Summary

Crystal Arthropathies are inflammatory joint diseases caused by the deposition of crystals within the synovial fluid. Gout is caused by Monosodium Urate (MSU) crystals (Needle/Negative) and typically affects the 1st MTPJ (Podagra) in men. It is driven by Hyperuricaemia. Pseudogout (CPPD) is caused by Calcium Pyrophosphate Dihydrate crystals (Rhomboid/Positive) and affects large joints (Knee/Wrist) in the elderly. Both present with acute, hot, swollen joints mimicking septic arthritis. Diagnosis requires Polarised Light Microscopy of synovial fluid. Management involves acute suppression (Colchicine/NSAIDs) and long-term prevention (Allopurinol for Gout). [1,2]

Key Facts

• Gout Crystal: Monosodium Urate (MSU).
• Gout Shape: Needle shaped.
• Gout Birefringence: Strongly Negative (Yellow when parallel).
• Pseudogout Crystal: Calcium Pyrophosphate (CPPD).
• Pseudogout Shape: Rhomboid (Brick) shaped.
• Pseudogout Birefringence: Weakly Positive (Blue when parallel).
• Commonest Joint:
  • Gout: 1st MTPJ (70%).
  • Pseudogout: Knee / Wrist.

Clinical Pearls

"Septic until Proven Otherwise": A hot, red, swollen joint is Septic Arthritis. You cannot distinguish Gout from Sepsis clinically (both have fever/raised CRP). You MUST aspirate. If there are crystals, it's Gout. If there is bacteria, it's Septic. (Rarely, you can have both!).

"Don't start Allopurinol in an Attack": Starting Allopurinol acutely causes fluctuations in urate levels, which can precipitate a fresh flare or prolong the current one. Wait 2-3 weeks after the inflammation settles. (However, if they are already on it, do NOT stop it).

"The Bed Sheet Sign": Gout pain is surprisingly exquisite. Patients cannot even tolerate the weight of a bedsheet touching the toe.

"The 5 Hs of Pseudogout": If a youngish person (<50) gets Pseudogout, look for a metabolic cause:

1. Hyperparathyroidism.
2. Haemochromatosis.
3. Hypomagnesaemia.
4. Hypophosphatemia.
5. Hypothyroidism.

        HOT SWOLLEN JOINT
                ↓
    ASPIRATE (Rule out Sepsis)
                ↓
    CONFIRMED CRYSTAL ARTHRITIS
                ↓
           ACUTE PAIN
    ┌───────────┼───────────┐
  NSAIDs    COLCHICINE   STEROID
(Naproxen)    (Oral)   (Intra-articular/Oral)
    ↓           ↓           ↓
  First Line   If NSAID    If both
              contraind.  failed

        RECURRENT ATTACKS (>2/year)
        OR TOPHI / EROSIONS
                ↓
      URATE LOWERING THERAPY (ULT)
                ↓
        ALLOPURINOL 100mg od
    (Cover with Colchicine/NSAID for 6m)
                ↓
        TITRATE EVERY 4 WEEKS
      Target Urate < 300 umol/L
                ↓
        NOT AT TARGET?
                ↓
    INCREASE DOSE (Max 900mg)
      OR SWAP TO FEBUXOSTAT

1. Acute Flare

• NSAIDs: High dose Naproxen/Indomethacin. Need PPI cover.
• Colchicine: 500mcg bd/tds. Side effect: Diarrhoea (dose limiting).
• Corticosteroids: Prednisolone 30mg for 5 days. Or Intra-articular depot (if sepsis excluded).

2. Chronic Prevention (Gout only)

• Allopurinol:
  • Mechanism: Xanthine Oxidase Inhibitor.
  • Rules: "Start Low, Go Slow". Titrate to target.
• Febuxostat:
  • Mechanism: Non-purine Xanthine Oxidase Inhibitor.
  • Use: If Allopurinol intolerant or renal failure.

3. Surgical

• Tophi Removal: For ulcerated or massive tophi causing mechanical block. The material is "toothpaste-like".
• Resection Arthroplasty: For destroyed joints.

• Principle: Some crystals refract light into two rays (Birefringence).
• Compensator: A Red filter is added.
• Negative Birefringence (Gout):
  • When the needle is Parallel to the filter = Yellow. ("Parallel-Low").
  • When Perpendicular = Blue.
• Positive Birefringence (CPPD):
  • When the rhomboid is Parallel to the filter = Blue. ("Parallel-Blue").
  • When Perpendicular = Yellow.

Key Studies

1. FACT Study: Febuxostat vs Allopurinol. Febuxostat lowered urate faster but had cardiovascular safety signals.
2. CACTUS Study: Canakinumab (IL-1 blocker) effective for acute gout (very expensive).

What is Gout?

Gout is a condition where your body has too much Uric Acid (a waste product). Usually, you pee it out. If levels get too high, tiny sharp crystals form in your joints, like grit in a bearing.

Why does it hurt so much?

Your immune system sees the crystals as invaders and attacks them violently. This causes massive inflammation, redness, and pain.

What should I eat?

Avoid "Rich" foods:

• Beer (worse than wine/spirits).
• Shellfish / Oily fish.
• Red meat / Offal (Liver/Kidney).
• Drink plenty of water to flush the kidneys.

Do I need the daily tablet?

If you have more than 2 attacks a year, or if you have lumps (tophi) forming, YES. Allopurinol dissolves the crystals over time, but you have to take it for life.