Chronic inflammatory arthritis of the spine and sacroiliac joints; HLA-B27-associated; presents with inflammatory back pain in young adults.
Also known as: AS, ankylosing spondylitis, axial spondyloarthritis, axSpA, Bechterew disease
Overview
Chronic inflammatory disease of the axial skeleton — sacroiliac joints, spine, and hips — leading to enthesitis, syndesmophyte formation, and eventually bony ankylosis. The prototypical seronegative spondyloarthropathy.
Epidemiology
Onset typically 15-40; rarely begins after age 45. Male-to-female ratio ~2-3:1, though women often have milder, less radiographic disease and are underdiagnosed. Prevalence parallels HLA-B27 prevalence in the population (0.1-1%).
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Question 1MusculoskeletalMedium
A 27-year-old man has low back pain and morning stiffness improving with exercise. He also has limited lumbar flexion and alternating buttock pain. Which of the following imaging findings would most likely confirm the diagnosis?
AVertebral lytic lesions
BBilateral sacroiliitis
CLumbar disc herniation
DFacet joint osteophytes
Reveal answer & full explanation
Correct answer: B — Bilateral sacroiliitis
AVertebral lytic lesions
BBilateral sacroiliitis✓
CLumbar disc herniation
DFacet joint osteophytes
Why Bilateral sacroiliitis is correct
Inflammatory back pain in a young man, with morning stiffness that eases on activity and alternating buttock pain, suggests axial spondyloarthritis (ankylosing spondylitis).
Sacroiliitis on imaging is the defining and earliest structural finding of the disease.
The inflammatory pattern (better with exercise, worse with rest) is what separates it from mechanical causes.
Why the others are wrong
Lumbar disc herniation — A herniated disc produces radicular leg pain worsened by sitting and bending, not exercise-relieved inflammatory stiffness; this anchors on back pain generally.
Facet joint osteophytes — Degenerative facet change drives mechanical pain that worsens with activity and appears in older patients, the inverse of this picture.
Vertebral lytic lesions — Lytic destruction points to malignancy or infection with night pain and red flags, not a young man with classic inflammatory features.
Question 2MusculoskeletalMedium
A 24-year-old man has chronic inflammatory back pain that improves with exercise and morning stiffness. He also has limited lumbar flexion. Which of the following best explains the pathophysiology of this disease?
ACartilage degeneration from loading
BUrate crystal deposition in joints
CEnthesitis with axial inflammation
DImmune complex deposition in synovium
Reveal answer & full explanation
Correct answer: C — Enthesitis with axial inflammation
ACartilage degeneration from loading
BUrate crystal deposition in joints
CEnthesitis with axial inflammation✓
DImmune complex deposition in synovium
Why Enthesitis with axial inflammation is correct
A young man with inflammatory back pain that improves with activity, morning stiffness, and reduced lumbar flexion has ankylosing spondylitis, a seronegative spondyloarthritis.
The primary lesion is enthesitis (inflammation where ligaments insert into bone), driving sacroiliitis and progressive axial fusion.
HLA-B27 is strongly associated, and the inflammatory pattern is why NSAIDs and TNF inhibitors help.
Why the others are wrong
Urate crystal deposition in joints — Gout produces acute, episodic peripheral monoarthritis (classically the first MTP), not chronic inflammatory axial pain; this is the wrong-crystal trap.
Cartilage degeneration from loading — Osteoarthritis worsens with use and improves with rest, the opposite of this activity-relieved inflammatory pattern; this is the mechanical-versus-inflammatory trap.
Immune complex deposition in synovium — This is the rheumatoid arthritis mechanism, which drives symmetric peripheral small-joint synovitis rather than sacroiliitis with restricted lumbar flexion in a young man.
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Enthesitis — inflammation at insertions of tendons, ligaments, and joint capsules — is the unifying lesion. Driven by IL-17/IL-23 axis and TNF-alpha. Inflammation is followed by reactive new bone formation, producing syndesmophytes that bridge vertebral bodies and ultimately fuse the spine ('bamboo spine').
