Crystal Arthropathies

• Definition: Crystal arthropathies are inflammatory arthritides caused by deposition of endogenous crystals in and around joints, with monosodium urate (gout) and calcium pyrophosphate dihydrate (CPPD disease, pseudogout) accounting for nearly all board-relevant cases. Basic calcium phosphate (hydroxyapatite) deposition is a third, less commonly tested entity that causes periarthritis and the destructive Milwaukee shoulder.

• Why it matters: Crystal arthritis is the most common cause of acute inflammatory monoarthritis in adults, and it is clinically indistinguishable at the bedside from septic arthritis — a joint-destroying emergency. Every hot joint therefore earns an arthrocentesis, not a presumptive diagnosis. Gout is also a marker of cardiometabolic disease: it clusters with hypertension, obesity, insulin resistance, and chronic kidney disease, and uncontrolled hyperuricemia produces irreversible tophaceous joint destruction and urate nephrolithiasis.

Epidemiology worth recalling

• Gout: The most prevalent inflammatory arthritis in US adults, affecting roughly 4% of the adult population per NHANES data, with prevalence rising alongside obesity and CKD. Strongly male-predominant (roughly 3–4:1) in midlife because estrogen is uricosuric; incidence in women climbs sharply after menopause, so a woman with podagra is usually postmenopausal or on a diuretic.
• CPPD disease: A disease of aging. Radiographic chondrocalcinosis is uncommon before age 50 and becomes common in the very elderly, with roughly equal sex distribution. Onset of CPPD before age 55 should prompt a search for a metabolic cause (hemochromatosis, hyperparathyroidism, hypophosphatasia).
• Presentation spectrum: Both diseases run from asymptomatic crystal deposition, through self-limited acute flares, to chronic destructive arthropathy. Asymptomatic hyperuricemia is far more common than gout — most hyperuricemic people never flare, which is why the ACR 2020 Gout Guideline conditionally recommends against urate-lowering therapy for asymptomatic hyperuricemia alone.

Gout — overproduction of urate (minority of cases)

• Enzyme defects: Lesch-Nyhan syndrome (complete X-linked HGPRT deficiency — self-mutilation, dystonia, hyperuricemia) and PRPP synthetase overactivity increase de novo purine synthesis.
• High cell turnover: Myeloproliferative and lymphoproliferative disease, hemolysis, psoriasis, and tumor lysis syndrome after cytotoxic chemotherapy flood the purine salvage pathway.
• Dietary/metabolic: Purine-rich red meat, organ meats, shellfish, and beer; fructose-sweetened beverages consume ATP during phosphorylation and generate AMP → urate.

Gout — underexcretion of urate (majority of cases)

• Renal: CKD of any cause reduces filtered urate clearance. Volume depletion and lactic acidosis (alcohol, von Gierke disease) compete with urate at the proximal tubular URAT1 transporter.
• Drugs: Thiazide and loop diuretics, low-dose aspirin, cyclosporine and tacrolimus, pyrazinamide, ethambutol, and niacin. Chronic lead exposure causes saturnine gout.

CPPD — mechanistic groupings

• Metabolic disease: Hemochromatosis, primary hyperparathyroidism, hypophosphatasia, and hypothyroidism; disordered magnesium homeostasis is also classically implicated (pyrophosphatase is magnesium-dependent).
• Structural/degenerative: Prior joint trauma, meniscectomy, and osteoarthritis promote local pyrophosphate generation in cartilage.
• Hereditary: ANKH gene mutations cause familial early-onset chondrocalcinosis.
• Precipitants examiners plant: Recent surgery, acute medical illness, or intra-articular hyaluronate injection triggering crystal shedding.

Modifiable vs non-modifiable

• Modifiable: Obesity, alcohol (especially beer), sugar-sweetened beverages, purine load, diuretic use, dehydration, lead exposure, untreated hyperparathyroidism or hemochromatosis.
• Non-modifiable: Age, male sex, postmenopausal status, genetic urate transporter polymorphisms (SLC2A9, ABCG2), enzyme deficiencies, ANKH mutation, and established CKD.

