of experiencing other inflammatory disease processes.