Basic methodological principles of multimodal intraoperative monitoring during spine surgeries

In the very early stages of development Intraoperative Neurophysiology (ION), somatosensory evoked potentials (SEPs) was the only available modality for monitoring functional integrity of the spinal cord and was used for many years in USA, Europe and Japan. This uni-modality approach very soon showed many disadvantages: it was nonspecific for the motor tracts, sensitive to inhalation anesthetics, needed relatively long time to update data, and patients with spinal cord pathology would sometimes have low quality of SEPs. Neuroanesthesia at that time, 30 years ago, was mainly based on inhalational anesthetics, the “deadly enemy” for SEPs when used with a higher amount of minimal alveolar concentration. An early report by Lesser et al. [10], showed that injury to the motor tracts of the spinal cord can happen with no changes within the parameters of SEPs. This first observation is later on confirmed by excellent and very well documented examples by Pelosi et al. [16], Minahan et al.[ 12], Jones et al. [9]. To avoid the influence of anesthetics and to show other advantages of direct recordings of traveling and stationary waves of SEPs, a team led by Dr. S. Jones from the UK, introduced monitoring of SEPs by the catheter-electrode in Europe placed in the epidural space of the spinal cord [7]. As highly and reliable this technique was it didn’t gain popularity as an intraoperative monitoring method of SEPs because of the relative invasiveness. During the process of developing different methods for spinal cord monitoring, there were many trials and errors, but probably the most challenging one was when the intraoperative monitoring method of “neurogenic motor evoked potentials” (NMEPS) was introduced [13]. This method was based on the translaminar electrical stimulation of the spinal cord and recording time locked electrical activity over peripheral nerves, mainly of the low extremities. The author of this method claimed that activity recorded from limb muscles represented motor evoked potentials. Later on, collision studies [21] and unfortunate postoperative paraplegic patients with preserved NMEPS showed that this activity was mostly an antidromic stimulation of the dorsal columns. Therefore, it might be used for monitoring the functional integrity of the dorsal, but not for lateral columns [5, 12]. In search of a more specific monitoring method for the functional integrity of the spinal cord’s motor tracts, Machida et al. [11] introduced a method of electrical stimulation of the spinal cord and recording activity over the limb muscles. The breakthrough in ION of the spinal cord took place by introducing methods of intraoperative monitoring of the motor evoked potentials (MEPs), eliciting them by applying single transcranial electrical stimulus and recording D wave from the vicinity of the spinal cord [1]. Using a technique of short train of stimuli, instead of single stimulus, Taniguchi and colleagues, elicited MEPs by direct stimulation of the exposed motor cortex [20] or transcranially and recording them from the limb muscles [15]. An important milestone in the preservation of spinal roots is the development of methodology for electrical stimulation of roots during placement of pedicle screws [2, for review see 22]. In this paper we will describe the methodological aspects of: (a) motor evoked potentials (MEPs) elicited by transcranial electrical stimulation (TES), (b) muscle activity after electrical stimulation of the spinal cord and (c) pedicle screw stimulation. The methodological aspect of intraoperative monitoring with somato-sensory evoked potentials is described in this issue by MacDonald. The importance of combining the use of multimodal evoked potentials during spinal cord surgeries has been addressed by Sala et al., in this issue as well.