Ankylosing spondylitis: Year of the Zebra 2024

Ankylosing spondylitis, or AS for short, is a chronic inflammatory condition that primarily affects the spine and sacroiliac joints, which connect the pelvis to the lower spine. In AS, inflammation primarily affects the vertebral joints in the spine, specifically the points where ligaments, tendons, and joint capsules attach to bone.

Chronic inflammation causes new bone to form along the outer edges of the spine. Now the affected part of the spine becomes more rigid, limiting its range of motion. As the condition progresses, the bony outgrowths grow bigger until they form bridges among adjacent vertebrae, causing ankylosis—or fusion—of the entire spine.

The exact cause of inflammation in AS is unclear, but it’s thought to be due to an autoimmune process, which is when the immune system attacks its own tissue; in this case, the collagen found in the joints. While no autoantibody has been specifically linked to the disease, most individuals with AS carry the gene HLA-B27, which plays a key role in helping the body distinguish between its own cells and foreign substances.

The most common symptom of AS is chronic back pain and stiffness, especially in the lower back and buttocks.

The pain and stiffness are usually worse in the morning and improve with activity, not rest. Over time, pain can extend up the spine and cause neck or upper back pain and immobility.

Because the ribs and vertebrae are involved in breathing, stiffness in the thoracic, or chest, region of the spine can result in shortness of breath.

The autoimmune process underlying AS can lead to pain in tendons and peripheral joints such as the hips, knees, and ankles. It can also lead to several other extra-articular manifestations, such as anterior uveitis, causing painful, red eyes with increased sensitivity to light.

Other important manifestations include inflammatory bowel disease, pulmonary fibrosis, psoriasis, heart blocks, and aortic inflammation, which can potentially lead to aortic aneurysm and dissection.