Treatment of "on-off effect" with a dopa decarboxylase inhibitor.

Irregularities in motor response after continuing levodopa therapy of Parkinson disease (the "on-off effect") were assessed with the addition of L-alpha-methyldopa hydrazine (carbidopa) in a double-blind study. Thirteen of 20 patients improved while receiving carbidopa and levodopa while only four of 17 patients improved while receiving placebo and levodopa. Twenty-three of 37 patients improved in a subsequent non-blind trial of carbidopa plus levodopa. Improvement was not dependent on an increase in dose or frequency of levodopa administration. Adverse effects included dyskinesia, imbalance, and confusion; nausea was eliminated. On patient died of glomerulonephritis that predated the drug trial, but worsened progressively during and after it. Carbidopa's suppression of the "on-off effect" suggests that extracerbral factors may be important in this phenomenon.