Ankylosing Spondylitis

General Considerations

• Chronic inflammatory disease of unknown etiology primarily affecting spine
• Most common spondyloarthropathy
• Age-young adults
• 15-35 years
• Mostly male
• M:F = 4:1
• Mostly Caucasian
• Caucasians to Blacks = 3:1

Clinical findings

• Insidious onset of low back pain and stiffness
• Poor chest expansion
• Stiffness
• Exaggerated dorsal kyphosis
• HLA-B 27 positive in >90%

Location

• Axial skeleton and large, usually central, appendicular joints
• Sacroiliac joint involvement
• Hallmark of disease
• Only synovial portion of SI joint is involved
• Inferior and anterior portion of joint
• Other enthesopathies like DISH can cause bridging of upper, non-synovial part of joint
• Usually site of initial involvement
• Bilaterally symmetric
• Widened with erosions at first
• Then ankylosis
• Spine
• Usually begins at either thoracolumbar or lumbosacral junctions
• Extends symmetrically without skip areas
• Reiter’s and psoriasis characteristically are asymmetric and have skip areas
• Marginal syndesmophyte formation = thin vertical dense spicules bridging the vertebral bodies
• Ossification of outer fibers of annulus fibrosus
• Not anterior longitudinal ligament
• Trolley-track sign on AP view = central line of ossification (supraspinous and interspinous ligaments) with two lateral lines of ossification (apophyseal joints)
• Bamboo spine on AP view = undulating contour due to syndesmophytes
• Prone to fracture resulting in pseudarthrosis
• Straightening / squaring of anterior vertebral margins
• Osteitis of anterior corners
• Reactive sclerosis of corners of vertebral bodies = shiny-corner sign
• Symmetric erosions of laminar and spinous process at level of lumbar spine
• Apophyseal  and costovertebral ankylosis
• Periosteal whiskering
• Sites of tendinous insertion
• Ischial tuberosity
• Iliac crest
• Ischiopubic rami
• Greater femoral trochanter
• External occipital protuberance
• Calcaneus
• Patella
• Dorsal arachnoid diverticula in lumbar spine with erosion of posterior elements
• Atlantoaxial subluxation
• Peripheral joint involvement
• Hip is most frequently involved
• Concentric joint narrowing
• Few erosions
• Protrusio acetabuli
• Temporomandibular joint
• Joint space narrowing
• Erosions
• Osteophytosis
• Hand (30%)
• Target area
• MCP, PIP, DIP
• Exuberant osseous proliferation
• Osteoporosis
• Joint space narrowing
• Osseous erosions (deformities less striking than in rheumatoid arthritis)
• Chest
• Bilateral upper lobe pulmonary fibrosis (1%) with upward retraction of hila
• Resembles tuberculosis
• Cardiovascular
• Aortitis (5%) of ascending aorta ± aortic valve insufficiency
• Prognosis: 20% progress to significant disability
• Occasionally death from cervical spine fracture / aortitis

DDx

• Reiter syndrome (unilateral asymmetric SI joint involvement, paravertebral ossifications)
• Psoriatic arthritis (unilateral asymmetric SI joint involvement, paravertebral ossifications)
• Inflammatory bowel disease

Associated with:

• Ulcerative colitis
• Regional enteritis
• Clinically the SI joint involvement is identical to
• Inflammatory Bowel Disease (IBD)
• Iritis in 25%
• Aortic insufficiency and atrioventricular conduction defect 

 
Ankylosing Spondylitis. Note fusion of both SI joints and thin, symmetrical syndesmophytes bridging the intervertebral disc spaces.