Seronegative spondyloarthropathies
- Lesion
A family of arthritides without rheumatoid factor that inflame the spine and the sites where tendons attach, often in people with HLA-B27.
What it is
The spondyloarthropathies share a pattern that separates them from rheumatoid arthritis: no rheumatoid factor, inflammation of the entheses (where ligaments and tendons insert into bone) rather than mainly of the synovium, involvement of the sacroiliac joints and spine, asymmetric arthritis of large lower-limb joints, and a strong link with HLA-B27. Inflammation is followed by new bone formation that can fuse the spine. The group includes ankylosing spondylitis, reactive arthritis after gut or genital infection, psoriatic arthritis and the arthritis of inflammatory bowel disease.
Structure
- Enthesitis: inflammatory cells at the ligament-bone junction, bone erosion and then reactive new bone that ossifies the ligament.
- Sacroiliac joints: synovitis and subchondral inflammation, erosions, then fibrous and bony fusion.
- Spine: syndesmophytes bridge the vertebral bodies at the annulus fibrosus; fused spine is brittle and fractures easily.
- Synovium in peripheral joints resembles RA but with fewer plasma cells and less pannus.
Step by step
- HLA-B27 and other genes (ERAP1, IL23R) set the susceptibility.
- Microbes in the gut or urogenital tract may trigger immunity by molecular mimicry or misfolded HLA-B27 that activates stress pathways.
- IL-23 activates entheseal T cells and innate lymphoid cells that produce IL-17 and TNF.
- Inflammation erodes bone at the entheses and sacroiliac joints.
- Healing is followed by excess bone formation (Wnt and BMP pathways) that bridges joints and vertebrae.
The spondyloarthropathies compared
| Disease | Trigger or setting | Joints | Extra features |
|---|---|---|---|
| Ankylosing spondylitis | HLA-B27 (most patients); young men | Sacroiliac joints, spine, hips | Anterior uveitis, aortitis, restrictive chest |
| Reactive arthritis | Chlamydia, Shigella, Salmonella, Yersinia, Campylobacter | Knees, ankles, feet, asymmetric | Urethritis, conjunctivitis, keratoderma, balanitis |
| Psoriatic arthritis | Psoriasis | DIP joints, dactylitis, spine in some | Nail pitting, pencil-in-cup erosions, arthritis mutilans |
| Enteropathic arthritis | Crohn disease, ulcerative colitis | Large peripheral joints; sacroiliitis | Peripheral arthritis follows bowel activity |
Clinical notes
- Inflammatory back pain: starts before 40, insidious, morning stiffness, improves with exercise not rest, night pain.
- Look for uveitis (red painful eye), heel pain from Achilles enthesitis and a family history.
- MRI shows early sacroiliitis before X-ray changes.
Test yourself
Think of the answer, then tap the question to see it.
Which HLA allele is linked to the spondyloarthropathies?
HLA-B27.
What is enthesitis?
Inflammation where tendons or ligaments insert into bone, typical of spondyloarthritis.
Which infections trigger reactive arthritis?
Genital Chlamydia trachomatis and enteric Shigella, Salmonella, Yersinia and Campylobacter.
What X-ray sign of psoriatic arthritis is classic?
Pencil-in-cup erosions, typically at the DIP joints.
Elsewhere in DurgiPedia
- HLA associations with disease
- Molecular mimicry and post-infectious autoimmunity
- Interleukin-17
- IL-12 and IL-23
Connections
Microorganisms2
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Ankylosing Spondylitis (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.