Ankylosing Spondylitis

Ankylosing Spondylitis – High‑Yield Study Guide for Medical Students

Definition

Ankylosing spondylitis (AS) is a chronic, immune-mediated, seronegative spondyloarthritis that primarily affects the axial skeleton, especially the sacroiliac joints and spine, leading to inflammatory back pain, progressive spinal stiffness, and possible structural ankylosis. It is part of the broader axial spondyloarthritis (axSpA) spectrum, which includes non-radiographic axSpA and radiographic axSpA (traditional AS).

Epidemiology

Ankylosing spondylitis typically begins in late adolescence to early adulthood (peak onset 20–40 years). There is a male predominance, though female cases are increasingly recognized as part of the axSpA spectrum. Prevalence varies by ethnicity and parallels the distribution of HLA-B27; rates are higher in Northern European and certain Asian populations. AS often affects individuals during their most productive working years and can substantially impact employment and work capacity, with disease activity, spinal mobility limitation, and functional impairment being major determinants of work disability.^[1]

Pathophysiology

The pathogenesis of ankylosing spondylitis is multifactorial, involving genetic susceptibility, environmental triggers, and dysregulated immune responses in the entheses and axial skeleton.

• Genetic factors: Strong association with HLA-B27, which contributes to aberrant antigen presentation, misfolding stress responses, and altered gut-immune interactions. Other non-HLA genes related to IL-23/IL-17 and TNF pathways also contribute.
• Enthesitis and chronic inflammation: Inflammation starts at the entheses (sites where ligaments and tendons attach to bone), especially in the sacroiliac joints and spinal ligaments. Persistent enthesitis leads to bone marrow edema and erosions.
• Pathologic new bone formation: Chronic inflammation is followed by repair processes with aberrant osteoproliferation, leading to syndesmophyte formation and eventual vertebral fusion ("bamboo spine").
• Immune pathways: Upregulation of TNF-α and IL-17/IL-23 pathways is central to disease activity, which is why targeted biologic agents (TNF and IL‑17 inhibitors) are effective in axSpA/AS.^[2]

Clinical Presentation

The hallmark symptom is chronic inflammatory back pain with gradual onset before age 45. Extra-axial and extra-articular manifestations are common and important to recognize for exams and clinical practice.

Axial manifestations

• Inflammatory back pain (≥3 months) with features such as:
  • Onset <40–45 years
  • Insidious onset rather than acute
  • Improves with exercise, not with rest
  • Nocturnal pain, often in the second half of the night
  • Morning stiffness >30 minutes
• Reduced spinal mobility:
  • Decreased lumbar flexion (Schober test)
  • Reduced chest expansion due to costovertebral involvement
  • Loss of lumbar lordosis and development of thoracic kyphosis in advanced disease
• Sacroiliitis: Buttock pain that can alternate sides; tenderness over SI joints.

Peripheral and extra-articular manifestations

• Peripheral arthritis: Asymmetric oligoarthritis, often involving hips, shoulders, and sometimes knees or ankles.
• Enthesitis: Heel pain (Achilles tendon, plantar fascia), tibial tuberosity, iliac crest, and other tendon insertion sites.
• Dactylitis: "Sausage" digits due to combined joint and tendon sheath inflammation (less common than in psoriatic arthritis but can occur).
• Ocular involvement: Recurrent acute anterior uveitis (iritis) presenting with painful red eye, photophobia, and blurred vision.
• Cardiovascular: Aortitis with aortic root dilation and aortic regurgitation; conduction abnormalities.
• Pulmonary: Restrictive pattern due to chest wall restriction; apical fibrobullous disease in late stages (rare).
• Constitutional symptoms: Fatigue, low-grade fever, weight loss in some patients.

Impact on function and quality of life

AS often interferes with daily functioning, physical activity, and work participation. Factors such as higher disease activity, worse functional limitation, and lower educational attainment have been associated with reduced employment in patients with AS, highlighting the importance of early diagnosis and effective treatment to preserve function.^[1]

Diagnosis

Diagnosis is based on a combination of clinical features, imaging of the sacroiliac joints and spine, and laboratory markers. AS falls within the spectrum of axial spondyloarthritis, which includes both non-radiographic and radiographic forms. Recognition and early diagnosis are essential to prevent structural damage and functional loss; real-world cohorts emphasize that many patients experience prolonged diagnostic delays and require systematic assessment to identify active disease and guide therapy.^[2]

Classification and diagnostic criteria (exam focus)

