Most people hear “psoriasis” and think only of the skin disease characterized by itchy, scaly rashes and crumbling nails. But according to the Annals of Rheumatic Disease, somewhere between 6 and 42 percent of people with psoriasis also develop a form of inflammatory arthritis called psoriatic arthritis (PsA). A person with this often debilitating condition experiences the skin rashes and nail damage of psoriasis along with painful, swollen joints in various parts of the body, especially the lower spine, fingers, toes, and knees.

People with psoriatic arthritis may notice symptoms such as joint pain, stiffness or swelling before the telltale rash symptoms of psoriasis appear, or they may experience the rashes first and then develop joint pain. Given this duality of symptoms, people diagnosed with PsA may wonder if it’s a joint disease or a skin disease. The answer may be surprising. It’s neither.

Understanding Autoimmune Response

Psoriatic arthritis is an autoimmune disease, meaning it occurs as the result of the body’s immune system malfunctioning. A healthy immune system releases antibodies – agents that act as natural defenses against disease-causing invaders or injury – to heal the body in times of distress. But autoimmune diseases turn the body against its own tissues, often causing inflammation that can, if not treated early and aggressively, lead to permanent joint and tissue damage.

Here’s a look at what happens inside the joint of a person with an autoimmune condition such as rheumatoid arthritis or psoriatic arthritis. White blood cells travel to the synovium – tissue that lines the joint capsule and produces synovial fluid, a clear substance that lubricates the joint and helps it move smoothly. The inflammation process causes the synovium to thicken, making the joint feel puffy and swollen.

As the disease progresses, the inflamed synovium invades the cartilage, elastic tissue that covers the ends of the bones in that joint. Over time, the cartilage erodes, causing bones to rub against one another. As the joint weakens, so do the surrounding structures such as muscles, ligaments and tendons. In the past, doctors thought this damage occurred over time. Now they believe it takes place in the first few years following disease onset. That’s why catching the disease early on and treating it properly are so important.

Just what causes the immune system to go awry is also unclear. Experts believe some people may be predisposed to an autoimmune disease like psoriatic arthritis – in fact, studies show a stronger genetic or family link to this particular disease than in other autoimmune rheumatic diseases. The disease may lay dormant in the body until triggered by some outside influence, such as a common throat infection. Another theory suggesting that bacteria on the skin triggers the immune response that leads to joint inflammation has yet to be proven.

The Diagnosis Dilemma

Psoriatic arthritis is diagnosed primarily by examining skin and joints, not one or the other. Doctors start with a physical exam, checking the skin for rashes, nails for signs of pitting or detachment and joints for swelling. Other tests may include X-rays to determine the extent of bone and joint damage or skin biopsies to rule out other skin conditions. The doctor will also ask about family history of either psoriasis or psoriatic arthritis, as this disease often strikes more than one person in families.