Ankylosing spondylitis (AS) is a chronic inflammatory joint disease that can gradually affect the joints and ligaments of the spine, as well as other parts of the body. Classically, the primary site of involvement is both sides of the sacroiliac (SI) joints which then ascends the spine into the thoracolumbar region, rib articulations and the cervical spine. Fusion of the bones and ossification of ligaments in the spine generally leads to the loss of the natural curvature of the spine (becoming more hunched), and its mobility – this is also usually accompanied with pain due to inflammation. While AS commonly affects the axial skeleton, it is also known to affect the appendicular skeleton in approximately one-third of patients with AS, such as the knees and hips (arthritis), and Achilles tendon (enthesitis).
Meaning: ‘Ankylosis’ means the abnormal stiffening and reduction of mobility of a joint due to fusion of the bones. ‘Spondylitis’ can be better understood as the inflammation of the vertebrae (spinal bones).
The specific cause to AS is still unknown at the moment, though AS has been linked with a gene known as human leukocyte antigen B27 (HLA-B27). It is thought that carrying the gene may make someone more vulnerable to developing the disease, and it may also be triggered by one or more environmental factors, but it is still unknown what these are. Thus, testing positive for this gene does not necessarily mean that one would develop AS because only 1-2% of people with the gene will develop the disease.
However, AS has a strong familial tendency in which the HLA-B27 gene can be inherited from another family member. If a parent has AS and the HLA-B27 gene, then there is a 50% chance of passing the gene down to any offspring – about 5-20% of children with this gene will then go on to develop AS. Males are also more likely to develop this disease than females with a ratio of 3:1.