Neurophysiologic Monitoring
Judith A. Freeman
▪ INTRODUCTION
Surgery often involves operating in close proximity to peripheral nerves, the spinal cord, and the brain, or their blood supply. These structures can be damaged unintentionally from scalpels, retractors, electrocautery devices, or other surgical instruments. Neurophysiologic monitoring can be used during surgery to assess the status of peripheral nerves, the spinal cord, and the brain. It can serve as an early warning system to alert the surgeon that something is wrong while there is still time for corrections to be made before permanent injury occurs. The basic technique is to apply a stimulus in the central or peripheral nervous system and to measure the response. The health of the system is determined from the nature of the measured response. Neurophysiologic monitoring may also be used to monitor intracranial pressure (ICP) during neurosurgery or when the brain is injured. Finally, neurophysiologic monitoring can be used to assess the depth of general anesthesia to reduce the risk of intraoperative awareness. This chapter provides an introduction to the major neurophysiologic monitoring techniques and their implications for anesthesia.
▪ NEUROPHYSIOLOGIC STIMULUS AND RESPONSE
Neurophysiologic monitoring involves the measurement of electrical signals generated along the entire length of motor or sensory neural pathways from peripheral nerves to the brain. Needle or surface electrodes may be used to both initiate the stimulus and measure the response. There are two basic methods of performing this type of monitoring. First, an electrical stimulus is applied to a peripheral sensory organ or nerve and the response signal is measured as it travels to the brain. In the second method, the stimulus is applied to the scalp over a particular brain region and the response is measured as it travels along the nerve pathways to the periphery. Measurements are averaged with a computer, and the results are displayed on a screen as continuously changing waveforms (older systems recorded the signals on graph paper). Both the amplitude (the strength) and the latency (the time it takes to travel) of the signal yield important information about the health of the pathways (Fig. 41.1). Amplitude and latency are continuously measured during the surgery, and changes in either of these may indicate damage in the neuronal pathway.
This type of monitoring will help to detect impending nerve damage along any part of the pathway produced by surgical manipulation of the brain, the spinal cord, peripheral nerves, or the blood supply to these structures. It provides an early warning system of altered nervous tissue function, thus allowing the surgeon to take steps to avoid permanent postoperative neurologic damage. Some examples of surgeries where neurophysiologic monitoring is utilized include carotid artery surgery (interrupts blood flow to the brain), spine surgery (the operation is in close proximity to nerves), and operations directly involving nerves, the spinal cord, or the brain.
Although nerve damage causes changes in monitored waveforms, they are also affected by changes in the physiologic milieu. Hypoxia, hypotension, hypothermia, and anesthetic drugs (see below) all can alter signal latency and amplitude. These variables must be controlled as much as possible during surgery to avoid affecting neurophysiologic monitoring. Any changes in signal latency or amplitude must be interpreted by taking into account any changes in physiologic parameters or the administration of anesthetic agents.
In most institutions, neurophysiologic monitoring is carried out by certified neurophysiology
technicians under the guidance of an expert neurophysiologist and ultimately overseen by a physician. Arrangements are usually made between the surgeon, the anesthesiologist, and the neurophysiologist regarding the appropriate monitoring for the specific case. The following sections will briefly describe the major neurophysiologic monitoring techniques.
technicians under the guidance of an expert neurophysiologist and ultimately overseen by a physician. Arrangements are usually made between the surgeon, the anesthesiologist, and the neurophysiologist regarding the appropriate monitoring for the specific case. The following sections will briefly describe the major neurophysiologic monitoring techniques.
 ▪ FIGURE 41.1 An example of the mixed peripheral nerve somatosensory evoked potential on stimulation of the posterior tibial nerve. The amplitude or strength of the response is represented by the height of the wave. The latency of the response is represented by the time in milliseconds that it takes for the response to be detected. (From Frymoyer JW, Wiesel SM, et al. The Adult and Pediatric Spine. Philadelphia, PA: Lippincott Williams & Wilkins; 2004, with permission.) |
▪ TYPE OF STIMULUS RESPONSE NEUROPHYSIOLOGIC MONITORING
Somatosensory Evoked Potentials
Repeated electrical stimuli are delivered to a peripheral nerve and the signal is recorded as it passes up the spinal cord into the cerebral cortex. For example, the stimulus could be applied to the posterior tibial nerve near the ankle because the surgeon is working on the spine near the spinal nerve roots. The roots may be difficult to see, and they contribute to the makeup of the tibial nerve. Somatosensory evoked potential (SSEP) waveform changes would alert the surgeon to potential injury to the nerve roots. In another example, the blood flow to the carotid artery must be interrupted during surgery. SSEP waveform changes could indicate that the brain is not receiving enough blood flow.
Motor Evoked Potentials
Electrical stimulation of the scalp overlying the motor cortex of the brain produces a response passing through the spinal cord to the peripheral nerve and finally to the muscle. The response can be detected at any point in this pathway. Of particular importance to anesthesia providers is that stimulation of the motor cortex in the brain intending to stimulate nerves to the legs often stimulates the facial nerve as well. This may lead to jaw clenching during stimulation. A soft bite block should be placed, so that the tongue does not get bitten.
