Monosodium Urate | Negatively Birefringent | Needle-Shaped Crystals
STAGES OF GOUT
Critical Must-Knows
- MSU crystals are needle-shaped and negatively birefringent (yellow parallel to polarizer)
- Podagra (1st MTP involvement) is the classic presentation
- Joint aspiration is gold standard for diagnosis - even during acute attack
- Urate-lowering therapy target is serum urate less than 6 mg/dL (360 micromol/L)
- Surgical indications: mechanical symptoms, ulceration, infection, nerve compression
Clinical Pearls
- "Negatively birefringent = yellow when parallel to polarizer axis
- "Acute attack: do NOT start allopurinol - may prolong attack
- "Dual-energy CT can identify urate deposits non-invasively
- "Tophi surgery: avoid primary closure over large defects
Critical Gout Exam Points
Crystal Identification
MSU crystals: Needle-shaped, negatively birefringent (yellow parallel, blue perpendicular). CPPD crystals: Rhomboid, positively birefringent (blue parallel, yellow perpendicular). This distinction is fundamental to diagnosis.
Acute Attack Management
Do NOT start urate-lowering therapy during acute attack - may prolong symptoms. Treat with NSAIDs, colchicine (within 12 hours), or corticosteroids. Continue existing ULT if already established.
Surgical Indications
Orthopaedic involvement for: mechanical symptoms from tophi, skin ulceration over tophi, secondary infection, nerve compression (carpal tunnel), joint destruction requiring arthroplasty.
Imaging Features
Radiographic signs: Punched-out erosions with overhanging edges ("rat bite"), preserved joint space until late, soft tissue tophi with calcification. Dual-energy CT shows urate deposits as green.
Crystal Arthropathy Comparison
| Feature | Gout (MSU) | Pseudogout (CPPD) |
|---|---|---|
| Crystal shape | Needle-shaped | Rhomboid/rod-shaped |
| Birefringence | Negative (yellow parallel) | Positive (blue parallel) |
| Classic joint | 1st MTP (podagra) | Knee, wrist |
| Radiographic sign | Punched-out erosions | Chondrocalcinosis |
| Risk factors | Purine-rich diet, alcohol, obesity | Age, OA, metabolic disease |
| Urate-lowering therapy | Yes - allopurinol, febuxostat | No specific therapy |
NEEDLEGout Crystal Features
| N | Needle-shaped crystals Long, thin morphology |
| E | Erosions with overhanging edges Rat bite appearance on X-ray |
| E | Elevated uric acid Serum urate greater than 6.8 mg/dL |
| D | Definitely negatively birefringent Yellow when parallel to polarizer |
| L | Locations: 1st MTP classic Podagra is pathognomonic |
| E | Early morning attacks common Nocturnal crystal precipitation |
| N | Needle-shaped crystals Long, thin morphology | E | Elevated uric acid Serum urate greater than 6.8 mg/dL | L | Locations: 1st MTP classic Podagra is pathognomonic |
| E | Erosions with overhanging edges Rat bite appearance on X-ray | D | Definitely negatively birefringent Yellow when parallel to polarizer | E | Early morning attacks common Nocturnal crystal precipitation |
Hook:NEEDLE crystals cause NEEDLE-sharp pain in gout!
COINSAcute Gout Treatment
| C | Colchicine Within 12 hours of onset most effective |
| O | Oral NSAIDs Indomethacin, naproxen - first line |
| I | Intra-articular steroids If monoarticular and NSAIDs contraindicated |
| N | No allopurinol initiation Do not start ULT during acute attack |
| S | Systemic steroids If polyarticular or contraindications |
| C | Colchicine Within 12 hours of onset most effective | N | No allopurinol initiation Do not start ULT during acute attack |
| O | Oral NSAIDs Indomethacin, naproxen - first line | S | Systemic steroids If polyarticular or contraindications |
| I | Intra-articular steroids If monoarticular and NSAIDs contraindicated |
Hook:Spend your COINS wisely on acute gout treatment!
