Electrical stimulation of motor cortex in experimental cortical ischaemia: pyramidal responses at C5 and the surface EMG.

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 C5 level and the EMG was recorded from the contralateral hand or foot muscle using subdermal needle electrodes. Cortical ischaemia was produced by transorbital occlusion of the common anterior cerebral artery, and regional cortical blood flow was measured by the hydrogen clearance method. In the normally perfused brain, the later I waves of the C5 response required a lower stimulus strength to elicit them than the earlier I1 wave. It was more difficult to record the EMG from the hand than from the foot following stimulation of the corresponding cortex even though the C5 responses were always obtained in both cases. With moderate ischaemia, the later I waves were selectively abolished, leaving the D and I1 waves. EMG amplitude was significantly correlated with cortical blood flow (r = 0.88, P less than 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.