Psoriatic arthritis (PsA) is an autoimmune disease - a form of inflammatory arthritis that can cause pain, swelling and sometimes damage to any joint in the body. It is a rarer form of arthritis compared to other forms of the disease (Osteoarthritis and Rheumatoid Arthritis). About 1 in 50 people have psoriasis and up to one-third of them may develop PsA (note that prevalence rates and figures vary considerably), so having psoriasis does not automatically mean you will have PsA. In fact, most people with psoriasis never develop it.
Although PsA generally occurs after psoriasis develops there are cases where the arthritis develops first. PsA usually begins slowly spreading to other joints over a period of a few weeks or even months. In rare instances PsA can develop quickly and can be quite severe. It is an unusual arthritis because it can look very different from person to person.
What happens in psoriatic arthritis?
To explain what happens in PsA it helps to understand how both arthritis and psoriasis can affect the body.
A healthy immune system releases antibodies agents that act as natural defences against injury or disease that heal the body in times of distress. Sometimes however an autoimmune disease occurs and turns the body against its own tissues. Psoriasis is an autoimmune disease that occurs when the immune system becomes confused and decides to attack the skin. It leads to a condition that causes a red, scaly rash, especially on the elbows, knees, back, buttocks and scalp. However, some people develop PsA before the condition while others will never develop the skin condition.
PsA is a type of inflammatory arthritis and an autoimmune disease. When it occurs, the joints become the target of the PsA attack.
What causes psoriatic arthritis?
At present the exact cause is not known. Research has shown that a particular combination of genes makes some people more likely to get psoriasis and PsA. However, having genes that predispose you to PsA does not necessarily mean you will develop this disease. Some people think that an event has to occur to trigger it. Unfortunately, we don't know what that trigger is. It could be a viral infection, trauma or something else in the environment. There may be more than one trigger.
Which parts of the body are affected?
Any of the 78 joints in the body can be affected by PsA but some are more likely to be affected than others. These are outlined in the diagram below: PsA also affects the body in certain patterns. They have been categorised into 5 different groups:
- Asymmetric Pattern This is the mildest form where PsA affects 1-3 joints on different sides of the body.
- Symmetric Pattern PsA involves many more joints and looks very much like rheumatoid arthritis.
- Distal pattern PsA affects the small joints in the fingers and toes closest to the nail.
- Spinal Pattern PsA affects the spinal column and may cause inflammation and stiffness in the neck, lower back, spinal vertebrae, or sacroiliac region (pelvic area), making motion difficult.
- Destructive Pattern PsA is a severe, painful deforming type of arthritis. It primarily affects the small joints in the fingers and toes closest to the nail leading to lost function of the involved joints. It also is frequently associated with lower back and neck pain. This pattern is very rare and is also known as arthritis mutilans.
While it is most commonly associated with joints, psoriatic arthritis is a systemic condition, meaning that over time the inflammation that characterises it can affect multiple joints and even organs.
Early signs and symptoms of psoriatic arthritis
As with other forms of arthritis, the symptoms of PsA vary among different people. Many symptoms are common to other forms of arthritis, making the disease tricky to diagnose. Here's a look at the most common symptoms and the other conditions that share them
- A red scaly skin rash.
- Thickening, discoloration and pitting of the nails.
- Stiff, painful, swollen joints. PsA typically affects the ankle, knees, toes and lower back. The joints at the tips of the fingers may also swell confusing it with gout, a form of inflammatory arthritis that typically affects only one joint.
- Dactylitis: This is a sausage- like swelling of the fingers or toes. This symptom is one that often helps differentiate PsA from RA, in which the swelling is usually confined to a single joint.
- Enthesitis: People with PsA often develop tenderness or pain where tendons or ligaments attach to bones. This commonly occurs at the heel (Achilles tendinitis) or the bottom of the foot (plantar fasciitis), but it can also occur in the elbow (tennis elbow). Each of these conditions could just as easily result from sports injuries or overuse as from PsA.
- Pain and swelling at the back of the heel.
- Eye inflammation (less frequent).
How is psoriatic arthritis diagnosed?
There is no single test for psoriatic arthritis, but the diagnosis is based on your symptoms and a physical examination. It is easier to diagnose if you have psoriasis along with red swollen fingers or toes. Psoriasis and PsA occur more frequently in some families than in others so your doctor may ask about your family's medical history.
Sometimes the doctor may make a diagnosis by eliminating other conditions with similar symptoms. He may therefore take a sample from the joint to rule out the presence of gout crystals.
Some of the symptoms of PsA are also similar to other forms of arthritis and it can sometimes be difficult to distinguish between psoriatic arthritis and rheumatoid arthritis (RA). Your doctor may therefore take a blood test for rheumatoid factor to eliminate the possibility of RA. If several joints are affected, your doctor will consider features such as the pattern of the arthritis that is, which joints are affected. Your doctor may also send you for x-rays, MRIs and blood tests to rule out these conditions.