• Urate supersaturation: Humans lack uricase, so urate is the terminal purine metabolite. Above roughly 6.8 mg/dL urate exceeds its solubility limit in physiologic fluid, and crystallization is favored further by low temperature and low pH — which is exactly why the coolest, most acral, most acidic joint in the body, the first MTP, is the classic first site, and why flares begin at night when the peripheral joints cool and the patient is relatively volume-depleted.
• Crystal shedding, not crystal presence, causes flares: Crystals sit silently in cartilage and synovium for years. An abrupt shift in urate concentration — a purine or alcohol binge, dehydration, a new diuretic, or the initiation of allopurinol — destabilizes deposits and sheds crystals into the joint space. This is the mechanistic reason urate-lowering therapy paradoxically triggers flares and requires concurrent anti-inflammatory prophylaxis.
• Inflammasome activation: Opsonized MSU crystals are phagocytosed by resident macrophages and activate the NLRP3 inflammasome, which cleaves procaspase-1 to caspase-1, which in turn converts pro-IL-1β to mature IL-1β. IL-1β drives endothelial activation, IL-8/CXCL8 release, and massive neutrophil influx — producing the erythema, warmth, tense effusion, and exquisite tenderness of a flare. This pathway explains why IL-1 blockade works in refractory gout and why colchicine, which disrupts microtubule-dependent neutrophil chemotaxis and inflammasome assembly, aborts attacks.
• Self-limitation: Neutrophil apoptosis, crystal coating with apolipoprotein B, and macrophage transition to an anti-inflammatory phenotype terminate the attack over days to weeks even untreated — hence the classic "resolves completely between attacks."
• Chronic disease: Persistent supersaturation produces tophi, granulomatous aggregates of crystals ringed by macrophages and giant cells, which release matrix metalloproteinases and drive osteoclast activation, generating punched-out erosions with overhanging margins.
• CPPD: Excess extracellular pyrophosphate, generated by chondrocyte ectoenzymes (ENPP1) and exported by the ANKH transporter, complexes with calcium in degenerate cartilage. The identical NLRP3/IL-1β pathway is triggered when these crystals shed, which is why the acute attacks look clinically indistinguishable from gout.

Acute gout flare — the classic stem

• Abrupt nocturnal monoarthritis: An obese middle-aged man, often hypertensive and on hydrochlorothiazide, awakens with excruciating pain in the great toe hours after a steak-and-beer dinner. Peak intensity within 12–24 hours reflects the explosive IL-1β-driven neutrophil influx.
• Podagra: First MTP involvement in the majority of first attacks (coolest joint, lowest solubility). Midfoot, ankle, and knee follow.
• Exam: Dusky red, hot, tensely swollen joint with such tenderness that the weight of a bedsheet is intolerable; overlying desquamation as the flare resolves. Low-grade fever and leukocytosis are common and do not exclude gout — nor do they exclude concurrent sepsis.
• Between attacks: Complete resolution (intercritical gout) with a normal joint exam — a feature that distinguishes gout from RA or septic arthritis.

Chronic tophaceous gout

• Tophi: Firm, non-tender, chalky-white subcutaneous nodules on the helix of the ear, olecranon bursa, Achilles tendon, and finger pads, appearing after years of untreated hyperuricemia. May ulcerate and extrude toothpaste-like material.
• Polyarticular presentation: Older women on diuretics with CKD may present with tophi in Heberden nodes, mimicking nodal osteoarthritis or RA.

Acute CPPD arthritis (pseudogout)

• Older patient, larger joint: An elderly patient develops an acutely hot, swollen knee or wrist, frequently a few days after hip surgery, an acute illness, or a hospital admission that shifted calcium and volume status.
• Chronic CPPD patterns: A "pseudo-osteoarthritis" pattern involving joints OA spares — MCP joints, wrists, elbows, shoulders — plus a "pseudo-RA" pattern with symmetric small-joint synovitis; both are hallmarks that should prompt screening for hemochromatosis and hyperparathyroidism.
• Crowned dens syndrome: CPPD around the odontoid causing acute neck pain, fever, and neck stiffness — a meningitis mimic in the elderly.

Step 1 — arthrocentesis, always

• Synovial fluid analysis is both the initial and the gold-standard test. Send cell count with differential, Gram stain, bacterial culture, and polarized light microscopy. The single most important reason is that septic arthritis is the can't-miss alternative and can coexist with crystals in the same joint.
• Cell count interpretation: Inflammatory fluid typically runs in the tens of thousands of WBC/µL with neutrophil predominance. Counts above roughly 50,000/µL with >90% neutrophils raise concern for infection, but crystal flares can reach this range — culture, not cell count, settles it.

Step 2 — polarized light microscopy

• MSU: Needle-shaped, strongly negatively birefringent, often intracellular within neutrophils during an acute flare (intracellular crystals confirm the crystals are causing the attack rather than incidental).
• CPPD: Rhomboid/rod-shaped, weakly positively birefringent — weak birefringence means they are easy to miss, so a negative smear does not exclude CPPD.

