Background
In a previous paper [[1]], the response of the compound motor action potential (CMAP) produced by peripheral
nerve stimulation was studied during a pure compression injury of the nerve. Although,
this is one mechanism by which a nerve might be injured during surgery, nerves can
also be injured as a consequence of stretch. In order to use the CMAP as a means of
warning a surgeon that a nerve is undergoing significant stretch during a surgical
procedure a number of criteria must be met. First, those characteristics of the CMAP
that can be measured in real time must be identified and their changes during stretch
must be understood. Second, optimal means of classifying whether there is impending
injury to the nerve based upon these parameters must be found. Finally, the sensitivity
and specificity of these changes in predicting injury must be determined. These are
the primary goals of this paper.
It is well known that stretching a peripheral nerve can cause injury. Many studies
have demonstrated that stretch can damage the myelin [[2],[3],[4]]as well as the cytoskeleton [[5],[6]]. The neurophysiology of stretch injury has also been investigated but primarily
in regard to the subacute injury caused by limb lengthening [[7],[8],[9],[10]] rather than the acute injury that may occur during a surgical procedure. In particular,
the electrophysiologic characteristics of these subacute injuries may be quite different
from acute injuries especially since it has been shown that longitudinal stretching
of the nerve for prolonged periods is associated with a greater chance of injury at
the same stretching force [[11]] than a brief period of stretch. Electrophysiologic studies of stretch have shown
both reductions in conduction velocity and decreased CMAP amplitudes but have not
evaluated the criteria that could be used to determine which electrophysiologic changes
provide the first indication of acute stretch related injury.
The specific goal of this paper is to study the changes in the CMAP during acute nerve
stretch and compare them to the changes seen during acute compression. In particular,
conduction velocities, CMAP amplitudes, CMAP duration, and the area under the curve
for the CMAP will all be studied as well as the presence of spontaneous electromyographic
(EMG) activity.
Methods
Use of animals
Under protocol #401 approved by the Marshall University IACUC, 21 sciatic nerves from
13 normal male golden Syrian hamsters were analyzes. The data were compared with data
obtained in a previous study [[1]] from 16 sciatic nerves from 10 normal male golden Syrian hamsters were subjected
to pure compression. Of the 21 nerves in this study, 5 nerves were taken from animals
sedated with pentobarbital (75 mg/kg ip) and 16 from animals sedated with isoflurane
(2-3.5% titrated to maintain sedation). All hamsters were purchased from BioBreeders
(Watertown, MA).
Recording the CMAP
Recordings of the CMAP were made from the stainless steel subdermal needle electrodes
(Model E2-48, Astro-Med, Inc., West Warwick,) placed in the muscles of the hind paw.
The sciatic nerve was stimulated proximally at the spine using similar subdermal needle
electrodes placed in tripolar fashion along the nerve with approximately 2 mm separation
between the electrodes. Stimulation was accomplished with a Grass S88 stimulator connected
to a Grass PSIU6 constant current isolation unit. The intensity of the stimulus was
increased in the range of 2-15 mA until further increases in the stimulus intensity
produced no apparent increase in the amplitude of the CMAP at the beginning of the
experiment. This stimulus intensity was used throughout the remainder of the experiment.
The duration of each stimulus was chosen as 0.01 msec in order to minimize stimulus
artifact.
The signal from the recording electrodes was amplified by Grass Model 12 amplifiers
(Astro-Med, Inc., West Warwick, RI) with the high frequency filter set at 3 kHz and
the low frequency filter set at 0.3 Hz and a gain of 500. Continuous recordings of
spontaneous muscle activity were amplified and directed to a loudspeaker so that spontaneous
electromyographic activity could be documented as they occur in synchrony with the
recorded CMAP data. The signal was digitized using a NI-USB-6259 16 bit, 1.25 MHz
data acquisition module (National Instruments, Austin, TX) with a sampling rate of
30,000 Hz/channel. Stimulation was performed at a rate of 5/sec and the average of
20 traces was computed prior to saving the response. Thus, CMAP’s were recorded every
4 seconds.
