6.1.3.1 Epidemiology

Ankylosing spondylitis has a prevalence of about 1% in the general population. It is seen mainly in Caucasians and is exceptionally rare in Japanese and black Africans. Onset is more frequent between 15 and 40 years of age, and clinical evidence of AS is 3Ð4 times more frequent in men than in women. However, if radioisotope studies that detect subclinical sacroiliitis are taken into account, AS occurs almost as frequently in females as in males (although with milder and more peripheral disease) [227]. There is a deÞnitive correlation between the prevalence of the disease and the presence of the histocompatibility antigen HLA-B27 [228]. More than 90% of AS Caucasian patients and 52% of their Þrst-degree relatives are HLA-B27 positive, compared with only 6% of a control population [229]. Between 1 and 10% of the HLA-B27-positive adults in the general population and 20Ð30% of the HLA-B27- positive Þrst-degree relatives of spondylitis patients are likely to suffer from AS [230].

6.1.3.2 Systemic Manifestations

Articular Involvement

The most characteristic early manifestation of AS is low back pain, which is of insidious onset, dull in character, unilateral and intermittent at Þrst, and initially felt deep in the gluteal region. Because symptoms often are ascribed to lumbar disk disease, diagnosis is usually delayed. The pain becomes bilateral, persistent, and localized in the lumbar spine, with occasional irradiation to the iliac crest or to the dorsal thigh within a few months of onset of symptoms. Morning stiffness, which usually lasts longer than is seen in mechanical spinal problems, is typical. Both the pain and the stiffness improve with exercise and worsen with rest. The lumbar spine progressively loses its normal lordosis, leading to limitation of movement (Fig. 6.9). If the thoracic spine is affected, costovertebral and sternomanubrial joint and tendon involvement may cause kyphosis and chest pain, especially on inspiration, sneezing, or coughing. Cervical spine spondylitis may result in cervical arthralgias, limitation of motion, or cord compression. A sudden exacerbation of back pain, especially in the cervical region, may follow a fracture after minimal or even unrecognized

Fig. 6.10 Abdominal ßat plate X ray of patient with ankylosing spondylitis. Note the calciÞcation in the anterior and lateral vertebral ligaments, the Òbamboo spine,Ó and the syndesmophytes. Note also the complete obliteration of the sacroiliac joints, erosion and reactive sclerosis in the pubic symphysis and adjacent bone, and the narrowing of the hip joints, with juxtaarticular sclerosis and cystlike erosions in the acetabular and femoral heads

injury [231]. Only a few patients progress to the end stage of Òbamboo spineÓ now because of the earlier recognition and better treatment of AS today compared to 30 years ago (Fig. 6.10). Bamboo spine is caused by the fusion of the calciÞed annulus Þbrosus with the vertebral bodies through the characteristic syndesmophytes.

Peripheral arthritis is present at some stage of the disease in 35% of AS patients. Although any joint may be involved, the hips, shoulders, and knees are most frequently affected. Pain, swelling, and effusion may be transient, but crippling changes similar to those found in RA may occur after a disease duration of ten or more years. Peripheral arthritis may be the initial manifestation in 20% of patients with AS.