Seronegative Spondyloarthropathies
A high-yield overview of the HLA-B27-associated seronegative spondyloarthropathies (ankylosing spondylitis, reactive, psoriatic, and enteropathic arthritis), covering shared IL-23/IL-17 pathophysiology, classic vignette buzzwords, sacroiliac imaging and synovial-fluid workup, and the NSAID → DMARD → biologic management ladder with board-style next-best-step decisions.
Overview & Pathophysiology
The seronegative spondyloarthropathies (SpA) are a family of chronic inflammatory arthritides unified by four features: (1) RF-negative AND anti-CCP-negative ("seronegative"), (2) strong HLA-B27 association, (3) axial + asymmetric peripheral joint disease with a hallmark of enthesitis (inflammation where tendons/ligaments insert into bone), and (4) shared extra-articular findings — anterior uveitis, mucocutaneous lesions, aortitis, and IBD.
The four classic members: ankylosing spondylitis (AS), reactive arthritis, psoriatic arthritis, and enteropathic (IBD-associated) arthritis.
Pathophysiology: HLA-B27 is an MHC class I molecule presenting peptide to CD8+ T cells. Proposed mechanisms include HLA-B27 heavy-chain misfolding and molecular mimicry, which drive the IL-23/IL-17 axis that fuels enthesitis and pathologic new bone formation. Unlike RA (synovial pannus → pure erosion), SpA produces both erosion AND bone proliferation — syndesmophytes, ankylosis, and the "bamboo spine."
- Seronegative = RF-negative AND anti-CCP-negative (key contrast with RA)
- HLA-B27 positive (highest in AS, ~90–95%)
- Inflammatory back pain buzzwords: onset age <40, insidious, morning stiffness >30 min, improves with exercise / worse with rest, night / early-morning pain, responds to NSAIDs
- Enthesitis = defining lesion — Achilles tendon, plantar fascia (heel pain)
- Dactylitis = "sausage digit" (psoriatic, reactive)
- Anterior uveitis = most common extra-articular finding (painful red eye, photophobia, blurred vision) → urgent ophthalmology referral
- Sacroiliitis = earliest and most characteristic imaging finding
Seronegative SpA vs Rheumatoid Arthritis
| Feature | Seronegative SpA | Rheumatoid arthritis |
|---|---|---|
| RF / anti-CCP | Negative | Positive (anti-CCP specific) |
| Genetic marker | HLA-B27 | HLA-DR4 |
| Joint distribution | Asymmetric, large/lower-limb joints; axial | Symmetric, small joints (MCP/PIP); spares DIP |
| Sacroiliac / spine | Sacroiliitis, syndesmophytes ("bamboo spine") | Cervical only (C1–C2 subluxation); SI spared |
| Enthesitis / dactylitis | Present | Absent |
| Synovial fluid | Inflammatory, sterile (WBC ~2,000–50,000, PMN-predominant, no crystals) | Inflammatory, sterile |
| Bone changes | Erosion + new bone formation | Erosions, pannus, periarticular osteopenia |
Vignette: A 24-year-old man reports 6 months of low back and buttock pain with morning stiffness lasting >1 hour that improves with exercise, worsens with rest, and wakes him at night. Exam: reduced lumbar flexion (positive Schober test) and decreased chest expansion. ESR/CRP elevated; RF negative.
Diagnosis: Ankylosing spondylitis.
Next best step (diagnosis):
- Plain radiograph of the sacroiliac joints first — look for sacroiliitis (erosions, sclerosis, pseudo-widening → fusion).
- If X-ray non-diagnostic but suspicion high → MRI of SI joints (detects early bone marrow edema/osteitis).
- Supportive: HLA-B27, ESR/CRP.
Management:
- First-line: NSAIDs + physical therapy/exercise (cornerstone).
- Refractory axial disease: TNF-α inhibitors (adalimumab, etanercept, infliximab) or IL-17 inhibitors (secukinumab).
- Conventional DMARDs (sulfasalazine, MTX) help peripheral but NOT axial disease.
Watch for: acute anterior uveitis, aortic regurgitation, restrictive lung disease (costovertebral fusion), and spinal fractures in the fused, osteoporotic "bamboo spine."