Each hamster’s rectal temperature was monitored continuously and controlled using
a warming lamp. The mean temperature for all nerves was 31°C with a standard deviation
of 2.3°C. In addition, continuous recordings were made of the output of a Shimpo DFS-1
force gauge (Shimpo Instruments, Itasca, IL) with a measurement accuracy of 0.1 g.
The actual force exerted on the nerve is properly measured in Newtons with the conversion
being the weight measured by the force gauge divided by 102. For the sake of simplicity,
the weight in grams will often be used instead of the force in Newtons in the remainder
of this paper. The in-house software controlling each experiment also allowed the
experimenter to make annotations that were synchronous with the CMAP recordings and
enabled both manual and automatic marking of the CMAP’s.
After dissection of the sciatic nerve, standard 1.3 mm wide vascular loops were wrapped
around the nerve as shown in [Figure 1] and then around the force gauge as the nerve was lifted out of the incision site.
It should be noted that the part of the nerve subject to stretch was exposed to atmospheric
oxygen throughout the experiment. Measurements were made of the height of the nerve
above the incision (h in [Figure 1]) and the length of the open incision (L in [Figure 1]). It is important to be aware that this is not a model that involves pure stretch.
Since the nerve is pulled away from the body, there is a component of both stretch
and compression. It is also important to be aware that this stretching produces an
elongation of the nerve which was estimated as 
([Figure 2]).
Figure 1
Schematic diagram of the nerve stretch experiment.
Figure 2
Computation of the degree of elongation of the nerve during stretch.
Before recording data, the stimulus intensity was adjusted to obtain a supramaximal
stimulus and the recording and stimulating electrodes were adjusted to obtain a high
amplitude (> 500 μV) response.
Each experiment occurred in the stages noted in [Table 1]. [Figure 3] shows a typical CMAP along with the typical points that are marked
Table 1
Stages of nerve stretch experiment and comparison with the nerve compression experiment
|
Stretch
|
Compression
|
|
Stage
|
Description
|
Maximum Force (gm)
|
Duration
|
Stage
|
Description
|
Maximum Force (gm)
|
Duration
|
|
1
|
Baseline
|
0
|
|
1
|
Baseline
|
0
|
|
|
2
|
First Stretch
|
10
|
3 min*
Mean 3.01
|
|
|
|
|
|
3
|
First Recovery
|
0
|
3 min
|
|
|
|
|
|
4
|
Second
Stretch
|
20
|
3 min*
Mean 2.87
|
2
|
First
Compression
|
20
|
3 min*
Mean 3.5
|
|
5
|
Second
Recovery
|
0
|
3 min
|
3
|
First
Recovery
|
0
|
3 min
|
|
6
|
Third
Stretch
|
40
|
3 min*
Mean 1.78
|
4
|
Second Compression
|
80
|
3 min*
Mean 1.78
|
|
7
|
Third Recovery
|
0
|
3 min
|
5
|
Second
Recovery
|
0
|
3 min
|
|
8
|
Fourth
Compression
|
Until 0 Amplitude
|
3 min*
Mean 4.41
|
6
|
Third
Compression
|
Until 0 Amplitude
|
3 min*
Mean 1.91
|
|
9
|
Fourth
Recovery
|
0
|
3 min
|
7
|
Third
Recovery
|
0
|
3 min
|
This table also shows sequence of force application during an experiment. It should
be noted that in stretch stages 2 4 and 6 if the CMAP amplitude fell to half of its
baseline, then the stretch was immediately released. In stage 8, when the CMAP amplitude
reached zero, the stretch was immediately released. Note that leg 8 is longer than
the other legs because of the extended time it took to gently create the higher stretch
forces.
*Planned duration. The number below this is the actual mean duration of the given
stage.
Figure 3
Typical CMAP along with the points marked on that CMAP. Note the definitions of the duration and amplitude.
Statistical analysis
The term latency always refers to the time delay between the stimulus and the onset
of the CMAP (marker 1 in [Figure 3]) and the term amplitude refers to the maximum peak to peak amplitude. Computation
of conduction velocities assumed a synaptic delay of 0.5 msec [[12]]. All latencies were corrected to the values corresponding to 37°C according to
the relation derived from an analysis of baseline latencies [[1]]:
where T is the rectal temperature at the time of the latency measurement and the corrected
latency is that expected at 37°C. In addition, a “corrected” velocity is also computed
using instead of the linear distance from the point of stimulation to the point of
recording that distance plus the amount the nerve is lengthened by the stretch.
