Ankylosing Spondylitis

What is it?

HLA-B27-associated enthesitis at sacroiliac and spinal insertions progresses to bony ankylosis.

Commonly affected: Progresses from SI joints → lumbar → thoracic → cervical

How it develops

1. Genetic PredispositionHLA-B27 increases risk
2. SacroiliitisInitial inflammation at SI joints
3. EnthesitisLigament/tendon insertions inflame
4. Spinal AnkylosisInflammation ossifies, spine fuses (bamboo)

Symptoms

• Inflammatory Back PainMorning stiffness >1 hour, improves with activity
• Night PainPain wakes patient in second half of night
• Reduced Chest ExpansionCostovertebral involvement limits chest expansion
• UveitisAnterior uveitis in 30–40%
• Postural ChangeChronic disease causes thoracic kyphosis

How it is examined

• Schober TestReduced lumbar flexion (<5 cm skin mark expansion)
• SI Joint TendernessBilateral SI tenderness
• Chest ExpansionNormal ≥5 cm; reduced in AS
• Occiput-to-WallInability to touch occiput to wall indicates kyphosis

Imaging

Pelvis X-ray is key — bilateral symmetric sacroiliitis characteristic.

• Bilateral symmetric sacroiliitis
• Joint narrowing/ankylosis
• Bamboo spine
• Syndesmophytes

MRI essential for early AS — detects SI bone marrow edema before X-ray changes.

• Active SI bone marrow edema (STIR+)
• Fat deposition (chronic)
• Vertebral corner edema (Romanus lesion)

When surgery is considered

Severe spinal deformity or hip ankylosis

Procedures that may be discussed

• Pedicle subtraction osteotomy
• Total hip arthroplasty

Outlook

Early diagnosis and biologics significantly improve prognosis and quality of life.