Calcium Pyrophosphate Arthropathy
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Direct answer
Calcium pyrophosphate arthropathy, or pseudogout, is an arthritis caused by deposition of calcium pyrophosphate dihydrate crystals in cartilage and joint fluid. It is confirmed by rhomboid, weakly positively birefringent crystals and by chondrocalcinosis on X-ray, most often in the knee. Acute flares resemble gout and respond to low-dose colchicine, NSAIDs or steroids, but a search for metabolic causes such as hyperparathyroidism and haemochromatosis is essential.
What you must remember
- Crystal: calcium pyrophosphate dihydrate, rhomboid-shaped and weakly positively birefringent, appearing blue when aligned parallel to the compensator.
- Chondrocalcinosis: linear calcification of fibrocartilage, seen in the knee menisci, the triangular fibrocartilage of the wrist and the symphysis pubis.
- Commonest sites: the knee is the most frequent acute site, with the wrist and metacarpophalangeal joints also involved.
- Metabolic associations: hyperparathyroidism, haemochromatosis, hypomagnesaemia, hypophosphatasia and hypothyroidism should be screened for, especially when onset is young.
- Acute flare: rapid onset with knee effusion mimics gout and septic arthritis, so aspiration with crystal study and culture is essential.
- Treatment: low-dose colchicine, NSAIDs, or aspiration with intra-articular steroid, with low-dose colchicine prophylaxis for recurrent flares.
- Imaging limit: chondrocalcinosis supports the diagnosis but is not seen in every patient, so the synovial fluid remains the definitive test.
A hot knee after a minor bump
A 71-year-old woman with a long-standing stiff knee wakes with a hot, swollen knee after a minor knock. Her knee X-ray shows linear calcification of the menisci.
- Aspirate and examine the fluid. Cell count, Gram stain, culture and polarised microscopy distinguish crystal disease from septic arthritis, which can look identical.
- Identify the crystal. Rhomboid, weakly positively birefringent crystals of calcium pyrophosphate dihydrate, appearing blue when parallel to the compensator, confirm the diagnosis.
- Look for chondrocalcinosis. Radiographs of the knee and wrist showing linear calcification of the menisci or the triangular fibrocartilage support the diagnosis.
- Screen the metabolism. Serum calcium, phosphate, magnesium, parathyroid hormone, ferritin with iron saturation and thyroid function are checked for associated conditions.
- Treat the flare. Low-dose colchicine, NSAIDs, aspiration and intra-articular steroid are used, chosen by renal function, comorbidity and the number of joints affected.
- Prevent recurrence. Low-dose colchicine or cautious NSAID cover may be given for recurrent flares, while the underlying metabolic cause is corrected.
Pseudogout in the examination room
Most traps come from treating calcium pyrophosphate disease as gout.
- Trap: calling a pseudogout flare gout because the picture is similar Correct: the crystal shape and birefringence differ, since CPPD crystals are rhomboid and positively birefringent, so serum urate does not settle the diagnosis.
- Trap: missing hyperparathyroidism or haemochromatosis in a young patient with CPPD Correct: CPPD at a young age prompts a metabolic work-up, including iron studies and calcium and parathyroid testing.
- Trap: starting allopurinol for pseudogout Correct: allopurinol does not treat CPPD crystals, so management is colchicine, NSAIDs and steroids with correction of any metabolic cause.
Frequently asked questions
What distinguishes CPPD crystals from urate crystals under polarised light?
CPPD crystals are rhomboid and weakly positively birefringent, appearing blue when parallel to the compensator. Urate crystals are needle-shaped and strongly negatively birefringent, appearing yellow in the same orientation, so the two can be separated on fluid examination.
Which joints show chondrocalcinosis most often?
Chondrocalcinosis is most often seen in the knee menisci, the triangular fibrocartilage of the wrist and the symphysis pubis. Its distribution on radiographs supports the diagnosis of calcium pyrophosphate arthropathy, although it is absent in some patients.
Which metabolic disorders should be screened for in calcium pyrophosphate arthropathy?
Hyperparathyroidism, haemochromatosis, hypomagnesaemia, hypophosphatasia and hypothyroidism are the associations most often screened for in calcium pyrophosphate arthropathy. Screening is especially important when the disease starts at a young age.
How is an acute calcium pyrophosphate flare treated?
An acute flare is treated with low-dose colchicine, NSAIDs, or aspiration with intra-articular corticosteroid, chosen by renal function and comorbidity. Recurrent flares may need low-dose colchicine prophylaxis while the underlying metabolic disorder is corrected.