Chapter 123

### INTRODUCTION

The spinal cord is a complex portion of the nervous system involved in receiving, sending, and processing information from the outside world to the brain and vice versa. Many neurologic monitoring modalities exist to aid in the preservation of spinal cord integrity during surgical intervention. One of these modalities is spinal cord evoked potentials. Spinal cord evoked potentials are electrical activity generated in response to either a sensory or motor stimulus: hence, they are categorized as sensory evoked potentials and motor evoked potentials (MEPs). Monitoring requires special training, as well as appropriate equipment and sufficient operating room space.

### SENSORY EVOKED POTENTIALS

Somatosensory evoked potentials require electrodes placed near peripheral nerves. An electric signal stimulates the electrodes that are transmitted to the sensory cortex where electrodes placed in the scalp measure the potential. Sensory evoked potentials measure the integrity of the dorsal columns.

### MOTOR EVOKED POTENTIALS

With MEPs, electric or magnetic signals are sent through transcranial stimulation or stimulation directly on the spinal cord to peripheral nerves, spinal column, or muscle, where the potential is measured. Motor evoked potentials measure the integrity of the ventral column and associated motor pathways. Transcranial stimulation can involve electric or magnetic stimulation. Electric stimulation consists of electrodes being placed in the scalp over the motor cortex, whereas magnetic stimulation consists of a magnetic stimulator being placed over the motor cortex.

### CLINICAL APPLICATION

Both sensory and motor potentials are measured in terms of latency and amplitude. Latency is the time period from the stimulus to the measured response, and amplitude is a measurement of the voltage of the response. Monitoring of spinal cord evoked potentials can be beneficial in numerous surgeries, including spinal fusion with instrumentation, spinal cord resection, cerebral tumor resection, thoracoabdominal aortic aneurysm repair, epilepsy surgery, and brachial plexus surgery.

#### Anesthetic Agents and Evoked Potentials

Because multiple anesthetic agents can affect the parameters measured (latency and amplitude), anesthesia management and technique is usually modified during evoked potential monitoring. Volatile agents, such as sevoflurane, desflurane, and isoflurane, cause a dose-dependent decrease in amplitude and increase the latency in somatosensory evoked potentials (SSEPs). When monitoring is being used, a minimum alveolar concentration (MAC) of 0.5-0.75 is ideal. Nitrous oxide can further interfere with monitoring and is typically avoided. In general, the intravenous anesthetic agents also decrease amplitude and increase latency in SSEPs, except for etomidate and ketamine, which can increase amplitude. Opioids, when given in large doses, can cause a transient interference with signal transmission. However, the clinical doses of intravenous agents and opioids, particularly when given as infusions, have negligible effects on electrophysiologic (EP) monitoring. Balanced anesthetic techniques using IV agents (eg, propofol) and opioids should be considered when developing the anesthetic plan. Clonidine and dexmedetomidine have negligible effects on EP monitoring and can be used to decrease anesthetic ...

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