Intraoperative Cranial Nerve Monitoring: Anatomy, Electrophysiology and Technique Jeffrey R. Balzer, Ph.D., DABNM, FASNM Associate Professor of Neurological Surgery, Neuroscience and Acute and Tertiary Care Nursing Director, Clinical Services, Center for Clinical Neurophysiology Director, Cerebral Blood Flow Laboratory University of Pittsburgh Medical Center Electromyographic (EMG) Recordings • Passive recordings (free-run EMG) made from muscles innervated by cranial nerves of interest. Used as a real-time indication of cranial nerve manipulation or insult. • Direct stimulation (triggered-EMG) of cranial nerves for: • The purpose of identification and mapping course of nerve through tumor or bone • Testing functional integrity • Predicting outcome Free-Run EMG Characterization of Free-Run EMG • EMG bursts or trains can be classified and analyzed via several parameters: – Number – Duration – Frequency – Peak-to-Peak Amplitude High Frequency Bursts: Neurotonic Discharge • High-frequency (50-100 HZ) bursting activity or neurotonic discharges are thought to reflect axonal damage or loss and portend poor outcome. • Associated with medial traction or pressure put on cranial nerve. • Audio typically likened to “dive-bomber” or “helicopter” sounding high frequency bursts. f-EMG Activity Patterns • Early studies assumed that any “train activity” 1 or 2 “neurotonic discharge” indicates potential injury to CN VII 1. Prass RL & Luders H (1986) Neurosurgery 19:392-400 2. Harner SG, Daube JR & Ebersold MJ (1987) Mayo Clin Proc 62:92-102 • Associated intensity of EMG activity, based on duration and volume of loudspeaker, with potential for injury. Romstöck et al., J. Neurosurgery, 93:586-593 2000 • Reviewed 30 patients undergoing large acoustic neuroma/meningioma resection. • Recorded and reviewed “characteristic EMG discharges” during surgery and compared to typical surgical maneuvers and postoperative facial nerve function. • Measured the number, duration, frequency, peak-to-peak amplitude, EMG waveform pattern (bursts or trains) and occurrence at a specific time. EMG activity recorded in the form of Spikes, Bursts, B-Trains and C-Trains “did not exhibit any correlation with pre- and post-operative paresis.” A-trains, regardless of their amplitude, duration and frequency, was “highly predictive of additional (new) postoperative morbidity”, specifically facial paresis. A sensitivity of 86% and a specificity of 89% were calculated for the A-Train activity Prell et al., J. Clin Neurophysiol 25:225-232, 2008 • Reviewed 15 patients undergoing microvascular decompression in trigeminal neuralgia. • Recorded and reviewed “EMG waveform patterns known from vestibular schwannoma surgery”. • Detected and measured EMG Spikes, Bursts and Trains (A, B and C) and occurrence at a specific time. Axonal Excitability • Irrigation fluids can cause a transient discharge of the musculature via neuronal depolarization. • This can be followed by a depression of discharge as the threshold for spike initiation is altered. • Even with warm irrigation fluid, changes in cranial motor neuron discharge frequently occurs. Conclusions • EMG trains of high-frequency and long duration are specific and sensitive indicators of cranial nerve injury. • The threshold train duration and frequency that predicts new post-op cranial nerve deficits: • Is lower for nerves demonstrating pre-operative deficits • Is lower for nerves involved in the surgical focus, i.e., tumor • The utilization of free-run EMG to predict nerves at risk of iatrogenic injury may be conditional: • On the specific surgical procedure and approach • On the extent of associated pathology Triggered-EMG Triggered-EMG • Method utilizes a stimulus/response paradigm whereby electrical stimulator is used to induce an evoked motor response in target musculature. • Hand-held probes with specific electrical output characteristics can be used to “search and map” the surgical field. Choice of Stimulation Parameters • For initial probing of the surgical stimulation duration should be 0.2 ms and applied at an intensity of 1-2 mA. • If the nerve in question is covered by tissue or tumor, stimulus strength might need to be increased. • Once a positive response in observed, stimulation intensity can be reduced to approximate proximity to neural structure. Techniques to Facilitate Finding a Nerve • Nerve location direction and course can be determined by slowing moving the probe and assaying triggered-EMG amplitude. • Method requires frequent fine adjustments of the stimulus strength and close collaboration between neurophysiologist and surgeon. • Assumes that tissue surrounding the nerve is isotropic with respect to resistance. Stimulation Pitfalls Current Shunting Current “Jumping” Kartush and Bouchard, 1992 