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IONM Modalities

IONM is designed to forewarn the surgeon or anesthesiologist of a problem with dysfunction in the nervous system before it leads to permanent neurological injury. Since the time to permanent injury from an insult depends upon the type of insult, its duration, and its location in the nervous system, the window of opportunity to correct the insult and limit its negative effects following a warning also varies. Prompt reporting of IONM events to the surgeon followed by swift remediation of any identified causes should lead to the best patient outcomes.



  • Evidence for efficacy of IONM is greatest in complex spinal surgery.
  • Many anesthetic agents directly affect the ability to record evoked potential responses and therefore may limit the usefulness of IONM if not given properly.
  • TCeMEPs (transcranial motor evoked responses) are highly dependent on the level of paralytic agent used. A train of four yielding at least 3 twitches is required for reliable use.
  • So called ‘neurogenic’ motor evoked responses elicited by direct cord stimulation appear to actually produce responses primarily from sensory tracts and therefore add no additional protection over standard SSEPs.

Using IONM
There are a variety of surgeries where IONM is indicated and its use supported by the scientific literature. There are some surgical procedures that should not be attempted without monitoring. The list of surgical procedures where IONM is indicated or may be highly useful include:

  • Surgery of the aortic arch, its branch vessels, or thoracic aorta, including carotid artery surgery, when there is risk of cerebral ischemia resection of epileptogenic brain tissue or tumor.
  • Resection of brain tissue close to the primary motor cortex and requiring brain mapping.
  • Protection of cranial nerves: tumors that affect optic, trigeminal, facial, auditory nerves, cavernous sinus tumors, microvascular decompression of cranial nerves, oval or round window graft, endolymphatic shunt for Meniere's disease.
  • Correction of scoliosis or deformity of spinal cord involving traction on the cord.
  • Protection of spinal cord where work is performed in close proximity to the cord as in the placement or removal of old hardware, or where there have been numerous interventions.
  • Spinal instrumentation requiring pedicle screws or distraction.
  • Decompressive procedures on the spinal cord, or cauda equina carried out for myelopathy or claudication where function of spinal cord or spinal nerves is at risk.
  • Spinal cord tumors.
  • Neuromas of peripheral nerves of brachial plexus, when there is risk to major sensory or motor nerves.
  • Surgery for intracranial AV malformations.
  • Surgery for arteriovenous malformation of spinal cord.
  • Cerebral vascular aneurysms.
  • Surgery for intractable movement disorders.
  • Arteriography, during which there is a test occlusion of the carotid artery.
  • Circulatory arrest with hypothermia [does not include surgeries performed under circulatory bypass (e.g., CABG, ventricular aneurysms)].
  • Distal aortic procedures, where there is risk of ischemia to spinal cord.
  • Leg lengthening procedures, where there is traction on sciatic nerve or other nerve trunks.
  • Basil ganglia movement disorders.
  • Surgery as a result of traumatic injury to spinal cord/brain.
  • Deep brain stimulation.

If you are unfamiliar with intraoperative monitoring, take time to browse our patient information, along with this physician resource areas. We strive to make intraoperative monitoring understandable, accurate and of real benefit to patients and their physicians. If you have questions or comments we would love to hear them here.    Contact Us