The duration of the CMAP is measured as the difference between the time of the first
and last noticeable deflection of the CMAP (the time difference between points 1 and
4 in [Figure 3]). Another characteristic of the CMAP is the area under the curve (AUC) Since the
CMAP generally has components above and below baseline, the area under the curve is
computed using Simpson’s rule applied to the absolute value of the CMAP
where tstart is the shortest time after stimulation at which reliable data is available and tstop is the latest time (> point 4 in [Figure 3]) for which a CMAP is present. Because the CMAP shape and amplitude depend on the
exact placement of the recording electrodes, the actual value of the measured parameters
is divided by the mean value of that parameter in the baseline state (Stage 1) to
arrive at “normalized” parameter values.
A number of statistical techniques are important in analyzing the data from this experiment.
A Spearman rank correlation analysis (Statistica, Tulsa OK) is used to determine how
independent the 5 CMAP measurements described above are. High rank correlation coefficients
between two measurements would suggest that they contain similar information and are
redundant descriptors of the data. In addition, a repeated measures ANOVA using the
5 measurements (MEASURE) as a repeated measure and the stage (STAGE) as an independent
variable will be used to determine whether there is a statistically significant difference
between the different measures in different stages. This analysis is not based upon
the raw data set because this data set has many measurements for each condition and
may thus produce a false statistical significance because of the large number of data
points. Instead, prior to the ANOVA analysis, a reduced file is created that has the
mean value of each normalized measure in each leg for each nerve. This is the file
that is subjected to statistical analysis. A similar (STAGE × MEASURE × ANESTHESIA)
repeated measures ANOVA is used to determine whether anesthesia has any effect on
the measures and whether that effect is dependent on the degree of stretch.
From the neurophysiologic monitoring standpoint, it was important to determine the
time at which the first statistically significant changes in one of the above discussed
CMAP parameters occurred during the experiment. A simple method to determine this
time involved performing a repeated measures ANOVA in the normalized variable under
study starting with the first two stages of the experiment and then adding successive
stages to the ANOVA until a statistically significant effect is noted. The reduced
size file is used for this analysis.
Finally, it was important to investigate the neurophysiologic parameters that distinguished
nerves subjected to different stretching forces. This was done by carrying out linear
discriminant analyses (Statistica, Tulsa OK) with the dependent variable being the
stage and the independent variables being all or a subset of the normalized measurements.
When more than one independent variable was used a linear stepwise analysis was carried
out with an F to enter of 3 and an F to remove of 1. Accuracy of the classification
was recorded as were the classification functions. Multiple such analyses were carried
out to compare the baseline CMAP data from that in each stage where there was nerve
compression. This was carried out separately for each of these stages since the criteria
for detection were likely to be different. These same analyses were carried out on
the data obtained in a previous set of experiments on the changes in the CMAP during
pure nerve compression [[1]].
Results
Nerve Breakage
For 16 nerves, information was available on the force at which the nerve breaks into
two different segments. This occurs at a mean force of 331 gm with a standard deviation
of 55 gm. In 14 nerves, the nerve broke at the distal incision, in one case the nerve
broke at the proximal incision site and in 1 case, the nerve broke at the location
of the vascular loops.
Force Required to Abolish the CMAP
It should be noted that the CMAP reached zero amplitude at a mean of 73 gm force with
a range of 41-120 gm and a standard deviation of 18 gm. This is roughly 22% of the
force required to break the nerve.
Changes in CMAP during Nerve Stretch
Independent Variables
There are a large number of potentially interesting variables describing the CMAP.
Because of this, it was important to know which variables contained unique information.
To achieve this, a Spearman rank correlation analysis ([Table 2]) is performed with all of the normalized measured variables both when the entire
data set and when the data set contained only the first 7 segments of the experiment.