MUSICSurgical Indications for Tophi
| M | Mechanical symptoms Limited ROM, tendon dysfunction |
| U | Ulceration of overlying skin Risk of secondary infection |
| S | Secondary infection Requires debridement |
| I | Impingement on nerves Carpal tunnel, ulnar neuropathy |
| C | Cosmetic concerns Large visible tophi |
| M | Mechanical symptoms Limited ROM, tendon dysfunction | I | Impingement on nerves Carpal tunnel, ulnar neuropathy |
| U | Ulceration of overlying skin Risk of secondary infection | C | Cosmetic concerns Large visible tophi |
| S | Secondary infection Requires debridement |
Hook:When tophi cause problems, it's time to make MUSIC with surgery!
Overview and Epidemiology
Gout is the most common inflammatory arthritis in adults, caused by deposition of monosodium urate (MSU) crystals in joints and soft tissues. It results from prolonged hyperuricemia leading to crystal formation when serum urate exceeds its saturation point of 6.8 mg/dL (404 micromol/L).
Epidemiology:
- Prevalence: 1-4% of adults in developed countries
- Male to female ratio: 4:1 (equalizes after menopause)
- Peak incidence: Males 40-50 years, females post-menopause
- Increasing prevalence due to obesity, metabolic syndrome, aging population
Risk Factors:
- Dietary: Purine-rich foods (red meat, seafood), alcohol (especially beer), fructose-sweetened beverages
- Medications: Thiazide diuretics, low-dose aspirin, cyclosporine
- Comorbidities: Chronic kidney disease, metabolic syndrome, hypertension, obesity
- Genetic: Variants in urate transporters (URAT1, GLUT9)
Pathophysiology Pearl
MSU crystals trigger the innate immune system via the NLRP3 inflammasome, leading to IL-1beta release. This explains why IL-1 inhibitors (anakinra, canakinumab) are effective in refractory cases.
Pathophysiology
Understanding the pathophysiology of gout is essential for both diagnosis and management. The disease results from a complex interplay of uric acid metabolism, crystal formation, and inflammatory responses.
Uric Acid Metabolism
Production:
- Uric acid is the end product of purine metabolism in humans
- Purines derived from dietary intake (exogenous) and cellular turnover (endogenous)
- Key enzyme: Xanthine oxidase converts hypoxanthine to xanthine to uric acid
- Humans lack uricase enzyme (present in most mammals) - cannot break down uric acid further
Excretion:
- 70% renal excretion via complex tubular handling
- 30% gastrointestinal excretion
- Key transporters: URAT1 (reabsorption), ABCG2 (secretion), GLUT9
Crystal Formation
Saturation point:
- Monosodium urate (MSU) saturates at 6.8 mg/dL (404 micromol/L)
- Below this level, crystals gradually dissolve
- Above this level, crystals can precipitate in tissues
Factors promoting crystallization:
- Lower temperature (explains predilection for peripheral joints)
- Lower pH (trauma, exercise-induced acidosis)
- Presence of nucleating agents
- Connective tissue matrix components
Inflammatory Response
NLRP3 Inflammasome activation:
- MSU crystals are phagocytosed by macrophages
- Crystals destabilize lysosomal membranes
- Cathepsin B released into cytoplasm
- NLRP3 inflammasome assembly triggered
- Caspase-1 activation
- Pro-IL-1beta cleaved to active IL-1beta
- Massive inflammatory cascade initiated
Inflammasome Mechanism
The NLRP3 inflammasome pathway explains why IL-1 inhibitors (anakinra, canakinumab) are effective in refractory gout. This is increasingly tested in fellowship exams as it bridges basic science with clinical application.
