What is Psoriatic Arthritis?

Psoriatic arthritis (PsA), is a chronic, progressive inflammatory disease. PsA causes painful, swollen, and stiff joints¹. It is one of a group of conditions called spondyloarthritis, which is a term used to describe various types of arthritis caused by inflammation.

Without proper treatment, PsA can get worse, which is known as a PsA flare. If it is left untreated, PsA can cause permanent joint damage^2,3.

Who can get psoriatic arthritis?

PsA is equally common in men and women⁴. It can occur at any age, but most people develop it when they are around 40-50 years old⁵. Sometimes it can also start in childhood⁶.

PsA is more likely to occur in people who already have psoriasis, which is an inflammatory skin condition that causes thick, raised, scaly patches⁴. Around one in five people with psoriasis also have PsA⁷. PsA is particularly likely to develop in people who have psoriasis in their nails, scalp, or around their anus⁸, and in people who have psoriasis in multiple areas of their body⁵.

Most people with both PsA and psoriasis have psoriasis for a long time before developing joint problems. Around 15 in every 100 people with both conditions develop PsA first, and another 15 in every 100 develop both psoriasis and PsA at the same time⁴.

What areas of the body are affected by PsA?

PsA can affect different parts of the body, including¹:

Peripheral joints (located in the arms and legs, including distal joints in the fingers and toes).
Axial joints (joints of the spine, including the sacroiliac joint where the spine connects to the pelvis).
Entheses (the connections between tendons or ligaments and bones).

Some people with PsA have symptoms in only one or two joints, while others have multiple joints and entheses affected. However, PsA tends to follow a pattern by affecting specific joints, including⁴:

Distal arthritis: only affecting fingers and
Asymmetric oligoarthritis: fewer than five small or large joints affected, but not on both sides (e.g. one elbow but not the other).
Symmetric polyarthritis: five or more joints on both sides of the body (e.g. both knees). Symptoms are similar to rheumatoid arthritis.
Arthritis mutilans: this deforms and destroys the joints, and often shortens affected fingers or toes.
Spondyloarthritis: affects axial joints.

Polyarthritis is the most common pattern⁴.

Illustration of body parts affected by PsA

What causes PsA?

PsA is not caused by overuse of joints or aging. It is an autoimmune disease, meaning the immune system, which normally protects against infections, attacks the body’s own tissues. Inflammatory proteins called cytokines are released, which cause inflammation and damage, in this case to the joints^1,4. PsA seems to develop due to a combination of genetic, immunological, and environmental factors^1,4.

Genetic factors
Around four in 10 people with psoriasis or PsA have a family history of one or either disease⁴. The risk of developing PsA if a family member also has it is around the same as the risk of developing PsA for someone who already has psoriasis⁴. Particular genes are linked to an increased risk of developing PsA, and also to the severity of the disease^1,4.

Immunological factors
People with PsA have an overactive immune system. This includes increased numbers of specific proteins (cytokines) that drive inflammation, triggering an anti-inflammatory cascade. This cascade leads to joint inflammation and damage^1,4. In people who have a genetic risk, environmental factors can trigger this immune inflammatory response^1,4.

Environmental factors
Environmental factors can trigger PsA in patients with a genetic risk. These factors include imbalance in the gut microbiome, obesity, mechanical stress or injury to joints, smoking, and certain infections (including streptococcal and HIV infection)^1,4.

References

Azuaga AB, Ramírez J, Cañete Psoriatic Arthritis: Pathogenesis and Targeted Therapies. Int J Mol Sci. 2023;24(5).
Haroon M, Gallagher P, FitzGerald Diagnostic delay of more than 6 months contributes to poor radiographic and functional outcome in psoriatic arthritis. Ann Rheum Dis. 2015;74(6):1045-50.
Tillett W, Jadon D, Shaddick G, Cavill C, Korendowych E, de Vries CS, et Smoking and delay to diagnosis are associated with poorer functional outcome in psoriatic arthritis. Ann Rheum Dis. 2013;72(8):1358-61.
Gladman D, Ritchlin UpToDate Patient education: Psoriatic arthritis (Beyond the Basics) 2024 [Available from: https://www.uptodate.com/contents/psoriatic-arthritis-beyond-the-basics#H1.
Ocampo DV, Gladman Psoriatic arthritis. F1000Res. 2019;8. Faculty Rev-1665.
Ogdie A, Weiss P. The Epidemiology of Psoriatic Arthritis. Rheum Dis Clin North Am. 2015; 41(4): 545–568.
Alinaghi F, Calov M, Kristensen LE, Gladman DD, Coates LC, Jullien D, et Prevalence of psoriatic arthritis in patients with psoriasis: A systematic review and meta-analysis of observational and clinical studies. J Am Acad Dermatol. 2019;80(1):251-65.e19.
Wilson FC, Icen M, Crowson CS, McEvoy MT, Gabriel SE, Kremers Incidence and clinical predictors of psoriatic arthritis in patients with psoriasis: a population-based study. Arthritis Rheum. 2009;61(2):233-9.