When the total data set was used, there was significant statistical correlation between
all of the normalized outcome variables at the p < 0.001 level. The strongest correlations
were between the area under the curve (AUC) and the normalized amplitude (R = 0.82)
and adjusted normalized velocity and normalized velocity (R = .58). The lowest correlation
was between the duration ratios and the amplitude and between the amplitude measures
and the velocity variables. Overall correlations are lower but still significant when
only the data from the first 7 experiment phases are used. Although this analysis
indicates that the normalized outcome variables are strongly correlated, the Spearman
rank correlation coefficients all being less than 0.82 suggests that each of the variables
contains at least some unique information.
Table 2
Correlations between measured variables
|
Normalized Amplitude
|
Normalized AUC
|
Normalized Velocity
|
Normalized Corrected Velocity
|
Normalized Duration
|
|
Normalized Amplitude
|
|
.82 (.63)
|
.14 (.03)
|
.06 (-.06)
|
.21 (.11)
|
|
Normalized AUC
|
.82 (.63)
|
|
.20 (.15)
|
.13 (.07)
|
.24 (.19)
|
|
Normalized Velocity
|
.14 (.03)
|
.20 (.15)
|
|
.62 (.56)
|
.31 (.20)
|
|
Normalized Corrected Velocity
|
.06 (-.06)
|
.13 (.07)
|
.62 (.56)
|
|
.35 (.27)
|
|
Normalized Duration
|
.21 (.11)
|
.24(.19)
|
.31 (.22)
|
.35(.27)
|
|
The entries in the table are Spearman rank correlation coefficients. All are significant
at p < .001 using all of the stages. Using data only from stages 1-7 gives the data
in parentheses.
The statistical difference between the 5 outcome measures during the stretch experiment
can also be estimated using a repeated measures ANOVA with stage as the independent
factor and the normalized outcome variables as 5 repeated measures. There was a significant
main effect of STAGE (F(6,140) = 4.1 p < .001) and outcome variable (MEASURE) (F(4,560)
= 8.7 p < .001) as well as a significant interaction term (F(24,560) = 1.75; p < .02).
This again suggests that the 5 outcome measures have different dependence on the experimental
stage.
General Trends
The overall results of the experiments are summarized in [Figures 4], [5] and [6]. [Figure 4] shows the changes in the CMAP peak to peak amplitude and AUC during each stage of
the experiment. In this figure it is evident that the AUC drops about 5% at 10 gm
stretch, 10% at 20 gm stretch and 20% at 40 gm stretch while recovering to baseline
after 10 and 20 gm stretch but not after stretch with 40 gm or greater. With stretch
forces less than 40 gm, the peak to peak amplitudes show significant rebound with
higher amplitudes during the recovery periods than baseline although each compression
does produce a relative decrease in amplitude from its pre-compression baseline. [Figure 5] shows that there are significant reductions in the normalized raw velocity even
at the 10 gm and 20 gm stretch conditions but even with the maximal compression, as
long as response is recordable, the conduction velocity is always greater than 70%
of baseline. Of course, since the nerve lengthens with stretch, the length of nerve
traversed by the nerve impulses increases. Correcting for this, the actual speed of
nerve conduction may be increased above baseline for stretch forces less than 40 gm.
However, at the 40 gm or more stretch even the corrected velocities decline. [Figure 6] shows that the duration of the CMAP increases slightly at the lowest stretch tension
and then declines at 40 gm and above.
Figure 4
Changes in the normalized peak to peak amplitude (AMP) and the normalized area under
the curve (AUC) during the stretch experiments.
Figure 5
Changes in the normalized nerve conduction velocity during various phases of the nerve
stretch experiment.
Figure 6
Changes in the normalized CMAP duration during the stretch experiments.
Individual Variability
The above summary results belie the complexity of the results from individual nerves.
[Figure 7a] shows the changes in CMAP’s during a typical experiment while [Figure 7b] shows the actual CMAP waveforms during this experiment. [Figures 7c] and [7d] show the dependence of the normalized peak to peak amplitude and the normalized
AUC in two other nerves experiments. It is clear that the amplitude of the CMAP changes
can exhibit many different patterns for stretch at < 40 gm but, for stretching forces
above 40 gm, the CMAP reliably declines precipitously. The changes in velocity are
more consistent from nerve to nerve than those of the CMAP amplitude or AUC, but the
effects of stretch on CMAP duration also show significant variability.
