Psoriatic arthritis (PsA) is a type of inflammatory arthritis that causes joint pain, swelling, and stiffness. It frequently co-occurs with psoriasis, an autoimmune skin disease that typically causes plaques of thick, inflamed red skin covered with silvery scales. In about 70 percent of PsA cases, people get psoriasis first and then later develop the joint pain and other symptoms indicative of psoriatic arthritis. About 15 percent of the time, people develop joint symptoms first and then psoriasis later. In about another 15 percent of the time, skin and joint problems show up at the same time.
While psoriatic arthritis is a type of inflammatory arthritis like rheumatoid arthritis (RA), it’s a mistake to think of PsA as just “rheumatoid arthritis with skin involvement,” according to rheumatologist Alexis Ogdie, MD, in an interview with CreakyJoints.
In fact, PsA falls under the umbrella of a type of arthritis known as spondyloarthropathy — a family of inflammatory rheumatic diseases that cause arthritis. In addition to psoriatic arthritis, other kinds of spondyloarthritis include ankylosing spondylitis, reactive arthritis, and peripheral spondyloarthritis. These diseases have some unique traits that distinguish them from RA in terms of both symptoms and how they respond to treatment.
“We’re identifying more and more differences between rheumatoid arthritis and psoriatic arthritis,” says Dr. Ogdie. “The treatments for these diseases are starting to split and go in different directions.”
What Causes Psoriatic Arthritis
Psoriatic arthritis happens when your immune system mistakenly attacks healthy cells and tissue. This abnormal immune response causes joint inflammation and overproduction of skin cells. It’s not clear why the immune system turns on healthy tissue, but a combination of genetic and environmental factors likely play a role. Many people with psoriatic arthritis have a family history of psoriasis, psoriatic arthritis, or other autoimmune diseases. Researchers have discovered certain genetic markers that may be linked to psoriatic arthritis. Physical trauma or environmental factors (like a viral or bacterial infection) may trigger psoriatic arthritis in people with a family history.
It’s important to understand potential risk factors and causes of psoriatic arthritis so scientists and researchers can continue to identify specific treatments that work uniquely well for PsA, and for doctors and patients to be aware of what people can do to reduce their risk, particularly if they already have psoriasis.
While we don’t know for sure what causes psoriatic arthritis, here’s a look at what we do know about causes and risk factors for psoriatic arthritis:
Psoriatic arthritis risk factor: Family history
Psoriatic arthritis is considered one of the more inheritable autoimmune diseases. About 40 percent of people with psoriasis or psoriatic arthritis have a family member with one of these diseases. According to UptoDate, close relatives of people with psoriatic arthritis are about 55 times more likely to develop the disease than an unrelated person. Studies indicate that psoriatic arthritis is more strongly tied to family history than psoriasis alone. You’re three to five times more likely to inherit psoriatic arthritis than you are to inherit psoriasis.
Psoriatic arthritis risk factor: Genes
Certain gene mutations are more commonly found in people with psoriatic arthritis than in those without. Certain gene variations are also unique to people with psoriatic arthritis compared to those with psoriasis.
The most well-known of these is the HLA-B gene, which gives instructions for making a protein that plays an important role in the immune system. It’s part of a family of genes called the human leukocyte (HLA) complex. The HLA complex helps the immune system differentiate between the body’s own proteins and those made by invaders like bacteria or viruses. The HLA-B gene has hundreds of variations, each differentiated by number, so your body can address lots of foreign proteins.
One variation of the HLA-B gene is HLA-B27. It’s not known exactly how the HLA-B27 gene increases the risk of development of psoriatic arthritic. Of people with psoriatic arthritis, 50 to 70 percent have HLA-B27, says Stanford Shoor, MD, a clinical professor of medicine and rheumatology at Stanford University in Palo Alto, California. However, simply having the HLA-B27 gene doesn’t mean you’ll get psoriatic arthritis or another type of arthritis. (HLA-B27 is even more prevalent among people with ankylosing spondylitis.)
Other variations of HLA, such as HLA-B17 and HLA-B13, occur more often in people with psoriatic arthritis than in the general population.
Aside from HLA, scientists are currently studying many other genes for their potential role in psoriatic arthritis and psoriasis. As more genetic markers become recognized, this could lead to better ways to diagnosing PsA as well as more targeted and effective treatment options in the future.
But despite their association with PsA, having these or other genetic mutations doesn’t mean you’ll definitely get psoriatic arthritis. It’s thought that environmental triggers are needed to ‘switch on’ the disease in those with a genetic disposition.
