Iatrogenic spinal cord injury secondary to spinal surgery is a devastating event. In an effort to minimise the prevalence of this complication multimodal Intraoperative Monitoring (IOM) using a combination of Somatosensory Evoked Potentials (SEP) and Transcranial Electrical Motor Evoked Potentials (MEP) signals is increasingly used in this setting during corrective procedures for scoliosis. The SEP/ MEP signals are continuously monitored throughout the procedure, especially during placement of instrumentation and deformity correction.

Immediate action by the surgeon is required when damage to the spinal cord is suspected at any time during the procedure in response to changes of >50% amplitude and >10% latency in the SSEP/ MEP signals.

The cost calculation of these complications can be exorbitantly high. Still, considering the enormous costs of health care and the human suffering related to a severe injury to the spinal cord that results in paraplegia/paraparesis or quadriplegia/quadriparesis, there is enough evidence to prove that costs of performing IOM surely do not exceed those of health care for the injured patients.

Somatosensory Evoked Potentials (SSEPs)
Electrical stimulation of the posterior tibial nerve at the ankle results in ascending volleys (non-synaptically) via the dorsal columns to the thalamus and then (synaptically) to the somatosensory cortex. SSEPs can be recorded either from the spinal cord (epidural electrodes) or from the scalp. Signal averaging techniques are used to extract the small evoked signal (0.5-3 µV) from the EEG and background noise; in the theatre typically 200-300 trials (sweeps) are necessary. The SSEP reflects the functional status of the sensory tracts which are mainly located in the dorsal columns. The major limitation of SSEP is the relative insensitivity to spinal cord ischaemia, especially when confined to the anterior part of the cord (anterior spinal artery territory). Therefore, when only SSEP are monitored, ischaemia limited to the motor tracts or anterior horn may go undetected, which makes SSEP less suitable for assessment of spinal cord function during procedures where the main mechanism of injury is spinal cord ischaemia (thoracic aortic procedures).

Motor Evoked Potentials (MEPs)
A single transcranial electrical stimulus applied the motor cortex results in descending volleys in the cortico-spinal tract and large amplitude muscle responses are recordable from subdermal needle electrodes placed in the muscle of lower limbs (typically Tibialis Anterior) and/or upper limbs (First Dorsal Interossei), provided that the motor neuron pool is sufficiently excitable. As many anaesthetics enhance GABA-currents and induce neuronal hyperpolarisation, it is necessary to apply a series of transcranial stimuli in quick succession (e.g. a ‘train’ of 5 pulses, separated by 2 msec) to elicit an action potential. In this way, temporal summation of excitatory postsynaptic potentials allows the hyperpolarised neuron to reach firing threshold.

Because the transcranial motor evoked potential (MEP) recorded from muscle traverses the spinal motor neuron pool in the anterior horn, it is extremely sensitive to an acute reduction of spinal cord blood flow. Experimental and clinical data indicate that myogenic transcranial motor evoked potentials (MEP) disappear within 1-2 minutes after the onset of acute ischaemia (residual flow < 25%). This technique is sufficiently rapid to allow timely interventions aimed at correcting ischemic conditions and restoration of spinal cord blood flow. In contrast, EP modalities that rely on axonal conduction such as SSEP may require 15 min or more for a significant change after the onset of ischaemia.

Stim Box

Effects of anaesthetics on intraoperative SSEPs AND MEPs
The success of intraoperative monitoring is critically dependent on anaesthetic management. Almost all anaesthetic drugs depress synaptic function both in the cerebral cortex and in the spinal cord grey matter. As a result, progressively increasing concentrations of inhaled or intravenous anaesthetics may result in EP changes that are indistinguishable from neuronal ischaemia. For spinal cord monitoring with SSEPs and MEPs, the margins are quite narrow. While spinal and subcortical responses are relatively resistant to depression by anaesthetic drugs, cortical responses are sensitive to volatile agents, and a standard isoflurane/N2O technique may depress amplitudes to such an extent that reliable monitoring becomes impossible. SSEPs and MEPs can be consistently recorded during total intravenous anaesthetic (TIVA) techniques, including propofol/opioid/air, etomidate/opioid and ketamine/midazolam.

Electrode placement on the head is carried out according to international 10-20 system, in the anaesthetic room as soon as the patient is put to sleep:

  • SSEP recording electrode (subdermal ‘corkscrew’ needle electrode) is placed over the midline, 2cm posterior to vertex Cz, with reference electrode over the frontal midline, Fz area.
  • SSEP stimulating electrode (pre-gelled disposable surface electrode) is placed over the posterior tibial nerve behind and proximal to the medial malleolus bilaterally and secured using tegaderm.
  • MEP recording electrodes (monopolar subdermal needle electrodes) are inserted subdermally in the Tibialis Anterior muscle bilaterally and secured with tegaderm. The active cathode is placed over the motor-point of the muscle and the reference over the cutaneous border of the tibia.
  • MEP stimulating electrodes (subdermal ‘corkscrew’ needle electrodes) are placed on the head over the left and the right motor strip, C1 and C2 respectively (C1 situated 1cm anterior to vertex Cz and 3cm laterally over the left hemisphere; C2 situated 1cm anterior to vertex Cz and 3cm laterally over the right hemisphere).
  • A ground (earth), large 2” x 4”, pre-gelled surface electrode, is placed on the thigh and secured with tegaderm.