Vignette: A 27-year-old man develops asymmetric arthritis of the knee and ankle, conjunctivitis, and dysuria ~2–4 weeks after an episode of bloody diarrhea. Exam: a swollen "sausage toe" (dactylitis), heel pain (enthesitis), and hyperkeratotic plaques on the soles.
Diagnosis: Reactive arthritis (post-infectious).
- Post-enteric (classic four): Shigella, Salmonella, Yersinia, Campylobacter.
- Post-venereal: Chlamydia trachomatis.
- Classic signs: keratoderma blennorrhagicum, circinate balanitis, oral ulcers.
Next best step: Arthrocentesis of the affected joint to exclude septic arthritis — reactive-arthritis fluid is inflammatory but sterile (no organisms, no crystals). Test for Chlamydia (NAAT).
Management:
- NSAIDs first-line; intra-articular steroids for mono/oligoarthritis.
- Treat active Chlamydia with doxycycline (antibiotics do not shorten post-enteric arthritis).
- Chronic/refractory → sulfasalazine or TNF inhibitors.
Most cases are self-limited (weeks–months); a minority relapse or become chronic.
Psoriatic arthritis (PsA):
- Occurs in ~30% of psoriasis patients; skin disease usually precedes arthritis
- Nail pitting / onycholysis correlates with joint involvement
- DIP involvement + dactylitis + enthesitis
- Radiograph: "pencil-in-cup" deformity; severe form = arthritis mutilans (telescoping "opera-glass" digits)
- Five patterns: asymmetric oligoarthritis, DIP-predominant, symmetric (RA-like), axial/spondylitis, mutilans
- Treatment: NSAIDs, methotrexate (peripheral/skin), TNF / IL-17 / IL-23 inhibitors
Enteropathic (IBD-associated) arthritis:
- Seen with Crohn disease & ulcerative colitis
- Peripheral arthritis often parallels bowel disease activity; axial disease (sacroiliitis) runs an independent course
- Manage the underlying IBD; TNF-α inhibitors treat both gut and joints
- Caution with NSAIDs — may trigger IBD flares
The Four Subtypes Compared
| Subtype | HLA-B27 | Joint pattern | Distinctive clues |
|---|---|---|---|
| Ankylosing spondylitis | ~90–95% | Axial (SI joints, spine) | Bamboo spine, ↓ chest expansion, aortic regurgitation, uveitis |
| Reactive arthritis | ~30–50% | Asymmetric oligoarthritis, lower limb | Post-GI/GU; conjunctivitis + urethritis + arthritis; keratoderma |
| Psoriatic arthritis | ~40–50% (axial) | DIP, asymmetric, mutilans, axial | Psoriasis, nail pitting, pencil-in-cup, dactylitis |
| Enteropathic | ~30–70% (axial) | Peripheral (parallels gut) + axial | Crohn/UC; erythema nodosum, pyoderma gangrenosum |
"PAIR" — the four seronegative spondyloarthropathies:
- Psoriatic arthritis
- Ankylosing spondylitis
- IBD-associated (enteropathic) arthritis
- Reactive arthritis
Reactive arthritis triad — "Can't see, can't pee, can't climb a tree":
- Can't see → conjunctivitis / uveitis
- Can't pee → urethritis
- Can't climb a tree → arthritis
(Formerly "Reiter syndrome" — the eponym is now avoided.)
Diagnostic Workup & Management Ladder
Diagnostic approach (boards logic):
- Recognize inflammatory back pain or asymmetric lower-limb arthritis plus an extra-articular clue (uveitis, psoriasis, recent GI/GU infection, IBD).
- Confirm seronegativity: order RF and anti-CCP (expected negative); check ESR/CRP (often elevated).
- Image the sacroiliac joints: X-ray first; if normal with high suspicion, MRI for early sacroiliitis.
- HLA-B27 supports but does not confirm or exclude the diagnosis (neither necessary nor sufficient).
- Arthrocentesis when a hot joint could be septic or crystal-induced — SpA fluid is inflammatory and sterile.
Management ladder (shared principles):
- NSAIDs + exercise/PT = first-line, especially for axial disease.
- Conventional DMARDs (sulfasalazine, methotrexate) for peripheral disease; ineffective for axial AS.
- Biologics — TNF-α inhibitors or IL-17 inhibitors — for refractory or axial disease.
- Treat the associated condition (Chlamydia, IBD) and screen/refer promptly for uveitis.
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