Hala El-Habashy et al.
Role of Motor Evoked Potential in The Predictability of
Surgical Outcome of Herniated Disc Induced Cervical
Myelopathy Under Different Anesthetic Techniques
Hala El-Habashy1, Ayman M. El-Badrawy2, Mohamed Esawy2,
Hisham Hozian3, Yasser El-Gohary2
Departments of Clinical Neurophysiology1, Anesthesia2, Neurosurgery3, Cairo University
ABSTRACT
Background: Intraoperative motor evoked potentials (MEPs) application provides a method for
monitoring the functional integrity of motor pathways. However, these potentials are sensitive to suppressive
effect by most anesthetic agents. The aim of the work: To evaluate the role of transcranial electrical motor
evoked potentials (TceMEPs) in predicting motor system affection and surgical outcome during anterior
cervical microdiscectomy operations and comparing the outcome under two anesthetic regimens (propofolfentanyl) versus (sevoflurane-N2O). Subjects and Methods: This study included 60 patients divided into two
groups (I, II). Anesthetic plane was induction by propofol and use of atracurium for intubation and muscle
relaxation in predetermined doses. Maintenance of anesthesia in group (I) was by (propofol-fentanyl)
infusion and in group II by (sevoflurane-N2O). During operation TceMEPs monitoring with myogenic
recoding of compound motor action potentials (CMAPs) from the four limbs was done. Results and
Conclusions: we found that; (CMAPs) recorded under (sevoflurane-N2O) anesthesia had statistically
significant lower amplitude than that recorded during (propofol-fentanyl) one, whereas both regimens had
minimal non-significant prolongation of latencies. However, multipulse TceMEPs monitoring with myogenic
recording remained stable throughout the operation under both regimens provided that the degree of
anesthesia and analgesia were kept constant. Also, TceMEPs monitoring with myogenic recording of CMAPs
alone could only point to harmful surgical manipulations but could not determine how much these
manipulations will progress to permanent postoperative motor deficit. Recommendations: It is suggested
that multipulse TceMEPs with epidural and myogenic recordings will show high degree of accuracy in
predicting post-operative motor deficit. (Egypt J. Neurol. Psychiat. Neurosurg., 2006, 43(1): 49-62)
INTRODUCTION
Concerning MEPs, monitoring of the
functional integrity of motor tract within the
spinal cord is a technique with great benefit. Loss
of motor function during spinal or vascular
surgery without loss of sensory function or change
in somatosensory evoked potentials (SSEPs) can
occur1. As SSEPs recorded from the brain are
carried solely within the dorsal column of the
spinal cord, SSEP monitoring may fail to detect
damage to the spinal cord motor pathways, and
techniques for directly monitoring the motor
pathways have been developed, Transcranial
magnetic brain stimulation is useful for
extraoperative evaluation of the motor system but
anesthetic effects on cortical synaptic activity
limit its usefulness for intraoperative monitoring2.
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Egypt J. Neurol. Psychiat. Neurosurg.
Multipulse transcranial electrical stimulation
uses a short train of high-frequency stimuli
producing several corticospinal volleys that
summate to depolarize spinal motor neurons,
producing muscle responses specific to motor
pathways3-7. This method provides individual limb
assessment throughout the entire procedure, and is
applicable to all levels. These responses benefit
from Total Intravenous Anesthesia (TIVA) using
narcotic with propofol, midazolam, ketamine, or
etomidate8. In neurosurgical spinal endovascular
and
thoracoabdominal
aortic
aneurysm
procedures, multiple-pulse TceMEPs has been
shown to provide rapid and sensitive motor
assessment and correlated with outcome, but has
been applied infrequently in orthopedics9,8.
Anatomical, theoretical, experimental, and clinical
foundations and the absence of reported adverse
effects have prompted the authors institution to
adopt transcranial electrical stimulation for
intraoperative motor pathway monitoring.
Contrary to SSEP methods, Transcranial
electrical stimulation for MEPs does not use
supra-maximal stimuli. Consequently, when
anesthesia deepens or accumulates, additional
lower motor neuron suppression may cause fading
or disappearance of muscle responses to the
initially chosen stimulus parameters. Therefore, it
is not surprising that increasing intensity or pulse
number may sometimes be necessary to maintain
responses in the face of evolving systemic effects.
