Intraoperative Neurophysiologic Monitoring - Types, Features and Methods

Introduction:

The ideal goal of a preoperative assessment of a patient is to decrease the number of unwanted and unexpected illnesses and deaths associated with the administration of anesthesia and surgery. It is also important to consider how quickly and for how long a patient takes to recover and resume normal functions.

A preoperative assessment is essential to determine these factors. There is an ever-growing evolution in the field of anesthesia toward formulating a standard protocol to detect the risks related to the nervous system while operating on it. If the damage caused by the surgery is irreversible, the neural components of the body at risk must be identified and saved in patients with neurological problems.

What Are the Types of Intraoperative Neurophysiological Monitoring Techniques?

The operating doctor should have a basic understanding of the principles of monitoring and managing neurological functions during surgery. The anesthetist should have the required experience and understanding of how anesthesia affects neurological functions. There are a few techniques used to monitor them. There are two types of techniques. These techniques can be used as a single modality or in combination.

1. The modalities used to identify spontaneous activity are:

1. Electroencephalography (EEG).

2. Electromyography (EMG).

2. The modalities used to measure the induced electrical reaction of a particular neural tract following an active stimulation are:

1. Somatosensory evoked potential (SSEP).
2. Motor-evoked potential (MEP).
3. Brainstem auditory evoked potential (BAEP).

What Are the Features of a Good Intraoperative Neurophysiological Monitoring Modality?

1. It should have a higher rate of sensitivity and specificity.

2. The time of response should be fast.

3. It should possess the ability to detect injured tissues so that timely measures can be taken to preserve them from irreversible injury.

4. It should protect the normal function of the target organs.

5. It should provide information on the prognosis of surgery which will help plan postoperative care.

How Does Intraoperative Neurophysiological Monitoring Work During Spinal Cord Surgery?

1. Somatosensory Evoked Potential (SSEP):

• It is the most commonly used modality.

• A peripheral nerve is electrically stimulated through the skin to monitor the ascending sensory pathways.

• There is a transmission of electrical impulses from the dorsal root nerve following stimulation and then ascending towards the spinal cord through the dorsal columns.

• SSEPs from the upper limbs are produced by stimulation of the ulnar or median nerve at the wrist. While the SSEPs from lower limbs are produced by stimulation of the posterior tibial nerve at the popliteal fossa or ankle.

• The signals are recorded through the length of the nerve pathway and several simulations are given. Their average is calculated.

• The electrodes are placed at landmarks namely Erb’s point, the cervical spine during subcortical SSEPs, the popliteal fossa during peripheral SSEPs, and the somatosensory cortex during cortical SSEPs.

• It can be done simultaneously during the surgery and live monitoring of the neural function is done.

2. Motor Evoked Potential (MEP):

• This potential is used to assess the descending corticobulbar and corticospinal tracts.

• The stimulation is given through the scalp called Transcranial electrical stimulation (TES) or directly to the brain called Direct electrical stimulation (DES).

• The stimulation starts from the motor cortex, then moves along the descending motor tract then onto the anterior horn of the spinal cord or cranial nerve nuclei through a synapse. Then it moves along any cranial or peripheral nerve and passes through the neuromuscular junction to produce muscle contraction.

• The electrodes are placed on the muscles innervated by the nerves which are thought to be at risk of damage.

• The muscles present in extremities are usually monitored like the tibialis anterior, abductor hallucis, and the thenar muscles.

• The drawback of this type of monitoring is that it is done only at intervals and there is a delay in identifying an injured nerve.

• Motor impairment is a crucial deficit after surgery. This modality is used when motor functions are at risk. The motor neurons demand more nutrition, but in the spinal cord, they are supplied by only one artery-anterior spinal artery. They are used along with SSEP during spinal cord surgery to detect any physical or hypoxic injury to the spine.

3. Brainstem Auditory Evoked Potential (BAEP):

• It is used to supervise the function of the eighth cranial nerve or the vestibulocochlear nerve and brainstem.

• Acoustic stimulus is given at the ear canal by a device. This stimulus generates electrical impulses from the cochlear that travels to the vestibular nerve and then to the nucleus and various brainstem components.

• The recording electrode is placed on the ear lobe or mastoid (behind the ear).

4. Electromyography (EMG):

• EMG checks the functioning of motor cranial nerves, spinal nerves, and peripheral nerves that are at risk by assessing the compound muscle action potentials of their respective innervated muscles.

• There are two types of electromyography produced.

• One is spontaneous EMG which is of low amplitude and frequency and recorded as free running. When there is any damage to a motor nerve, like during stretching or irritation of a nerve, high-frequency neuro tonic discharges are produced which will alert the whole operating team.

• Triggered EMG is when an intentional direct stimulation of a cranial or peripheral nerve is done. It is used when the visualization of a nerve is challenging, in case of tumor removal.

What Are the Anesthesia Concerns Regarding Intraoperative Neurophysiological Monitoring?

• Some of the anesthetic agents have an impact on the evoked potentials. They tend to suppress the potential making it difficult to monitor. So an appropriate anesthetic regimen should be planned.

• Anesthetics should create a secure anesthetic and physiological environment, and facilitate easy detection of signal deviations during surgery.

• Anesthetics should be aware of a definite signal change and the other causes of signal change other than surgery and should act on time to prevent further damage.

• Anesthetics should follow proper precautions to prevent nerve injuries that happen due to Intraoperative monitoring of neural pathways.

• Generally, the suppression of evoked potentials is greater with inhalational anesthetics than with intravenous anesthetics.

How Do the Anesthetic Agents Interfere With Intraoperative Neurophysiological Monitoring?

1. Volatile Agents

• Inhalational against decrease the amplitude and delays the evoked response according to their dose.

• SSEPs can be noted at alveolar concentrations less than 1.

• MEPs are affected at a lesser concentration than SSEPs. BAEPs and EMGs do not tend to change because of inhalational agents.

2. Intravenous Agents

• Barbiturates: SSEP signals are resistant to high doses whereas MEP signals tend to change.

• Ketamine: It is beneficial for the supplementation of anesthesia and analgesia by enhancing the MEP and SSEP signals.

• Propofol: It does not affect the SSEP and MEP at concentrations used in clinics. But affect the signals at a higher concentration.

• Etomidate: They enhance the amplitude of the cortical SSEPs but do not affect the peripheral evoked signals.

• Benzodiazepines: When used at premedication doses do not affect SSEPs and MEPs.

• Opioids: The evoked action potentials are resistant to opioids even at higher concentrations. So, short-acting opioids can be used to lower the anesthetic requirement when neurological monitoring is involved.

• Muscle Relaxants: MEP and EMG recordings are sensitive to muscle relaxants because muscle contraction is hindered at the neuromuscular junction when stimulated.

Conclusion

Initially, when intraoperative monitoring systems for neurological functions were developed, the modalities were occasionally followed by some anesthesiologists. However, an integrated and dedicated team of neurosurgeons, anesthetists, nurses, and neurologists now regularly monitor the procedures. A successful approach to dealing with surgical complications is to establish clear and good communication and trust among the members of the operating team. Then, reducing the disturbance of anesthesia and technical issues during signal transmission. The team should be aware of the timing, nature, and location of any potential neurological injury so that an appropriate monitoring procedure can be implemented. Finally, they should be able to identify and interpret the changes in the signals from neurons, communicate the changes to the team, and devise an organized and timely response to the signal variation.