Intraoperative Monitoring
Behrouz Zamanifekri, MD
Neurophysiology Fellow
KUMC
March 2013
Intraoperative monitoring
The most primitive method of monitoring the patient
50 years ago were continuous palpation of the radial
pulsations throughout the operation or wake up test!!
History

1921, Dr Penfield, intraoperative neurophysiology research

1950s, Dr Penfield, electrical stimulation to find epileptic foci

1970s, Dr Brown used SSEP for scoliosis operation
1974 , among 7,800 operations conducted with Harrington
instrumentation, 87 patients had subsequently developed
significant spinal cord problems



Early 1980, IOM in operations for large skull base tumors
1980, the American Society for Neurophysiological
Monitoring [ASNM])
Introduction
What is IOM ?

use of neurophysiological recordings for detecting
changes caused by surgically induced insults

assess the function of specific parts of the nervous
system continuously during an operation

It is becoming part of standard medical practice
What is the purpose of IOM?
1. Reduce the risk of postoperative neurological
deficits
2. Identify specific neuronal structures and
landmarks that cannot be easily recognized
3. Research purposes in basic science,
pathophysiology and therapeutic management
What are the most common types of
recording?

Spontaneous activity
EEG
EMG

Evoked responses (through external stimulation of a neural
pathway)
Sensory :
1- visual
2-auditory
3-somatosensory
Motor

The type of test to be used and the sites of recording and stimulation are chosen on
a case by case
Complications during surgery
 ischemia
 mechanical
insult
PRACTICAL ASPECTS OF
MONITORING SPINAL CORD
Spinal Monitoring

Spinal cord, nerve roots, and blood vessels
are frequently placed at risk for injury

Electrophysiological modalities for monitoring:
SSEPs
MEPs
free run or spontaneous EMG (sEMG)
triggered EMG (tEMG)
Spine Surgery:
surgical insults to the ventral parts of the
cord, using motor evoked potentials
(MEPs)
 dorsal columns of the spinal cord , SEP
 the purpose of IOM is to detect
response changes due to surgery, not to
make a clinical diagnosis

Monitoring of Somatosensory
Evoked Potential
Earliest used method in IOM
 1970s in operations for scoliosis
 Stimulation of peripheral nerve and
recorded from scalp
 Only monitor dorsal(sensory) spinal cord
 patient sensory examination for position
and vibration is recommended prior to
surgery

SSEP

By electrical stimulation of peripheral nerves

Median nerve at wrist for injury above C8
Posterior tibial nerve at ankle for injury below C8


spinal cord through the dorsal roots,
ascending pathways, thalamus
and, finally, to the
primary sensory cortex
Recording

P9 from
P11
P14-16
P20
brachial plexus
Dorsal horn
Dorsal column nuclei
Primary sensory cortex(contralat.)
upper limb SSEP

N37
Primary sensory cortex(contralat.)
lower limb SSEP



Location of the stimulating and recording
posterior tibial nerve SEPs.

It is important to note
- Earlier peaks tend to be less sensitive to
anesthesia
- used to differentiate SSEP monitoring
changes resulting from anesthetic effects
from surgical manipulation.
Alarm criteria
-50% reduction in amplitude
-10% increase in latency
Factors that affect the SSEP amplitude
include halogenated agents, nitrous
oxide, hypothermia, hypotension, and
electrical interference
Normal SSEPs from median nerves and posterior
tibial nerves
SSEP in Peripheral nerves?

sciatic nerve injury during pelvic fracture

Injuries to brachial plexus in positioning
of pt is common

Prolong latency of all peaks and
decrease amplitude
Nerve root

SSEP: insensitive to changes
in nerve root function

Why?
SSEP in nerve root injury?
SSEP used during placement of pedicle
screws
 Risk of spinal nerve root injury


If one root damaged, no change in
SSEP
Dermatomal stimulation is better

Inhalational anesthetics, cortical responses

Intravenous Agents
- Propofol increases the latency by approximately10%
- Benzodiazepines reduce the amplitude of cortical SEP
- Etomidate : cortical SEP amplitude augmented 200–600%,
increases SEP latencies
- Opiates, cause a slight increase in SEP latency
- Muscle relaxants, not affect SEPs

SEP changes due to surgical maneuvers
(e.g., spinal distraction) or ischemia (e.g.,
after placement of an artery clamp) are
abrupt and localized and only one side of
the body may be affected

whereas changes due to anesthesia or
body temperature changes are relatively
slower
Detection of cord injury due to
misplaced instrumentation



just after placement of
instrumentation,
both the cortical (peak N45) and
cervical (peak N30) responses
disappear
SEPs obtained after cross-clamping of the internal carotid ,which
resulted in ischemia (time 9:45) that later deteriorated (9:55). After
placement of a shunt,
response amplitude is restored to within normal limits (time 10:01).

