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The Effect of TCI on
China Medical University Hospital
Evoked potential monitoring includes somatosensory
evoked potentials (SSEP), brainstem auditory evoked
potentials (BAEP), motor evoked potentials (MEP),
neurogenic MEP (NMEP)and visual evoked potentials
Electromyography (EMG) also is used extensively
during operative cases.
Scalp electroencephalography (EEG) provides data
for analysis monitor cerebral function during carotid or
other vascular surgery.
Electrocorticography (ECoG),EEG recorded directly
from the pial surface.
SSEPs are recorded by stimulating peripheral
afferent nerves, recorded with scalp electrodes
and averaged to improve signal-to-noise ratio.
Median nerve at the wrist is the most common
stimulation site for upper extremity monitoring.
In the lower extremity, the posterior tibial
nerve just posterior to the medial malleolus
is used most commonly. Other sites that can be
utilized include the ulnar and peroneal nerves.
Needle electrodes generally are used to
reduce artifactual signals. Recording
electrodes are placed on the scalp and on the
cervical spine. Additionally, electrodes can be
placed at the Erb point for upper extremity
SSEP recording and over the lumbosacral
spine for lower extremity recording.
If the operative field affords exposure,
recording electrodes may be placed directly in
the epidural space. Typically, the electrodes
are placed just proximal to the lesion of
Operating rooms are inundated with
equipment that emits electromagnetic
interference, which is greatest at the
frequency of alternating current (60 Hz in
the United States).
Adequate filtering to remove this artifact
Additionally, shielding to reduce
interference is essential.
Amplitude, shape, and latencies of the
responses are monitored.
Serially recorded responses are compared
with laboratory norms.
Establishing a reproducible baseline
recording prior to any positioning or surgical
manipulation is important.
Changes from the baseline responses are
the indicators of neurological dysfunction.
Keep in mind that anesthetics can alter the
evoked responses significantly.
Table 1. Typical Responses to Median Nerve Stimulation
Name Site Latency Probable Anatomical
Erb point Erb point 9 ms Trunks of brachial plexus
Cervical A Cervical 11 ms Dorsal root entry zone of cervical
Cervical B Cervical 12-13ms Posterior columns
Cervical C Cervical 14 ms Brain stem
N18 Scalp 18 ms Subcortical structures
N20 Scalp 20 ms Somatosensory cortex
Table 2. Typical Responses to Tibial Nerve
Name Site Latency Probable Anatomical
N20 Lumbar 20 ms Spinal roots/cord
P27 Cervical spine 27 ms Nucleus gracilis
N35 Scalp 35 ms Somatosensory
P40 Scalp 40 ms Somatosensory
Spinal surgery: Changes in latency and
amplitude can be monitored during positional
manipulations, including open or closed
reduction of spinal deformities.
Extradural manipulations, including surgery on
disk or vertebral segments, or on epidural
abscess or neoplasm, can be monitored with
Resection of intradural and intramedullary
lesions, including tumors and arteriovenous
malformations, also can be monitored.
Recognizing that SSEP recording
monitors primarily the integrity of dorsal
columns is important. Inability to test
motor pathways, which probably are
more important clinically than dorsal
column integrity, is a significant
limitation of the technique.
Carotid surgery including endarterectomy:
Changes in SSEP recordings are sensitive for
detection of cerebral ischemia. SSEP
monitoring can be helpful in determining the
need for shunting during the surgical procedure.
Aortic cross-clamping: Changes in SSEP
indicate a high risk of neurological injury,
especially if the changes are immediate.
somatosensory evoked potentials,
carotid stump pressure,
transcranial Doppler sonography,
jugular bulb oximetry
near infrared spectroscopy
during carotid artery surgery for monitoring
clamp related ischemia during carotid artery
Cerebral aneurysm surgery:
Changes may indicate occlusion of parent
vessel branches, which potentially could be
reversed by repositioning of aneurysm clips.
SSEP monitoring can signal changes prior to
irreversible cerebral ischemia.