Paired pulses or a train of pulses have been
effectively used for stimulation in intraoperative
MEP monitoring10,11.
PATIENTS AND METHODS
Patients
This study included two groups of patients
each 30 in number and carried out in the
neurosurgical theater in Kasr El-Aini Hospital
after achieving the consent of the patients and the
approval of the Ethics and Research Committee
(ERC) of Anesthesia Department of faculty of
medicine in Cairo University. The age of patients
in group I (propofol- fentanyl) ranges from 30-66
year (mean 43.10, standard deviation 8.45). The
50
age of patients in group (II) (Sevoflurane – N2O )
ranges from 25-60 year (mean 43.90 SD 10.57).
Group I included 21 ♂, 9♀. Group II included 20
♂, 10♀ .
The patient in this study fulfilled a number
of inclusion, exclusion criteria and certain
preparation which were:
I. Inclusion Criteria:
 Patient with motor affection due to
herniated
disc
induced
cervical
myelopathy.
 The surgery was anterior cervical
microdiscectomy.
 Age between 20-70 years old.
 American Society of Anesthesiologist
(ASA) class I or Π.
II.
Exclusion Criteria:
 History of epilepsy.
 Hypertensive patient.
 Neurological disorder other than cervical
compression myelopathy.
 Previous cranial operation.
 Patient with intracardiac pacing devices.
III. Preoperative Preparation:
 All patients would be cannulated with
18- gauge peripheral venous cannula.
 Premedication with midazolam 2 mg I.V.
15 minutes before induction of
anesthesia.
 Site of insertion of stimulating and
recording devices would be cleaned out
by alcohol 70% then brushed with Ether.
IV. Anesthetic Management:
A. Induction of Anesthesia
Induction of anesthesia in all patients in the
two groups would be the same as follows:
 Propofol 1-2 mg/kg I.V.
 Fentanyl 2-3 µg/kg I.V.
 Muscle relaxation with atracurium 0.30.5 mg/kg to facilitate tracheal intubation
followed by mechanical ventilation with
tidal volume of 7-10 ml/kg, respiratory
rate 10-12 breaths/min, inspiration to
expiration ration (I:E ratio) 1: 3
Hala El-Habashy et al.
B.
Maintenance of Anesthesia:
Patients divided into two groups each group
includes 30 patients:
1. Group I: In this group, anesthesia would
be maintained with propofol infusion
(200 µg/kg/min. for 10 min.), then 150
µg/kg/min for another 10 min. followed
by 100 µg/kg/min till end of operation.
Analgesia would be provided with
incremental doses of fentanyl 50 µg I.V.
when needed as indicated.
2. Group II: Anesthesia in this group will
be maintained by sevoflurane 1%, N2O
60% and oxygen 40%.
N.B.: In the two groups, putting a paded oral
airway with suitable size would be a must as
during stimulation the patient might pit his
or her tongue due to contraction of masseter
muscles.
V.
Monitoring:
All patients in this study monitored as
following:
 ECG: 5 lead ECG.
 Blood pressure: non-invasive.
 Pulse oximetry: Oxygen saturation, heart
rate.
 End tidal CO2.
 Temperature: Core temperature through
nasopharyngeal probe.
 Neuromuscular block monitoring:
Partial neuromuscular blockade was
necessary in order to prevent patient
movement (either from transcranial
stimulation or surgical maneuvers) that
may interfere with the micro surgical
procedure.
VI. Evaluation and Grading of Motor Power of
Patients:
Each patient was evaluated pre and
postoperatively. It was a must to find one grading
system which was familiar to all persons
participated
in
this
study
(anesthetist,
Neurosurgeon, neurophysiologist). This system of
grading was obtained from British Medical
Research Council Scale12. It would applied on
each limb
Equipments:
I.
II.
The Stimulating device:
 The stimulating device in this study was
EPOCH
2000
(Axon
Systems
Hauppauge, New York).
The stimulating electrodes:
 We
used
cork-screw
subdermal
stimulating electrodes.
