Reversible spastic paraparesis induced by high-dose intravenous methadone.

High doses of parenteral opioids can cause multifocal myoclonus and seizures. Spasticity has been reported in patients receiving intraspinal opioids. In this article, we describe a patient who developed reversible spastic paraparesis with prominent extensor spasms in the legs while receiving an infusion of intravenous methadone at 100 mg/hr. We discuss clinical presentation and possible pathophysiologic mechanisms of opioid side effects on the somatic motor system.