CPPD Secondary Causes — SCE Rheumatology MCQ

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ModerateCrystal ArthropathyCPPD Secondary CausesSCE Rheumatology

A 50-year-old woman has recurrent acute CPP crystal arthritis confirmed by identification of CPP crystals in synovial fluid. Investigation for an associated metabolic disorder shows an albumin-adjusted calcium of 2.78 mmol/L, phosphate of 0.62 mmol/L and parathyroid hormone above the reference range. Her renal function is normal. Which condition best explains these findings and is a recognised association of CPPD?

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Correct answer: C — Primary hyperparathyroidism

The correct answer is primary hyperparathyroidism. An elevated albumin-adjusted calcium with an inappropriately elevated PTH indicates PTH-dependent hypercalcaemia; increased renal phosphate loss accounts for the low phosphate. Primary hyperparathyroidism is a recognised metabolic association of CPPD and is particularly important to identify in a relatively young patient. Vitamin D deficiency may cause secondary hyperparathyroidism and hypophosphataemia, but calcium is usually low or normal rather than elevated. Hypothyroidism does not explain this biochemical pattern, while diabetes and gout are not causes of PTH-dependent hypercalcaemia. Other established metabolic associations of CPPD include haemochromatosis and hypomagnesaemia. Treating hyperparathyroidism is important for its systemic complications, but established CPP deposition should not be expected to regress after parathyroid treatment.

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/21216817/