Intraoperative neuromonitoring

Intraoperative neuromonitoring (IONM) is a monitoring technique which is used to assess the integrity of the brain, brainstem, spinal cord, cranial, or peripheral nerves.

Types of IONM

Type	Stimulate	Signal	Measures
Electromyography (EMG)	Nerves	Muscle response (twitches)	Functional integrity of nerve root and peripheral nerves
Motor Evoked Potentials (MEP)	Cerebral cortex	Muscle response (twitches)	Descending motor pathways (e.g. corticospinal tract)
Somatosensory Evoked Potentials (SSEP)	Peripheral nerve (e.g. median nerve, ulnar nerve, posterior tibial nerve)	Cerebral cortex	Sensory pathways (e.g. dorsal columns)
Electroencephalography (EEG)	Cortex	Cortex	Cortical electrical activity to detect cerebral ischemia
Visual Evoked Potentials (VEP)	Light under eyelid	Visual cortex	Optic pathway integrity
Brainstem Auditory Evoked Response (BAER)	Sound emission in ear	Auditory cortex	Auditory pathway integrity
Direct Stimulation of Specific Nerve	Specific nerve (e.g. facial nerve)	Muscle response (e.g. orbicularis oculi, orbicularis oris, frontalis, mentalis)	Specific nerve function

Anesthetic Considerations

• Most to least affected by anesthetics
  • VEP (Very)
  • MEP (Mostly)
  • SSEP (Somewhat)
  • BAER (Barely)
• Effects of anesthetic medications on neuromonitoring
  • Volatile ↓↓
  • Propofol ↓
  • Benzodiazepines ↓
  • Nitrous ↓
  • Opioids minimal effect
  • Etomidate ↑
  • Ketamine ↑
• Effect of patient factors on neuromonitoring
  • Anemia, ischemia, hypoxia ↓

Types of Neuromonitoring Used in Specific Surgeries

Surgery	IONM
Intracranial tumor removal, aneurysm clipping	SSEP, EEG
Acoustic neuroma	SSEP, EMG, BAER, CNVII direct stimulation
Chiari malformation	MEP, BAER, CNVII direct stimulation
ACDF	SSEP, +/- MEP, +/- EMG
Thoracic spine surgery	SSEP, MEP
Lumbar spine surgery	EMG