• Modified New York criteria for ankylosing spondylitis (radiographic axSpA):
  • Radiologic criterion: Definite sacroiliitis on plain X‑ray:
    • Grade ≥2 sacroiliitis bilaterally, or
    • Grade 3–4 sacroiliitis unilaterally.
  • Clinical criteria (any one of):
    • Low back pain and stiffness for >3 months that improves with exercise but is not relieved by rest.
    • Limitation of lumbar spine motion in sagittal and frontal planes.
    • Reduced chest expansion (compared with normal values for age and sex).
  • Definite AS requires radiologic criterion plus ≥1 clinical criterion.
• ASAS criteria for axial spondyloarthritis (high-yield concept, even if not memorized verbatim):
  • Age at onset <45 years and ≥3 months of back pain, plus either:
    • Imaging arm: Sacroiliitis on imaging (X‑ray or MRI) plus ≥1 spondyloarthritis feature, or
    • Clinical arm: HLA-B27 positivity plus ≥2 other spondyloarthritis features (e.g., inflammatory back pain, arthritis, enthesitis, uveitis, dactylitis, psoriasis, IBD, good response to NSAIDs, family history, elevated CRP).

Laboratory findings

• Inflammatory markers: Elevated ESR and CRP in many, but not all, patients; normal values do not exclude disease.
• HLA-B27: Positive in 80–95% of many Caucasian AS cohorts, but sensitivity and specificity vary by ethnicity. It supports the diagnosis but is not diagnostic on its own.
• Serology: Rheumatoid factor (RF) and anti-CCP antibodies are typically negative (seronegative pattern).

Imaging

• Plain radiographs:
  • Sacroiliac joints: Early changes include erosions, sclerosis, and pseudo-widening; later, joint-space narrowing and ankylosis.
  • Spine: Squaring of vertebral bodies, Romanus lesions (shiny corners), syndesmophytes bridging vertebral bodies, and eventual "bamboo spine" appearance in advanced cases.
• MRI:
  • More sensitive for early disease; detects bone marrow edema and active sacroiliitis before structural changes appear on X‑ray.
  • Useful in non-radiographic axSpA and when X‑rays are normal but clinical suspicion is high.
• CT:
  • Excellent for structural detail but involves higher radiation; reserved for specific indications (e.g., complex spinal fractures in fused spines).

Assessment of disease activity and function

Disease activity is commonly assessed with indices such as the BASDAI (Bath Ankylosing Spondylitis Disease Activity Index) and functional status with the BASFI. Although developed for AS, BASDAI has also been studied in related conditions such as fibromyalgia and axial spondyloarthritis, where symptom overlap can complicate interpretation.^[3] To evaluate flare risk and guide treatment decisions, prediction models and nomograms have been developed for axSpA populations, highlighting the role of composite assessment of symptoms, inflammatory markers, and imaging in routine care.^[4]

Management

Management aims to control pain and stiffness, reduce inflammation, maintain spinal mobility and function, prevent structural damage, and optimize quality of life and work participation. Treatment is individualized based on disease activity, structural damage, comorbidities, and patient preferences.

Non-pharmacologic management

• Patient education: Explain the chronic nature of the disease, importance of adherence, and role of exercise and posture.
• Physical therapy and exercise (core of management):
  • Daily stretching and posture exercises to maintain spinal extension, chest expansion, and overall mobility.
  • Supervised group physiotherapy programs can improve function and pain.
  • Swimming and low-impact aerobic activities are particularly beneficial.
• Lifestyle modifications:
  • Smoking cessation (smoking is associated with more severe structural damage).
  • Ergonomic adjustments at work and home; sleeping on a firm mattress, avoiding pillows that promote neck flexion.

Pharmacologic management

• NSAIDs (first-line pharmacologic therapy):
  • Examples: naproxen, indomethacin, diclofenac, celecoxib.
  • Often provide marked relief of inflammatory back pain and may reduce radiographic progression when used continuously in some patients.
  • Use the lowest effective dose; monitor for GI, renal, and cardiovascular adverse effects.
• Biologic DMARDs (for active disease despite NSAIDs):
  • TNF inhibitors: Adalimumab, etanercept, infliximab, golimumab, certolizumab.
    • Highly effective for axial symptoms, peripheral arthritis, and extra-articular features such as uveitis and some IBD manifestations.
  • IL‑17 inhibitors: Ixekizumab, secukinumab.
    • Demonstrated efficacy in both clinical trials and real-world practice for axSpA/AS, improving disease activity scores, patient-reported outcomes, and maintaining effectiveness in TNF‑inadequate responders.^[2]
  • Choice between TNF and IL‑17 inhibitors is guided by comorbidities (e.g., IBD, psoriasis), prior biologic exposure, safety profile, and patient preference.
• Targeted synthetic DMARDs (JAK inhibitors):
  • Agents such as upadacitinib have shown efficacy in axial spondyloarthritis in clinical trials, though their use requires careful risk-benefit assessment given class-specific risks (e.g., infection, thrombosis). Data from other immune-mediated diseases (e.g., atopic dermatitis) indicate increased risk of herpes zoster with JAK inhibitors, underlining the importance of vaccination status and monitoring.^[5]
• Conventional synthetic DMARDs:
  • Sulfasalazine: Limited benefit for axial symptoms; may be considered for prominent peripheral arthritis.
  • Methotrexate: Generally not effective for pure axial disease; sometimes used in peripheral involvement or concomitant psoriasis.
• Glucocorticoids:
  • Systemic steroids are not recommended for long-term axial disease control due to limited sustained benefit and significant side effects.
  • Local steroid injections can be used for focal enthesitis or peripheral joint flares.

