NITROUS-OXIDE DEPRESSES THE H-REFLEX IN CHILDREN WITH CEREBRAL-PALSY

Hoffmann's reflex or H-reflex (HR) is an electrically elicited reflex that measures excitability of motoneurons and shares some physiologic properties with the deep tendon reflex. Children with tendon hyperrefl exia due to cerebral palsy usually have higher amplitude HRs. Nitrous oxide (N2O) depresses the HR in patients with normal spinal reflexes, although the effect of N2O in conditions with hyperreflexia such as ce rebral palsy is not known. We propose to determine the effect of N2O o n the amplitude of the HR under general anesthesia in children with hy perreflexia due to cerebral palsy. We studied eight children undergoin g selective dorsal rhizotomy (SDR) for the relief of spasticity. The m aximum amplitudes of the HR (HR(max)) and direct motor response (MR(ma x)) were routinely evoked under the following anesthetic conditions: 1 ) sufentanil and 66% N2O/33% oxygen; and 2) sufentanil and 100% oxygen . The HR(max) amplitude was significantly lower when N2O was part of t he inspired gas mixture. The differences between the no N2O and the 66 % N2O groups were significant. The MR(max) did not change significantl y. Abnormal spinal reflexes seen in spastic diplegia can be abolished by inhaled N2O. This finding also suggests that N2O-induced depression of spinal reflexes should be a consideration during physiologic monit oring of the spinal cord under general anesthesia.