Psoriasis and psoriatic arthritis: how are these two conditions related?

Psoriasis is a complex chronic inflammatory disease, which can manifest itself in many forms that vary from patient to patient, as well as for the same patient throughout his or her life. There are four types of psoriasis, namely plaque psoriasis, guttate psoriasis, pustular psoriasis and erythrodermic psoriasis.

Plaque psoriasis, the most common form of psoriasis, is characterised by well-defined red plaques covered with whitish scales or a thin, silvery film that peels off easily. The plaques appear on different parts of the body, most commonly on the elbows, knees and scalp, and leave the skin red. This chronic disease progresses in cycles with periods of remission. It is not contagious and can be well controlled with treatment.

Joints can also be affected by this disease. Patients with psoriasis are at risk of developing a type of arthritis called psoriatic arthritis. This is a chronic condition characterised by inflammatory pain in the joints, tendons and bones, and its symptoms vary from person to person.

It is also characterised by swelling of the joints (particularly of the hands and feet), heel pain, and sometimes pelvic and back pain. In some cases, there may be extra-articular manifestations involving the eye (uveitis) or the gastrointestinal tract (Crohn's disease or ulcerative colitis), and 80% of patients have their nails affected (discolouration or peeling).

Psoriatic arthritis is often a consequence of psoriasis, but 20% of people with psoriatic arthritis have no signs of psoriasis.

What is the link between psoriasis and psoriatic arthritis? 

About 2% of the UK population is thought to suffer from psoriasis, or about 1.8 million people. The disease can take many forms and can vary from "local" to "diffuse", depending on the size of the skin surface covered by plaques. Up to 30% of psoriasis patients may experience joint pain (arthralgia) or joint inflammation (arthritis).

Psoriatic arthritis most often occurs after psoriasis. This condition can cause irreparable damage to the joints.

A common symptom in people with psoriasis (70-80% of cases) is nail psoriasis (the presence of numerous small pits on the surface of the nails).

What are the similarities between psoriasis and psoriatic arthritis? 

Psoriasis and psoriatic arthritis are both chronic diseases, the skin lesions they cause may be permanent or intermittent. Plaques may grow and worsen, diminish or temporarily disappear, until the next relapse; the same is true for joint symptoms.

The location of psoriasis plaques can affect the chances of developing psoriatic arthritis, and the risk is highest if they spread to the scalp or the gluteal crease. If the nails are affected and the fingers are inflamed (dactylitis), there is also an increased risk of developing joint inflammation. 

Psoriatic arthritis usually affects people with mild psoriasis (less than 10% of the body surface), regardless of the form of the disease (plaques, glands, pustules, etc.). 

Pathological factors in psoriatic arthritis are genetic, environmental and immunological. The disease affects both men and women, with a peak in clinical manifestations between the ages of 30 and 50. 

Symptoms of psoriatic arthritis usually begin around the age of 40 and are rare in children.

40% of patients have a family history of psoriasis or arthritis, suggesting an inheritance risk of 5-10% if one parent is affected.

Obesity, type 2 diabetes, high blood pressure, HIV and metabolic syndrome are all associated with an increased risk of psoriasis. Psoriatic arthritis can result from infections that provoke an immune response, or from streptococcal infection.

Physical or psychological stress can also trigger a relapse of psoriasis or psoriatic arthritis.

What are the differences between psoriasis and psoriatic arthritis? 

Unfortunately, the severity of psoriasis symptoms does not necessarily indicate the severity of rheumatism that may (or may not) develop. About one-third of people with psoriatic arthritis have fairly mild psoriasis. 

Psoriatic arthritis usually occurs 5-10 years after the onset of skin symptoms of psoriasis, but sometimes there is no link between the two conditions. In such cases, doctors may look for localised signs of the disease (behind the ears, buttock crease, scalp, and nails) or a family history of psoriasis.

Psoriasis may occur before or after the onset of rheumatism. The rash can sometimes go unnoticed because it may be hidden under the hair on the head, in the navel or in the folds of the skin, for example at the junction between the buttocks and the thighs.

People with psoriatic arthritis suffer from painful inflammation of the joints or spine. Some patients also suffer from fibromyalgia, which causes muscle pain, joint stiffness and discomfort.

The diagnosis of psoriatic arthritis is based on the identification of joint inflammation which is characteristic of people with both arthritis and psoriasis. The doctor may also look for cases of psoriasis in the patient's family history.