Spondyloarthropathies

The spondyloarthropathies (SpA) are related inflammatory conditions defined by enthesitis, axial involvement, asymmetrical peripheral arthritis and an association with HLA-B27.

They include axial spondyloarthritis (axSpA: ankylosing spondylitis, AS, and non-radiographic axSpA), psoriatic arthritis (PsA), reactive arthritis and enteropathic arthritis. AxSpA affects similar numbers of women and men, and diagnosis is often delayed by years.

Inflammatory back pain in a young person should prompt consideration of axSpA. Do not rule it out on any single finding: HLA-B27, inflammatory markers and X-rays can all be normal in active early disease.

The enthesis, where tendon, ligament or capsule inserts into bone, is the primary target. The 'enthesis organ' concept (Benjamin and McGonagle) includes the adjacent fibrocartilage, bone and periosteum, and explains dactylitis as inflammation around the digital flexor tendon sheath.

This is the key distinction from rheumatoid arthritis, which is synovitis-driven. SpA also forms new bone (syndesmophytes in the spine, enthesophytes peripherally) alongside erosion, and bridging syndesmophytes can fuse the spine into the 'bamboo spine' of advanced AS. IL-23, IL-17 and TNF-alpha are the main cytokines.

HLA-B27 is present in about 8% of the White UK population and in around 90% of people with AS, but in only about 20% with PsA. Most carriers never develop SpA, and a negative result does not exclude it. The sacroiliac joints are the earliest site of axial disease.