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 typically appears in people who have psoriasis, a chronic disease characterized by a scaly, reddish skin rash that usually appears on the elbows, knees and scalp.

A healthy immune system releases antibodies – agents that act as natural defenses against injury or disease-causing invaders – to heal the body in times of distress. But autoimmune diseases such as psoriatic arthritis turns the body against its own tissues, sending white blood cells to and inflaming the synovium – tissue that lines the joint capsule and produces synovial fluid, which lubricates the joint and keeps it moving smoothly. This inflammation causes the synovium to thicken, resulting in a puffy, swollen joint.

Over time, the synovium invades the cartilage, elastic tissue that covers the ends of the bones in a joint. The cartilage in turn erodes, causing bones to rub together. As the joint weakens, so do its the surrounding structures, such as muscles, ligaments and tendons. Because this joint damage can occur early in the disease process, diagnosing psoriatic arthritis as quickly as possible and treating it properly are important.

And while it’s most commonly associated with joints, psoriatic arthritis is a systemic condition, meaning that over time the inflammation that characterizes it can affect multiple joints and even organs.

What are the symptoms of psoriatic arthritis?

As with other forms of arthritis, 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:

Painful, swollen joints: Psoriatic arthritis typically affects the ankle, knees, fingers, toes and lower back. However, the joint at the tip of the finger may swell, making it easy to confuse with gout, a form of inflammatory arthritis that typically affects only one joint.

Dactylitis: Many people with PsA experience this sausage-like swelling along the entire length of their fingers or toes. This symptom is one that helps differentiate psoriatic arthritis from rheumatoid arthritis (RA), in which the swelling is usually confined to a single joint.

Enthesitis: People with psoriatic arthritis 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 psoriatic arthritis.