Resolution:
- Acute attacks are self-limiting (7-14 days)
- Aggregated neutrophil extracellular traps (NETs) help resolve inflammation
- Coating of crystals by proteins reduces immunogenicity
- Anti-inflammatory macrophage phenotype emerges
Clinical Presentation
Acute Gouty Arthritis
Classic presentation:
- Rapid onset over 6-12 hours, often waking patient from sleep
- Exquisitely painful - unable to tolerate bedsheet contact
- Monoarticular in 85-90% of initial attacks
- Podagra (1st MTP) is the classic location - 50% of first attacks
- Signs of inflammation: Erythema, warmth, swelling mimicking cellulitis
Common joint involvement:
- First metatarsophalangeal joint (podagra) - 50%
- Ankle and midfoot - 25%
- Knee - 15%
- Wrist, fingers, elbow - 10%
Chronic Tophaceous Gout
Features:
- Tophi: Chalky deposits of MSU crystals in soft tissues
- Common locations: Fingers, olecranon bursa, Achilles tendon, ears
- Joint destruction: Erosive arthropathy with preserved joint space initially
- Tendon involvement: Can cause rupture (Achilles, patellar, extensor tendons)
Physical Examination
Inspection:
- Swelling and erythema over affected joint
- Tophi visible as subcutaneous nodules (white-yellow through skin)
- Skin ulceration over tophi in advanced cases
Palpation:
- Extreme tenderness - even light touch is painful
- Warmth over affected joint
- Tophi are firm, irregular nodules
Investigations
Laboratory Studies
Synovial fluid analysis (Gold Standard):
- MSU crystal identification under polarized microscopy
- Needle-shaped, negatively birefringent crystals
- WBC count: 10,000-70,000/microL (predominantly neutrophils)





Serum uric acid:
- May be normal during acute attack (paradoxical decrease)
- Elevated greater than 6.8 mg/dL (360 micromol/L) supports diagnosis
- Target for ULT: less than 6 mg/dL (360 micromol/L)
Additional labs:
- Renal function (CKD common comorbidity)
- Lipid profile and glucose (metabolic syndrome)
- CBC (elevated WBC during acute attack)
Imaging

Plain Radiographs:
- Early: Soft tissue swelling, normal bone
- Established: Punched-out erosions with overhanging edges ("rat bite" or "mouse ear")
- Preserved joint space (unlike OA)
- Soft tissue tophi may calcify


Ultrasound:
- Double contour sign: Hyperechoic line on hyaline cartilage surface
- Aggregates and tophi visible
- Useful for guided aspiration
Dual-Energy CT (DECT):
- Color-codes urate deposits (typically green)
- Pooled sensitivity approximately 0.87 and specificity 0.84 (Ogdie meta-analysis)
- Lower sensitivity in early/acute gout and small deposits (false negatives in disease under 6 weeks)
- Useful when aspiration not possible; can detect occult tophi
- Beware artefacts (nail-bed, skin, beam hardening) mimicking urate
Management

Acute Attack Management
Do NOT Initiate ULT During Acute Attack
Starting allopurinol or febuxostat during an acute gout flare can prolong the attack by mobilizing urate crystals. Treat the acute attack first, then initiate ULT 2-4 weeks after resolution. However, continue ULT if already established.
First-line options:
- NSAIDs: Indomethacin 50mg TDS, naproxen 500mg BD - continue until attack resolves
- Colchicine: Most effective within 12 hours of onset. Loading dose 1mg, then 0.5mg 1 hour later. Low-dose regimen preferred.
- Corticosteroids: Prednisolone 30-40mg daily for 5-7 days, or intra-articular injection
Refractory cases:
- IL-1 inhibitors (anakinra) for patients with contraindications to all above
- Joint aspiration alone provides significant relief
Urate-Lowering Therapy (ULT)
Indications for ULT:
- Recurrent acute attacks (2 or more per year)
- Presence of tophi
- Radiographic changes
- Chronic kidney disease stage 2 or greater
- Urolithiasis
Agents:
- Allopurinol: First-line xanthine oxidase inhibitor. Start low (50-100mg, lower in CKD), titrate to target. Consider HLA-B*58:01 testing before starting in high-risk ethnicities (Han Chinese, Thai, Korean with CKD) to reduce severe cutaneous adverse reactions (SCAR/SJS-TEN).
- Febuxostat: Alternative if allopurinol intolerant or target not reached. More potent urate lowering; cardiovascular safety debated (see Controversies).
- Probenecid / benzbromarone (uricosurics): Require adequate renal function and urine output; avoid with urolithiasis. Useful for under-excretors when xanthine oxidase inhibitors fail.
