Surgical Procedures
Patients benefit from neuromonitoring during certain surgical procedures, namely any surgery where there is risk to the nervous system. Most neuromonitoring is utilized by spine surgeons or neurosurgeons, but vascular, orthopedic, otolarygologists and urology surgeons have all utilized neuromonitoring as well.
The most common applications are in neurosurgery such as spinal surgery; selected brain surgeries; carotid endarterectomy; ENT procedures such as acoustic neuroma (vestibular schwanoma) resection, parotidectomy; and 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.
For example, during any surgery on the thoracic or cervical spinal column, there is some risk to the spinal cord. Since the 1970s, 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 made from muscles in the limbs, or from 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.
EEG measures taken during anesthesia exhibit stereotypic changes as anesthetic depth increases. These changes include complex patterns of waves with frequency slowing accompanied by amplitude increases which typically peak when loss of consciousness occurs (loss of responses to verbal commands; loss of righting reflex). As anesthetic depth increases from light surgical levels to deep anesthesia, the EEG exhibits disrupted rhythmic waveforms, high amplitude burst suppression activity, and finally, very low amplitude isoelectric or 'flat line' activity. Various signal analysis approaches have been used to quantify these pattern changes and can provide an indication of loss of recall, loss of consciousness and anesthetic depth. Monitors have been developed using various algorithms for signal analysis and are commercially available, but none have as yet proven 100% accurate. This is a difficult problem and an active area of medical research.
EMG is used for cranial nerve monitoring in skull base cases and for nerve root monitoring and testing in spinal surgery. ABR (aka BSEP, BSER, BAEP, etc.) is used for monitoring of the acoustic nerve during acoustic neuroma and brainstem tumor resections.
Read more about this topic: Intraoperative Neurophysiological Monitoring
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