Ankylosing Spondylitis (AS)

Frontal radiograph of the lumbar spine shows fusion of the sacroiliac joints (arrowheads) and thin syndesmophytes along the lateral borders of the lumbar spine.

Facts: AS

• Prototype of seronegative spondyloarthropathies (SNSA), which is a group of disorders of chronic inflammation of sacroiliac joints and spine
• Frequency: about 0.1% - 6% across different population (most likely toward the lower end)
• Genetic risk factor = human leukocyte antigen (HLA)-B27
• Principal bone/joint abnormalities = sacroilitis, synovitis and enthesitis

Imaging Findings

• Imaging findings are incorporated into the modified New York criteria for ankylosing spondylitis, as one of the four criteria.
• Sacroilitis: pseudowidening, sclerosis, erosions and later ankylosis
• Grading of sacroilitis on radiographs can be viewed here (link to Google Document provided by Schering-Plough Ply)

Modified New York Criteria for AS

Definite AS if 4a or 4b AND any clinical criteria (1-3)

1. Low back pain for at least 3 months' duration improved by exercise and not relieved by rest

2. Limitation of lumbar spine motion in sagittal and frontal planes

3. Chest expansion decreased relative to normal values for age and sex

4a. Unilateral sacroilitis grade 3-4

4b. Bilateral sacroilitis grade 2-4

Reference

Kippel JH. Primer on the rheumatic diseases, 13rd ed, 2008.