Alois Ebner

Influence of anesthetics--nitrous oxide in particular--on electromyographic response evoked by transcranial electrical stimulation of the cortex.

The influence of anesthetics usually used for neuroleptic anesthesia--nitrous oxide, fetanyl, flunitrazepam, and thiopental sodium--on motor evoked potentials (MEP) was examined in 15 patients during neurosurgical operations on the spinal cord, in 16 patients in traumatic coma, and in 6 healthy volunteers. MEP were recorded from the contralateral thenar and anterior tibial muscles in response to single transcranial electrical stimuli on the motor cortex. Intraoperatively, during neuroleptic anesthesia we found the amplitudes to be reduced to an average of 11% of the preoperative baselines for the thenar potentials, and to 7% of the preoperative baselines for the anterior tibial muscle potentials, despite a maximum stimulus strength of 750 V. A similar reduction of MEP amplitudes was observed in 6 volunteers during breathing of an oxygen/nitrous oxide mixture (34%/66%), whereas fentanyl, flunitrazepam, and thiopental had only a minor effect on MEP. We conclude that with respect to anesthesia-related suppression of amplitudes, an average of 5 to 15 electromyographic responses should be evaluated for intraoperative monitoring of MEP using the technique described here.

Nitrous oxide suppresses the electromyographic response evoked by electrical stimulation of the motor cortex

The influence of nitrous oxide on motor evoked potential (MEP) elicited in rats by cortical and midcervical electrical stimulation was studied and compared with early components of somatosensory evoked potential (SEP) following stimulation of the posterior tibial nerve in 6 rats. We found that nitrous oxide gradually suppresses MEP, depending on the concentration of this inhalation agent. At a concentration of 66 vol% of nitrous oxide, the MEP was completely abolished, whereas the initial component N1-P1 of the SEP was only slightly reduced. We conclude that the descending impulse elicited by electrical stimulation of the corticospinal tract is mainly inhibited at the level of the spinal neuronal or interneuronal system, since (1) neuromuscular transmission is not blocked by nitrous oxide, and (2) MEP suppression is the same following cortical and midcervical stimulation.

Frontal and parietal components of enhanced somatosensory evoked potentials: a comparison between pathological and pharmacologically induced conditions.

Pathologically enhanced somatosensory evoked potentials (giant SEPs) were recorded in 10 patients with cortical myoclonus of various origins. With non-cephalic reference electrodes a giant frontal negativity corresponding to normal N30 was found over the contra- and ipsilateral hemispheres which was not simply a phase reversal of the well-known enhanced parietal P25. The preceding far-field P14, parietal N20 and frontal P22 were of normal size. A similar result was found when SEPs were studied during the action of etomidate, an ultrashort-acting non-barbiturate hypnotic which produced a marked increase of the parietal P25 and frontal N30 after intravenous administration. These increased components, on the other hand, were abolished when recording was repeated immediately after application of electroconvulsive shock whereas P14, N20, and P22 remained more or less unchanged in both conditions. Our results indicate that there are neuronal elements in the sensorimotor cortex which are more resistant to influences such as narcotic drugs and seizure activity than others, being highly modifiable by these alterations. It is speculated whether these highly modifiable cortical systems are those in which giant SEPs, as well as pharmacologically increased SEP components, arise.

Prognostic value of somatosensory- and motor-evoked potentials in patients with a non-traumatic coma.

SummaryA total of 28 patients with non-traumatic coma were studied both with somatosensory- and motor-evoked potentials. While somatosensory-evoked potentials (SEP) have proved to be useful in predicting the outcome in patients with severe brain damage, the aim of this study was to find out whether the additional evaluation of motor-evoked potentials (MEP) could contribute to a better prediction of the outcome than SEP alone. Our results clearly indicate that in terms of prognostic value, SEP are superior to MEP. Nine patients with bilaterally preserved MEP died, while all of the patients with bilaterally preserved SEP and a central conduction time ≤ 6.5 ms survived, with a Glasgow outcome score of 1 to 3. Therefore, we cannot recommend the inclusion of MEP in the prognostic evaluation of patients with non-traumatic coma.

Event-related brain potentials (P300) and neuropsychological deficit in patients with focal brain lesions

Sixteen patients with chronic focal brain lesions were investigated with an acoustic P300 test and psychological tests of spatial abilities (Maze Tracing Speed Test, Form Board Test), cognitive speed (Sequential Number Connection Test), categorization (Figure Sorting Test), verbal fluency and vigilance. Neither the psychological battery nor P300 analysis discriminated frontal from retrorolandic brain lesions. Abnormalities of P300 significantly correlated with impairments in those psychological tests which had a spinal component in common. We suggest that abnormality of the P300 with focal brain damage rather indicates higher mental function impairment than direct effects of the lesion.

Differences of cortical activation in spontaneous and reflex myoclonias

Frontal and parietal components of somatosensory evoked potentials (SEPs) following median nerve stimulation and scalp potentials preceding myoclonic jerks (jerk-locked averaging, JLA) were compared in 6 patients with cortical reflex myoclonus. Giant potentials were found over the parietal cortex in both conditions. Prominent frontal activity was detected following median nerve stimulation which, however, was absent in jerk-locked averages. Therefore an identical generator of the giant SEP and the JLA is unlikely. As the frontal component is lacking in jerk-locked averaging, the spontaneous jerks produced in our experimental paradigm are believed to be due to spontaneous hyperactivity of the parietal cortex rather than to pathologically enhanced transcortical reflexes.

Motor evoked responses recorded epidurally in a patient with Guillain-Barré syndrome

SummaryThe case of a 75-year-old man with Guillain-Barré syndrome is presented. By means of transcranial electrical stimulation and epidural recording at the spinal level L2-3, distinct potentials with a latency of 21ms were obtained when the patient was tetraplegic. At the same time electromyographic responses of the thenar and anterior tibial muscles were absent following both transcranial and peripheral nerve stimulation. The patient recovered partially within 4 weeks. It is concluded that epidurally recorded motor evoked responses allow electrophysiological assessment of the descending pathways even in severe cases of Guillain-Barré syndrome and might contribute to a more accurate prediction of outcome.