Adjunctive testing

• Serum urate: Frequently normal or low during an acute flare (urate is an acute-phase reactant that falls with IL-6). A normal level never excludes gout. Recheck two or more weeks after resolution to establish the baseline for treat-to-target therapy.
• 24-hour urine uric acid on a regular diet distinguishes overproducers from underexcretors and identifies patients in whom a uricosuric would be inappropriate.
• Radiographs: Chronic gout shows punched-out juxta-articular erosions with overhanging edges ("rat-bite") and preserved joint space; CPPD shows chondrocalcinosis — linear calcification of knee menisci, triangular fibrocartilage of the wrist, and pubic symphysis.
• Ultrasound may show the double contour sign (urate coating hyaline cartilage) and dual-energy CT can color-code urate deposits when aspiration is not feasible.

Named criteria

• The 2015 ACR/EULAR gout classification criteria are a weighted point system (joint pattern, flare characteristics, tophus, serum urate, imaging) used when synovial fluid crystal identification is unavailable. ACR/EULAR have likewise published classification criteria for CPPD disease. Both are classification, not diagnostic, tools.

Before anything else

• Exclude septic arthritis. If Gram stain, culture, or clinical suspicion supports infection, the answer is joint drainage plus empiric antibiotics, not anti-inflammatories.

Acute flare (ACR 2020 Gout Guideline)

• The ACR treats NSAIDs, colchicine, and glucocorticoids as equally effective first-line options; choose by comorbidity rather than by hierarchy. A monoarticular flare in a patient with CKD, heart failure, or GI bleeding risk is best treated with intra-articular glucocorticoid (e.g., triamcinolone) after infection is excluded; polyarticular flares in such patients get oral prednisone.
• Colchicine is most effective when started early; ACR endorses a low-dose regimen (a 1.2 mg loading dose followed by 0.6 mg one hour later, then usual dosing), which is as effective as and far less toxic than historical high-dose schedules.
• Escalation: For refractory or multi-comorbid flares, the ACR conditionally supports IL-1 inhibition (anakinra), targeting the inflammasome pathway directly.
• Do not stop established urate-lowering therapy during a flare — stopping and restarting shifts urate and prolongs the attack.

Urate-lowering therapy (ULT)

• Indications (ACR 2020, strong): ≥2 flares per year, ≥1 tophus, or radiographic damage from gout; conditional indications include CKD stage ≥3, serum urate markedly elevated, or urolithiasis. ACR conditionally recommends against ULT for asymptomatic hyperuricemia.
• Allopurinol is first-line for all patients, including CKD, started at a low dose and titrated to a treat-to-target serum urate <6 mg/dL. Test **HLA-B*5801** before starting in patients of Southeast Asian or African descent because of allopurinol hypersensitivity risk.
• Anti-inflammatory prophylaxis (low-dose colchicine or NSAID) for at least 3–6 months during titration prevents mobilization flares.
• Second line: Febuxostat (FDA boxed warning for cardiovascular death — avoid with established cardiovascular disease), probenecid (avoid in nephrolithiasis and reduced GFR), and pegloticase with immunomodulator co-therapy for refractory tophaceous disease.
• Contraindicated combinations: allopurinol with azathioprine or 6-mercaptopurine (fatal marrow suppression); colchicine with strong CYP3A4/P-glycoprotein inhibitors such as clarithromycin or cyclosporine.
• Adjuncts: Substitute losartan for a thiazide when possible; weight loss and reduced alcohol/fructose intake.

Disease complications

• Chronic tophaceous gout: Years of supersaturation produce tophi that erode bone via MMP release and osteoclast activation; the signal is a deforming, nodular arthropathy with punched-out erosions and overhanging edges. Tophi may ulcerate and become secondarily infected.
• Uric acid nephrolithiasis: Radiolucent stones forming in persistently acidic urine — suspect in a gout patient with flank pain and hematuria whose CT shows a stone invisible on plain film. Prevention is hydration and urinary alkalinization.
• Urate nephropathy: Chronic interstitial deposition, and in tumor lysis syndrome, acute urate nephropathy with oliguric AKI, hyperkalemia, hyperphosphatemia, and hypocalcemia — an emergency managed with aggressive hydration and rasburicase (contraindicated in G6PD deficiency).
• Concomitant septic arthritis: Crystals do not protect against infection. Persistent fever, rigors, or a flare that fails to improve on adequate anti-inflammatory therapy demands repeat aspiration — an emergency.
• CPPD-specific: Chronic destructive pyrophosphate arthropathy with severe joint-space loss, and crowned dens syndrome presenting as fever with neck rigidity that can be mistaken for meningitis.

