6.3 Gout: Pathophysiology & Acute Management
Key Takeaways
- Gout, pseudogout and hydroxyapatite deposition disease are the three enumerated crystal arthropathy topics.
- Monosodium urate crystals are needle-shaped and negatively birefringent, appearing yellow when aligned parallel to the polarizer axis.
- Serum urate is frequently normal or low during an acute flare, so a normal value does not exclude gout.
- Septic arthritis can coexist with crystal disease, so synovial fluid must also be sent for Gram stain and culture.
- Colchicine, non-steroidal anti-inflammatory drugs and corticosteroids are all effective for acute flares, and the choice is driven by comorbidity.
Last updated: August 2026
Crystal arthropathies and osteoarthritis represent the most common causes of acute monoarthritis and chronic joint disability encountered in internal medicine. Accurate differentiation through synovial fluid analysis, recognition of secondary underlying metabolic and endocrine disorders, and evidence-based pharmacologic management are core ABIM exam competencies.
1. Gout
Pathophysiology & Triggers
Gout is an autoinflammatory crystal deposition disease resulting from prolonged hyperuricemia, defined as a serum urate concentration exceeding the physiological solubility limit (>6.8 mg/dL [404 µmol/L] at 37°C). Uric acid is the end product of purine catabolism, catalyzed by xanthine oxidase.
- Hyperuricemia Mechanisms:
- Underexcretion of Urate (~90% of patients): Impaired renal tubular clearance via URAT1 / GLUT9 transporters, worsened by chronic kidney disease, volume depletion, hypertension, metabolic syndrome, and medications (thiazide and loop diuretics, low-dose aspirin, cyclosporine, tacrolimus, pyrazinamide, ethambutol).
- Overproduction of Urate (~10% of patients): High-purine diet, excessive alcohol intake, inherited enzyme defects (HPRT deficiency / Lesch-Nyhan syndrome, PRPP synthetase overactivity), myeloproliferative/lymphoproliferative disorders, psoriasis, and tumor lysis syndrome.
- NLRP3 Inflammasome Activation: Precipitation of Monosodium Urate (MSU) crystals in the joint space triggers toll-like receptor (TLR2/TLR4) recognition and phagocytosis by synovial macrophages. This activates the NLRP3 inflammasome complex, driving caspase-1 cleavage of pro-IL-1β into active Interleukin-1β (IL-1β). IL-1β initiates massive endothelial activation and neutrophil recruitment, producing intense, excruciating synovitis.
- Acute Flare Triggers: Rapid fluctuations in serum urate levels (either sudden increases OR sudden drops), alcohol ingestion (especially beer due to high purine/guanosine content and liquor; alcohol metabolism generates lactate, which competes with urate for renal excretion), purine-heavy meals (red meat, seafood, organ meats), high-fructose corn syrup beverages, surgical stress, systemic infection, trauma, dehydration, and abrupt initiation or cessation of urate-lowering therapy.
Arthrocentesis & Synovial Fluid Polarized Light Microscopy
- Gold Standard: Polarized light microscopy of aspirated synovial fluid remains the definitive diagnostic test.
- Microscopy Findings: Needle-shaped, strongly negatively birefringent crystals:
- When the crystal's long axis is parallel to the red compensator filter, the crystal appears yellow.
- When the crystal's long axis is perpendicular to the compensator, the crystal appears blue.
- Synovial Fluid Characteristics: Inflammatory fluid with cloudy/turbid appearance, WBC count typically 2,000 to 50,000/µL (can exceed 100,000/µL in severe acute flares), with >75–90% polymorphonuclear neutrophils (PMNs).
- Board Rule: Always perform Gram stain and bacterial culture on every arthrocentesis aspirate; crystal confirmation does NOT rule out concurrent septic arthritis!
- Serum Urate during Acute Flare: Serum uric acid levels may be falsely normal or low in up to 30–40% of patients during an acute gout flare due to inflammatory IL-6 mediated renal uricosuria. A normal serum uric acid level during an acute attack NEVER excludes the diagnosis of gout.
Chronic Tophaceous Gout & Imaging
- Tophi: Nodular, chalky aggregates of MSU crystals surrounded by granulomatous chronic foreign-body giant-cell inflammation. Common locations: helix and antihelix of the ear, olecranon bursa, prepatellar bursa, Achilles tendon, and distal interphalangeal joints.
- Radiographic Findings: Late manifestation showing punched-out, well-circumscribed periarticular bony erosions with overhanging sclerotic margins ("rat-bite" or "martini-glass" erosions), with relative preservation of the joint space until end-stage disease.
- Dual-Energy CT (DECT): High-sensitivity non-invasive imaging modality that color-codes uric acid crystal deposits (green) in tendons and soft tissues.
Management of Acute Gout Flares
Pharmacotherapy should be initiated within 24 hours of flare onset. Continue therapy until the flare is completely resolved (typically 5–10 days).
| Acute Drug Class | Dosing Regimen | High-Yield Clinical Indications & Cautions |
|---|---|---|
| Oral Colchicine | 1.2 mg PO at symptom onset, followed by 0.6 mg 1 hour later (total 1.8 mg on day 1), then 0.6 mg daily or BID starting 12 hours later until flare resolution. | Mechanism: Binds tubulin, inhibiting microtubule polymerization and neutrophil chemotaxis/inflammasome activation.<br/>Renal Adjustment: Reduce dose if eGFR <30 mL/min.<br/>Contraindications: Severe renal/hepatic impairment; co-administration with strong CYP3A4 inhibitors (clarithromycin, ketoconazole) or P-glycoprotein inhibitors (cyclosporine, verapamil) due to fatal colchicine toxicity (rhabdomyolysis, pancytopenia, neuromyopathy). |
| NSAIDs | Indomethacin 50 mg PO TID, Naproxen 500 mg PO BID, or Celecoxib 400 mg initial then 200 mg BID for 5–7 days. | First-line in young, healthy patients without comorbidities.<br/>Contraindications: Chronic kidney disease (CKD stage >=3), active peptic ulcer disease / GI bleeding, decompensated heart failure, cirrhosis, or severe hypertension. |
| Systemic Glucocorticoids | Prednisone 30–40 mg PO daily for 3–5 days, then tapered over 7–10 days; or IV/IM Methylprednisolone. | Drug of choice in patients with renal impairment (CKD), heart failure, anticoagulation, or elderly patients where NSAIDs and colchicine are contraindicated. |
| Intra-Articular Glucocorticoids | Triamcinolone acetonide 20–40 mg injected directly into the joint. | Ideal for acute monoarticular flares involving an easily accessible large joint (knee, shoulder, ankle) once infection is excluded. |
| IL-1 Inhibitors | Anakinra (100 mg SC daily x 3 days) or Canakinumab. | Off-label rescue therapy for refractory, frequent flares in patients with severe contraindications to NSAIDs, colchicine, and steroids. |
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