What is Psoriatic Arthritis?
Psoriatic arthritis is an autoimmune disease whereby the joints are under attack, causing swelling, pain, warmth, redness and stiffness as well as fatigue. Psoriatic arthritis can appear in any joint or where ligaments and tendons connect to the bone. It can also cause neck and back pain and stiffness.
The primary locations where psoriatic arthritis can occur can be broken down into three categories.
Refers to the inflammation of the entheses, the site where ligaments and tendons attach to the bone, causing discomfort, stiffness and swelling. Common areas for this to occur are the back and bottom of the heels, around the elbows and knees, ribs, spine and pelvis.
Also referred to as “sausage digits” due to the inflammation that occurs in a whole finger or toe. Dactylitis typically involves a few digits and is the result of the entheses, tendons and small joints in the finger or toe becoming inflamed.
This refers to the inflammation of the spine, including the joints that connect the spine to the pelvis (sacroiliac joints) or one or more vertebrae. This can typically cause lower back and neck pain and stiffness.
People with psoriatic arthritis can experience the following five patterns.
Oligoarthritic psoriatic arthritis
Typically affects one to four joints, small or large.
Distal psoriatic arthritis
Primarily affects the small joints closest to the nail in your fingers and toes (this is characteristic of psoriatic arthritis).
Spinal psoriatic arthritis
This type of psoriatic arthritis affects the joints located on the spine and sacroiliac joints, making motion very difficult.
Destructive psoriatic arthritis
Also known as Arthritis Mutilans, this is a very severe, painful, deforming type of psoriatic arthritis that can affect the shape of the joint and tissue and bone that surrounds it. This type of psoriatic arthritis is rare.
In a healthy joint (see figure below), the synovial membrane, which lines the surface of the joint, produces synovial fluid which is lubricating and nourishing.
In contrast, in an inflamed joint the synovial membrane is filled with immune cells ( T cells and other chronic inflammatory cells) and these cells begin to divide excessively. The immune cells within the inflamed joint produce many pro-inflammatory cytokines (such as TNF, IL-17A) resulting in the recruitment of more immune cells to the membrane and production of cytokines .
A combination of genetics and environmental factors trigger proinflammatory pathways in psoriasis, particularly through Th1 and Th17 cells. These cells release a variety of cytokines, resulting in increased activation of proinflammatory cells in the skin that in turn perpetuate this process. A similar process occurs in the joints in psoriatic arthritis, resulting in initiation and perpetuation of inflammation within as well as near the joints such as at the entheses (see figure).
An important feature of psoriatic arthritis is inflammation at the insertion of the tendons and ligaments to bone, termed enthesitis. In the human body, such insertion sites are numerous, e.g., the insertion of the Achilles’ tendon to the heel. Achilles’ enthesitis is thus a relatively common feature of PsA as is enthesitis at other sites.
If left uncontrolled, the disease progresses, and the proliferating synovial membrane cells can become invasive and begin to break down cartilage and bone at the joint. The inflammation can also cause deposition of new bone near the joint, sometimes leading to joint fusion.