- Pegloticase (recombinant uricase): For severe refractory tophaceous gout; rapidly dissolves tophi but immunogenic.
Target:
- Serum urate less than 6 mg/dL (360 micromol/L)
- Less than 5 mg/dL if tophi present for faster dissolution
Surgical Management
Indications for Surgery
- Mechanical symptoms: Large tophi limiting joint motion or tendon function
- Skin ulceration: Over tophi with risk of secondary infection
- Secondary infection: Debridement of infected tophi
- Nerve compression: Carpal tunnel, ulnar neuropathy from tophi
- Joint destruction: Arthroplasty for end-stage arthropathy
- Tendon rupture: Repair or reconstruction
Surgical Principles
Tophus Excision
Preoperative:
- Optimize medical management (ULT established)
- Assess skin viability and plan closure
- Consider staged procedures for large tophi
Technique:
- Incision planned to allow adequate exposure and closure
- Identify and protect neurovascular structures
- Debulk tophaceous material - chalky white deposits
- Curette affected bone if involved
- Assess tendon integrity
- Irrigate thoroughly
Closure considerations:
- Primary closure if possible
- Negative pressure wound therapy for large defects
- Skin grafting or flap coverage may be required
- Avoid tension on skin closure
Postoperative:
- Continue ULT to prevent recurrence
- Wound care and monitoring for healing
- Physiotherapy for ROM
This completes the tophus excision approach.
Complications
Disease Complications
- Chronic erosive arthropathy: Joint destruction, subluxation
- Tendon rupture: Achilles, patellar, extensor tendons
- Carpal tunnel syndrome: From tophaceous deposits
- Renal complications: Uric acid stones, urate nephropathy
- Cardiovascular disease: Independent risk factor
Surgical Complications
- Wound healing problems: Skin necrosis, delayed healing over tophi
- Infection: Risk increased with ulcerated tophi
- Recurrence: If ULT not optimized
- Tendon injury: During tophus excision
Differential Diagnosis
The single most important mimic of acute gout is septic arthritis - the two can coexist, and a hot, swollen joint must be aspirated to exclude infection before attributing it to crystals.
Acute Monoarthritis - Key Differentials
| Feature | Gout (MSU) | Pseudogout (CPPD) | Septic Arthritis | Reactive/Psoriatic |
|---|---|---|---|---|
| Typical joint | 1st MTP, midfoot, knee | Knee, wrist, MCP | Knee, hip; any joint | Knee, ankle, sacroiliac |
| Onset | Hours, often nocturnal | Subacute over days | Hours to days, systemically unwell | Days to weeks |
| Synovial WBC | 10,000-70,000/microL | 10,000-50,000/microL | Often greater than 50,000, up to 100,000+ | 5,000-50,000/microL |
| Crystals | Needle, negatively birefringent | Rhomboid, positively birefringent | None | None |
| Gram stain/culture | Negative | Negative | May be positive (definitive) | Negative |
| Key discriminator | Crystals + podagra | Chondrocalcinosis on X-ray | Fever, raised CRP, positive culture | Enthesitis, skin/eye/GU features |
Never assume crystals exclude infection
Crystals and sepsis can occur in the same joint. If synovial WBC is very high, the patient is febrile, or CRP is markedly raised, treat as septic arthritis until cultures return - send fluid for urgent Gram stain and culture, not just crystal analysis.
Controversies & Areas of Uncertainty
- Febuxostat cardiovascular safety: CARES (29527974) showed higher all-cause and CV mortality with febuxostat versus allopurinol in patients with established CV disease, prompting an FDA boxed warning. The later EMA-mandated FAST trial (33181081) found febuxostat non-inferior with no excess mortality. The discrepancy (high CARES drop-out and on-treatment vs intention-to-treat handling) leaves the true CV risk unresolved; allopurinol remains the pragmatic first-line.
- Asymptomatic hyperuricaemia: routine urate-lowering for asymptomatic hyperuricaemia is not recommended by ACR or EULAR; whether high-risk subgroups (CKD, CV disease) benefit remains debated.
