Neuromonitoring for Spine Surgery

Aim

To ensure the integrity of the spinal cord and nerve roots especially at the site of surgery, while providing safe and adequate anesthesia.

The Problem

The nervous system is suppressed by anesthetics, hypothermia, hypotension and anemia; depending on anatomical site, specific drug & dose, site of stimulation, and site of monitoring. Discuss the monitoring plan and anesthetic techniques with the surgeon AND the neuromonitoring team prior to starting the case.

EMG

The EMG assesses the integrity of cranial/spinal nerve roots and indirectly peripheral nerves from the site of stimulation to the muscle. Anesthetic Implications: Avoid neuromuscular blocking drugs during the period of monitoring i.e. after intubation and positioning. High dose vapor may have a suppressive effect, and may skew sensory vs motor nerve root thresholds.

Somatosensory Evoked Potentials (SSEP)

SSEPs monitor the integrity of sensory pathways from peripheral nerves to the sensory cortex. Disruption along any part of this pathway may disrupt normal SSEP responses.

Anesthetic Implications: SSEPs are progressively suppressed by inhaled anesthetic (vapor or N2O) > 0.5 MAC. All intravenous agents (propofol, barbiturates, midazolam, opioids, ketamine) have minimal effect and are preferred. Standard dose neuromuscular blockers can be used.

Auditory Evoked Potentials (AEP)

AEPs monitor the integrity of auditory pathways from ear through brain stem to cortex. They are also referred to as brainstem auditory evoked potentials (BAEP or BAER).

Anesthetic Implications: Similar to SSEP but are much less sensitive to inhaled anesthetics so that most anesthetic options are acceptable.

Motor Evoked Potentials (MEP)

MEPs involve transcranial motor cortex stimulation to elicit a response from muscles and thereby assess the integrity of motor pathways.

Anesthetic Implications: MEPs are easily suppressed by inhaled anesthetics (vapor >N2O). These should usually be kept to a minimum or not used. In patients with preexisting neuropathy <0.3MAC is preferred while in patients without neuropathy <0.5MAC is usually acceptable. IV hypnotics are less suppressive. Propofol up to 120
mcg/kg/min is usually acceptable. Ketamine & etomidate have minimal effect or increase amplitude. There is insufficient literature about dexmedetomidine but it seems
to work well in many patients.