 The sites of these stimulating electrodes
were as follow:
A. For upper limb stimulation:
 The negative electrode was at Cz.
 The positive electrode of right upper
limb was at 1 cm in front of C3.
 The positive electrode of left upper
limb was at 1 cm in front of C4.
B. For lower limb:
 The negative electrode was at Cz.
 The positive electrode of both lower
limbs was at 6 cm in front Cz.
N.B.: Cz, C3, C4 from international 10-20
system of electrode placement for recording
EEG
III. Recording Electrodes:
 For recording, we used subdermal needle
electrodes (1.5 cm stainless steel needle)
 The sites of these recording electrode
were as follows:
A. Upper limb:
 Two needles would be inserted in each
thenar muscle 3 cm a part.
 The positive electrode is distal to
reference electrode
B. Lower limbs:
 Two needle electrodes would be
inserted in each tibialis anterior muscle
3 cm apart.
 The positive electrode is distal to
reference electrode
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Vol. 43 (1) – Jan 2006
Egypt J. Neurol. Psychiat. Neurosurg.
N.B.:
 The recording electrodes would be 50 cm
apart from the earthing pad of diathermy.
 The earthing cable of the stimulating
device would be away from the earthing
pad of diathermy by 50 cm.
IV. Stimulus Parameters:
The stimulus parameters were as follows:
 Type: Train
 Stimulation
mode:
manual
(non
repetitive)
 Polarity: Normal
 Train rate (pulse/sec.): 200
 Train duration: 25 m.sec
 Pulse duration: 0.5 m.sec
 Train count: 5
 Inter pulse duration: 4.5 m.sec
 Voltage: 350 volt
 Current: 100 mA
 Time between each train  30 seconds as
safety margin locking time
V.
The recording parameters
The recording parameters were as follows:
 Sweep length: 100 m.sec.
 Time base: 5-10 m.sec/div.
 Delay: Zero
 Signal to Noise Ratio (SNS) 100/10 µv
 Number: 1
 Filtering:
- Low frequency filter LFF: 100 Hz
- High frequency filter HFF: 5000 Hz
 Artifact rejection level: 10 µv.
Methodology & Data Collection and Data
Analysis:
I.
Methodology and Data Collection
The stimuli and so the recordings of (CMAPs)
have collected at five different time intervals:
- 1st reading (baseline I): before induction of
anesthesia.
- 2nd reading (baseline II): 5 min. after induction
of anesthesia and before surgical manipulation
and instrumentation
52
-
3rd reading: During surgical manipulation and
instrumentation
4th reading: after completion of surgical
manipulations on the spine.
5th reading: 5 min. after reversal of muscle
relaxant
N.B.: As the transcranial electrical stimulation is
severely painful the first stimulus (before
induction of anaesthesia) has been done under
analgesic dose of ketamine 0.3-0.5 mg/kg I.V. as
ketamine in the only drug which does not affect
the amplitude or latency of MEP (CMAPs)13.
II.
Statistical Analysis:
Data will be analyzed on an IBM compatible
computer using SPSS win, Ver. 9 statistical
package. Numerical data will be described in
terms of means and medians for central tendency
and standard deviation and range for dispersion.
Categorical data will be described in terms of
count and percentages. For comparison between
the two study groups student t-test, Chi-square or
their nonparametric counterpart will be used as
appropriate. Probability (p-value) of less than
0.05 will be considered to be significant.
RESULTS
Data Description:
I. Age: On comparison between the group I
(propofol-fentanyl)
and
group
II
(Sevoflurane-N2O) as regard the age P. value
was 0.747 (i.e. > 0.05)
II. Sex: On comparison between group I, group
II as regard sex the P. value was 0.781 (i.e.
>0.05).
III. ASA class: On comparison between group I,
II as regard ASA class P. value was 0.606
(i.e. >0.05)
N.B.: For the following records (latency &
amplitude & motor Power grading) we
consider each limb as a separate entity i.e.
each patient considered four patient (as
every patient had four limbs), so by this
method we had two groups each contained
Hala El-Habashy et al.