Figure 7
Illustration of the differences in the responses of various nerves to stretch and
the typical CMAP waveforms recorded.
In order to find the first stage for which statistically significant changes in one
of the parameters describing the CMAP occurs, a sequence of one-way ANOVA’s was carried
out using each different parameter as the dependent variable and STAGE as the independent
variable. Although the value of STAGE began at 2 for each ANOVA, the largest value
of STAGE ranged from 3 to 9. In particular, the reduced data file in which only 1
data point is available for each stage is used in order to avoid the false statistical
elevations that might occur as the result of multiple measurements in the same stage.
[Table 3] indicates that the velocity measures are much more sensitive to changes at low stretch
forces than the amplitude or duration measures. In addition, the AUC ratio is more
sensitive than peak to peak amplitude ratios at low stretch forces and the duration
alone does not show statistically significant changes until the highest levels of
stretch force.
Table 3
First experiment phase in which a significant change is noted in the given variable
|
Variable
|
First Stage Significant
|
Significance at First Significant Stage
|
Significance at Stage 9
|
|
Normalized Amplitude
|
6
|
.05
|
< .001
|
|
Normalized AUC
|
6
|
.01
|
< .001
|
|
Normalized Velocity
|
2
|
.001
|
< .001
|
|
Normalized Corrected Velocity
|
2
|
.001
|
< .001
|
|
Normalized Duration
|
8
|
.002
|
< .001
|
Anesthesia Effects
One important question is whether the variability seen in individual stretch experiments
is related to the anesthesia used. In order to see if this were true, a MEASURE ×
STAGE × ANESTHESIA 5 × 9 × 2 repeated measures ANOVA was performed. There were significant
main effects of STAGE (F(8,154) = 17, p < .001), ANESTHESIA (F(1.154) = 4.8, p = .03)
and MEASURE (F(4,616) = 27, p < .001). There was a significant effect of anesthesia
on MEASURE (p < .001) but no significant triple interaction of MEASURExSTAGExANESTHESIA.
In fact, the velocities and durations are similar with both anesthesia types but the
peak to peak amplitude and AUC were significantly lower with pentobarbital anesthesia.
The sequential ANOVA analysis described above was repeated on only the group of nerves
from which data was collected under isoflurane anesthesia and statistically significant
changes were not found at earlier points in the experiment.
Predictability
Clinically, it is important to know what changes in the CMAP predict injury to the
nerve and to know the sensitivity and specificity of these predictions. In order to
answer these questions, multiple linear discriminant analyses were used with all or
specific subsets of the four outcome variables that would be available in real time
(normalized peak to peak amplitude, normalized AUC, normalized velocity, and normalized
duration) to classify CMAPs as either from baseline or from one of the compression
stages (2, 4, 6 or 8). As seen in [Table 4], discriminating between baseline and any of the compression states can be done with
85-95% accuracy. The specificity and sensitivity of the classifier for stage 8 versus
stage 1 is 100% and 84% respectively. When a low stretch force is applied, the normalized
velocity is the primary contributor to the classification function and better as a
univariable predictor than any of the amplitude related variables. With the larger
stretch forces (> 40 gm), the normalized peak to peak amplitude or AUC are better
univariable classifiers than the velocity. The duration used alone cannot provide
as good a classification as the other outcome variables.