Psoriatic arthritis risk factor: Psoriasis
In most cases of psoriatic arthritis, the autoimmune skin disease psoriasis shows up first. Up to 30 percent of people with psoriasis wind up developing psoriatic arthritis, estimates show. However, it’s possible to develop psoriatic arthritis without first having psoriasis; some patients experience joint pain and later go on to develop psoriasis skin plaques.
While anyone with psoriasis is at an increased risk of developing PsA compared to someone without psoriasis, there are some specific psoriasis features that make developing PsA more likely. This includes psoriasis that affects the scalp and groin. People with nail involvement in psoriasis, such as nail pitting, are especially likely to develop psoriatic arthritis.
Psoriatic arthritis can occur in people with mild, moderate, or severe psoriasis. But the general thinking is that more skin and nail involvement there is with psoriasis, the greater the risk of developing PsA, says Maureen Dubreuil, MD, assistant professor of rheumatology at Boston University School of Medicine.
Psoriatic arthritis risk factor: Obesity
Up to 40 percent of people with PsA are obese. Studies show that being overweight increases the risk of developing PsA and that losing weight can help improve PsA symptoms. For example, British researchers at the University of Bath studied more than 90,000 cases of people diagnosed with psoriasis from 1998 to 2015 to see who was later diagnosed with psoriatic arthritis and what they might have in common compared with people who didn’t develop PsA. Psoriasis patients who were overweight or obese were found to be significantly more likely to develop psoriatic arthritis compared to those whose weight was normal.
A separate study, presented at the annual European Congress of Rheumatology (EULAR 2019) in Madrid, specifically looked at the impact of weight on disease activity. When researchers at the University of Glasgow in Scotland analyzed data on 917 PsA patients, they found that those with a higher body mass index (BMI) were more likely to have greater PsA disease activity and more PsA-related disability.
“Obesity is associated with the production of inflammatory chemicals in the body,” says Rebecca Haberman, MD, clinical instructor in the department of medicine’s division of rheumatology at NYU Langone Health in New York City. “It may be that this underlying inflammatory environment helps predispose the body to the development of PsA. Why obesity is so strongly linked to PsA [as compared with other inflammatory conditions] is unknown.”
Obesity is a modifiable risk factor, which means you have some control over it. If you have psoriasis or psoriatic arthritis, one of the most important lifestyle changes you can make is to maintain a healthy weight or lose weight if you’re overweight.
Psoriatic arthritis risk factor: Smoking
We all know smoking is harmful. Multiple studies have shown that smoking is a risk factor for the development of PsA. A 2011 study from Harvard researchers found that women smokers were three times as likely to develop psoriatic arthritis as those who never smoked. Past smokers had a 50 percent greater risk compared to those who had never smoked. The greatest risk of developing PsA was among those who smoked the most.
Smoking also interferes with PsA treatment. A 2014 Danish study in the journal Annals of Rheumatic Diseases found that people with PsA who smoke were less likely to get the best response from treatments such as anti-tumor necrosis factor (TNF) biologic medications compared to people with PsA who didn’t smoke.
Researchers don’t know exactly how smoking contributes to the onset and severity of PsA, but it’s thought that chemicals in tobacco increase the amount of inflammatory chemicals in the body.
Psoriatic arthritis risk factor: Age
Psoriatic arthritis can start at any age. However, it occurs most often in adults ages 30 to 50. For the majority of patients, PsA starts five to 10 years after the development of psoriasis, says Dr. Haberman. “The reason younger people tend to get psoriatic arthritis, compared with other types of arthritis, isn’t well known,” says Bharat Kumar, MD, associate rheumatology fellowship program director in the division of immunology at the University of Iowa Hospitals and Clinics in Iowa City. People with certain genetic risk factors tend to get symptoms earlier, even in childhood in some cases.
Psoriatic arthritis risk factor: Environmental factors
Exposure to certain infections may contribute to the development of psoriatic arthritis. Some experts believe there is a link between streptococcal infection and the development of psoriasis and psoriatic arthritis, although the link hasn’t been proven. Psoriatic arthritis also occurs more commonly in people with the human immunodeficiency virus (HIV) than in the general population. Physical trauma has also been identified as a risk factor for developing psoriatic arthritis among patients with psoriasis. Other possible PsA triggers include extreme emotional stress, stressful life events, and certain skin infections like staph.
Getting Diagnosed with Psoriatic Arthritis
If you suspect your joint pain or other symptoms could be PsA, and you have some psoriatic arthritis risk factors (or even if you don’t), see your health care provider and ask for a referral to a rheumatologist. Read more about how PsA is diagnosed.