What is ankylosing spondylitis?

Ankylosing spondylitis is a disorder that primarily affects the spine. It is a form of chronic inflammatory arthritis characterized by back pain and stiffness. These symptoms typically appear in adolescence or early adulthood. As the condition progresses, back movement can gradually become limited as the bones of the spine (vertebrae) fuse together. Joint stiffness or a limited range of motion in certain joints is called ankylosis.

The earliest symptoms of this disorder result from inflammation of the joints between the base of the spine (the sacrum) and the hipbones (the ilia). These joints are called sacroiliac joints, and inflammation in this region is known as sacroiliitis. The disorder also causes inflammation of the joints between vertebrae, which is called spondylitis. Ankylosing spondylitis can involve other joints as well, including the shoulders, hips, and, less often, joints in the limbs. Over time, this disorder can affect the joints between the spine and ribs, restricting movement of the chest and making it difficult to breathe.

Ankylosing spondylitis affects the eyes in up to 40 percent of cases, leading to episodes of eye inflammation called acute iritis. Acute iritis causes eye pain and increased sensitivity to light (photophobia). Rarely, ankylosing spondylitis can also have serious complications involving the heart and lungs.

How common is ankylosing spondylitis?

Ankylosing spondylitis affects about 0.5 percent of people of Western European descent. This disorder occurs twice as often in men as in women, and symptoms tend to be more severe in men.

What genes are related to ankylosing spondylitis?

Variations of the HLA-B gene increase the risk of developing ankylosing spondylitis.

Ankylosing spondylitis is likely caused by a combination of genetic and environmental factors, most of which have not been identified. Researchers have determined, however, that a particular version of the HLA-B gene (called HLA-B27) increases the risk of developing this disorder.

The HLA-B gene provides instructions for making a protein that plays an important role in the immune system. HLA-B is part of a family of genes called the human leukocyte antigen (HLA) complex. The HLA complex helps the immune system distinguish the body's own proteins from proteins made by foreign invaders (such as viruses and bacteria). The HLA-B gene has many different normal variations, allowing each person's immune system to react to a wide range of foreign invaders. Although many patients with ankylosing spondylitis have the HLA-B27 variation, most people with this version of the HLA-B gene never develop the disorder. It is not known how HLA-B27 increases the risk of developing ankylosing spondylitis.

Other genes are believed to affect the chances of developing ankylosing spondylitis and influence the progression of the disorder. Some of these genes likely play a role in the immune system, while others may have different functions. Researchers are working to identify these genes and clarify their role in ankylosing spondylitis.

How do people inherit ankylosing spondylitis?

Although ankylosing spondylitis occurs in more than one person in some families, it is not a purely genetic disease. Multiple genetic and environmental factors likely play a part in determining the risk of developing this disorder. Inheriting the HLA-B27 variation from a parent does not mean that a person will develop ankylosing spondylitis, even in families where more than one family member has the disorder. In fact, about 80 percent of children who inherit HLA-B27 from a parent with ankylosing spondylitis do not develop the disorder.

Source: National Institutes of Health

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