Reversible myeloneuropathy of nitrous oxide abuse: serial electrophysiological studies. Detailed electrophysiological studies were performed in 4 patients with myeloneuropathy induced by abuse of nitrous oxide for 1 to 4 years. All presented with paresthesias, weakness, and Lhermitte's phenomena, and exhibited signs of sensorimotor polyneuropathy, ataxia, and arreflexia. Two had subnormal serum vitamin B12 levels. Baseline electrophysiologic testing revealed reduced motor unit potentials, prolonged F wave latencies, absent H reflexes, denervation potentials, and delays in motor and sensory conduction. Three had peripheral and nuchal delay after median nerve stimulation. All were reevaluated after 3 to 12 months' abstinence and treatment with vitamin B12, and all showed substantial clinical improvement. Parallel improvement in electrophysiologic findings occurred, but residual minor conduction delays, loss of H reflexes, electromyographic evidence of denervation, or abnormalities of posterior tibial SEP were noted. These findings confirm the reversibility of myeloneuropathy of nitrous oxide abuse and describe the profile of electrophysiologic recovery in subjects who abstain from further neurotoxic exposure. 