The prevalence in different ethnic groups is related to the frequency of HLA-B27 in these populations. Bony ankylosis is most frequent and the sacro-iliac joints often become fused. The disease is characterised by bilateral sacro-iliitis. As the disease extends up to the intervertebral joints, there is 'squaring' of the vertebral bodies radiologically, seen on the lateral view and calcification of the annulus fibrosus giving the characteristic syndesmophytes which fuse to form the classic 'bamboo spine'. The disease most commonly begins between 16 and 40 years of age.
Back pain is the usual presenting symptoms.
Its onset is gradual and is often accompanied by constitutional disturbances. The pain usually disturbs sleep and is associated with morning stiffness and stiffness after immobility. In the late stages of the disease, there may be reduction in spinal pain as the axial skeleton becomes ankylosed. Symptoms improve with exercise. Diagnosis is often delayed, symptoms being ascribed to lumbar disc disease.