Gout and Other Crystal-Associated Arthropathies¶
Chapter 384 | Gout and Other Crystal-Associated Arthropathies
KEY CLINICAL POINTS¶
- Gout is a hyperuricemic metabolic disorder caused by chronic hyperuricemia leading to monosodium urate (MSU) crystal deposition in joints and tissues.
- Diagnosis requires synovial fluid analysis to identify MSU crystals (negative birefringence) or other crystals (e.g., calcium pyrophosphate [CPP], hydroxyapatite).
- Urate-lowering therapy (allopurinol, febuxostat, probenecid) is essential for long-term management to prevent recurrent flares and tophi.
- Crystal-associated arthropathies (e.g., CPPD, apatite) share overlapping clinical features but differ in pathogenesis, imaging, and treatment approaches.
- Acute gout flares are managed with NSAIDs, colchicine, or corticosteroids, while chronic management focuses on urate control.
1. DEFINITION & OVERVIEW¶
Gout is a metabolic disorder characterized by recurrent inflammatory arthritis due to monosodium urate (MSU) crystal deposition. Other crystal-associated arthropathies include calcium pyrophosphate (CPP) deposition disease and hydroxyapatite (apatite) crystal deposition. These conditions share common mechanisms of crystal-induced inflammation but differ in etiology, clinical presentation, and management.
Table 384-1: Musculoskeletal Manifestations of Crystal-Induced Arthritis¶
| Acute arthritis (episodic) | Chronic arthropathy | Destructive arthropathies | Peculiar type of osteoarthritis |
|---|---|---|---|
| Mono-, oligo-, or polyarthritis | Chronic inflammatory arthritis | Chronic erosive monoarthritis (resembling erosive osteoarthritis) | Spinal arthritis |
| Periarticular inflammation | Destructive arthropathies | Hydroxyapatite pseudopodagra | Carpal tunnel syndrome |
| Bursitis | Tumoral calcinosis | Tophaceous deposits | |
| Tendinitis | Spinal stenosis | Spinal arthritis | |
| Enthesitis | Calciphylaxis (renal failure/long-term dialysis) | Calcific periarthritis (e.g., supraspinatus tendon apatite deposit rupture) |
1.1 Pathogenesis¶
Gout arises from chronic hyperuricemia, leading to MSU crystal formation. Urate crystals activate the NLRP3 inflammasome, triggering IL-1 β release and inflammation. CPP and apatite crystals also induce similar inflammatory pathways but with distinct crystal structures and clinical manifestations.
1.2 Diagnostic Criteria¶
Diagnosis requires synovial fluid analysis to identify crystals (MSU, CPP, or apatite) and exclusion of other causes. Imaging (ultrasound, CT) and clinical features (e.g., acute monoarthritis, tophi) support the diagnosis.
2. EPIDEMIOLOGY¶
Gout affects ~1–2% of adults globally, with higher prevalence in men and postmenopausal women. Risk factors include obesity, metabolic syndrome, alcohol consumption, and diuretic use. CPPD is more common in the elderly, with prevalence increasing with age (up to 50% in those >85 years). Apatite deposition is associated with chronic renal failure and connective tissue diseases.
2.1 Demographics¶
Gout predominantly affects middle-aged to elderly men and postmenopausal women. CPPD is more common in the elderly, while apatite deposition is linked to chronic renal failure and metabolic disorders.
2.2 Risk Factors¶
Obesity, insulin resistance, Western diet, alcohol, diuretics, and genetic predisposition (e.g., HLA-B*5801) increase risk. Hyperparathyroidism, hypomagnesemia, and thiazide diuretics are associated with CPPD.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Gout results from chronic hyperuricemia due to impaired urate excretion or overproduction. CPPD arises from abnormal pyrophosphate metabolism, while apatite deposition is linked to hyperphosphatemia and tissue damage. All conditions involve crystal-induced inflammation via the NLRP3 inflammasome.
Table 384-2: Factors and Conditions Associated with Calcium Pyrophosphate Deposition Disease¶
| Endocrine-Metabolic Conditions | Genetic Factors | Drugs | Other Conditions |
|---|---|---|---|
| Primary hyperparathyroidism | Familial hypocalciuric hypercalcemia | Thiazide and loop diuretics | Hemochromatosis |
| Hypophosphatasia | X-linked hypophosphatemic rickets | Proton pump inhibitors | Gitelman’s syndrome |
| Hypomagnesemia | Familial-genetic | Malabsorption | Connective tissue diseases (e.g., systemic sclerosis) |
3.1 Urate Metabolism¶
Urate is produced from purine metabolism. Hyperuricemia results from reduced excretion (90% of cases) or overproduction (10%). Xanthine oxidase inhibitors (allopurinol, febuxostat) reduce urate synthesis.
3.2 Crystal-Induced Inflammation¶
MSU, CPP, and apatite crystals activate the NLRP3 inflammasome, leading to IL-1 β release and neutrophil recruitment. This drives acute inflammation and chronic joint damage.
