Crystal Arthropathies

Crystal Clear Overview - Sparkly Invaders

Crystal arthropathies: Group of inflammatory joint diseases. Caused by deposition of microscopic crystals in/around joints, leading to acute or chronic inflammation.

• Key culprits:
  • Monosodium Urate (MSU): Gout. Needle-shaped, negatively birefringent. (📌 Negative Needles)
  • Calcium Pyrophosphate Dihydrate (CPPD): Pseudogout. Rhomboid/rod-shaped, weakly positive birefringence. (📌 Pseudogout = Positive)
  • Basic Calcium Phosphate (BCP): Apatite deposition disease. Non-birefringent, small, pleomorphic.

⭐ Endogenous crystals like MSU and CPPD trigger inflammation primarily by activating the NLRP3 inflammasome complex, leading to IL-1β release.

Gout - The King's Complaint

• Metabolic disease: Monosodium Urate (MSU) crystal deposition from chronic hyperuricemia (serum urate >6.8 mg/dL or >400 µmol/L).
• 📌 GOUT: Great toe, One joint, Uric acid, Tophi.
• Features:
  • Acute: Sudden, severe monoarthritis (podagra - 1st MTP).
  • Chronic: Tophi, erosions, nephropathy (urolithiasis, urate nephropathy).
• Diagnosis:
  • Arthrocentesis: Needle-shaped, negatively birefringent MSU crystals (gold standard).
  • Serum Urate: Often ↑, but can be normal/low in acute flare.
• Management:
  • Acute Attack: NSAIDs, colchicine (low-dose), or corticosteroids (oral/IA/IM). Initiate within 24h.
  • Chronic (ULT): Indications: tophi, ≥2 attacks/yr, CKD stage ≥2, urolithiasis.
    • Goal: Serum urate <6 mg/dL (<360 µmol/L); <5 mg/dL (<300 µmol/L) if tophi/severe disease.
    • Agents: Allopurinol (start 100mg/day, lower in CKD), febuxostat.
    • Anti-inflammatory prophylaxis (colchicine/NSAID) for 3-6 months when starting ULT.

⭐ The HLA-B*5801 allele is a strong predictor of severe cutaneous adverse reactions (SCARs) to allopurinol, particularly in Han Chinese, Thai, and Korean populations.

Pseudogout (CPPD) - The Great Imitator

• Calcium Pyrophosphate Dihydrate (CPPD) crystal deposition in/around joints.
• Crystals: Rhomboid/rod-shaped; weakly positively birefringent under polarized light.
  • 
• X-ray Hallmark: Chondrocalcinosis (cartilage calcification). Common: knee, wrist, symphysis pubis.
  • 
• Clinical: "The Great Imitator"
  • Acute pseudogout: sudden, severe monoarthritis (often knee).
  • Chronic CPP crystal arthritis: polyarticular, mimics RA/OA.
• Diagnosis: Synovial fluid analysis: definitive, shows CPPD crystals.
• Management:
  • Acute flares: NSAIDs, colchicine, corticosteroids (intra-articular/systemic).
  • Chronic: Symptomatic relief; treat associated conditions.

⭐ CPPD deposition disease is frequently associated with underlying metabolic conditions: hyperparathyroidism, hemochromatosis, hypophosphatasia, and hypomagnesemia (📌 Triple H M).

Crystal Comparisons - Spot the Sparkle

• Definitive Diagnosis: Synovial fluid analysis for crystal identification.
• Acute Attack Management: NSAIDs, colchicine, corticosteroids. Prophylaxis aims to ↓ crystal load.

Diagnostic Algorithm: Acute Monoarthritis

High‑Yield Points - ⚡ Biggest Takeaways

• Gout: MSU crystals (needle, negatively birefringent). Classic: Podagra (1st MTP).
• Pseudogout (CPPD): CPPD crystals (rhomboid, positively birefringent). Affects knee, wrist. Chondrocalcinosis on X-ray.
• Acute Gout Attack: Treat with NSAIDs, colchicine, or corticosteroids.
• Chronic Gout: Manage with allopurinol or febuxostat (target urate <6 mg/dL).
• Tophi: Pathognomonic for chronic gout; MSU crystal deposits.
• Colchicine: For acute attacks & prophylaxis; inhibits microtubule function.