Once the patient is moved onto the oper ating table all the recording and stimulating electrodes are connected appropriately to the headbox and the stimulator box, attached to the operating table.

Whilst the patient is being prepared and draped several consistent runs of baseline SSEPs are acquired and reference trace set. The stimulus intensity is usually set to 45mA supra-maximally.

Similarly several consistent runs of baseline MEPs are acquired and reference trace set. The cortical stimulator (Digitimer D185 MultiPulse) is initially set at a pulse rate of 5 with an inter-stimulus-interval (ISI) of 4, ensuring that the starting sensitivity for all traces is 50uV/Div. Stimulation is started sequentially bilaterally at an initial voltage of 50V; increasing in increments of 50V until a supramaximal level is found. The stimulus level is never allowed to exceed 500V otherwise there is a high risk of burn injury to the scalp.

Subdermal Needle Electrode Corkscrew Electrode

A post-anaesthetic / pre-incision baseline recording is now obtained for both SSEP and MEP, ready for monitoring the entire operation.

Typically, SSEP is recorded continuously throughout the operation with a pause period of 20 sec after each run of 256 sweeps.

MEP is recorded once the first screw is inserted and then typically every 5 min until the completion of all spinal instrumentation.

Synergy IOM System

Alarm Criteria
In most cases, a combination of SSEP and MEP monitoring provides optimal safety. The operating room is an electrically hostile environment, and electrical interference and other artefacts can complicate interpretation of the monitoring potentials. Variations in the depth of anaesthesia, and changes in body temperature and blood pressure can also influence potentials. The monitoring personnel must be able to detect and understand the source of such variables in order to deal with them appropriately. It is not necessary or desirable to alert the surgeon to every change in the SSEP or MEP recordings, particularly if they are not physiologically meaningful. On the other hand, it is absolutely crucial to warn the surgeon when a change in the potentials that may reflect neural injury is observed. Since there can be moment-to-moment changes in any of the potentials, any variation in the recordings must be sustained and reproducible. In practical terms, this means that a physiologically meaningful change in the potentials should be seen in more than one recording and, preferably in more than one monitoring modality. A complete loss of potentials in the appropriate monitoring channels in either SSEPs or MEPs, or both, is clearly an indication of significant disturbance of spinal cord function. The surgeon should be alerted to this occurrence in as timely a manner as possible. Given the fact that SSEP recordings require up to 5-min lag time, MEP recordings are often more useful for confirming a loss of signals.

Before and After X-Rays
  • SSEPs:
    Alarm criteria of a 50% reduction in amplitude of cortical response and a 10% increase in latency are generally used as guidelines for notifying the surgeon of a potential deficit. Factors that potentially affect the SSEP amplitude include halogenated agents, nitrous oxide, hypothermia, hypotension, and electrical interference. A common factor affecting SSEP latency readings is temperature. Any SSEP changes with amplitude reduction of more than 50% is also considered relevant if they are temporally associated with a specific surgical intervention, such as during placement of spinal instrumentation or during correction of a spinal deformity.
  • MEPs: Current criteria used for MEP responses:
    • The ‘all-or-nothing’ criterion.
      This is the most widely cited and used method, given the inherent variability of signals in MEP monitoring. Based on this approach, a complete loss of the MEP signal from a preliminary baseline recording is indicative of a clinically significant event.
    • The amplitude criterion. A modification of the ‘all-or-nothing’ approach involves measuring the CMAP amplitude at baseline, then measuring relative changes in amplitude to determine if a clinically significant change has occurred. The amplitude criterion uses greater than 50% amplitude decrement is regarded as a clinically significant change.

Throughout the operation good communication, particularly with the surgeon and the anaesthetist, is maintained by informing and reassuring of any changes to the SSEP and or MEP waveforms. All relevant observation and communication is documented in the annotation area of the monitoring software, which is then saved with the patient’s report in the database. The monitoring is continued until all spinal instrumentation is complete and skin closure is commenced.

Intraoperative multimodality spinal cord monitoring has a sensitivity and specificity close to 100%. As opposed to the SSEP alone, which could provide a false negative, SSEP along with MEP can provide the surgical team a constant feedback on the state of the spinal cord. For the spine surgeon, the neurophysiologist and the anaesthetist there is a learning curve to master all the tricks of the trade of this complex technology and its surgical and anaesthetic applications.