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Q1: Definition and clinical presentation (2 min)
CPPD disease is a crystal-deposition arthritis caused by calcium pyrophosphate (CPP) crystals forming in articular cartilage/fibrocartilage (chondrocalcinosis) and shedding into the joint. Name the clinical patterns: (1) asymptomatic chondrocalcinosis; (2) acute CPP crystal arthritis ("pseudogout") — acute mono/oligoarthritis, classically the knee or wrist, often after surgery/illness; (3) chronic CPP crystal inflammatory arthritis (RA-like); (4) OA with CPPD (pseudo-OA, atypical joints — MCPs, wrists). Commonest in older adults; strongly linked with OA and ageing.
Q2: Diagnosis and crystal identification (2 min)
Definitive diagnosis is synovial fluid aspiration with compensated polarised-light microscopy: rhomboid, weakly positively birefringent crystals (blue when parallel to the compensator axis) — contrast with gout's needle-shaped, strongly negatively birefringent urate crystals. Always send Gram stain and culture to exclude septic arthritis (which can coexist). X-ray shows chondrocalcinosis — calcification of knee menisci, wrist triangular fibrocartilage (TFCC), symphysis pubis, acetabular/glenoid labra. Ultrasound/CT/MRI can aid detection (2023 EULAR imaging recommendations).
Q3: Metabolic associations and screening (2 min)
In any younger patient (under 55) or atypical case, screen for the metabolic associations — the 4H mnemonic: Hyperparathyroidism, Haemochromatosis, Hypothyroidism, Hypomagnesaemia — plus hypophosphataemia, Wilson disease, ochronosis. Tests: calcium/PTH, iron studies/ferritin, magnesium, TSH, phosphate. Haemochromatosis arthropathy classically affects 2nd/3rd MCPs with hook-like osteophytes and chondrocalcinosis — finding it matters because venesection can reduce flares.
Q4: Management (2 min)
Acute CPP arthritis mirrors gout: NSAIDs (full dose; caution renal/GI/CV in elderly), colchicine (low dose, best within 24 h; renal caution), intra-articular corticosteroid (single large joint, after excluding sepsis), or oral glucocorticoids (short course; preferred in elderly/renal). Chronic/recurrent: low-dose colchicine prophylaxis, NSAIDs with PPI, methotrexate in selected refractory cases. Prophylaxis of peri-operative flares: colchicine. Treat any metabolic cause. State clearly: no drug reliably dissolves CPP crystals.
Q5: Common pitfalls and exam pearls (1 min)
- Always aspirate acute monoarthritis — pseudogout, gout and sepsis can coexist.
- Crystal "opposites": CPPD = rhomboid, weakly positive; gout = needle, negative.
- Joint "opposites": CPPD = knee/wrist; gout = first MTP.
- Don't give NSAIDs in elderly CKD — use intra-articular/oral steroid.
- Pseudogout and gout can coexist in the same joint.