120 patient. (30 X 4=120). Also we divide
each group (120 limb) into two subgroup (a,
b) (a: upper limbs, b: Lower limbs) each of
them contain
IV. Latency: Readings of latencies in each
group shows that:
a. On comparison between mean latency
change after all stimuli in group I (a, b)
p. Value was < 0.05.
b. On comparison between mean latency
change after all stimuli in group II (a, b)
p. value was <0.05
c. On comparison between percentage of
mean latency change in group I after all
stimuli, results were as follow, table 1
and 2.
d. On comparison between percentage of
mean latency change in group II after all
stimuli results were as follow in table 3
and 4.
e. On comparison of percentage of mean
latency change between group I, II before
and after use of anesthetics p. value was
> 0.05 (insignificant).
V. Amplitude: Readings of amplitudes of
(CMAP) in each group shows that:
a. On comparison between mean amplitude
after all stimuli in group I (a, b) p.
Value was < 0.05.
b. On comparison between mean amplitude
after all stimuli in group II (a, b) p.
Value was <0.05.
c. On comparison between percentage of
mean amplitude change in group I after
all stimuli, results were as follow, table
5 and 6.
d.
On comparison between percentage of
mean amplitude change in group II after
all stimuli, results were as follow, table
7 and 8.
e. On comparison of percentage of mean
amplitude change between group I,II
before and after use of anesthetics, p.
value was < 0.05 (significant) with
incidence of decrease in mean amplitude
of (CMAPs) in group II more than that
group I.
VI. Motor Power grading:
The records of motor power grading for each
group (group I: Propofol-fentanyl / Group II:
sevoflurane-N2O) are listed as follow:
Important Observations:
1. There was one patient (4 limbs) in whom we
could not perform this test (TceMEPs) with
myogenic recording, this most probably
attributed to technical errors (earthing,
amplification, filtering,…. etc).
2. On comparison between patient (limbs) as
regard changes in latencies recorded after
stimulus (2), stimulus (3) and stimulus (4)
we found that as in Table (13).
3. On comparison between patient (limbs) as
regard changes in amplitude recorded after
stimulus (2), stimulus (3) and stimulus (4)
we found as in Table (14).
The forty patients deteriorated during
surgical manipulation progress as follows in Table
(15).
Table 1. Shows% of mean latency change in group I for upper limbs.
Group I (Propofol –fentanyl) a: upper limbs
Latency %change 1 (from stim. 1 to stim. 2)
Latency %change 2 (from stim. 2 to stim. 3)
Latency %change 3 (from stim. 3 to stim. 4)
Latency %change 4 (from stim. 4 to stim. 5)
Latency %change 5 (from stim. 1 to stim. 5)
Latency %change 6 (from stim. 2 to stim. 4)
Median %
3
0.00
-4.05
-3.71
-5.72
-5.41
Minimum %
-9.38
-11.76
-26.83
-26.67
-34.15
-26.83
Maximum %
23.08
6.90
10.00
6.67
14.29
10.00
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Vol. 43 (1) – Jan 2006
Egypt J. Neurol. Psychiat. Neurosurg.
Table 2. Shows % of mean latency change in group I for lower limbs.
Group I (Propofol –fentanyl) a: lower limbs
Latency %change 1 (from stim. 1 to stim. 2)
Latency %change 2 (from stim. 2 to stim. 3)
Latency %change 3 (from stim. 3 to stim. 4)
Latency %change 4 (from stim. 4 to stim. 5)
Latency %change 5 (from stim. 1 to stim. 5)
Latency %change 6 (from stim. 2 to stim. 4)
Median %
3.26
0.00
-4.42
-5.00
-3.52
-5.26
Minimum %
-5.26
-10.00
-28.57
-19.05
-30.95
-28.57
Maximum %
13.33
12.00
13.33
25.00
10.00
6.25
Minimum %
-25.93
-5.88
-37.50
-20.00
-17.07
-16.67
Maximum %
12.12
45.00
9.09
100.00
87.50
45.00
Minimum %
-23.08
-16.67
-23.81
-19.51
-25.00
-33.87
Maximum %
20.00
35.00
19.05
68.42
100.00
30.00
Minimum %
-13.01
-62.96
-65.71
-44.55
-49.59
-67.57
Maximum %
6.06
21.87
150.00
245.83
18.29
19.05
Table 3. Shows % of mean latency change in group II for upper limbs.