Treatment complications

• NSAIDs: Prostaglandin-dependent afferent arteriolar constriction causes AKI; COX-1 inhibition causes peptic ulceration and GI bleeding; sodium retention decompensates heart failure.
• Colchicine: Microtubule poisoning yields diarrhea first, then myopathy with elevated CK, neuropathy, and pancytopenia — risk rises steeply with renal impairment or CYP3A4/P-gp inhibitors. Overdose is potentially fatal.
• Glucocorticoids: Hyperglycemia, especially in the diabetic patient in whom they were chosen to spare the kidneys.
• Allopurinol hypersensitivity syndrome/DRESS: Fever, eosinophilia, rash progressing to SJS/TEN, hepatitis, and AKI — an emergency; associated with HLA-B*5801 and with starting at excessive doses.
• Allopurinol plus azathioprine/6-MP: Blocked xanthine oxidase catabolism causes profound myelosuppression.
• Pegloticase: Anaphylaxis and infusion reactions (loss of response heralded by rising urate), and hemolysis/methemoglobinemia in G6PD deficiency.
• ULT initiation: Mobilization flares in the first months — expected, and the reason for prophylaxis.

• The single best next step in any acute hot joint is arthrocentesis. Not serum urate, not an x-ray, not empiric colchicine. Send cell count, Gram stain, culture, and polarized microscopy. Septic arthritis and crystal arthritis coexist often enough that the stem will punish a presumptive diagnosis.
• A normal serum urate does not exclude gout. Urate commonly falls during a flare. The classic distractor is a stem with textbook podagra and a urate of 5.5 mg/dL — still gout. Recheck weeks later to set the treat-to-target baseline.
• Negative = Needle = urate. MSU crystals are needle-shaped and negatively birefringent (yellow when parallel to the compensator); CPPD is rhomboid and positively birefringent (blue when parallel). Intracellular crystals within neutrophils confirm the crystals are driving the attack.
• Never start — or stop — urate-lowering therapy based on the flare. Do not initiate allopurinol as the answer to an acute attack without concurrent anti-inflammatory cover, and never discontinue chronic allopurinol during a flare. Cover ULT titration with low-dose colchicine or an NSAID for months, per the ACR 2020 Gout Guideline.
• The association examiners love: allopurinol plus azathioprine or 6-mercaptopurine. Xanthine oxidase blockade prevents 6-MP catabolism → life-threatening pancytopenia. Check **HLA-B*5801** before allopurinol in patients of Southeast Asian or African descent.
• Early-onset or atypical-joint CPPD is a metabolic screen. Chondrocalcinosis in a patient under 55, or a "pseudo-OA" pattern hitting the MCPs, wrists, or shoulders, should trigger evaluation for hemochromatosis and primary hyperparathyroidism.
• Lesch-Nyhan: X-linked complete HGPRT deficiency — self-mutilation, dystonia, intellectual disability, hyperuricemia with orange "sand" in the diaper.
• Renal-sparing flare therapy: In CKD or heart failure, the correct answer is usually intra-articular or systemic glucocorticoid, not indomethacin.

• Gout: Monosodium urate (MSU) crystals; needle-shaped, negatively birefringent
• Pseudogout: Calcium pyrophosphate dihydrate (CPPD) crystals; rhomboid, positively birefringent
• Diagnosis: Synovial fluid analysis with polarized light microscopy is gold standard
• Acute attacks triggered by dehydration, alcohol, purine-rich foods, diuretics, chemotherapy (tumor lysis)
• Both present with acute monoarticular/oligoarticular arthritis, warmth, erythema, severe pain

1. 1.Confusing birefringence: MSU is negatively birefringent (appears yellow parallel to compensator); CPPD is positively birefringent (appears blue). Reverse orientation = opposite color.
2. 2.Starting urate-lowering therapy during acute attack: Triggers crystal shedding and worsens inflammation. Use NSAIDs/colchicine/corticosteroids first; start allopurinol/febuxostat after attack resolves.
3. 3.Assuming all acute monarthritis is gout: Septic arthritis must be ruled out; both can coexist. Always culture synovial fluid and check WBC.

ACUTE ATTACK

• NSAIDs (indomethacin, naproxen) - first-line if no contraindications
• Colchicine - use early (within 24-36 hours); 0.6 mg BID × 3 days
• Corticosteroids - oral or intra-articular if NSAIDs/colchicine contraindicated

CHRONIC MANAGEMENT (start after acute resolves)

• Allopurinol (xanthine oxidase inhibitor) - target urate <6 mg/dL; start low, titrate up
• Febuxostat - alternative xanthine oxidase inhibitor
• Uricosuric agents (probenecid) - if underexcretion
• Uricase (pegloticase) - for refractory gout