Psoriasis

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Question Received:

  1. Is there a link between psoriasis and arthritis?

Response:

Is there a link between psoriasis and arthritis?

13th August 2000

Psoriasis is fairly common in Western countries, affecting about 2% of people. The skin over the knees, elbows and scalp is most commonly affected, but lesions can appear anywhere on the body. The nails frequently appear thickened and yellowish, and they may separate from the nailbed. The severity of the condition waxes and wanes. It is thought that psoriasis is caused by several factors including both genetic and environmental factors.

Studies of populations, families, and twins all strongly suggest an important genetic (hereditary) component. Genetic-linkage studies have shown that there are multiple chromosomal locations linked with a predisposition to psoriasis (Menter, 1998). This inherited susceptibility is thought to be activated by environmental triggers such as infections and minor injuries to the skin (Espinoza et al, 1998). Exposure to bacteria such as streptococci from the gut, tonsils and psoriatic plaques results in a continuing stimulation of cells of the immune system (eg: monocytes, macrophages and dendritic cells) which then migrate throughout the body (Vasey et al, 1982; Rahman et al, 1990). Small injuries to the skin and joints cause these activated cells to migrate to the sites of damage. Interaction there with genetically hyperactive cells results in the release of growth factors and the development of psoriatic skin plaques and arthritis (Troughton and Morgan, 1994). Thus, psoriasis is intimately linked with the immune system, and has features in common with other auto-immune diseases such as Crohn's disease and rheumatoid arthritis (Christophers, 1996).

Arthritis develops in up to a third of people with psoriasis and in the initial stages particularly affects the joints of the fingers. Although psoriatic arthritis is similar to rheumatoid arthritis in many ways, it can be distinguished by the absence of rheumatoid antibodies in the blood. Although the relationship between psoriasis and arthritis was once thought to be largely coincidental, current evidence suggests that similar structural changes and immunological mechanisms are involved in both the skin lesions and synovial membrane changes in the affected joints - the conditions are linked. Usually the skin changes precede the development of psoriatic arthritis, but they may appear concurrently or, more rarely, the arthritis begins before the skin lesions.

Recent research has focused on the role of T cells (specialised white blood cells involved in immune reactions), cytokines (signalling molecules), adhesion molecules (that cause cells to ‘stick’ together), and blood-vessel changes in the affected skin and synovial membrane (O'Neill and Silman, 1994; Veale et al, 1994; Veys and Mielants, 1994; Christophers, 1996; FitzGerald and Kane, 1997; Ortonne, 1999; Scarpa and Mathieu, 2000).

The earliest detectable changes in the skin and joints occur in the blood vessels in those regions (Kapp, 1993). They are also the first changes to resolve with treatment. Tests such as the erythrocyte sedimentation rate and bone scintigraphy are useful in the evaluation and follow-up of patients with psoriatic arthritis (Espinoza, Cuellar, and Silveira 1992). The similarity of the changes in psoriatic skin and joints means that similar treatments can be effective for both. Most of the treatments for psoriasis aim to calm down the immune system - they are immunosuppressive. Non-steroidal anti-inflammatory drugs are the mainstay of treatment for most patients, and there is a good response in 75-85%. For those who respond less well, agents such as methotrexate can be effective. Potentially more toxic drugs such as cyclosporin A, bromocriptine, and retinoids have been effective in some patients (Barker, 1994; Cuellar, Citera, and Espinoza, 1994; Cuellar, Silveira, and Espinoza, 1994; Cuellar and Espinoza, 1995; Ruzicka, 1996; Salvarani et al, 1998).

Recently there has been interest in the involvement of arachidonic acid and its metabolites in the development of psoriasis. The metabolites are called eicosanoids, and include biologically important molecules such as prostaglandins, thromboxane, and leukotrienes. They are thought to influence a number of physiological processes that might contribute to psoriasis (Ikai, 1999). Dietary modification may be able to influence this aspect of psoriasis (Gladman, 1992).

References

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