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Intra-operative Neurophysiologic

Monitoring (IONM)

Principles & the Anaesthesiologist’s Role

Chong Chin Ted Neuroanaesthesia Symposium 2012

Neuroanaesthesia Symposium 2012


Chong Chin Ted

MMed(Anaes), FANZCA

Consultant

Dept of Anaesthesiology, Intensive Care and Pain Medicine

Tan Tock Seng Hospital

National Healthcare Group, Singapore

Clinical Senior Lecturer

Yong Loo Lin School of Medicine

National University of Singapore


What is IONM?


Intra-operative neurophysiologic monitoring (IONM) is a technique of

measuring electrical potentials from the nervous system, that is

directly aimed minimize neurological morbidity from operative

manipulations


Provides feedback regarding changes in brain, spinal cord, and

peripheral nerve function prior to irreversible damage


Localize anatomical structures, including peripheral nerves and

sensorimotor cortex, which helps guide the surgeon during dissection

(neuro-identification)


Anaesthetic drug dosing


How neural injury occurs intra-op?


Mechanical causes : direct neural injury (contusion/surgical trauma),

excessive traction or distortion, patient malpositioning


Vascular causes: stretching of critical spinal cord vasculature,

ligation/clamping of vessels, prolonged hypotension


What can we monitor?


Electroencephalograhy (EEG)

raw/processed, and

Electrocorticography (ECoG)


Evoked Potentials (EP)

Sensory evoked

potentials

(SEP)

1. Somatosensory

(SSEP)

2. Brainstem Auditory

(BAEP)

3. Visual (VEP)

Motor evoked

potentials

(MEP)

1. Trans-cranial

(tcMEPs

tcMEPs)

2. Electromyography

(EMG):

spontaneous vs.

triggered


Electroencephalography (EEG)


EEG is the summation and recording of postsynaptic potentials from

the pyramidal cells of the cerebral cortex; typically classified by

frequency

In order of frequency, brain waves are classified as:

delta


EEG


Burst suppression & electrical silence


Sudden unexpected burst

suppression and electrical

silence, or delta waves (0-4 Hz)

indicate depressed functions

(coma, deep anesthesia,

hypothermia, hypoxia,

ischemia, infarction, poor

metabolism)

delta


EEG Indications


Craniotomy for cerebral aneurysm clipping when a temporary clip is

used


Carotid Endarterectomy (under general anesthesia)


Extracranial-intracranial bypass procedures


Pharmacologic depression of brain for “cerebral protection” eg

induced barbiturate coma


Evoked Potentials (EP)




Baseline recordings - Controls (not from normal populations)

Most commonly the opposite side of the body, before start of surgery

Shifts in latency, amplitude, frequency, duration, and morphology of

waveforms depending on modality being monitored


SSEP recording




Cortical/subcortical responses to repetitive electrical stimulation of a

mixed peripheral nerve.

Signal-averages, as each potential very small

Slight delay from real time (< 1 min)


Amplitude loss & Latency prolongation


Injurious insult → Ion channels affected → axonal transmission

blocked → oxidative phosphorylation uncoupled → loss of cellular

function and structural integrity → voltage drop in evoked potential

amplitude


Latency prolongation rarely occurs without amplitude loss except for

shifts related to increased anaesthetic concentrations, hypothermia,

maybe hypercarbia


SSEP alerts


Amplitude decrease ≥ 50% of baseline


Latency prolongation ≥ 5% of baseline


SSEP Indications


Intra/extracranial vascular surgery (aneurysm clipping/carotid

endarterectomy)


Spine surgery


Brain tumours located in or near sensorimotor or parietal cortex


Effect of Anaesthetics on SSEP


SSEPs are affected by: physiological events, surgical

manipulation/pathological events, and anaesthetics


Generally most anaesthetics decrease the amplitude and increase the

latency of waveforms


Anaesthetic effects on SSEP

Halogenated Inhalational agents (> MAC 0.5)



