Ankylosing spondylitis

Overview

Ankylosing spondylitis (AS) is the prototype of seronegative spondyloarthropathies. It is a chronic, progressive inflammatory disorder primarily affecting the axial skeleton and sacroiliac joints.

• Epidemiology: Young adults (onset 15–35 years). M:F ratio approx 3:1 (radiographic disease); approaches 1:1 in non-radiographic axial SpA.
• Genetics: Strong association with HLA-B27 (>90% in Caucasians; slightly lower in Asian populations but typically >85%).
• Pathology: Enthesitis (inflammation at tendon/ligament insertion) $\to$ Osteitis $\to$ Sclerosis/Erosions $\to$ Ossification (Ankylosis).

Clinical features

Diagnostic criteria

• Modified New York criteria (1984): The historical standard. Requires radiographic sacroiliitis (grade 2 bilateral or grade 3-4 unilateral) + clinical criteria.
• ASAS criteria (Assessment of SpondyloArthritis international Society): Modern standard. Allows diagnosis via MRI (active sacroiliitis on STIR) in patients without plain film changes (non-radiographic axial spondyloarthritis, nr-axSpA).

The 5 'A's of extra-articular involvement

1. Anterior uveitis (Acute, unilateral, recurrent). Most common extra-articular feature (~25–30%).
2. Achilles tendinitis (and plantar fasciitis). Manifestation of peripheral enthesitis.
3. Apical lung fibrosis (1–2%). Upper lobe fibrobullous disease; mimics TB or fungal infection.
4. Aortic regurgitation / Aortitis. Due to inflammation of the aortic root/valve ring.
5. Amyloidosis. Secondary (AA type); a rare cause of renal failure in long-standing disease.

Imaging features

1. Sacroiliitis (Hallmark)

• Distribution: Typically bilateral and symmetrical.
• Radiographic grading (New York criteria):
  • Grade 0: Normal.
  • Grade 1: Suspicious.
  • Grade 2: Minimal sclerosis, some erosion ("pseudo-widening"), joint margins distinct.
  • Grade 3: Definite sclerosis, significant erosions, joint space narrowing.
  • Grade 4: Total ankylosis (fusion).

2. Spine

Progression typically ascends: Lumbar $\to$ Thoracic $\to$ Cervical.

• Romanus lesion:
  • MRI (Early): Anterior vertebral corner oedema (enthesitis at annulus fibrosus insertion).
  • X-ray (Late): "Shiny corner" (reactive sclerosis following the osteitis).
• Vertebral squaring: Loss of normal anterior concavity due to corner erosion and sub-ligamentous bone formation.
• Syndesmophytes:
  • Marginal: Thin, vertical ossification of the outer layers of the annulus fibrosus (Sharpey's fibres).
  • Crucial distinction: Must differentiate from non-marginal (bulky/gap from endplate) seen in Psoriatic arthritis and flowing (anterolateral candle wax) seen in DISH.
• Bamboo spine: Complete fusion of vertebral bodies via marginal syndesmophytes.
• Ligamentous ossification signs:
  • Dagger sign: Ossification of supraspinous and interspinous ligaments (appears as a single central dense line on frontal view).
  • Trolley track sign: Dagger sign + ossification of facet joint capsules (three vertical dense lines).
• Andersson lesion: Aseptic inflammatory spondylodiscitis. Appears as disc space destruction/irregularity. Can mimic infection.

3. MRI findings (Early diagnosis)

• Protocol: STIR or T2 fat-sat is mandatory to detect active inflammation.
• Active inflammatory lesions (ASAS positive):
  • Bone marrow oedema (BMO): Hyperintense on STIR/T2FS. Must be peri-articular (SI joints) or at vertebral corners.
• Chronic structural changes:
  • Fatty metaplasia (T1 hyperintensity).
  • Sclerosis / Erosions / Ankylosis.

Complications

• Spinal fractures:
  • Chalk stick fracture: Trans-vertebral or trans-discal fractures of the fused spine.
  • Location: Most common at the cervicothoracic or thoracolumbar junctions.
  • Risk: Can occur with trivial trauma. High risk of epidural haematoma and cord injury due to the spine acting as a long lever arm.
• Atlanto-axial subluxation: Due to transverse ligament laxity/pannus (similar to RA).
• Cauda equina syndrome: Rare. Caused by dural ectasia and arachnoiditis (diverticula formation eroding into posterior elements).

Differential diagnosis (The "Spinal outgrowths")

Management summary

• First line: NSAIDs and physiotherapy (maintain mobility).
• Resistant disease: Biologics (TNF-alpha inhibitors e.g., Infliximab, Etanercept; IL-17 inhibitors e.g., Secukinumab).
• Surgery:
  • Vertebral osteotomy (e.g., pedicle subtraction) for severe fixed kyphosis (chin-on-chest deformity).
  • Stabilization for chalk stick fractures (often requires long-segment fixation).