Psoriatic arthritis, also called psoriatic spondylitis, is a type of inflammatory arthritis. PsA occurs in about one-third of people with psoriasis, causing pain and inflammation in joints throughout the body. The causes of PsA are not fully understood, but researchers have identified several genetic and environmental risk factors.
Psoriatic arthritis is an autoimmune disease, meaning the body’s immune system mistakenly attacks healthy cells and tissues as if they were viruses or bacteria. In psoriatic arthritis, the immune system attacks the ligaments and joints, leading to inflammation and damage.
Psoriatic arthritis is part of a family of related autoimmune disorders collectively called spondylitis, or spondyloarthritis.
The cause of psoriatic arthritis is not understood. However, rheumatologists and researchers believe that the onset of psoriatic arthritis can be triggered by a combination of biological and environmental factors.
Research has identified genetic factors associated with the development of PsA. Like other types of spondyloarthritis, PsA has been linked to specific variations on certain inherited genes — such as HLA-B27 — that are involved in the immune system. Researchers have identified several such genes.
An abnormal immune response is responsible for causing symptoms of all types of inflammatory arthritis. They can involve a variety of inflammatory mediators, substances in the body that promote inflammation.
T cells, a type of immune cell, are heavily involved in psoriatic arthritis. These cells release proteins called cytokines — chemical messengers that stimulate inflammation. This inflammation causes joint swelling, pain, and damage. Studies show that higher numbers or activity of these T cells and the cytokines they release are linked to PsA symptoms and severity.
Drugs such as Orencia (abatacept) work by controlling T cell activity, turning down the immune response mediated by T cells and cytokines. Other biologic drugs inhibit tumor necrosis factor (TNF), a potent proinflammatory cytokine. TNF inhibitors include drugs such as Humira (adalimumab), Enbrel (etanercept), and Remicade (infliximab).
A newer class of biologics, called Janus kinase (JAK) inhibitors, are also being used to treat PsA. Drugs such as Rinvoq (upadacitinib) and Xeljanz (tofacitinib) treat PsA by blocking the action of a variety of pro-inflammatory cytokines.
Research has found that certain infections may contribute to the development of PsA.
In particular, infection with Streptococcus pyogenes, which causes strep throat, seems to correlate with increased PsA risk. Other infections believed to trigger PsA include the sexually transmitted disease chlamydia and bacteria that cause infectious diarrhea, including species of Campylobacter and Yersinia.
Several viral infections may possibly be linked to PsA development, including HIV and hepatitis C. Viruses believed to cause rheumatoid arthritis may also cause PsA, including parvovirus B19, cytomegalovirus, and Epstein-Barr virus.
People with psoriasis can experience what is known as the Koebner phenomenon. This phenomenon causes new skin lesions to appear at the site of skin injuries, including scratches, cuts, and burns. Even minor injuries can induce a mild immune response, but in susceptible individuals, this can cause new autoimmune lesions to appear.
This Koebnerization effect also appears to affect tissues such as tendons in people with PsA. This may explain how work-related PsA develops. Research has found that people who work in jobs that involve repeated heavy lifting (100 pounds per hour or more) and pushing heavy loads (more than 200 pounds per hour) are more likely to develop PsA.
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Many risk factors have been associated with PsA. Additional research is needed, however, to determine why and how they contribute to disease.
Psoriasis, a skin condition characterized by scaly lesions (plaques), is the most significant risk factor for PsA. However, people without skin psoriasis can also develop psoriatic arthritis. PsA develops in nearly 30 percent of people with psoriasis, most commonly about 10 years after skin symptoms arise. In approximately 10 percent to 15 percent of people, psoriatic arthritis symptoms begin before skin psoriasis develops.
Psoriatic arthritis runs in families. Nearly 40 percent of people with psoriatic arthritis have a family member with psoriasis or arthritis. Combined with the known association between PsA and certain inherited genes, this supports the assertion that genetics play a significant role in the development of psoriatic arthritis.
Psoriatic arthritis is significantly more prevalent in white people. However, studies have shown that Asian and Hispanic people tend to present with more extensive and severe PsA. Additionally, research has found that Pakistani populations have a higher risk of developing PsA based on family history, even higher than white populations.
PsA can occur at any age. Psoriatic arthritis usually appears between the ages of 30 and 50, although children with psoriasis are also at risk.
Other environmental risk factors for PsA include:
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