4. CLINICAL FEATURES¶
Acute gout flares present as monoarthritis with severe pain, swelling, and erythema. CPPD often mimics gout but may involve multiple joints. Apatite deposition causes chronic joint damage and calcific periarthritis. Tophi and chronic arthritis are common in long-standing disease.
4.1 Acute Flares¶
Sudden onset of pain, swelling, and redness in a single joint (podagra). Synovial fluid shows MSU crystals with negative birefringence. Fever and systemic symptoms may occur in CPPD.
4.2 Chronic Manifestations¶
Tophi, joint destruction, and erosive arthritis. CPPD may present with chronic polyarticular arthritis resembling osteoarthritis. Apatite deposition causes calcific periarthritis and joint instability.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include septic arthritis, rheumatoid arthritis, pseudogout (CPPD), and osteoarthritis. Key differentiators: MSU crystals (gout), CPP crystals (CPPD), and apatite crystals (apatite) in synovial fluid. Systemic symptoms and imaging findings help distinguish these conditions.
5.1 Septic Arthritis¶
Acute joint pain with fever, elevated WBC, and positive cultures. Differentiated by synovial fluid analysis and imaging.
5.2 Pseudogout¶
CPP crystals with weak positive birefringence. Often affects knees and wrists, with less severe pain than gout.
6. INVESTIGATIONS & DIAGNOSIS¶
Synovial fluid analysis (polarized light microscopy) is the gold standard. Imaging (ultrasound, CT) detects crystal deposits and joint damage. Laboratory tests include serum urate levels, renal function, and markers of inflammation.
Table 384-3: Clinical Manifestations of Apatite Crystal Deposition¶
| Articular | Periarticular | Other |
|---|---|---|
| Hemorrhagic shoulder effusions in the elderly (Milwaukee shoulder) | Calcific periarthritis (e.g., supraspinatus tendon apatite deposit rupture) | Tumoral calcinosis |
| Chronic destructive arthropathy | Bursitis and tendinitis | Hyperparathyroidism |
| Chronic erosive monoarthritis (resembling erosive osteoarthritis) | Hydroxyapatite pseudopodagra | Calciphylaxis (renal failure/long-term dialysis) |
6.1 Synovial Fluid Analysis¶
MSU crystals show negative birefringence; CPP crystals are weakly positive or nonbirefringent. Apatite crystals require electron microscopy for identification.
6.2 Imaging¶
Ultrasound detects double-contour sign (gout), chondrocalcinosis (CPPD), and calcific deposits (apatite). CT and MRI assess joint damage and crystal distribution.
7. MANAGEMENT & TREATMENT¶
Acute flares are managed with NSAIDs, colchicine, or corticosteroids. Long-term urate-lowering therapy (allopurinol, febuxostat, probenecid) prevents recurrence. CPPD and apatite deposition require anti-inflammatory agents and management of underlying conditions.
7.1 Acute Gout¶
NSAIDs (e.g., ibuprofen, naproxen), colchicine (1.2 mg initially, 0.6 mg/h), or corticosteroids (oral or intra-articular). Avoid rapid urate lowering to prevent flare.
7.2 Chronic Management¶
Urate-lowering therapy to maintain serum urate <6 mg/dL. Monitor for drug interactions (e.g., allopurinol with HLA-B*5801). SGLT2 inhibitors may reduce urate levels and comorbidities.
8. PROGNOSIS & COMPLICATIONS¶
Untreated gout leads to chronic arthritis, tophi, and kidney stones. CPPD and apatite deposition may cause joint destruction and disability. Complications include renal failure, infection, and drug toxicity (e.g., allopurinol hypersensitivity).
8.1 Long-Term Outcomes¶
Chronic gouty arthritis with joint damage and tophi. CPPD may progress to destructive arthritis. Apatite deposition leads to calcific periarthritis and joint instability.
8.2 Complications¶
Urate nephrolithiasis, renal failure, septic arthritis, and drug-related adverse effects (e.g., allopurinol hypersensitivity, febuxostat hepatotoxicity).
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Allopurinol is safe, but febuxostat is contraindicated. Elderly patients require cautious dosing due to renal impairment. Diuretics increase gout risk; avoid in acute flares. Monitor for drug interactions (e.g., thiazides with hyperuricemia).
9.1 Pregnancy¶
Allopurinol is preferred; avoid febuxostat. Monitor for fetal complications and adjust dosing based on renal function.
9.2 Renal Impairment¶
Reduce allopurinol doses in CKD. Avoid probenecid in severe renal failure. Use pegloticase cautiously in patients with renal disease.
10. KEY POINTS & CLINICAL PEARLS¶
- Urate-lowering therapy is essential for long-term gout management. 2. Synovial fluid analysis is critical for crystal identification. 3. Acute flares require anti-inflammatory agents, while chronic management focuses on urate control. 4. CPPD and apatite deposition require tailored approaches based on crystal type and comorbidities. 5. Monitor for drug interactions and renal function in all patients.