Group II (sevoflurane –N2O) a: upper limbs
Latency %change 1 (from stim. 1 to stim. 2)
Latency %change 2 (from stim. 2 to stim. 3)
Latency %change 3 (from stim. 3 to stim. 4)
Latency %change 4 (from stim. 4 to stim. 5)
Latency %change 5 (from stim. 1 to stim. 5)
Latency %change 6 (from stim. 2 to stim. 4)
Median %
2.94
2.00
-3.13
-3.77
-3.23
0.00
Table 4. Shows % of mean latency change in group II for lower limbs.
Group II (sevoflurane –N2O) a: lower limbs
Latency %change 1 (from stim. 1 to stim. 2)
Latency %change 2 (from stim. 2 to stim. 3)
Latency %change 3 (from stim. 3 to stim. 4)
Latency %change 4 (from stim. 4 to stim. 5)
Latency %change 5 (from stim. 1 to stim. 5)
Latency %change 6 (from stim. 2 to stim. 4)
Median %
3.28
0.00
-4.45
-3.77
-5.28
-2.99
Table 5. Shows % of mean amplitude change in group I for upper limbs.
Group I (Propofol –fentanyl) a: upper limb
Amplitude %change 1 (from stim. 1 to stim. 2)
Amplitude %change 2 (from stim. 2 to stim. 3)
Amplitude %change 3 (from stim. 3 to stim. 4)
Amplitude %change 4 (from stim. 4 to stim. 5)
Amplitude %change 5 (from stim. 1 to stim. 5)
Amplitude %change 6 (from stim. 2 to stim. 4)
54
Median %
-9.12
0.00
7.40
8.73
1.64
5.52
Hala El-Habashy et al.
Table 6. Shows% of mean amplitude change in group I for lower limbs.
Group I (Propofol –fentanyl) b: lower limb
Amplitude %change 1 (from stim. 1 to stim. 2)
Amplitude %change 2 (from stim. 2 to stim. 3)
Amplitude %change 3 (from stim. 3 to stim. 4)
Amplitude %change 4 (from stim. 4 to stim. 5)
Amplitude %change 5 (from stim. 1 to stim. 5)
Amplitude %change 6 (from stim. 2 to stim. 4)
Median %
-9.38
0.00
7.57
7.85
2.51
6.48
Minimum %
12.30
-64.81
-1.33
0.00
-1.20
-5.56
Maximum %
-6.47
2.40
168.47
23.17
16.67
15.71
Minimum %
-19.85
-79.63
-39.58
-57.75
-62.96
-64.86
Maximum %
0.00
5.83
120.00
123.08
20.00
18.45
Minimum %
-18.60
-81.48
-8.89
-39.16
-50.00
-64.81
Maximum %
-2.80
6.80
114.29
100.00
55.00
42.86
Table 7. Shows % of mean amplitude change in group II for upper limbs.
Group I (sevoflurane –N2O) a: Upper limb
Amplitude %change 1 (from stim. 1 to stim. 2)
Amplitude %change 2 (from stim. 2 to stim. 3)
Amplitude %change 3 (from stim. 3 to stim. 4)
Amplitude %change 4 (from stim. 4 to stim. 5)
Amplitude %change 5 (from stim. 1 to stim. 5)
Amplitude %change 6 (from stim. 2 to stim. 4)
Median %
-11.99
-0.85
10.10
11.37
1.41
1.99
Table 8. Shows% of mean amplitude change in group II for lower limbs.
Group I (sevoflurane –N2O) b: lower limb
Amplitude %change 1 (from stim. 1 to stim. 2)
Amplitude %change 2 (from stim. 2 to stim. 3)
Amplitude %change 3 (from stim. 3 to stim. 4)
Amplitude %change 4 (from stim. 4 to stim. 5)
Amplitude %change 5 (from stim. 1 to stim. 5)
Amplitude %change 6 (from stim. 2 to stim. 4)
Median %
-12.00
-0.54
8.16
13.81
0.62
1.84
Table 9. Preoperative motor power grading (group I).
