Clinically Determined Asterixis Is Markedly Increased in Patients Taking Clozapine.

To the Editor: A sterixis, also known as negative myoclonus, is caused by a 50to 200-ms silence in electrical impulses to a tonically contracting muscle. It was originally described in the hands as a “flapping” tremor, but it may be seen in other parts of the body. It commonly appears in toxic and metabolic encephalopathies, but may be present with medications at non-toxic doses, seizures, central nervous system infections, and other disorders. It may be co-existent with myoclonus. Possible asterixis has been rarely reported in patients treated with clozapine (Cloz), but most articles reported poorly described episodes lasting under 2 minutes, of “knee buckling,” “dropping things,” and “cataplexy.” This study was undertaken to determine how often asterixis was present in the hands and arms in patients taking Cloz, even at the extremely low doses (usually 6.25–50 mg) used in people with Parkinson disease (PD).