Electrical stimulation of motor cortex in experimental cortical ischaemia: pyramidal responses at C 5 and the surface EMG

Abstract

In 7 baboons maintained under propofol anaesthesia, pyramidal tract responses were related to the corresponding peripheral EMG evoked by electrical stimulation of the motor cortex under conditions of focal cortical ischaemia. Pyramidal responses were recorded epidurally at the C 5 level and the EMG was recorded from the contralateral hand or foot muscle using subdermal needle electrodes. Cortical ischaemia was produced by transorbial occlusion of the common anterior cerebral artery, and regional cortical blood flow was measured by the hydrogen clearance method. In the normally brain, the later I waves of the C 5 response required a lower stimulus strength to elicit them than the earlier I 1 wave. It was more difficult to record the EMG from the hand from the foot following stimulation of the corresponding cortex even though the C 5 responses were always obtained in both cases. With moderate ischaemia, the later I waves were selectively abolished, leaving the D and I 1 waves. EMG amplitude was significantly correlated with cortical blood flow ( r = 0.88, P < 0.005), and the threshold of cortical flow for the EMG was 10–13 ml/100 g/min. Our results indicate that changes in amplitude of the late I waves and particularly of the EMG are sensitive indicators of cortical ischaemia.