Gout Study Guide

📖 Core Concepts Gout – inflammatory arthritis caused by deposition of needle‑shaped monosodium urate (MSU) crystals in joints/tissues. Hyperuricemia – serum urate > 420 µmol/L (7 mg/dL) in men, > 360 µmol/L (6 mg/dL) in women. Uric acid metabolism – end product of purine catabolism; humans lack uricase, so uric acid is not further degraded. Crystal formation – when urate concentration exceeds its solubility, MSU crystals precipitate (supersaturation). Inflammatory cascade – MSU crystals → NLRP3 inflammasome activation → caspase‑1 → IL‑1β → acute neutrophilic inflammation. Podagra – classic first‑metatarsophalangeal (MTP) joint attack; accounts for ½ of gout flares. Tophi – chronic, painless aggregates of MSU crystals in soft tissue, often near joints, ear helices, or olecranon. Underexcretion vs. overproduction – ≈ 90 % of hyperuricemia is renal underexcretion; < 10 % is excess production. --- 📌 Must Remember Serum urate thresholds – > 420 µmol/L (men) / > 360 µmol/L (women) = hyperuricemia. Target for chronic therapy – < 300–360 µmol/L (5.0–6.0 mg/dL); < 300 µmol/L (5 mg/dL) if tophi present. Indications to start urate‑lowering therapy (ULT) – ≥ 2 attacks/yr, tophi, chronic urate nephropathy, or erosive changes on X‑ray. First‑line acute meds – NSAIDs (e.g., indomethacin), low‑dose colchicine, or glucocorticoids. First‑line ULT – allopurinol; febuxostat if allopurinol intolerant; probenecid for pure underexcretion. HLA‑B58:01 – strong predictor of allopurinol hypersensitivity; screen high‑risk ethnic groups. Crystal identification – needle‑shaped MSU crystals with strong negative birefringence under polarized light. 90 % renal underexcretion → prioritize uricosuric agents when renal function allows. --- 🔄 Key Processes Crystal‑Induced Inflammation ↑ Serum urate → supersaturation → MSU crystal precipitation → phagocytosis by macrophages → NLRP3 inflammasome → caspase‑1 → IL‑1β → neutrophil influx → acute gout attack. Acute Flare Management (step‑wise) Assess comorbidities → ✅ No contraindications → NSAID (indomethacin) ± PPI. ❌ NSAID contraindicated → low‑dose colchicine (≤ 1 mg loading, then 0.5 mg q6‑12 h). ❌ Colchicine contraindicated → oral or intra‑articular glucocorticoid. Chronic Urate‑Lowering Initiation Confirm indication → start allopurinol 100 mg daily; titrate every 2–4 weeks to target SUA, adjusting for renal function. If allopurinol not tolerated → switch to febuxostat (40–80 mg daily). Add uricosuric (probenecid 250 mg bid) only if renal underexcretion and eGFR ≥ 30 mL/min/1.73 m². Monitoring Schedule 2–4 weeks after dose change: check SUA, renal & hepatic labs. Every 3–6 months thereafter: SUA, renal function, tophi assessment, side‑effects. --- 🔍 Key Comparisons NSAIDs vs. Colchicine vs. Steroids NSAIDs: rapid pain relief; GI & renal risks. Colchicine: effective at low dose; GI toxicity & drug interactions (CYP3A4/P‑gp inhibitors). Steroids: useful when NSAIDs/colchicine contraindicated; must exclude infection. Allopurinol vs. Febuxostat Allopurinol: xanthine oxidase inhibitor; dose limited by renal function; HLA‑B58:01 risk. Febuxostat: non‑purine XO inhibitor; no renal dose‑adjustment; boxed warning for CV mortality. Uricosuric (Probenecid) vs. Xanthine Oxidase Inhibitors Uricosuric: ↑ renal excretion; ineffective if eGFR < 30 mL/min. XO inhibitors: ↓ production; work regardless of renal excretion capacity. Gout vs. Pseudogout Gout: MSU crystals, negative birefringence, needle shape, often