Monitoring and long-term care

• Regular assessment of symptoms, physical function, and disease activity indices (e.g., BASDAI, BASFI).
• Periodic measurement of CRP/ESR to track inflammation, acknowledging that some patients have normal markers.
• Follow-up imaging (X‑ray or MRI) as clinically indicated to monitor structural progression.
• Screening and management of comorbidities: osteoporosis, cardiovascular risk factors, mood disorders, and work-related disability.

Telemedicine and novel care models

For axial spondyloarthritis, including AS, telehealth approaches have been explored to reduce diagnostic delay and improve access to specialist care. Stepwise asynchronous telehealth assessment has shown feasibility for identifying suspected axSpA patients and triaging them for in-person rheumatology evaluation, potentially shortening the prolonged diagnostic delays typical for these conditions.^[6]

Complications and prognosis

• Structural complications:
  • Spinal fusion and severe kyphosis leading to fixed forward posture.
  • Increased risk of spinal fractures due to reduced flexibility and associated osteoporosis; even low-impact trauma can cause unstable fractures.
• Extra-articular complications:
  • Recurrent uveitis, which can threaten vision if not promptly treated.
  • Aortic regurgitation, conduction abnormalities, and rarely aortitis.
  • Restrictive lung disease due to costovertebral involvement and chest wall rigidity.
• Functional prognosis:
  • Early diagnosis and initiation of targeted therapy (TNF or IL‑17 inhibitors) can significantly improve symptoms, maintain function, and potentially limit structural damage.
  • Higher disease activity, delayed diagnosis, and inadequate treatment are associated with worse functional outcomes, greater work disability, and reduced quality of life.^[1]

Key Clinical Pearls for Exams and Practice

• Think AS in young adults (<45 years) with >3 months of back pain that improves with exercise and worsens with rest, especially with morning stiffness and night pain.
• Ask about extra-articular features such as uveitis, psoriasis, IBD, and family history of spondyloarthritis.
• Physical findings: Reduced lumbar flexion (Schober test), reduced chest expansion, tenderness over sacroiliac joints, and postural changes (loss of lumbar lordosis, thoracic kyphosis).
• Imaging is crucial: MRI of sacroiliac joints is more sensitive than X‑ray in early disease and can reveal bone marrow edema consistent with active sacroiliitis before structural damage.
• HLA-B27 supports but does not confirm the diagnosis; its predictive value depends on disease prevalence and ethnicity.
• NSAIDs are first-line for symptoms and should be tried at adequate doses before escalation to biologic therapy.
• TNF and IL‑17 inhibitors are key biologic options for persistent active axSpA/AS; real-world data confirm substantial improvements in disease activity and quality of life with agents like ixekizumab.^[2]
• Conventional DMARDs do not treat axial disease; sulfasalazine may be used for peripheral arthritis but not as primary therapy for spinal involvement.
• Always consider osteoporosis and fracture risk in long-standing AS with spinal fusion.
• Early diagnosis changes trajectory: Reducing diagnostic delay via structured assessment and telehealth triage can improve outcomes and preserve employment and function.^[6]

Summary

Ankylosing spondylitis is a prototypical axial spondyloarthritis characterized by inflammatory back pain, sacroiliitis, and progressive spinal ossification. Early recognition using clinical features, HLA-B27 testing, and especially MRI-based detection of sacroiliitis allows timely initiation of NSAIDs, biologic agents (TNF or IL‑17 inhibitors), and structured physiotherapy. Modern management, including emerging care models such as telehealth triage and individualized biologic therapy, can substantially improve symptoms, preserve function, and limit structural damage in this historically disabling disease.^{[2], [6]}