Neurology Case Diagnosis: Involuntary Contractions and Lip Twitching

The diagnosis of the case presented on February 19, 2013, and a discussion of reader responses

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 sites are then occupied by serum calcium ions. Both mechanisms result in net lowering of ionized calcium, which produces overactivity of nerve and muscle. Magnesium serum levels are probably similarly affected and act similarly on nerve and muscle to produce hyperexcitability. Correcting the alkalosis by rebreathing expired air in a small bag corrects all the symptoms in a few minutes. Hyperventilation also causes Chvostek sign, in which the facial nerve is hyperirritable and responds to light tapping by firing and causing the innervated muscles to contract.

Like most patients, this woman was not aware that she was overbreathing. It is important for clinicians to consider this diagnosis, which may be elicited by voluntary hyperventilation for 2 minutes. This patient now carries a small bag with which to correct the symptoms. I have found that very low doses of benzodiazepines are effective in many patients to prevent anxiety-driven hyperventilation. This patient was not anorexic and had not been given agents that can produce acute dystonic reactions, alternative diagnoses suggested by some readers.