Table 4
Various linear models to predict stretch injury from the outcome variables
|
Comparison Stages
|
Normalized Peak-Peak Amplitude
|
Normalized AUC
|
Normalized Velocity
|
Normalized Duration
|
Best Classification
|
Classifier For Compression Stage
|
|
1-2
|
Yes
|
Yes
|
Yes
|
Yes
|
87% (96,77)
|
VEL-0.33DUR < 0.62
|
|
Yes
|
Yes
|
No
|
No
|
63% (81,46)
|
AUC < 0.94
|
|
Yes
|
No
|
No
|
No
|
63% (77,50)
|
AMP < 0.94
|
|
No
|
Yes
|
No
|
No
|
64% (95,75)
|
AUC < 0.95
|
|
No
|
No
|
Yes
|
No
|
85% (82,46)
|
VEL < 0.96
|
|
No
|
No
|
No
|
Yes
|
63% (76,49)
|
DUR > 1.02
|
|
1-4
|
Yes
|
Yes
|
Yes
|
Yes
|
84% (96,71)
|
-0.25AMP+VEL
+0.45AUC-0.75DUR < 0.35
|
|
Yes
|
Yes
|
No
|
No
|
67% (85,49)
|
-0.65AMP+AUC < 0.26
|
|
Yes
|
No
|
No
|
No
|
67% (98,32)
|
AMP > 1.2
|
|
No
|
Yes
|
No
|
No
|
65% (97,70)
|
AUC < .90
|
|
No
|
No
|
Yes
|
No
|
84% (92,37)
|
VEL < .95
|
|
No
|
No
|
No
|
Yes
|
72% (88,54)
|
DUR > 1.04
|
|
1-6
|
Yes
|
Yes
|
Yes
|
Yes
|
93% (100,81)
|
-0.074AMP+VEL+
0.24AUC+0.23DUR
< 0.97
|
|
Yes
|
Yes
|
No
|
No
|
79% (97,48)
|
-0.33AMP+AUC < 0.52
|
|
Yes
|
No
|
No
|
No
|
63% (100,0)
|
——
|
|
No
|
Yes
|
No
|
No
|
65% (100,82)
|
AUC < .74
|
|
No
|
No
|
Yes
|
No
|
93% (98,31)
|
VEL < .85
|
|
No
|
No
|
No
|
Yes
|
70% (99,17)
|
DUR < .86
|
|
1-8
|
Yes
|
Yes
|
Yes
|
Yes
|
96% (100,84)
|
0.48AMP+VEL+
0.74AUC-0.98*DUR
< 0.68
|
|
Yes
|
Yes
|
No
|
No
|
96% (100,93)
|
0.66AMP+AUC < 0.95
|
|
Yes
|
No
|
No
|
No
|
96% (100,93)
|
AMP < .30
|
|
No
|
Yes
|
No
|
No
|
95% (100,61)
|
AUC < .59
|
|
No
|
No
|
Yes
|
No
|
89% (99.8,92)
|
VEL < .75
|
|
No
|
No
|
No
|
Yes
|
81% (100,32)
|
DUR < .82
|
VEL is the normalized velocity, AMP is the normalized peak to peak amplitude, DUR
is the normalized duration and AUC is the normalized area under the curve. Under best
classification the top number is the total number of correctly classified cases. The
two numbers in parentheses below this are the specificity and sensitivity.
Using multiple different criteria to classify the CMAP is important in clinical neurophysiology.
[Figure 8] is a graphical representation of the percentage of the traces in each stage that
have normal velocities and amplitudes using the univariable classifiers developed
by the linear discriminant analysis (normalized velocity abnormal if < 0.95 and normalized
peak to peak amplitude < 0.57). This figure shows that the probability that both velocity
and amplitude are normal (V+A+) is very low for stretch > 40 gm. The number where
both are abnormal (V-A-) becomes high only when during the terminal stretch stage.
Figure 8
Fraction of traces in each stage fitting the amplitude and voltage criteria or both. V+ means normalized velocity > 0.95, V-means normalized velocity < = 0.95, A+ indicates
peak to peak amplitude > 0.57, A-means peak to peak amplitude < .57.
For comparison, the same analysis is carried out with the compression data from the
previous paper [[1]]. These results are summarized in [Table 5]. This table demonstrates that, for nerve compression, amplitude is a better predictor
of compression induced changes than velocity even at low compressive forces, although
the predictability increases with higher compression forces.