podagra. Pseudogout: calcium pyrophosphate crystals, positive birefringence, rhomboid shape, knees common. --- ⚠️ Common Misunderstandings “Uric acid must be high during an attack.” – Serum urate can be normal during an acute flare; diagnosis relies on crystal identification. “Only the big toe can have gout.” – Other joints (ankle, knee, wrist, hand) are frequent, especially with chronic disease. “Allopurinol prevents attacks immediately.” – It lowers SUA; flares may worsen initially (urate‑lowering paradox). “Diet alone cures gout.” – Lifestyle changes are essential but must be combined with pharmacotherapy for most patients. --- 🧠 Mental Models / Intuition Supersaturation = “Sugar‑in‑tea” – Just as sugar stops dissolving when the tea is saturated, urate stops dissolving once its solubility limit is exceeded → crystals form. “Danger‑signal” model – MSU crystals act like a fire alarm (NLRP3 inflammasome) that summons neutrophils; removing the alarm (crystals) stops the fire. Treat‑to‑Target = “Blood pressure goal” – Aim for a specific SUA number just as you aim for a BP < 130/80 mmHg; adjust meds until goal is reached. --- 🚩 Exceptions & Edge Cases Refractory gout – Consider pegloticase (IV recombinant uricase) after failure of XO inhibitors + uricosurics. Renal underexcretion only – Probenecid or lesinurad (with XO inhibitor) preferred; avoid high‑dose allopurinol if eGFR < 30. Allopurinol hypersensitivity – Screen HLA‑B58:01 in Asian, African‑American, or Hispanic patients before initiation. Pregnancy & lactation – Colchicine and NSAIDs (except low‑dose aspirin) are generally avoided; corticosteroids are safer. --- 📍 When to Use Which | Clinical Situation | Preferred Acute Therapy | Preferred Chronic Therapy | |--------------------|--------------------------|---------------------------| | No comorbidities | NSAID (indomethacin) + PPI | Allopurinol, titrate to SUA < 300 µmol/L | | CKD, NSAID‑contraindicated | Low‑dose colchicine (≤ 1 mg) | Allopurinol (dose reduced for eGFR) or febuxostat | | Cardiovascular disease, allopurinol‑intolerant | Colchicine or steroids | Febuxostat (monitor CV risk) | | Documented renal underexcretion, eGFR ≥ 30 | NSAID/colchicine as tolerated | Probenecid ± allopurinol; add lesinurad if needed | | Refractory to XO inhibitors & uricosurics | Steroids or IL‑1 inhibitor (anakinra) | Pegloticase (IV) with pre‑treatment antihistamine/ prednisone | --- 👀 Patterns to Recognize Podagra at night – sudden, excruciating pain in first MTP joint, maximal within 24 h. Negative birefringence – under polarized light, MSU crystals appear yellow when aligned with the slow axis. Tophaceous deposits – firm, non‑tender nodules over helix, olecranon, or Achilles tendon. Uric acid stones – radiolucent on X‑ray, often associated with low urinary pH (< 5.5). Flare triggers – alcohol, high‑purine meals, dehydration, rapid urate changes, diuretics. --- 🗂️ Exam Traps “Serum uric acid is always > 7 mg/dL in gout.” – May be normal during an acute attack; crystal analysis is decisive. “Positive birefringence means gout.” – Positive birefringence indicates calcium pyrophosphate (pseudogout). “Allopurinol is contraindicated in CKD.” – It is used with dose adjustment, not contraindicated. “Target SUA is 6 mg/dL for everyone.” – Goal is < 5 mg/dL when tophi are present or for aggressive control. “Colchicine should always be given at 1.2 mg loading then 0.6 mg every hour.” – Modern practice favors low‑dose regimens to avoid GI toxicity. ---