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The patient presented in the case “Involuntary Contractions and Lip Twitching” (JW Neurol Feb 19 2013) was, as most readers surmised, diagnosed with carpopedal spasm, caused by hyperventilation resulting in respiratory alkalosis and decreased serum ionized calcium. Rebreathing her expired air from a small paper bag alleviated all symptoms in a few minutes.
Carpopedal spasm is a form of neuromuscular hyperactivity related to low levels of ionized calcium in serum. The low ionized calcium results from two mechanisms. Protons leave the intracellular compartment to buffer the alkalosis, and, in exchange, calcium enters the cell. In addition, hydrogen ions bound to circulating proteins are released to neutralize the alkalosis and their binding si…