- Start ULT during a flare? ACR 2020 permits initiating allopurinol during an acute flare (with prophylaxis) on the basis it does not prolong the attack; traditional teaching and many other guidelines still defer ULT until the flare settles. Either way, never stop established ULT during a flare.
- Surgery for tophi: timing and technique are not standardised. Wide excision risks unclosable defects; many now favour debulking with optimised ULT to let residual deposits dissolve medically rather than aggressive resection.
- Diet vs drugs: dietary modification lowers urate only modestly (roughly 1 mg/dL); it supports but does not replace ULT, and over-emphasis can stigmatise patients and delay effective treatment.
Evidence Base
AGREE Trial - Low-dose vs High-dose Colchicine
- 50% or greater pain reduction at 24h: 37.8% low-dose vs 32.7% high-dose vs 15.5% placebo
- Diarrhoea in 77% of high-dose vs 23% of low-dose patients
- Most effective when started early (within roughly 12-24 hours of flare onset)
CARES Trial - Febuxostat vs Allopurinol CV Safety
- Primary MACE endpoint non-inferior (HR 1.03, upper 98.5% CI 1.23)
- All-cause death HR 1.22 (95% CI 1.01-1.47); CV death HR 1.34 (95% CI 1.03-1.73)
- About 45% of participants discontinued follow-up, a major limitation
Imaging Modalities for Gout Classification (Meta-analysis)
- DECT pooled sensitivity 0.87 (95% CI 0.79-0.93), specificity 0.84 (95% CI 0.75-0.90)
- Ultrasound double contour sign sensitivity 0.83, specificity 0.76
- Most evidence derived from longstanding established disease (mean duration over 7 years) - sensitivity is lower in early gout
Nurse-led Treat-to-Target Care for Gout (RCT)
- Urate target achieved: 95% nurse-led vs 30% usual care (RR 3.18)
- All secondary outcomes (flares, tophi, quality of life) favoured nurse-led care at 2 years
- Education and shared decision-making drove high ULT uptake and adherence
FAST Trial - Long-term CV Safety of Febuxostat
- Primary CV endpoint HR 0.85 (95% CI 0.70-1.03), non-inferior to allopurinol
- No increase in all-cause death (febuxostat 7.2% vs allopurinol 8.6%)
- Open-label design and high discontinuation are limitations, but lower drop-out than CARES
Global Epidemiology of Gout
- Prevalence under 1% to 6.8%; higher in men, with age, and in some ethnic groups (e.g. Taiwanese aboriginal, Maori, Pacific peoples)
- Only one-third to one-half of patients receive ULT; fewer than half adhere
- Established links to CKD and CV disease, plus newer associations (AF, OSA, VTE)
2020 ACR Guideline for the Management of Gout
- Allopurinol preferred first-line over febuxostat (cost and CV safety)
- Start ULT low (allopurinol 100 mg/day or less; lower in CKD) and titrate to target
- Treat-to-target serum urate below 6 mg/dL with serial monitoring
Exam Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
Scenario 1: Acute Gouty Arthritis
"A 52-year-old obese man presents with sudden onset severe pain and swelling of his right first MTP joint. He woke at 3am unable to tolerate the bedsheet touching his foot. He drinks beer regularly. Examination shows an exquisitely tender, erythematous, swollen 1st MTP joint."
Scenario 2: Chronic Tophaceous Gout
"A 65-year-old man with a 15-year history of gout presents with a large tophus over his left olecranon causing skin breakdown. He has limited elbow flexion. Serum urate is 9.2 mg/dL despite allopurinol 300mg daily."
Scenario 3: Crystal Identification
"You are shown a polarized microscopy image of synovial fluid showing needle-shaped crystals that appear yellow when aligned parallel to the polarizer. What is your diagnosis?"
Guidelines, Registries & Global Practice
Global Epidemiology
- Prevalence: ranges from under 1% to 6.8% of adults depending on population (Dehlin et al., Nat Rev Rheumatol 2020). Highest in Taiwanese aboriginal, Maori and Pacific Island populations (often greater than 6-10%), where reduced renal urate excretion and genetic urate-transporter variants combine with diet.