Amplitude and latency of the N20 peak, central
conduction time (CCT), and latency difference
between the N14 and N20 peaks are reliable
indicators of cerebral hemispheric function in
Localization of sensorimotor
Localization of the motor cortex is important to
minimize the risk of contralateral motor deficits
resulting from surgical procedures in its vicinity.
When recording SSEP, the primary sensory
cortex and motor cortex generate potentials that
are mirror images of each other.
This “phase reversal” across the central sulcus
is a highly reproducible characteristic that can
aid in the localization of primary motor cortex.
Intraoperative photograph showing
orientation of monitoring electrode
for intraoperative SSEP. 1,
Location of phase reversal
Imaging studies obtained in a patient
with metastatic adenocarcinoma.
Preoperative axial contrast-enhanced MR
The red lines indicate the central sulcus
based on radiographic landmarks.
Intraoperative photograph showing the
location of tumor within sensorimotor
1. Paper tickets identify areas of
2. 1, motor for thumb;
3. 2, sensory for hand;
4. 3, motor for face.
Brainstem auditory evoked potentials
(BAEP) record cortical responses to
This allows monitoring of the function of
the entire auditory pathway including
acoustic nerve, brain stem, and cerebral
Recordings are obtained by stimulating with
auditory clicks in the ear.
Standard EEG cortical montage is used with
recordings obtained from scalp electrodes.
Best responses are obtained from electrodes
near the ears (A1, A2) referenced to the vertex
Auditory clicks are delivered in a repetitive
pattern, often at 11 Hz, with a frequency that
does not coincide with the 60-Hz noise of
electrical AC current.
Positive deflections are termed waves I-VII.
Waves I, III, and V are the waves most
consistently seen in healthy subjects (obligate
Wave V is the most reliably seen wave,
particularly in patients with hearing impairment
or undergoing surgery.
A shift in latency of 1 millisecond or a drop in
amplitude of 50% could be significant and
should be reported to the surgeon.
Cerebellopontine angle surgery:
This includes surgery for acoustic neuroma or
meningioma, or for microvascular decompression for
tic douloureux or hemifacial spasm.
Important parameters to monitor include peak
amplitude of waves III and V, latency of wave V,
latency of waves I-V, and latency of waves I-III.
If changes occur, they may be due to improper
retraction on the cerebellum and brain stem; these
may be reversible with a change of position of the
retractors by the surgeon
Monitoring of visual pathways has potential utility in
surgery performed in proximity to the visual apparatus,
especially in the parasellar region.
VISUAL EVOKED POTENTIALS
Tumors that arise in this area include
craniopharyngiomas, pituitary adenomas, and
suprasellar meningiomas. Resection of these
tumors carries significant risk of visual
It has potential usefulness in assessing
integrity of visual pathway including
optic nerves; however, it cannot detect
the presence of visual field defects.
Visual stimulation is given by flashing
light-emitting diodes (LED) or strobe
Potentials are recorded with scalp
Signal-averaging and noise-reduction
techniques are used.
Typically 3 negative peaks (N1, N2, N3)
and 3 positive peaks (P1, P2, P3) are
The P1-N2-P2 complex typically is
monitored during surgery.
Latency and amplitude changes are
Experiences described in the literature
regarding the clinical utility of intraoperative
VEP have been conflicting.
Monitoring has been performed in tumor
resections that require manipulation of the optic
apparatus, but its use has not yet become
MOTOR EVOKED POTENTIALS
SSEP has been the standard of intraoperativ monitoring,
with excellent ability to assess dorsal column and
lateral sensory tract function; it probably also can
detect changes in function of anterior motor tracts by
stimulating mixed sensorimotor peripheral nerves.
However, significant motor deficits have been seen
in patients undergoing spinal surgery despite
MEPs were developed to better assess the motor
neurophysiological pathways. Note that anesthetic
agents can severely diminish the motor evoked
MEPs are elicited by either electrical or
magnetic stimulation of the motor
cortex or the spinal cord.
