Which crystals are characteristic of pseudogout?

Pseudogout, also known as calcium pyrophosphate dihydrate (CPPD) crystal disease, is characterized by the presence of CPPD crystals in the synovial fluid. The distinguishing feature of these crystals when viewed under polarized light microscopy is that they are positively birefringent and appear as rhomboid-shaped crystals.

This characteristic helps differentiate pseudogout from gout, which is caused by monosodium urate crystals that are negatively birefringent and shaped like needles. The specific morphology and birefringence of the crystals are crucial in making a correct diagnosis of the type of crystal arthritis present. In the case of pseudogout, the positive birefringence of the rhomboid-shaped crystals is a key diagnostic criterion.