Cause:Genetic, HLA association suggests close link to primary gene or immune interaction between antigen and agent, eg, klebsiella antigens (Bull Rheum Dis 1989;39[2]:1)
Pathophys:Tendonitis, periostitis, and ligamentous inflammation and calcification lead to bony hyperplasia and ankylosis
Sx:
Hip and foot arthritic sx, though can involve any joint; onset age 15-35 yr, or in late childhood
Back pain, which improves with exercise, worsens with inactivity, unlike chronic low-back syndrome; sacroiliitis early and constant
Anorexia and weight loss, fever, sciatica (10% have it when 1st present)
Si:
10-cm mark on LS spine stretches on flexion only to <15 cm; chest expansion 
2.5 cm (Ann IM 1976;84:1)
Painful SI joints on palpation
Iridocyclitis (25%); aortic insufficiency (5%); mitral insufficiency (Nejm 1978;299:1448) and abnormal cardiac conduction including heart block (8%); kyphosis, fracture of cervical spine; pneumonitis (1%) that can look like old tbc
r/o other seronegative spondyloarthropathies all with similar HLA B27 association and clinical syndromes, eg, Reiter’s (reactive arthritis), regional enteritis, ulcerative colitis, psoriatic arthritis, post-Yersinia colitis (Nejm 1989;321:16)
Lab:
Hem:ESR increased (80%)
Serol:HLA B27-positive; 10% false negative, 8% false positive (have the gene but don’t get the disease)
Xray:Bilaterally symmetric sacroiliitis with subchondral sclerosis (100%), osteitis of symphysis pubis, anterior spinal ligament calcification, late “bamboo spine” with bony fusion of spine and osteophytes are parallel not perpendicular to spine. Calcification of tendons and heels (cf Reiter’s)