With the standardization of criteria for the diagnosis of spondyloarthropathy (2), better epidemiologic studies have been completed. Initially, spondyloarthropathy was thought to be a disease predominantly affecting men (M:F = 10:1), but more recent studies suggest that women are affected quite commonly, although with a milder form of the disease. AS usually has its insidious onset during the ages of 20 to 35 years and is rare in onset after the age of 40 years.

The progress of the lesion and its major pathologic features are well demonstrated on repeated radiographic examinations. Because the disease does not have a clear clinical presentation, a criteria approach to diagnosis is used (Tables 7-1 and 7-2).

The cause of AS is unknown, except that individuals who have inherited the HLA-B27 gene are predisposed to develop the syndrome. The overall incidence of AS in North American Caucasians is 0.1% to 0.2%, whereas the incidence of AS in Caucasians with the HLA-B27 gene is 10% to 20%. There is a 20-times greater incidence of AS among relatives of persons with AS. These relatives have a much higher incidence of HLA-B27 than the normal population. Whether the B27 gene/antigen is the primary cause or whether it acts as a receptor for an infective and/or environmental agent that triggers the disease is unknown.

AS affects both synovial and fibrous joints; the pathologic changes take the form of chronic synovitis. The chronic synovitis is followed by cartilage destruction, erosions, sclerosis of underlying bone, and finally, fibrosis and ankylosis of the affected joints. The SI joints are involved most of
the time; the intervertebral discs, symphysis pubis, and manubriosternal joints are frequently involved. Two categories of extra-articular involvement are characteristic:

The usual presentation is in a young (adolescent or early adulthood) man who reports the insidious onset of grumbling low back pain. Characteristically, there are remissions and exacerbations over the months. The pain may refer to the buttocks and upper thigh and may even be unilateral, leading readily to confusion with the diagnosis of a ruptured disc. Typical of AS is the absence of neurologic symptoms and the presence of good straight leg raising, which should make the diagnosis of a disc rupture immediately suspect. The next most prevalent complaint is stiffness of the lumbosacral area, especially in the morning, with a hot shower and/or the morning’s activity alleviating this complaint. Prolonged periods of inactivity worsen the back pain and stiffness. At times, back pain may awaken the patient at night, and often this pain is at the thoracolumbar junction as well as the low back. Eventually, the pain and stiffness affect the entire spine, causing clinically detectable loss of range of movement. Spread to the peripheral skeleton occurs and most often affects the shoulders and hips. Other symptoms are listed in Table 7-3.

Early in the disease, there is little to find on clinical examination, which leads to a delay in diagnosis. Probably the two most commonly mistaken diagnoses are to label the patient as having “fibrositis” or to mistake unilateral SI pain for a disc rupture. To the careful examiner, there will be detectable loss of lumbar motion in all three planes: flexion, extension, and lateral flexion. This is in contrast to a patient with a herniated nucleus pulposus, who usually has limited flexion, good backward extension, and limitation of lateral flexion to one side more pronounced than the other (determined by the location of the disc fragment on the nerve root). In AS, there may be pain on direct pressure on the SI joint, and various tests that stress the SI joint may increase the pain.

As the disease progresses, there is loss of lumbar lordosis and a decrease in ability to expand the rib cage. Contrary to other reports, these two changes occur early in the natural history of the disease (3). The loss of chest cage mobility occurs because of involvement of the posterior costovertebral and costotransverse articulations and the anterior costochondral junctions. The normal chest expansion (measured at the level of the fourth rib) is reduced from more than 5 cm to less than 2 cm.

The final stage of advancement in the disease is ankylosis of the spine, and if this occurs in a poorly supervised or poorly motivated patient, severe flexion deformities of the spine may occur (Fig. 7-1).

Extra-articular manifestations include ocular inflammation, psoriasis and psoriasiform nails, keratodermia blennorrhagicum, mucosal ulcers, balanitis aortitis and myocarditis, and apical pulmonary lesions.

Early in the disease, the enthesopathy may present as tenderness over bony prominences of the ischial tuberosity, greater trochanter, calcaneus, spinous processes, and other bony prominences.

The course of AS is unpredictable. Women tend to have a less severe form of involvement, as do men with later onset. The most severely involved tend to be younger men, but the variability in progression is striking. With proper supervision and an exercise regimen, even the most severely affected can maintain long-term, gainful employment.

Aside from severe extraskeletal involvement of the aorta and pulmonary tree, the most limiting symptoms come from ankylosis of the hips (25%), a spondylodiscitis causing severe back pain, or an atlantoaxial subluxation causing severe neck pain.

The patient with AS is very susceptible to accidents causing fractures of the spine. These fractures are more common in the cervical spine but can occur anywhere in the ankylosed spine. The trauma is usually minor, and the fracture is often missed on initial examination. If a neurologic lesion (paraplegia/quadriplegia) occurs at the time of the spinal fracture, the prognosis for recovery is dismal.

Ninety percent of symptomatic patients have positive blood test results for HLA-B27. The incidence of HLA-B27 in the normal Caucasian population is 6% to 8%. Most patients will have some
elevation in the erythrocyte sedimentation rate and C-reactive protein (CRP), although this elevation is rarely dramatic. There is no association with rheumatoid factor and antinuclear antibodies, thus the designation “seronegative spondylarthropathies.

The diagnosis of AS is confirmed by radiograph. The characteristic involvement of the SI joints in the presence of several of the clinical criteria listed in Table 7-2 confirm the diagnosis.

Stages of Radiographic Changes in the SI Joints. (The radiographic involvement is usually symmetric despite lateralization of symptoms.)

Early Stages (Fig. 7-2)

Both of these changes may occur throughout the SI joint but are seen earliest in the lower two thirds (the synovial portion) of the SI joint. The erosions eventually leave the appearance of widening (pseudowidening) of the SI joints.

Late Changes (Fig. 7-2)