Although the cause of spondylitis is unknown, scientists have discovered a strong genetic or family link. Most people with spondylitis have a genetic marker known as HLA-B27. Having this genetic marker does not mean a person will develop spondylitis, but people with the marker are more likely to develop the disease than people who do not have it. The start of AS often begins with an ache in the buttocks, in the back of the thighs and down the leg and in the lower part of the back. One side is commonly more painful than the other.
AGE of onset is mostly in the late teens and twenties.
The opposite is the case with mechanical back problems. Spondylitis affects more men than women. It affects people between the ages of 16 and 35. Less than one percent of the adult population have the disease. In women, spondylitis is generally mild and is not easily diagnosed.