Gout is an inflammatory disease characterized by deposition of sodium urate monohydrate crystals in cartilage, subchondral bone, and periarticular structures. An acute attack is characterized by sudden onset of monarticular arthritis, most commonly in the metatarsophalangeal (MTP) joint of the great toe, with swelling, erythema, and tenderness. Gout can also occur in other joints. The deposits of crystals in the tissues about the joint produce a chronic inflammatory response termed a tophus.
In pseudogout, calcium pyrophosphate dihydrate (rod- or rhombus-shaped, weakly birefringent) crystals are deposited. Although any joint may be involved, knees and wrists are the most common sites. The acute presenting signs and symptoms are identical with those of gout, but formation of tophi is not seen. Fever, pain, and erythema are common to both entities.
Cellulitis and septic arthritis must be excluded. In the cell count of the synovial fluid from an inflamed joint, 2,000 to 50,000 WBCs with polymorphonuclear neutrophil leukocyte (PMN) predominance is expected. The diagnosis is made by seeing urate or calcium pyrophosphate dihydrate crystals on polarized microscopy coupled with negative Gram stain and cultures.
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