How Is Intraoperative Neurophysiological Monitoring Performed?

Intraoperative neurophysical monitoring measures nerve activity through electrodes. This helps the surgeon know they aren't injuring nerves by accident.

Intraoperative neurophysiological monitoring is a procedure to continuously monitor the nervous system’s functional integrity during operations involving the brain and spinal cord. Electrodes or needles attached to the patient’s relevant anatomy continuously transmit information, which can be viewed on a monitor. A neurophysiologist analyzes and interprets the data transmitted and alerts the surgeon to any abnormal reading.

Intraoperative neurophysiological monitoring is performed using any of the following modalities:

Electromyography (EMG)

Electromyography is a procedure to continuously monitor and record the muscle’s electrical activity using needle electrodes. 

Technique

• The neurologist places needle electrodes into the muscle and monitors its spontaneous activity continuously.
• To test the muscle’s functional integrity, the associated nerve can be stimulated, which results in muscle contraction.

Clinical Uses

• Facial and cranial nerve monitoring: For nerve stimulation during surgeries involving the brain and face.
• Selective dorsal rhizotomy: To identify and selectively sever affected nerves to treat cerebral palsy.
• Tethered spinal cord release procedure: To identify and remove scar tissues that are attached to the spinal cord limiting its movement.
• Pedicle screw placement: For correct placement of surgical screws during spinal fusion surgery.

Electroencephalography (EEG)

Electroencephalography monitoring is performed during surgeries in the brain or major vascular surgeries.

Technique

• The neurologist places electrodes in the scalp and monitors the brain’s electrical activity with the EEG.

Clinical Uses

Electroencephalography reveals changes in blood flow in the brain and the surgeon can take immediate corrective action if there is a problem. The procedure is used during surgeries such as:

• Carotid endarterectomy (removal of plaque from carotid artery)
• Cerebral aneurysm repair
• Aortic cross-clamping for repair of narrowed aorta
• Cardiac surgery with blood circulation arrest

Electrocorticography

Electrocorticography monitoring is performed by placing the electrodes inside the cortex region of the brain. This method of monitoring is used during surgeries for epileptic seizures to:

• Identify the origin of seizures in the cortex
• Complete removal of the affected tissue accurately

Cortical Electrical Stimulation

Cortical electrical stimulation involves stimulating the cortical region of the brain. This procedure is useful in mapping the functional part of the nerve tissues in lesion removal surgeries for epileptic seizures.

Evoked Potentials

Evaluation of evoked potentials is a procedure to assess the response time taken by the brain to stimulus through touch, sight or sound. Several methods are followed to record evoked potentials depending on the procedure.

• Somatosensory Evoked Potentials (SSEP)
• Somatosensory function is the brain’s ability to interpret bodily sensations such as touch, pain, itch, temperature, vibration and pressure.

Technique

The neurologist

• Places recording electrodes in the scalp, in the cervical spine, or in the operation area, close to the lesion if possible.
• Places needle electrodes to stimulate peripheral nerves such as
• Median or ulnar nerve in the wrist
• Posterior tibial nerve or peroneal nerve near the ankle
• Stimulates the peripheral nerves and monitors the time taken for the brain’s response to the stimulus.

Clinical Uses

SSEP monitoring is performed to prevent nerve damage during:

• Spinal surgeries
• Cranial vascular surgeries
• Surgery in the cortex region of the brain

Brainstem Auditory Evoked Potentials (BAEP)

Brainstem auditory evoked potentials record the brain’s response to sound stimulus. This helps in monitoring the acoustic nerve, brainstem and cerebral cortex. The procedure is also known as brainstem auditory evoked responses (BAER), auditory evoked potentials (AEP) and auditory brainstem responses (ABR).

Technique

The neurologist monitors the auditory nerve response by:

• Placing electrodes in the scalp near the ears and the top of the head
• Making repetitive auditory clicks in the patient’s ears

Clinical Uses

BAEP monitoring is used during surgeries for conditions such as:

• Acoustic neuroma or meningioma (benign tumors in the ear nerves)
• Trigeminal neuralgia

Motor Evoked Potentials (MEP)

Motor evoked potentials monitoring is performed eliciting responses from the lower spinal cord and peripheral nerves by electrical or magnetic stimulation of the spinal cord or the motor cortex region of the brain.

Technique

The neurologist monitors muscle response by:

• Placing electrodes in the spine, scalp or inside the cortex if the skull is opened for surgery
• Delivering electrical or magnetic stimulation through the electrodes

Clinical Uses

• MEP monitoring is used during brain and spinal surgeries, solely or in combination with SSEP.

Visual Evoked Potentials (VEP)

Visual evoked potential monitoring during surgeries near the visual neural pathways prevents vision impairment.

Technique

The neurologist monitors the visual nerve function by:

• Placing electrodes in the scalp
• Flashing strobe lights or light-emitting diodes (LED) in the patient’s visual field.

Clinical uses

Visual evoked potentials monitoring is still developing and not in common use. This method of monitoring has been used for tumor removal surgeries performed close to the visual nerves.

The challenges in the intraoperative neurophysiological monitoring are:

• Electromagnetic interference: Typically, operating rooms are full of monitoring devices that emit electromagnetic waves. Adequate filtering and shielding from signal interference are essential to prevent incorrect recording of signals.
• Anesthetic effects: Anesthetic agents work by suppressing the brain’s metabolism. This can alter evoked responses in the brain and the EEG recordings. The anesthesia needs to be light enough to produce responses while deep enough to block pain.

Actual costs of intraoperative neurophysiological monitoring are difficult to assess as it involves personnel, electronic devices and disposable material. Intraoperative monitoring reduces the likelihood of permanent impairment but does not eliminate the risk. The procedure is still cost-effective, considering the health care costs of avoidable permanent brain disability caused by a lack of real-time intraoperative data on nerve function.