Intraoperative Neurophysiological Monitoring (IONM) is the use of electrophysiological methods (such as EEG, EMG, and evoked potentials) to monitor the functional integrity of neural structures (brain, nerves, spinal cord) during surgery.
The most common applications are in neurosurgery such as spinal surgery; selected brain surgeries; carotid endarterectomy; ENT procedures such as acoustic neuroma resection; and peripheral nerve surgery. Motor evoked potentials have also been used in surgery for TAAA (thoracic-abdominal aortic aneurysms).
Intraoperative monitoring is used to localize neural structures, for example to locate cranial nerves during skull base surgery; to test function of these structures; and for early detection of intraoperative injury, allowing for immediate corrective measures.
Since the 1970's, SSEP (somatosensory evoked potentials) have been used to monitor spinal cord function by stimulating a nerve distal to the surgery, and recording from the cerebral cortex or other locations rostral to the surgery. A baseline is obtained, and if there are no significant changes, the assumption is that the spinal cord has not been injured. If there is a significant change, corrective measures can be taken; for example, the hardware can be removed.
More recently, transcranial electric motor evoked potentials (TCeMEP) have also been used for spinal cord monitoring. This is the reverse of SSEP; the motor cortex is stimulated transcranially, and recordings are made from muscles in the limbs, or from the spinal cord caudal to the surgery. This allows direct monitoring of motor tracts in the spinal cord.
EEG (electroencephalography) is used for monitoring of cerebral function in neurovascular cases (cerebral aneurysms, carotid endarectomy) and for defining tumor margins in epilepsy surgery and some cerebral tumors.
EMG (electromyography) is used for cranial nerve monitoring in skull base cases and for nerve root monitoring and testing in spinal surgery.
ABR (auditory brainstem response, aka BSEP, BSER, BAEP, BAER, etc.) is used for monitoring of the acoustic nerve during acoustic neuroma and brainstem tumor resections.