Epidemiology & Risk Factors

PAIR: Psoriatic, Ankylosing spondylitis, IBD-associated, Reactive arthritis. c
Strong HLA-B27 link (AS >90%, ReA 50–80%).
Demographics: Typical onset 20–40 yo; male predominance (marked in AS/ReA).
Triggers (ReA): Preceding chlamydial urethritis (C. trachomatis) or enteric infection (Campylobacter, Salmonella, Shigella, Yersinia).

Clinical Features

Inflammatory Back Pain: Age <40, insidious, worse with rest, improves with exercise, morning stiffness >30 min.
Enthesitis: Heel pain at Achilles tendon or plantar fascia insertion.
Dactylitis: Diffuse tenosynovitis (“sausage digits”).
Peripheral Arthritis: Asymmetric, oligoarticular, predominantly lower extremities (PsA involves DIPs).
Extra-articular:
- Ocular: Acute anterior uveitis (pain, photophobia, ciliary flush).
- Cardiac: Aortic regurgitation, AV conduction block.
- Pulmonary: Restrictive lung pattern (costovertebral ankylosis).
- Subtype specifics:
  - ReA: Keratoderma blennorrhagica, circinate balanitis, sterile urethritis, conjunctivitis.
  - PsA: Psoriatic plaques, nail pitting, onycholysis.

Initial Test: Pelvic X-ray (detects bilateral sacroiliitis).
Most Sensitive (Early): MRI SI joints (detects active bone marrow edema).
Key Labs:
- Serology: Negative RF and negative anti-CCP (defines “seronegative”).
- Inflammatory markers: ↑ ESR and ↑ CRP (correlate with active disease).
- HLA-B27: Supportive in ambiguous cases; not sufficient alone for diagnosis (present in ~8% of healthy general population).
- Arthrocentesis (if peripheral effusion present): Inflammatory synovial fluid (WBC 2,000–50,000/mm³ with >50% PMNs, negative Gram stain/culture, no crystals).
Key Imaging Buzzwords:
- AS: “Bamboo spine”, marginal syndesmophytes, vertebral squaring.
- PsA: “Pencil-in-cup” deformity on hand/foot X-ray.
Bedside Test (AS): Modified Schober test (<5 cm increase with lumbar flexion).

Rheumatoid Arthritis: Diff by (+) RF/anti-CCP, symmetric polyarthritis, MCP/PIP involvement (spares DIPs), no sacroiliitis.
Osteoarthritis: Diff by non-inflammatory pattern (worse with use, improves with rest), brief morning stiffness (<30 min), normal ESR/CRP.
Septic Arthritis: Diff by acute fever, monoarticular joint swelling, synovial fluid WBC >50,000/mm³ with (+) Gram stain/culture.
Vertebral Osteomyelitis: Diff by focal vertebral percussion tenderness, unremitting rest/night pain, fever, elevated ESR/CRP.

1st-line: High-dose NSAIDs (indomethacin, naproxen) + physical therapy.
2nd-line (Axial Disease): TNF-α inhibitors (adalimumab, infliximab) or IL-17 inhibitors.
- High-Yield: Methotrexate/Sulfasalazine are ineffective for axial spine disease.
2nd-line (Peripheral Disease): csDMARDs (Methotrexate, Sulfasalazine).
Infection-specific: Doxycycline for ReA only if active C. trachomatis infection is ongoing.

Vertebral Fractures (“Chalk stick fracture”): Minor trauma causes severe spinal fracture with high risk of spinal cord injury; evaluate immediately with CT or MRI.
Restrictive lung disease (chest wall rigidity).
Vision loss (untreated anterior uveitis).
Aortic root dilation / aortic regurgitation.
Secondary (AA) amyloidosis (nephrotic-range proteinuria).

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