Group I
Grade
Grade
Grade
Grade
Grade
(Propofol + fentanyl)
3
44
4+
5
Total
Frequency
8
46
5
37
24
120
Percent
6.7%
38.3%
4.2%
30.8%
20%
100%
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Vol. 43 (1) – Jan 2006
Egypt J. Neurol. Psychiat. Neurosurg.
Table 10. Post operative motor power grading (group I).
Group I
Grade
Grade
Grade
Grade
(Propofol + fentanyl)
44
4+
5
Total
Frequency
40
16
36
28
120
Percent
33.3%
13.4%
30%
23.3%
100%
Frequency
8
24
28
18
16
26
120
Percent
6.7%
20%
23.3%
15%
13.3%
21.7%
100%
Frequency
6
13
37
26
12
22
4
120
Percent
5%
10.8%
30.8%
21.7%
10%
18.3%
3.3%
100%
Table 11. Preoperative motor power grading (group II).
Group II
Grade
Grade
Grade
Grade
Grade
Grade
(Sevoflurane + N2O)
2
3
44
4+
5
Total
Table 12. Post operative motor power grading group (II).
Group II
Grade
Grade
Grade
Grade
Grade
Grade
(Sevoflurane + N2O)
2
3
44
4+
5
Missing system
Total
Table 13.
No deterioration during
surgical manipulations
Deterioration during surgical
manipulations (latency   10%)
Return to 100% of base time II after
stopping surgical manipulations
230 (from 236)
97.46%
6 (from 236)
2.54%
6 (from 236)
2.54%
Table 14.
No deterioration during surgical manipulations
196
83.05%
56
Deterioration during surgical manipulations
(amplitude   50%)
40
16.95%
Hala El-Habashy et al.
Table 15.
Return to 100% of base line II after
stopping surgical manipulation
Return to 75% of base line after
stopping surgical manipulation
Sustained deterioration till
end of surgery
4
10%
20
50%
16
40%
Stim. 3: During Decompression and Retraction Group II (Sevoflurane + N 2O).
Stim. 4: After Decompression and Stoppage Group II (Sevoflurane + N2O).
57
Egypt J. Neurol. Psychiat. Neurosurg.
DISCUSSION
The intraoperative MEPs largely depends on
the stimulation pattern and anesthetic technique,
further improvement in intraoperative MEP
recording requires exact knowledge of the
modifying effects of each of these factors14. The
aim of this work is to evaluate the role of motor
evoked potentials in response to multipulse
transcranial electrical stimulation in predicting
motor system affection and surgical outcome
during anterior cervical microdiscectomy in
patients with herniated disc induced cervical
myelopathy and to compare the outcome under
two anesthetic regiments (Propofol-fentanyl)
versus (sevoflurane-N2O). This study included
two groups of patients each 30 in number; group
(I) under (Propofol-fentanyl) anesthesia and group
(II) under (sevoflurane-N2O) anesthesia. On
comparing between group I, group II there were
no significant effect of age, sex, and ASA class on
results as P. value was <0.05. There was one
patient (four limbs) in whom this test (TceMEPs
with myogenic recording) could not be performed,
but, this may be attributed to technical errors,
(earthing, amplification, filtering…etc). The
current study showed that, the presence of
preoperative paresis had no significant influence
on repetitive TceMEPs monitoring as a baseline
(I) recording at start of operation and baseline (II)
recording after induction of anesthesia could be
obtained. These two baseline recordings used as
reference for detecting any change after starting
anesthesia
and
after
starting
surgical
manipulations respectively. This is confirmed by
other study which assumed that; in the presence of
preoperative paresis, the TceMEPs monitoring
applicability is reduced but there is no influence
on the intraoperative pattern of MEPS15.
