Cranial Nerve Monitoring

Cranial Nerve Monitoring Brian Owens Minnesota Epilepsy Group

Cranial Nerves
EMG is used to monitor all cranial nerves accept I,II & VIII.
II & VIII are monitored by evoked potentials.
No current method for monitoring of CN I.

EMG (Electromyography)
EMG is the recording of electrical activity of muscle.
Free run EMG
Monitor for irritation or injury
Direct Stimulation
Identify nerves
Test their integrity

The Motor Unit
A single motor neuron and all the muscle fibers it innervates.
Muscle fibers contract in response to action potentials from the neuron.
When all the fibers contract this is called a motor unit action potential (MUAP).

Compound Muscle Action Potential (CMAP)
“ The summation of nearly synchronous muscle fiber action potentials recorded from a muscle, commonly produced by stimulation of the nerve supplying the muscle either directly or indirectly.”
-AAEM 2001

Recording Methods
Surface needle electrodes (most common)
Electrodes are placed subdermally over the muscle.
Can be placed in a monopolar or bipolar manner.

EMG Parameter Settings
Bandpass:
5Hz-5KHz
Time base:
250ms-1sec
Sensitivity:
50-100mV

Direct nerve stimulation
Bandpass:
5Hz-5kHz
Analysis time:
15-20ms
Intensity: 0.1mA-1mA
A threshold can be determined also for stim intensity.

EMG Intraoperative Interpretation
Based primarily on the presence of activity and partially on pattern.
Free run EMG should be made audible for instant feedback to the Surgeon and Neurophysiologist.

EMG Alarm Criteria
Sustained firing of high frequency train lasting for tens of seconds
Several large bursts of activity of complex morphology
Sudden bursts of high amplitude spikes followed by complete silence

EMG Examples Zouridakis, Papanicolaou, 2001 (a) Baseline EMG. Note the low amplitude background activity on ch2. (b) High amplitude spikes are present on ch3 indicating irritation of the nerve corresponding to that channel.

Anesthesia Requirements (EMG)
No muscle relaxants after anesthetic induction.
Depth of relaxation should be verified & monitored. (Train of four)
4/4 twitches optimal.
Gases have little or no effect on EMG.
Often you may be recording other modalities, plan accordingly.

CN I (Olfactory)
No current method for monitoring of the olfactory nerve.
In one study OEP’s, were recorded from the olfactory tract s/p craniotomy & stimulated from the olfactory mucosa.

CN II (Optic)
Flash VEP’s used intraoperatively.
LED goggles used for stimulation.
Typically 3 negative peaks and 3 positive peaks.
Responses do not reproduce well in surgery.
Responses heavily affected by inhalational gases.
Not commonly performed.

CN III, IV, & VI
Skull base tumor removal
Posterior fossa tumor removal
Clivus tumor removal
Use caution when placing needle electrodes near the eye.
III – inferior rectus, inferior oblique IV – superior oblique VI – lateral rectus

CN V (Trigeminal)
Microvascular decompression for trigeminal neuralgia
Clivus tumor removal
Large posteior foss tumor removal
Recorded from the masseter.

Masseter Muscle

CN VII (Facial Nerve)
Acoustic neuroma removal
Parotid gland tumor removal
Recorded from Obricularis Oris (lower branch) & Orbicularis Oculi (upper branch)

CN VII (Facial Nerve)

Orbicularis Oris, Oculi

CN VIII (Vestibular cochlear)
Sensory Nerve
BAERs (Brainstem Auditory Evoked Potentials) used to test function of auditory nerve and pathways in the brain stem.

CN VIII – Surgical Procedures
Skull based procedures
Acoustic Neuromas
Cerebello-pontine angle
Posterior fossa

CN VIII - Anesthesia
No requirements.
Anesthetic agents generally have minimal or no effect on BAERs.

CN VII - Stimulation
Auditory clicks delivered through foam ear inserts attached to an air tube.
Rarefaction is recommended for well defined peaks.
Intensity: 80dB – 100dB
Rate: 11.1Hz
Duration: 0.03 – 0.1 msec
Contralateral ear should be masked with a white noise 40dB less than stimulated ear.

CN VIII - Recording
Bandpass:
30-3000Hz
Sensitivity:
1uV
Time base:
10 -15msec
Trials:
1500-2000

CN VIII - Generators
I - Auditory nerve
II - Cochlear nucleus
III - Superior olive
IV - Lateral lemnisci
V - Inferior colliculus

CN VIII - Interpretation
Have baselines visible for comparison.
Correlate clinical hx, peripheral hearing loss may result in unclear or absent peaks.
BAERs are subcortical and therefore minimally affected by anesthesia.
The slightest changes in amplitude and latency can be significant.
Amplitude and latency of peaks I & V most important.
Interpeak latencies of I-III & III-V represent peripheral and central conduction time and are also critical.

CN IX
Large posterior fossa tumor removal (acoustic neuroma)
Radical neck dissection
Recorded from soft palate or Stylopharyngeus muscle (dilates the pharynx for swallowing)

CN X (Vagus)
Record from false vocal cords with needle electrodes
Or record from vocal cords with special wired endotracheal tube

CN XI (Spinal Accessory)
Jugular foramen tumor removal
Record from Trapezius muscle

CN XII (Hypoglossal)
Jugular foramen tumor removal
Large posterior fossa tumors
Radical neck disection
Recorded from the tongue

Cranial Nerve Monitoring

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