PSEUDOGOUT
Synonyms: Chondrocalcinosis Calcium pyrophosphate disease Calcium gout
Key features
Pseudogout is a form of arthritis that is due to deposits of calcium pyrophosphate dihydrate (CPPD) crystals within the joint
It most commonly affects the knee and other large joints of older persons with osteoarthritis
In aspirates of joint fluid, there are rhomboid CPPD crystals that exhibit weakly positive birefringence under polarized light
Pseudogout, which may be clinically indistinguishable from gout, was recognized as a distinct disease entity in 1962. It results when CPPD crystals (produced by nucleoside triphosphate pyrophosphohydrolase, a catalytic enzyme found in osteoarthritic cartilage) form within joint fluid and then lead to lysis of neutrophils that have ingested the crystals. As in gout, the formation of NETs plays a role in the inflammatory cascade. Although pseudogout is most commonly related to osteoarthritis, it has also been associated with metabolic abnormalities such as hyperparathyroidism and hemochromatosis.
Symptoms of pseudogout include intermittent attacks of pain and swelling of affected joints, most commonly the knee, wrist, and shoulder. The onset of symptoms in pseudogout is usually more insidious than in gout. Occasionally, extra-articular CPPD deposition (tophaceous pseudogout) may present as a hard subcutaneous nodule overlying joints, primarily of the head (especially the temporomandibular joint), neck, or distal extremities.
Microscopically, CPPD crystals appear shorter than urate crystals and are often rhomboidal in shape. In tophaceous pseudogout, rhomboid crystals as well as foci of calcification can be seen within the dermis. Polarizing microscopy of sections stained with a non-aqueous alcoholic eosin stain (NAES) allows the demonstration of CPPD crystals with positive birefringence. This is in contrast to the negative birefringence of urate crystals in gout and the lack of birefringence of calcium hydroxyapatite crystals in tumoral calcinosis. Treatment of the acute phase of pseudogout is identical to that of gout. In patients with chronic renal failure, the anti-IL-1β antibody canakinumab may be considered (see Fig. 45.13).