Concerning the effects of anesthesia on
intraoperative TceMEPs monitoring; this study,
showed that anesthesia in group I (PropofolFentanyl) did not prevent recording of (CMAPs)
after multipulse TceMEPs, but (PropofolFentanyl) anesthesia produced statistically
significant increase in latency of (CMAPs) [by
58
Vol. 43 (1) – Jan 2006
median (+3%) for upper limbs and (+3.26%) for
lower limbs]. Also (propofol-fentanyl) regiment of
anesthesia produced statistically significant
decrease in amplitude of (CMAPs) [by median (–
9.12%) for upper limbs and (-9.38%) for lower
limbs]. With anesthesia in group II (SevofluraneN2O), recording of (CMAPs) after multipulse
TceMEPs was not prevented but (SevofluraneN2O) produced statistically significant increase in
latency of (CMAPs) [by median (+2.94%) for
upper limbs and (+3.28%) for lower limbs]. Also
(Sevoflurane-N2O)
anesthesia
produced
statistically significant decrease in amplitude of
(CMAPs) [by median (-11.99%) for upper limbs
and (-12%) for lower limbs]. On comparing the
effects of anesthetics between group I and group
II, it is noticed that (Propofol-Fentanyl) regiment
produced statistically insignificant prolongation of
latency of (CMAPs) compared to the recorded
under
(Sevoflurane-N2O)
regiment.
Also
(Sevoflurane-N2O) regiment produced statistically
significant decrease in (CMAPs) amplitude more
than that recorded under (Propofol-Fentanyl)
regiment. These results are in agree with many
previes studies16-21.
In another study16 which performed
intraoperative TceMEPs monitoring using
multipulse stimulation under propofol anesthesia
and demonstrated that propofol anesthesia caused
a reduction of (CMAPs) amplitude to 7% of
baseline recording. It was found that propofol
reduced TceMEP amplitude in a dose dependant
manner when using a multipulse stimulation with
a constant intensity and number of stimuli. Indeed,
doubling the propofol concentration in blood from
0.7 mg litre-1 to 1.4 mg litre-1 may reduce
(CMAPs) amplitude by 30-50% 20. Previous
authors found that motor neuron excitability is
markedly impaired when target propofol
concentrations reaching 0.9 mg litre-1 21. On the
other hand TceMEP latencies were minimally
influenced by propofol concentrations commonly
used to maintain a deep anesthesia and remained
stable during the whole operations16.
Previous studies using multipulse TceMEPs
reported that nitrous oxide at concentrations
Hala El-Habashy et al.
above 50% reduced significantly the amplitude of
(CMAPs) but did not prevent monitoring in the
large majority of operations19. This is consistent
with earlier studies using multipulse TceMEPs18.
Short trains of anodal stimuli delivered to the
exposed motor cortex, succeeded in monitoring
myogenic
MEPs
during
sevoflurane-N2O
anesthesia22. It is likely that the depressant effect
of sevoflurane-N2O during multipulse TceMEPs
monitoring is dose dependant23. CMAPs recorded
under (Sevoflurane-N2O) anesthesia had lower
amplitude and higher trial to trial variability than
CMAPs recorded during (Propofol-Fentanyl)
anesthesia. Sevoflurane had minimal effect on
CMAP latencies 4.In conclusion it is assumed that
"provided that the degree of general anesthesia
and analgesia were kept constant" multipulse
TceMEPs with myogenic recording remained
stable throughout the operation under both
regiment of anesthesia (Propofol-fentanyl/
Sevofuran-N2O). Nevertheless CMAPs recorded
under (Sevoflurane-N2O) regiment of anesthesia
had lower amplitude and higher trial to trial
variability than CMAPs recorded during
(Propofol-fentanyl) one whereas both regiment
had minimal non-significant prolonging effect on
latencies. These data are comparable with data
obtained from current study except for effects of
both regiment of anesthesia on latencies of CMAP
that
demonstrate
statistically
significant
prolongation of latencies. This may be attributed
to difference in doses of given anesthetics or
differences in stimulus intensity. A dose effect
relationship was not investigated in current study.
Consequently, an exact knowledge of the effects
of increasing concentrations of these anesthetics
on TceMEPs with myogenic recording is needed
to avoid the erroneous conclusion that intraoperative changes in TceMEPs during surgical
maneuvers represent spinal cord injury.