4 Isoflurane is most potent in its effect on SSEPs

4 Sevoflurane and desflurane are similar to isoflurane at steady state,

but have a more rapid onset and offset of effect

Nitrous Oxide

4 Decrease in amplitude (in cortical responses)

4 Increase in latency

4 Above 50% acts with halogenated agents to significantly affect

waveforms

Intravenous Anaesthetic Agents

4 Dose related depression of amplitude, less than inhalational agents

4 Ketamine and Etomidate - increases amplitude in cortical SSEP


MEP

_

+


Trans-cranial

electrically stimulated,

myogenic motor evoked

potentials (tcMEPs)


Why MEP?


SSEP-monitors of the posterior

columns

MEP


“False negative” - reports of

patients becoming paralyzed

highlighted the need for

anterior cord monitoring

SSEP


Combination of MEP and SSEP

techniques provides global

monitoring coverage of spinal

cord


Why MEPs?



MEP amplitude

Onset latency



Lower Limbs (more critical for

thoracolumbar surgery)

Upper Limbs


Anaesthetic effects on MEP


Generally incompatible with volatile anaesthetics

Virtually abolished if MAC ≥ 0.5


Anaesthesia Requirements




TIVA anaesthetic from induction

4 remifentanil (0.05-0.2 mcg/kg/min), propofol (75-300 mcg/kg/min)

50% or less N 2 O

Minimal to none halogenated agents (tcMEPs) are exquisitely

sensitive to the depressive effects of volatile agents.

MEPs may even completely abolished with volatiles MAC ≥ 0.5,

especially if patient has pre-existing neurologic deficit





No muscle paralysis after intubating dose, check TOF

Ketamine and etomidate and opioids are minimally suppressive

Bite block (prevents damage to dentition, tongue,lips, ETT)

Head and neck movement warrants frequent checking of patients

position


Anesthesia Requirements


Contraindications

4 History of epilepsy, seizure disorder

4 Cardiac pacemaker, implanted neural stimulators or intracranial

implants

4 Previous craniotomy or skull defects


Stable anaesthetic depth (avoid rapid changes esp critical stages of

surgery)

TIVA regimen preferred

No neuromuscular blockers for MEP or EMG monitoring, except at the

start (for intubation, positioning, surgical access and exposure)

Remifentanil for surgical immobility, Opioids suitable

Critical Intraoperative Neurophysiologic Alert*

1. SSEP amplitude decrease >50%,

2. tcMEP amplitude decrease >75%

Isolated changes in latency without amplitude change

most likely unrelated to surgical insult


SSEP

BAEP

VEP

MEP

EMG

Volatile

anaesthetics

0.5-0.7 MAC

acceptable

No

limitations

Similar to

SSEP

Avoid, limit use

MAC < 0.3

No limitations

N 2 O

50-70%

acceptable if

baseline not

compromised

Up to 50%

acceptable

No limitations

Intravenous

Can be used

Preferred

No limitations

anaesthetics

Neuro

euro-

muscular

blockers

Can be used

Avoid, very

limited use*

Avoid, limited

use*

*In situations where patient safety and well-being require paralysis, the surgeon and technologist should be notified

immediately


Critical Intraoperative Neurophysiologic Alert

Principles of Response to a Significant Intra-operative

Neurophysiologic Alert at the Spinal Cord Level

1. Rule out technical factors (check electrodes in position

2. Exclude anaesthesia-related factors (eg bolus or rapid change in anaes conc)

3. Elevate MAP to > 90 mmHg

4. Increase FiO2

5. Discontinue spinal instrumentation, release distraction forces

6. Irrigate with warm saline

7. Assess ABG for unrecognized electrolyte abnormality or anaemia

8. If evoked potentials fail to return, perform a wake-up test, may need removal

of spinal instrumentation

9. Consider corticosteroids (acute SCI protocol)

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