Recordings are obtained either as
neurogenic potentials in the distal
spinal cord or peripheral nerve, or as
myogenic potentials from the
Transcranial electrical stimulation involves
stimulation of electrodes on the scalp, or if
the brain is exposed by a craniotomy,
stimulation of electrodes placed directly on the
Electrical stimulation also can be applied
directly over the spinal cord when a
laminectomy affords exposure proximal to the
lesion in question. Distal neurogenic
potentials then can be recorded.
Transcortical magnetic stimulation delivers a
pulsed magnetic field over the scalp in the region
of the primary motor cortex.
The basis for electrical stimulation generated by
applying a magnetic field is based on the
Faraday law, which states that a changing
magnetic field induces an electric current in a
Unfortunately, generating good signals in the
operating room with this technique is difficult;
also, the devices necessary to apply strong
magnetic fields can be a hindrance in surgery.
EFFECTS OF ANESTHETICS ON
EVOKED POTENTIALS AND EEG
Anesthetics exert their effects on the
brain by depressing cerebral
This results in alteration of EEG
recordings of the brain.
Each type of anesthetic agent alters the
evoked response in different ways
The volatile agents, which include the
halogenated anesthetics and nitrous oxide,
produce a dose-dependent depression of
They have the most potentially deleterious
effect of all anesthetics.
All cause similar depression of evoked
potentials and prolongation of latencies.
They affect cortically evoked responses more
than subcortical, spinal, or peripherally evoked
At high concentrations, most also can suppress
These may decrease evoked potential
amplitude and lengthen latency, but
typically recordings can be obtained
despite high doses.
They also increase beta frequency
In low doses, etomidate can increase
evoked potential amplitude but prolong
At induction doses, amplitude may be
Ketamine either does not affect or may
increase evoked potential amplitude.
Narcotics cause mild reduction in
amplitude of evoked potentials but
usually allow consistent monitoring.
Benzodiazepines usually result in decreased
amplitude with little effect on latencies.
Like barbiturates, they increase beta activity
(more over normally than abnormally
functioning cortex), but they typically decrease
rather than increase epileptiform activity.
These agents have no significant effect
on evoked potentials.
Muscle relaxation reduces artifactual
signals from spontaneous muscle
activity and, if complete, suppresses
evoked muscular responses as well.
Waters, A.; Mahmoud, M.; Goldschneider,
K.; Sadhasivam, S.: Comparison of patient
controlled analgesia with and without
dexmedetomodine following spine surgery
in children. Presented at the SPA Winter
Conference; February 16-19, 2006; Fort
Susceptibility of Motor-Evoked Potentials to Varying
Targeted Blood Levels of Dexmedetomidine
Reduction of the spinal cord injuries during scoliosis surgery is a major
goal of the anesthesia and surgical team. Despite improvement in
scoliosis surgery over the years, the development of neurological
deficits remains the most feared complication of spine surgery.
During scoliosis surgery it is very important to monitor the spinal cord to
detect spinal cord injury with surgical manipulation. Continuous or
intermittent intraoperative electrophysiological monitoring (neuron-
monitoring) is used routinely during these procedures to provide the
surgeon with information concerning the integrity of neurological
structures at risk. All neuron-monitoring modalities are affected by the
anesthetic regimen used. Of the various intravenous anesthetic drugs, the
combination of propofol, remifentanil and dexmedetomidine appear
to impact neuron-monitoring the least. The current anesthetic practice
is to use the three drugs in combination at doses that do not depress the
signals but there is no data relating targeted dexmedetomidine and
propofol blood levels to neuron-monitoring signals. The lack of data
results in wide variability in dosing with consequent variability in patient
response.Hypothesis: Clinically relevant blood levels of
dexmedetomidine will affect the amplitude of transcranial motor-
evoked potentials (TcMEP) either independently or by interaction
with propofol in a dose dependent manner.
NMEP with low dose propofol
NMEP - 3D Trend NMEP - 3D Trend
Lt Pop F Rt Pop F
0 13:07:29 0 13:07:29
0 25 50 0 25 50
5 ms/Div 5 ms/Div