= autoimmune disease of unknown etiology characterized by inflammation of multiple articular + paraarticular structures frequently resulting in bone ankylosis primarily affecting axial skeleton

Prevalence: 0.1–0.2% of general population

Peak age: 15–35 years; M÷F = 3÷1 to 10÷1; Caucasian÷Blacks = 3÷1

Associated with:

1. Ulcerative colitis, regional enteritis
2. Iritis in 25%
3. Aortic insufficiency + atrioventricular conduction defect

• HLA-B27 antigen positive in 96%
• insidious onset of low back pain + stiffness

Path: involves synovial + cartilaginous joints and sites of ligamentous attachment

Location:

1. axial skeleton: sacroiliac joints, thoracolumbar + lumbosacral junctions
2. HALLMARK is sacroiliac joint involvement!
3. peripheral skeleton (10–20%): sternal joint, symphysis pubis, hip, glenohumeral joint
4. tendinous insertions in pelvis + proximal femur

Temporal course: initial abnormalities of sacroiliac joints + thoracolumbar + lumbosacral junctions with gradual involvement of remaining spine

• Skull
  • temporomandibular joint space narrowing, erosions, osteophytosis
• Hand (30%)
  • exuberant osseous proliferation
  • osteoporosis, joint space narrowing, osseous erosions (deformities less striking than in rheumatoid arthritis)
• Sacroiliac joint / symphysis pubis
  • initially sclerosis of joint margins primarily on iliac side (bilateral + symmetric late in disease, may be unilateral + asymmetric early in disease)
  • later irregularities + widening of joint (= cartilage destruction)
  • bony fusion
• Pelvis
  • periostitic “whiskering”: ischial tuberosity, iliac crest, ischiopubic rami, greater femoral trochanter, external occipital protuberance, calcaneus
• Spine
  • squaring = straightened / convex anterior vertebral margins = erosive osteitis of anterior corners
  • “shiny corners” = reactive sclerosis of corners of vertebral body
  • diskitis = erosive abnormalities of diskovertebral junction
  • “diskal ballooning” = biconvex shape of intervertebral disk related to osteoporotic deformity + diskal calcification
  • marginal syndesmophyte formation (in 15%) = thin vertical radiodense spicules bridging the vertebral bodies = ossification of outer fibers of annulus fibrosus (NOT anterior longitudinal ligament):
    • “bamboo spine” on AP view = undulating contour due to syndesmophytosis
      Cx: prone to insufficiency fracture → pseudarthrosis
  • ankylosis of vertebral edges / center (with bony extension through disk)
  • asymmetric erosions of laminar + spinous processes of lumbar spine
  • ossification of supraspinous + interspinous ligaments:
    • “dagger” sign = single radiodense line on AP view
    • “trolley-track” sign on AP view = central line of ossification with two lateral lines of ossification (= apophyseal joint capsules)
  • apophyseal + costovertebral joint ankylosis (on oblique views)
  • dorsal arachnoid diverticula in lumbar spine with erosion of posterior elements (Cx: cauda equina syndrome)
  • atlantoaxial subluxation
  
  MR:
  • anterior / posterior / marginal spondylitis
  • spondylodiskitis
  • transdiskal / transvertebral insufficiency fracture
  • arthritis of zygapophyseal (facet), costovertebral, costotransverse joints (best seen on axial images)
  • bone marrow edema
  • joint effusion, synovitis, erosions
  • ankylosis in late stage → impairing chest excursion
  • enthesitis of interspinous ligaments ± osteitis of subjacent spinous processes
  • syndesmophytes + ankylosis of diskovertebral unit
• Chest
  Frequency: 1% of patients with ankylosing spondylitis, usually at an advanced stage of disease
  Histo: interstitial + pleural fibrosis with foci of dense collagen deposition, NO granulomas
  • bone manifestation obvious + severe
  
  Location: apices / upper lung fields
  • sternomanubrial joint irregularities + sclerosis
  • uni- / bilateral coarse upper lobe pulmonary fibrosis with upward retraction of hila (DDx: tuberculosis)
  • reticulonodular progressively confluent opacities in lung apices
  • apical bullae, cysts + cavitation (mimicking TB)
  
  HRCT:
  • peripheral interstitial lung disease
  • bronchiectasis
  • paraseptal emphysema
  • tracheobronchomegaly
  • apical fibrosis
  
  Cx: superinfection, especially with aspergillus (mycetoma formation) in 19–60% / atypical mycobacteria
  • hemoptysis
  
  DDx: other causes of pulmonary apical fibrosis (primary infection by fungi / mycobacteria; cancer)
• Cardiovascular (up to 80% paralleling duration of disease)
  Location: ascending aorta, aortic valve
  • aortic wall thickening (60%)
  • aortic valve thickening + nodularity → aortic valve insufficiency

Prognosis: 20% progress to significant disability; occasionally death from cervical spine fracture / aortitis

Rx: regular lifelong exercises + NSAID; tumor-necrosis factor (TNF)–α inhibitors

DDx:

1. Reiter syndrome (unilateral asymmetric SI joint involvement, paravertebral ossifications)
2. Psoriatic arthritis (unilateral asymmetric SI joint involvement, paravertebral ossification)
3. Inflammatory bowel disease
4. Sternoclavicular hyperostosis (pustulosis palmaris et plantaris)