Clinical presentation
Symptoms
Inflammatory back pain — insidious onset before age 45, duration >3 months
Morning stiffness >30 minutes
Pain improves with exercise, worsens with rest
Night pain, especially second half of night
Alternating buttock pain (sacroiliitis)
Peripheral enthesitis — Achilles tendon, plantar fascia
Psoriatic arthritis / IBD-associated arthritis / reactive arthritis — Other spondyloarthritis subtypes — extra-articular features distinguish
Diffuse idiopathic skeletal hyperostosis (DISH) — Older patients, flowing anterior osteophytes ≥4 levels, preserved SI joints, normal CRP
Osteoarthritis of spine — Older patients, mechanical pattern, marginal osteophytes
Fibromyalgia — Widespread pain, normal imaging, normal CRP
Infection (vertebral osteomyelitis) — Fever, focal severe pain, elevated WBC; MRI and biopsy diagnostic
Diagnostic workup
Diagnostic criteria
ASAS axial spondyloarthritis criteria (patients with ≥3 months back pain, age <45): sacroiliitis on imaging plus ≥1 SpA feature, OR HLA-B27 plus ≥2 SpA features.
Labs
HLA-B27 — strongly supportive when positive in the right clinical context
CRP, ESR — elevated in active disease but normal in many patients
CBC — anemia of chronic disease in active disease
RF, anti-CCP — negative (seronegative)
Imaging
Pelvic radiograph — sacroiliitis (sclerosis, erosions, joint space narrowing or fusion). Grade ≥2 bilateral or ≥3 unilateral required for modified New York criteria
MRI of sacroiliac joints — detects bone marrow edema (active sacroiliitis) years before radiographic changes; essential for non-radiographic axial spondyloarthritis
Lateral cervical spine radiograph before intubation or surgery to assess for fusion and fracture risk
Diagnostic algorithm
Inflammatory Back Pain Feature
Notes
Onset <45 years
Insidious onset
Duration >3 months
Chronic
Morning stiffness >30 min
Improves with movement
Improves with exercise
NOT with rest
Pain at night
Often second half of night
Alternating buttock pain
Sacroiliitis
HLA-B27
Positive in 85-90%
MRI SI joint
Bone marrow edema = active sacroiliitis
Inflammatory vs mechanical back pain — the cardinal AS history. Use to decide whom to image with sacroiliac MRI.
Treatment
First-line
Patient education and exercise — daily stretching, posture training, swimming or pilates
NSAIDs — naproxen, ibuprofen, indomethacin, meloxicam — full anti-inflammatory dose for 2-4 weeks, then maintenance; continuous use may slow radiographic progression in active disease
IL-17 inhibitors — secukinumab, ixekizumab — alternative when TNFi contraindicated or ineffective (avoid in IBD — may worsen)
JAK inhibitors — tofacitinib, upadacitinib — newer option
Sulfasalazine — useful for peripheral arthritis but ineffective for axial disease
Methotrexate — limited efficacy for axial disease; reserved for peripheral joints
Intra-articular corticosteroid injections for SI joints or enthesitis; systemic steroids generally avoided
Complications
Spinal fusion ('bamboo spine') with rigid posture
Vertebral fracture — fused spine is brittle; even minor trauma can cause unstable fractures; high suspicion required
Atlantoaxial subluxation
Anterior uveitis (recurrent)
Aortic regurgitation, conduction defects
Apical pulmonary fibrosis (rare)
Restrictive lung disease from fused thoracic cage
Osteoporosis paradoxically despite syndesmophytes
Cauda equina syndrome (rare, late)
PANCE pearls
Inflammatory back pain in a young adult that improves with activity is the cardinal history — opposite of mechanical pain.
MRI of SI joints with bone marrow edema diagnoses non-radiographic axSpA years before plain films change.
A fused spine fractures easily — image the entire spine, not just the symptomatic area, after any trauma.
IL-17 inhibitors can worsen IBD — choose TNFi if both AS and IBD coexist.
Acute anterior uveitis in a young person is HLA-B27 spondyloarthritis until proven otherwise — refer to ophthalmology urgently.
References
ACR/SAA/SPARTAN 2019 — 2019 Update of the ACR/SAA/SPARTAN Treatment Recommendations for Ankylosing Spondylitis and Non-radiographic Axial Spondyloarthritis (Ward et al., Arthritis Rheumatol 2019)
ASAS-EULAR 2022 — ASAS-EULAR Recommendations for the Management of Axial Spondyloarthritis (Ramiro et al., Ann Rheum Dis 2023)
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