Prognostic Value of Triggered-EMG • The ability to activate targeted musculature after stimulation of cranial nerves at the brainstem after tumor removal has been shown to correlate with good post-operative outcome. • Several studies, using different methods and criteria, have defined which measures are most sensitive in predicting outcomes. Optimal Anesthesia • No muscle relaxants should be given after the initial intubation dose is given. • Four out of 4 twitches are necessary; partial blockade is not acceptable. • A positive control for the presence of pharmacological paralytics should be used in every case if possible. • Care should be taken not to use local anesthetic mixtures locally for blood loss control especially in infratemporal and extracranial procedures. Techniques for Monitoring Cranial Nerves Cranial Nerves I & II • CN I – Olfactory Nerve – Collection of sensory nerve rootlets that extend from the olfactory bulb. – Not utilized in intraoperative monitoring. • CN II – Optic Nerve – Originates from the retinal ganglion cells which are connected to the specialized receptors in the retina (rod and cone cells). – Optic nerve exits the back of the eye and enters the optic canal and exits into the cranium. It enters the central nervous system at the optic chiasm (crossing) where the nerve fibers become the optic tract just prior to entering the brain. – Can be non-specifically monitored intraoperatively using flash visual evoked potentials (VEP). CN III, IV and VI: Oculomotor Muscles • Muscles that control movement of the eye. • Included in this group are the medial rectus, lateral rectus, superior rectus, inferior rectus, inferior oblique, superior oblique, musculus orbitalis, and levator palpebrae superioris. Cranial Nerve III: Oculomotor • Originates from motor neurons in the oculomotor (somatomotor) and Edinger-Westphal (visceral motor) nuclei in the brainstem. • Cell bodies give rise to axons that exit the ventral surface of the brainstem as the oculomotor nerve. • Innervates superior, medial and inferior rectus and inferior oblique muscles. • Recording electrodes: • Medial rectus at inner canthus or inferior rectus at infraorbital margin Cranial Nerve IV: Trochlear • Originates from cell bodies located in ventral part of the brainstem in the trochlear nucleus. • Innervates only superior oblique muscle. • Recording electrodes: • Superior oblique at supraorbital margin • One quarter out from inner canthus Cranial Nerve VI: Abducens • Originates from cell bodies located in the ventral pons. • Innervates the lateral rectus muscle for contraction. • Recording electrodes: • Lateral rectus at outer canthus CN IV CN III CN VI Electrophysiological Characteristics Latency to Onset of Oculomotor Responses Cranial Nerve III: 2-5 ms Cranial Nerve IV: 3-5 ms Cranial Nerve VI: 2-7 ms Stimulus-Triggered EMG: CN IV Expanded endonasal approach to skull base craniopharyngioma on the right. MEP of R CN IV Onset latency 4.1 ms Stimulus-Triggered EMG: CN III & VI Expanded Endonasal Approach to Skull base tumor eccentric to left side. MEP of L CN III – live stimulation MEP of L CN VI – live stimulation Free-run EMG recorded from left CN III quiet showing stimulus artifact CN VI quiet showing stimulus artifact Waterfall L CN III – recent responses L CN VI – recent responses Cranial Nerve V: Trigeminal • Sensory and motor components. • Composed of three large branches: • V1: ophthalmic (sensory) • V2: maxillary (sensory) • V3: mandibular (sensory and motor) • Motor branch is distributed to the muscles of mastication, the mylohyoid muscle and the anterior belly of the digastric. • The mandibular nerve also innervates the tensor veli palatini and tensor tympani muscles. • Recording electrodes: • Masseter or temporalis muscles Microvascular Decompression EEA Skull Base Surgery Electrophysiological Characteristics Latency to Onset of Trigeminal Nerve Response Cranial Nerve V: 3-5 ms (always earlier than VII) From Möller Cranial Nerve VII: Facial • Mixed nerve containing both sensory and motor components. • Emanates from the brain stem at the ventral part of the pontomedullary junction. • The main body of the facial nerve is somatomotor and supplies the muscles of facial expression. • Recording electrodes: • Orbicularis oculi • Orbicularis oris • Mentalis Electrophysiological Characteristics Parotidectomy Mastoidectomy Latency to Onset of Facial Nerve Response: Intracranial Procedures Cranial Nerve VII: 6-8 ms From Möller Retro-Sigmoid Craniectomy for Resection of Acoustic Neuroma Hand-held monopolar stimulator set at 1.5V and 0.5V and was stimulating at different sites in each slide. MVD of CN VII for HFS: Lateral Spread Response • Generated via stimulation of either the zygomatic or marginal mandibular branch of the facial nerve.