- Incidence: 0.58-2.89 per 1000 person-years; rising worldwide with obesity, ageing and metabolic syndrome.
- Sex/age: male predominance roughly 3-4:1, narrowing after menopause; peaks in men aged 40-60.
- Care gap: only one-third to one-half of patients ever receive urate-lowering therapy, and fewer than half adhere - the dominant problem in gout is implementation, not drug efficacy.
Side-by-side Guideline Comparison
Major Gout Guidelines
| Issue | ACR 2020 (US) | EULAR 2016 (Europe) | BSR 2017 (UK) |
|---|---|---|---|
| First-line ULT | Allopurinol (incl. CKD) | Allopurinol; adjust dose to renal function | Allopurinol first-line |
| Urate target | Below 6 mg/dL (360 micromol/L), treat-to-target | Below 6 mg/dL; below 5 mg/dL (300) in severe/tophaceous | Below 5 mg/dL initially, then below 6 long-term |
| Start ULT timing | May start during a flare (with prophylaxis) | Start after flare settles, plus prophylaxis | Once flare settled |
| Flare prophylaxis on starting ULT | At least 3-6 months | At least 6 months | Up to 6 months |
| Febuxostat positioning | Reserve for allopurinol failure (CV caution) | Second-line; caution if CV disease | Second-line if allopurinol intolerant |
| HLA-B*58:01 testing | Conditional in high-risk groups (e.g. Han Chinese, Thai, Korean CKD) | Consider in at-risk ethnicities | Consider in Han Chinese/Thai/Korean |
Registry & Surgical Outcome Notes
- There is no dedicated international gout registry equivalent to arthroplasty joint registries, but national arthroplasty registries (NJR UK, AJRR US, AOANJRR Australia, SHAR Sweden) capture gout as a comorbidity. Crystal arthropathy is associated with modestly higher periprosthetic joint infection and wound-complication rates in some series, reinforcing the value of preoperative urate control.
- Hand-surgery series of tophaceous excision report good functional gain but real risks of delayed wound healing and recurrence if ULT is not optimised.
High- vs Limited-Resource Practice Variation
- Well-resourced settings: ready access to polarised microscopy, ultrasound and DECT; HLA-B*58:01 genotyping where indicated; biologic IL-1 inhibitors (anakinra, canakinumab) for refractory disease.
- Limited-resource settings: diagnosis often clinical or via serum urate; polarised microscopy and DECT may be unavailable. Allopurinol and colchicine are inexpensive and on the WHO model essential-medicines landscape, so treat-to-target ULT remains achievable even where advanced imaging is not. Counterfeit colchicine and lack of monitoring are practical hazards.
- Across all settings, cardiovascular and renal risk screening is integral given the strong, consistent association of gout with CKD and cardiovascular disease.
GOUT AND CRYSTAL ARTHROPATHY
Clinical summary
Crystal Identification
- •MSU: Needle-shaped, negatively birefringent (YELLOW parallel)
- •CPPD: Rhomboid, positively birefringent (BLUE parallel)
- •Mnemonic: 'Yellow Parallel Gout' vs 'Blue Parallel Pseudogout'
Classic Presentation
- •Podagra: 1st MTP involvement (50% of first attacks)
- •Sudden onset, often nocturnal (3-4am)
- •Exquisitely tender - cannot tolerate bedsheet
Imaging Signs
- •Punched-out erosions with overhanging edges
- •Preserved joint space until late
- •Soft tissue tophi may calcify
- •DECT shows urate as green
Acute Treatment
- •NSAIDs: Indomethacin 50mg TDS or Naproxen 500mg BD
- •Colchicine: Low-dose regimen (1mg then 0.5mg)
- •Steroids: If NSAIDs/colchicine contraindicated
- •DO NOT start allopurinol during acute attack
ULT Targets
- •Serum urate less than 6 mg/dL (360 micromol/L)
- •Less than 5 mg/dL if tophi present
- •Start 2-4 weeks after acute attack resolution
Surgical Indications (MUSIC)
- •Mechanical symptoms from tophi
- •Ulceration of overlying skin
- •Secondary infection
- •Impingement on nerves
- •Cosmetic concerns