Procedures involves the ICA, MCA,
PCA, P.Com, or BA?
Median nerve SEPs
procedure involves the
ACA or the A.Com artery?
Posterior tibial nerve

MONITORING SPINAL
MOTOR SYSTEM
Introduction
SSEP for sensory pathway
 MEP for motor


SSEP + MEP: Small reversible changes
in SSEP that occur when motor pathway
are injured
MEP

1990s, TC-MEP as a method to monitor
the corticospinal tracts

Prior to MEP monitoring,
the only way to assess corticospinal
tract during surgery was wake-up test
TC-MEPs

stimulation through the skull with signal
recording at the level of
muscle (CMAP)
nerve (neurogenic MEP)
spinal cord ( D-wave )
-the newer technologies is continuous free-running
EMG throughout the surgery
Recording of Muscle Evoked
Potentials

Stimulation of cortex, activation of
coticospinal, EMG of distal( Hand m.,
abd hallucis, tibialis anterior)

Muscle relaxant can not be used
MEP
Interpretation of MEP Recording

4 methods :
1) all-or-nothing criterion: the most used method,
complete loss of the MEP signal from a baseline recording is indicative of
a significant event
2) amplitude criterion: 80% amplitude decrement in at least 1
out of 6 recording sites
3) threshold criterion: increases in the threshold of 100 V or more
required for eliciting CMAP responses that are persistent for 1 h or more
4) morphology criterion: changes in the pattern and duration of MEP
waveform morphology
TcMEP monitoring
contraindicated in
-deep brain stimulators or cochlear implants
Tongue biting is the most common complication
Normal MEPs
Recording of the response from
spinal cord(D, I wave)
Recording from epidural
electrodes
 D (direct activation of
corticospinal)
 I ( indirect, through transsynaptic)

Not affected by muscle relaxants, but latencies increase with cooling
Subdural electrodes can be substituted for epidural electrodes
Needle electrodes can be place in interspinous ligaments both sides of
surgery area
 major benefits reported during intramedullary spinal cord tumor
resection
 a complete loss of MEPs with at least 50% preservation of the D-wave
amplitude generally results in a transient paraplegia



Spontaneous EMG
monitor nerve roots
 recording electrodes placed in the
muscles
 no stimulation is performed
 monitoring of 2 muscles is
recommended
 C5 nerve root injury, The deltoid and
biceps brachii
 MEPs be obtained intermittently

sEMG

no paralytic agents

train-of-4 testing should indicate that at
least 3 out of 4

Myasthenia gravis, Botox treatments,
and muscular dystrophy are classic
conditions that interfere with EMG
Abnormal sEMG
spikes
 Bursts
 trains


Trains are continuous, repetitive EMG firing
caused by continuous force applied to the nerve
root.
Example of EMG activity
indicating irritation of the nerve
Baseline recordings. Note the low
amplitude background activity
High amplitude spikes are present
Artifacts may be mistaken for spikes or trains

a neurostimulator

the surgical table

the surgeon’s head light

bipolar electrocautery device
Triggered EMG (Pedicle Screw
Stimulation)

used to determine whether screws have breached
the medial or inferior pedicle wall and thus pose a
risk to the exiting nerve root at that level

When a pedicle screw is accurately placed, the
surrounding bone acts as an insulator to electrical
conduction, and a higher amount of electrical current
is thus required to stimulate the surrounding nerve
root.

When a medial pedicle wall breach occurs, the
stimulation threshold is significantly reduced
False negative response

muscles relaxants

fluid, blood, or soft tissue around the head of the
screw , shunt current away from the screw

it is important that the stimulation probe be
placed directly on the top of the screw and not
the tulip, as these 2 structures are not
structurally fused

Presence of preexisting nerve root injury. Injured
nerve roots will have higher triggering
thresholds,

Due to the variation in thickness and shape between
thoracic and lumbar pedicles, different stimulation
thresholds exist for these regions

A threshold < 10 mA for screw stimulation, suggest a
medial wall breach in the lumbar pedicles

A thresholds > 15 mA indicate a 98% likelihood of
accurate screw positioning

For thoracic pedicle screw placement, stimulation
threshold < 6 mA suggest a medial pedicle breach

In cervical and thoracic procedures, the
spinal cord are of greater importance

Conversely, in lumbar or sacral
procedures the nerve roots are at
greater risk of injury
Overview of IOM classified by spinal region
Conclusions

Multimodality neurophysiological
monitoring is extremely valuable in the
prevention of neurological injury

Knowledge of the benefits and
limitations of each modality helps
maximize the diagnostic value of IOM
during spinal procedures

Neurosurg Focus / Volume 27 / October
2009

A concise guide to intraoperative
monitoring / George Zouridakis, Andrew C.
Papanicolaou.2001

Intraoperative neurophysiological
monitoring / Aage R. Moller. -- 2nd ed.