Concerning the role of intraoperative
TceMEPs monitoring in prediction of new injury
to spinal cord as regard motor pathways: on
comparing between changes of mean latencies and
mean amplitudes of CMAPs recorded before and
during surgical manipulations (after stimuli 2 and
3 respectively). This study showed that the mean
latencies did not increase by more than 10% and
mean amplitude did nor decrease by more than
50%, so there were no significant risk of
deterioration during this type of surgery (Anterior
cervical microdiscectomy). This may be attributed
to the high degree of safety of surgical maneuver.
However, we could not calculate the sensitivity
and specificity of this test (TceMEPs with
myogenic recording) as we had no single patient
with
new
postoperative
motor
deficit.
Nevertheless we had less than 2% false positive
recordings during surgical manipulations. Also we
found that during decompression and surgical
manipulation, 2.5% of cases showed deterioration
by increase in latency of CMAP > 10% and all
these patient returned to their original recordings
of latency after completion of cervical spinal cord
decompression with no new postoperative motor
deficit. Also during decompression and surgical
manipulation, 17% of cases showed deterioration
by decrease in amplitude of CMAP > 50%, only
1.6% of them returned to 100% of their original
recordings, 8% returned to 75% of their original
recordings and 7.4% showed sustained
deterioration till end of surgery with no new
postoperative motor deficit in all patient.
This apparently makes TceMEPs with
myogenic recording a test that can only point to
harmful manipulations but can not alone
determine how much these harmful manipulations
will progress to permanent postoperative motor
deficit.
Several investigators have reported clinical
and experimental parameters of intraoperative
TceMEPs monitoring using CMAPs recording.
These authors referred to the threshold of the
changes in CMAPs as indicators for the detection
and prevention of neurologic injury24,25. They
insisted that an amplitude reduction of 50%
should be the threshold. In another study26, which
performed a detailed experiment using animal
models of scoliosis and concluded that a 10%
delay in onset latency would be an appropriate
threshold. Clinically the critical point should be
defined at a point yielding a 10% latency delay or
59
Vol. 43 (1) – Jan 2006
Egypt J. Neurol. Psychiat. Neurosurg.
disappearance of CMAPs. The parameter
monitored in muscle MEP (CMAP) recording was
the presence or absence of responses. This all-ornone concept has been adopted for two reasons.
Firstly in contrast to epidural MEPs that show
little amplitude variation27, the variability of
muscle MEP amplitude is tremendous28. Thus,
defining a threshold amplitude below which one
expects an intraoperative injury would be
extremely difficult. Secondly; the analysis of the
available reports indicated that a motor deficit
occurred only when the muscle response was
lost18.
The repetitive trans-cranial electrical
stimulation with epidural and myogenic recoding
was found to be highly accurate for predicting
postoperative motor deficit. The sensitivity and
specificity values of this technique were 1 and 1
respectively29. This is consistent with results of
current study which showed that TceMEPs with
myogenic recording of CMAPs alone could only
point to harmful manipulations but could not
determine how much these harmful manipulations
will progress to permanent postoperative motor
deficit. So it is assumed that multipulse TceMEPs
with epidural and myogenic recordings will show
high accuracy on predicting postoperative motor
deficit. Using this highly reliable method, existing
surgical procedures can be safely performed, and
the possibility of performing more aggressive
surgical procedures in the further is anticipated.
Finally we conclude that, TceMEPs, monitoring is
very labile test for many reasons, the most
important of them are technical difficulties;
therefore this test must be done with close
attendance of experienced personells.
A dose-effect relationship was not
investigated in this study. Consequently, an exact
knowledge of the effect of increasing
concentrations of these anesthetics on TceMEPs
with myogenic recording is needed to avoid the
erroneous conclusion that intraoperative changes
in TceMEPs during surgical maneuvers represent
spinal cord injury.
Also It is recommended to perform this test
(TceMEPs) on other types of surgery that require
60
spinal cord monitoring by both MEPs, SSEPs
such as, operations on thoracoabdominal aorta,
operations for correction of spinal deformity,
operations for post-traumatic spine injury and
operations within spinal cord (spinal cord tumors).
With knowledge of physics, techniques and
safety issues of neurophysiological monitoring,
the field will continue to proliferate and assist
surgeons alleviating human suffering.
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