Animal and human motor system neurophysiology related to intraoperative monitoring

1.1

Introduction

The uses of monitoring motor responses for stimulating the human cerebral cortex have clearly expanded beyond the clinical mapping studies of the exposed brain at operation . The discovery that transcranial electrical stimulation (TES) of human motor cortex could cause muscle activation and its replacement in (awake) humans by the less painful transcranial magnetic stimulation (TMS) greatly expanded the opportunities for motor pathway monitoring. The current uses include:

• 1.

  Intraoperative monitoring was the focus of this and other presentations during the Intraoperative Neurophysiological Monitoring in Neurosurgery (Second International Symposium in New York City, November 20–21, 2000) . Such recordings immediately impact on the surgical procedures and also aid in predicting the outcome (see Chapter 2 : Intraoperative neurophysiology and methodologies used to monitor the functional integrity of the motor system).

• 2.

  Preoperative, mapping of motor representation , for example, prior to cortical removal or implanting a grid for cortical stimulation. The combination of surface visualization of the cerebral cortex by using appropriate MRI software and mapping with focal TMS appears especially advantageous .

• 3.

  Diagnosis and pathophysiological research , for example, of lesions of the motor system such as multiple sclerosis, amyotrophic lateral sclerosis, strokes, myoclonus, etc., have yielded unique data on central motor delays . However, rarely has the clinical diagnosis depended on the electrophysiological monitoring. Rather, such monitoring has provided a quantitative functional measure of the effect of the lesion, which may assist in a correlating the imaged extent of the lesion and the results of neurological examination. Such comparison potentially aids in explaining the underlying pathophysiology of motor system disease, and the changes in brain function, that is, “reorganization” following lesions .

In intraoperative motor system monitoring, the physiological and pathophysiological mechanisms concern two types of response to stimulation of the cerebral cortex. The first, recorded from the corticospinal tract (CT), is relatively insensitive to anesthetic conditions, is “approximately” linearly related to stimulus intensity, but is of low amplitude and the recordings often include troublesome stimulus artifacts requiring special attention. In the second type of recording, the electromyography (EMG) of transcranially activated muscles is of greater amplitude than the CT response and therefore easily recorded by widely used techniques, but it is highly sensitive to anesthetic level and requires particular attention to the parameters of cortical stimulation, to which it is nonlinearly related and a high frequency short-train of stimuli usually being needed.

1.2

Corticospinal responses