Pseudogout — SCE Rheumatology MCQ

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EasyCrystal ArthropathyPseudogoutSCE Rheumatology

A 70-year-old woman presents with an acutely swollen knee. Synovial fluid microscopy demonstrates rhomboid-shaped crystals that are weakly positively birefringent under compensated polarised light. Knee radiography shows chondrocalcinosis. What is the composition of these crystals?

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Correct answer: D — Calcium pyrophosphate dihydrate

The crystals are calcium pyrophosphate dihydrate (CPPD). Acute large-joint arthritis in an older adult, chondrocalcinosis and rhomboid-shaped, weakly positively birefringent crystals are characteristic of acute CPP crystal arthritis. Monosodium urate crystals in gout are typically needle-shaped and strongly negatively birefringent. Basic calcium phosphate, including hydroxyapatite, is not birefringent and is generally not visible using routine compensated polarised microscopy. Calcium oxalate crystals may be positively birefringent but usually have bipyramidal or envelope-like morphology and occur particularly in advanced renal failure. Cholesterol crystals are large, plate-like structures with notched corners and are associated more often with chronic effusions.

Reference: Zhang W, et al. European League Against Rheumatism recommendations for calcium pyrophosphate deposition. Part I: terminology and diagnosis. Ann Rheum Dis. 2011;70:563–570. https://pubmed.ncbi.nlm.nih.gov/22303531/