Brainstem Auditory Evoked Potentials or Responses
Repeated clicking sounds are delivered via an earpiece placed in the auditory canal, and the responses are picked up by electrodes on the scalp. This technique monitors the condition of the ear, the cochlear nerve, and the pathway to the auditory cortex through the brainstem. It is most useful during resection of acoustic neuromas (tumor on a nerve leading from the ear to the brain), brain surgery close to the junction of the brainstem and the cerebellum, and during decompression of certain cranial nerves.
Visual Evoked Potentials
Lights are repeatedly flashed in front of the eyes, and the pathway to the visual cortex in the brain is recorded. Visual evoked potentials (VEPs) may be useful information in patients who have tumors involving the optic pathway (the optic nerve and the pituitary gland).
Electromyography
The sensing electrodes are placed in the muscles innervated by the nerve at risk for damage. When the surgeon touches the nerve, an electrical signal will be generated in the muscle. This method is used in spine surgery, surgery where the facial nerve is at risk of damage, and more recently in thyroid surgery with the introduction of the NIM electromyography (EMG) endotracheal tube. The NIM endotracheal tube has two sensing electrodes just proximal to the cuff. When the tube is placed into the trachea with the cuff just past the vocal cords, the sensors will detect signals from the vocal cords. If the surgeon irritates the recurrent laryngeal nerve, the electrodes will sense vocal cord signals. EMG is a test involving
motor nerves; therefore, muscle relaxants should not be used during the anesthetic.
motor nerves; therefore, muscle relaxants should not be used during the anesthetic.
▪ IMPLICATION FOR ANESTHESIA PERSONNEL
Anesthetic agents affect the evoked potential signals in varying degrees. Inhaled anesthetic gases have the greatest effect by depressing signal amplitude and prolonging latency. Low-dose intravenous (IV) agents have less effect on waveforms, but at higher doses, they can significantly decrease amplitude. Some IV agents (e.g., ketamine and etomidate) will even augment the signals. Not all evoked potential signals are equally susceptible to anesthetic agents.
For most cases in which neurophysiologic monitoring will be utilized, anesthesia will consist of a small amount of inhaled anesthetic gas, supplemented by IV infusions, primarily propofol (some institutions require total IV anesthesia when monitoring MEPs). Opioids are frequently used to supplement the anesthetic. Muscle relaxants should be avoided in all cases where the motor response of a nerve will be monitored visually or by EMG or MEP. Because propofol infusions often cause hypotension, it is important to maintain perfusion of the brain and the spinal cord. A phenylephrine infusion may be required.
Cases involving neurophysiologic monitoring are often complex and carried out for many hours; therefore, large amounts of propofol may be administered. At the end of the procedure, it may not be possible for the patient to rapidly emerge from anesthesia and undergo extubation of the trachea. Transport of an intubated patient to either the postanesthesia care unit or the intensive care unit (ICU) is always a possibility, and a transport monitor and an Ambu bag should always be available.
Anesthesia technicians should prepare for these cases with the following:
A multichannel infusion pump with appropriate tubing
100-mL propofol vials for infusion
Possible phenylephrine infusion
Soft bite block
Transport equipment for an intubate patient
▪ OTHER BRAIN MONITORS
Electroencephalography
Electroencephalography (EEG) measures brain activity through an array of 20 electrodes placed at specific locations on the scalp. It may also be measured directly during a craniotomy by electrodes placed on the brain (electrocorticography). The standard recording has 16 channels and requires special training and experience to interpret. The signal may be processed to produce a single number, which may be more easily interpreted and indicates in which general direction the EEG is going. The bispectral index (BIS) monitor is a form of processed EEG. Hypoxia, hypotension, temperature changes, carbon dioxide tension, and all anesthetic drugs may affect the EEG. EEGs may be used during neurosurgical ablation of a seizure focus, awake craniotomy for resection of a tumor or vascular malformation, or carotid surgery.
Bispectral Index (BIS Monitor)
Although anesthesia has been delivered safely for many years, there is no specific monitor for determining whether a patient is actually unconscious. Adequacy of anesthesia is based on a combination of knowledge of drug doses and monitoring of changes in heart rate and blood pressure. Many have argued that these are not reliable indicators of the depth of anesthesia. Multiple studies involving thousands of patients have estimated an incidence of awareness under anesthesia of between 1 in 1,000 and 1 in 10,000 patients anesthetized. Awareness mainly consists of remembering conversations and an inability to move or breathe while experiencing pain (this can happen if the patient is paralyzed with neuromuscular blocking agents). Subsequent significant long-term psychological sequelae including posttraumatic stress disorder may ensue in about 33% of these patients. The causes for awareness under anesthesia have been attributed to the following situations:
It was unsafe to administer deep anesthesia to the patient (e.g., very sick patients, severely injured trauma patients, emergency obstetric surgery where it is important to minimize drugs to the fetus).
Anesthesia machine malfunction (e.g., the vaporizer is not delivering the set amount of agent, problems with gas flows diluting a volatile agent)
Anesthetic has run out (e.g., an empty vaporizer or infusion pump that goes unrecognized).
Total IV anesthesia
Sedated patients where the patient experiences awareness. They sometimes do not understand that awareness is normal and common when a patient is sedated and not under general anesthesia (e.g., sedation only or sedation with regional anesthesia).
Partial awareness during emergence that is interpreted by the patient as intraoperative awarenessRelated posts:
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