Crystal arthropathies

Quick Overview

Crystal arthropathies-primarily gout and pseudogout-are common causes of acute monoarthritis in UK practice. Accurate diagnosis relies on synovial fluid analysis demonstrating characteristic crystals. Management follows NICE CG177: acute flares with NSAIDs/colchicine, and urate-lowering therapy (ULT) for recurrent gout. Distinguishing features and treatment thresholds are high-yield for clinical practice.

Core Facts & Concepts

Gout vs Pseudogout Key Features:

Diagnostic Criteria:

• Gold standard: Synovial fluid aspiration showing crystals (>80% sensitivity)
• Synovial fluid WCC: Typically 2,000-50,000 cells/mm³ (inflammatory but not infected)
• Serum urate: Target <300 µmol/L with ULT (NICE CG177)

Precipitants:

• Gout: Alcohol, red meat, seafood, diuretics, dehydration, surgery
• Pseudogout: Acute illness, surgery, hypocalcaemia, hyperparathyroidism

Problem-Solving Approach

Acute Monoarthritis Management (NICE CG177):

1. Aspirate joint before treatment if diagnosis uncertain (unless septic arthritis suspected-then aspirate + immediate antibiotics)
2. Exclude septic arthritis: Gram stain, culture (can coexist with crystals)
3. Acute treatment (start within 24 hours of flare):
   • 1st line: NSAIDs (e.g., naproxen 750mg then 250mg TDS) + PPI
   • 2nd line: Colchicine 500mcg BD-QDS (lower dose if eGFR <50; avoid if eGFR <10)
   • 3rd line: Oral/IM corticosteroids (prednisolone 30mg OD for 5 days)
4. Do NOT start/stop ULT during flare (precipitates prolonged attack)

🚩 Red Flags:

• Fever + joint pain = septic arthritis until proven otherwise
• Polyarticular gout = check for renal impairment, diuretic use

Urate-Lowering Therapy (ULT) Indications:

• ≥2 attacks in 12 months
• Tophi present
• Renal impairment (eGFR <60)
• Uric acid renal stones
• Prophylactic diuretic therapy

ULT Protocol:

• Start allopurinol 100mg OD (50mg if eGFR <60) 2-4 weeks post-flare
• Titrate by 100mg every 4 weeks to target urate <300 µmol/L
• Co-prescribe colchicine 500mcg BD or NSAID for 6 months (flare prophylaxis)

Analysis Framework

Differentiating Acute Monoarthritis:

Quick Decision Rule:

• Synovial fluid WCC >50,000 or fever >38.5°C → treat as septic arthritis (IV flucloxacillin 2g QDS)
• Crystals + WCC <50,000 → crystal arthropathy (but send culture-can coexist)

Visual Aid

Key Points Summary

✓ Synovial fluid analysis is gold standard: MSU crystals (negative birefringence, needle) = gout; CPPD (positive birefringence, rhomboid) = pseudogout

✓ Acute management (NICE CG177): NSAIDs 1st line, colchicine 2nd, steroids 3rd-start within 24 hours; never start/stop ULT during flare

✓ ULT indications: ≥2 attacks/year, tophi, renal impairment, stones-start allopurinol 100mg OD (50mg if eGFR <60) 2-4 weeks post-flare with 6-month colchicine prophylaxis

✓ Target serum urate <300 µmol/L (not <360 µmol/L)-titrate allopurinol by 100mg every 4 weeks

✓ Exclude septic arthritis: Can coexist with crystals; synovial WCC >50,000 or fever >38.5°C warrants immediate antibiotics

✓ Pseudogout triggers: Acute illness, surgery, metabolic disturbance (check calcium, PTH, thyroid)-no ULT indicated

✓ Common pitfall: Serum urate can be normal during acute gout flare (don't rule out diagnosis)