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Although ankylosing spondylitis (AS) affects each individual differently in symptom severity and disease progression, the primary symptoms of ankylosing spondylitis typically include back pain and back stiffness, especially in the morning. The chronic inflammation and irritation of the spinal joints (vertebrae) can eventually cause the vertebrae to fuse together, a process referred to as ankylosis. Ankylosis is a debilitating condition that in its later stages may cause total loss of spinal mobility and function and adversely impacts a person's quality of life. The degree of fusion in ankylosing spondylitis can range from partial fusion (may be limited to the pelvic bones) to fusion of the entire spine which is very rare. Since bone formed during fusion is inherently weak, there is an increased risk of spinal fracture for patients with ankylosing spondylitis.

The pain in ankylosing spondylitis is caused by enthesitis - inflammation at the junction where the tendons or ligaments attach to the bone. Enthesitis can take place in multiple sites called "hot spots" and may lead to swelling and tenderness. The sites of enthesitis are one of the determining factors as to how much impact AS will have on quality of life. For example, if enthesitis appears in the Achilles tendon at the back of the heel or in the plantar fascia at the base of the heel, walking can be significantly impaired. Enthesitis may deteriorate into enthesopathy - calcification of joints, tendons, and ligaments that can also impair movement and mobility.

In ankylosing spondylitis, subchondral tissue (smooth tissue at the ends of bones) becomes granulomatous (small or granular nodular inflammatory lesions) and is infiltrated with various types of immune cells such as plasma cells and lymphocytes. Joints that are affected show markings of sclerosis (hardening of tissue due to inflammation) and irregular erosion. The tissue is gradually replaced with fibrous cartilage and becomes ossified (bony). Outer fibers of the spine are replaced by bone which eventually fuses the vertebrae. When this process ascends the spine in the later stages of disease, the spine becomes a bony column and is often referred to as a "bamboo spine". Progression to this point, however, is rare.

The body's natural process of healing and repair following inflammation may cause the damage to the joint and the extra bone formation. Thus if a person has severe ankylosing spondylitis for many years, the continuous inflammation and process of repair leads to bony fusion of ligaments in the spine and sometimes other joints as well.

Because ankylosing spondylitis is also a systemic rheumatic disease, it can lead to inflammation that affects other parts of the body in addition to the spine. Patients with ankylosing spondylitis may develop arthritis, pain or stiffness in other location such as:

• Hips
• Knees
• Ankles
• Heels
• Shoulders
• Ribs

Some people with ankylosing spondylitis can develop other complications as a result of the chronic inflammation which can affect the eyes, heart, kidneys as well as other organs.