What are intraoperative neurophysical monitoring techniques?
Intraoperative neurophysiological monitoring is performed using any of the following modalities:
Electromyography is a procedure to continuously monitor and record the muscle’s electrical activity using needle electrodes.
- 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.
- 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 monitoring is performed during surgeries in the brain or major vascular surgeries.
- The neurologist places electrodes in the scalp and monitors the brain’s electrical activity with the EEG.
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 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.
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.
- 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.
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).
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
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.
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
- 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.
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.
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.