Ankylosing Spondylitis

Definition

Ankylosing Spondylitis (AS) is a chronic inflammatory rheumatic disease primarily affecting the axial skeleton, leading to progressive stiffness and fusion of the spine and sacroiliac joints. It may involve peripheral joints and extra-articular organs.

Epidemiology

• Prevalence varies worldwide: 0.1–1.4% in most populations
• More common in males than females (3:1 ratio)
• Onset typically in late adolescence or early adulthood (15–40 years)
• Strong association with HLA-B27 genotype
• Family history increases risk

Etiology

• Genetic predisposition, particularly HLA-B27
• Immune-mediated inflammation triggered by environmental or microbial factors
• Possible role of gut microbiota dysbiosis
• Inflammation predominantly at entheses (sites of ligament and tendon attachment to bone)
• No single causative pathogen identified
• Combination of genetic and environmental factors drives disease

Pathophysiology

• Chronic inflammation at entheses leading to bone erosion and subsequent new bone formation
• Cytokine dysregulation: TNF-α, IL-17, IL-23 pathways involved
• Progressive ossification leads to spinal fusion (ankylosis)
• Sacroiliac joint inflammation causes pain and stiffness
• Peripheral arthritis occurs in some patients (hips, shoulders, knees)
• Extra-articular manifestations may involve eyes, heart, lungs, and gastrointestinal tract
• Chronic inflammation can result in reduced spinal mobility and posture changes