Snapshot A 70-year-old man presents to the emergency department for severe left knee pain. Medical history is significant for hyperparathyroidism managed with bisphosphonates. On physical exam, the left knee is erythematous, warm, enlarged, and tender to palpation. Radiography of the affected joint demonstrates chondrocalcinosis. Joint aspiration demonstrates a leukocyte count of 2800/mm3 with 50% polymorphonuclear cells. Polarized microscopy shows weakly positively birefringent rhomboid crystals. Introduction Clinical definition a metabolic arthropathy due to deposition of calcium pyrophosphate dihydrate (CPP) in connective tissue Epidemiology risk factors radiographic CPP deposition increases with age Etiology CPP deposition in joints Pathogenesis aging and/or genetic factors may result in increased adenosine triphosphate breakdown producing inorganic pyrophosphate CPP is produced after inorganic pyrophosphate binds with calcium login to view 1 more bullet Associated conditions hemochromatosis hyperparathyroidism hypomagnesemia can be seen in Gitelman and Bartter syndrome joint trauma, surgery, and severe medical illness these conditions can provoke an acute attack Prognosis acute attacks typically resolve in 10 days patients may experience functional limitation due to joint damage may resemble osteoarthritis or rheumatoid arthritis Presentation Symptoms asymptomatic (most cases) acute attack (pseudogout) typically affects the wrists, knees, and metacarpophalangeal joints are clinically indistinguishable from gout symptoms and physical exam findings include login to view 5 more bullets "pseudo-rheumatoid arthritis" inflammatory arthritis symptoms login to view 1 more bullet pyrophosphate arthropathy resembles osteoarthritis Imaging Radiography indication to assess the affected joint findings chondrocalcinosis and degenerative changes login to view 1 more bullet Studies Arthrocentesis confirms the diagnosis Gram stain and culture should always be performed since infection could co-exist leukocyte count is 2,000-100,000/mm3 > 50% polymorphonuclear cells polarized microscopy demonstrates weakly positively birefringent rhomboid crystals blue when parallel to light and yellow when perpendicular to light Differential Gout differentiating factor polarized microscopy demonstrates negatively birefringent crystals more commonly affects the first metatarsophalangeal (MTP) joint Treatment Conservative observation indication login to view 1 more bullet Medical nonsteroidal anti-inflammatory drugs (NSAIDs) indication login to view 1 more bullet colchicine indication login to view 1 more bullet glucocorticoids indications login to view 2 more bullets Complications Joint damage