Surgical monitoring of motor pathways

Intraoperative monitoring of corticospinal function is no longer an experimental technique, having been introduced into routine practice in a number of centers, each of which has now accumulated large series of some hundreds of cases. Different techniques have been developed by these centers; each has advantages and disadvantages, and it is clear that no one technique in particular is optimal for all surgical procedures. The corticospinal system can be activated by transcranial stimulation of the motor cortex or by direct stimulation of the spinal cord with electrical or magnetic stimuli delivered singly or as double or multiple pulses. The evoked activity may be recorded directly from the spinal cord using epidural electrode, or as a postsynaptic volley in motor axons ("neurogenic motor evoked potentials," MEP), or as a compound muscle action potential (CMAP) from innervated muscles. For scoliosis surgery, we use transcranial electrical stimulation, recording the evoked volley from the spinal cord using epidural electrodes at two spinal levels. By simultaneously stimulating the tibial nerves in the popliteal fossae, descending corticospinal volleys and ascending somatosensory volleys can be recorded in the same sweep. Accordingly, this technique allows monitoring of two different modalities of function at two separate levels of the nervous system, a goal that is most desirable because it helps identify the earliest evidence of dysfunction and at the same time minimizes false-positive reports to the surgeon. Our technique has the advantage of being relatively immune to the depressant effects of anesthesia, and full muscle relaxation is possible--even desirable. More peripheral recordings of neurogenic MEP or CMAP, are sensitive to the choice of anesthetic, and the latter requires incomplete curarization. However, these techniques may be appropriate when the pathology is in the low spinal cord or nerve roots.