Table 5
Various linear models to predict compression injury from the outcome variables
|
Comparison Stages
|
Normalized Peak-Peak Amplitude
|
Normalized AUC
|
Normalized Velocity
|
Duration
|
Best Classification
|
Classifier
|
|
1-2
|
Yes
|
Yes
|
Yes
|
Yes
|
63% (58,66)
|
0.17AMP+VEL
-0.12AUC
-0.18DUR < .86
|
|
Yes
|
No
|
No
|
No
|
54% (29,75)
|
AMP < 1.05
|
|
No
|
Yes
|
No
|
No
|
49% (5,87)
|
AUC < .88
|
|
No
|
No
|
Yes
|
No
|
49% (39,72)
|
VEL < 1.0
|
|
No
|
No
|
No
|
Yes
|
62% (34,57)
|
DUR > .98
|
|
1-4
|
Yes
|
Yes
|
Yes
|
Yes
|
86% (92,77)
|
0.61AMP+VEL
+0.55AUC < 1.81
|
|
Yes
|
No
|
No
|
No
|
86% (99,65)
|
AMP < .69
|
|
No
|
Yes
|
No
|
No
|
81% (96,57)
|
AUC < .74
|
|
No
|
No
|
Yes
|
No
|
76% (98,36)
|
VEL < .93
|
|
No
|
No
|
No
|
Yes
|
52% (34,67)
|
DUR > 1.29
|
|
1-6
|
Yes
|
Yes
|
Yes
|
Yes
|
97% (99.7,91)
|
AUC-0.12DUR < 0.48
|
|
Yes
|
No
|
No
|
No
|
95% (99,89)
|
AMP < .57
|
|
No
|
Yes
|
No
|
No
|
96% (95,75)
|
AUC < .58
|
|
No
|
No
|
Yes
|
No
|
85% (100,62)
|
VEL < .89
|
|
No
|
No
|
No
|
Yes
|
82% (95,55)
|
DUR < .58
|
Under best classification the top number is the total number of correctly classified
cases. The two numbers in parentheses below this are the specificity and sensitivity.
Spontaneous EMG Activity
Clinically, the presence of spontaneous EMG activity is one of the factors used in
determining when there is a significant injury to a nerve. In order to understand
how the presence of spontaneous EMG activity depends on the stretching force, the
CMAP and anesthesia, a factorial ANOVA is performed with EMG activity as the dependent
variable and ANESTHESIA and STAGE as independent factors. In this analysis there were
significant main effects of STAGE (F(8,171) = 6.4, p < .001) but not ANESTHESIA (F(1,171)
= 3.2, p < .08) and there was no significant interaction (F(8,171) = .82, p < .58).
This is consistent with the observations of [Figure 9] that the presence of EMG activity mainly occurred during stretch at the higher force
levels and during recovery after a severe stretch injury. As in the previous paper
[[1]], EMG activity was more likely when the CMAP amplitude was significantly reduced
from baseline. In particular, the value of the normalized peak to peak amplitude was
0.14 when EMG activity was heard and 0.80 when no EMG activity was heard (t = 17.5
df = 8624 p < .001). Similarly EMG activity was significantly associated with reduced
normalized velocities (0.85 when spontaneous EMG present and 0.92 when such activity
was not present p < .001) and reduced duration ratios (0.93 when EMG present and 1.0
when EMG absent p < .001).
Figure 9
Changes in spontaneous electromyographic (EMG) activity during the experiment.
Does the Effect of Low Stretch Levels Predict the Response to High Stretch Levels?
Since this experiment involves multiple sequential stretches of a nerve, it is useful
to ask whether the response to a low level of stretch predicts the response to a higher
level of stretch. As a partial answer to this question, multiple Spearman rank correlation
analyses were performed between the value of the outcome variables in one stage and
other stages. Because of the large number of comparisons involved, a Bonferroni correction
was made and significance tested at the .001 level. The results are shown in [Table 6]. There was a strong positive correlation (R = 0.8 p < .001) between the minimum
velocity in stage 2 and the minimum velocity in stage 4 but not stage 6. Similarly,
there was a positive correlation (R = .85, p < .001) between the minimum AUC in stage
2 and stage 4 although a similar relation was not seen for the peak to peak amplitudes.
There was also a positive correlation between the duration in stages 2 and 4
Table 6
Significant correlations in outcome variables (minimum normalized amplitude, minimum
AUC, minimum normalized velocity, minimum duration) in different stages
|
Stage 2
|
Stage 4
|
Stage 6
|
Stage 8
|
|
Stage 2
|
–
|
(VEL,VEL)
|
N.S.
|
N.S.
|
|
|
(AUC,AUC)
|
|
|
|
|
(DUR,DUR)
|
|
|
|
Stage 4
|
|
–
|
N.S.
|
N.S.
|
|
Stage 6
|
|
|
–
|
(VEL, VEL)
|
|
|
|
|
(VEL, DUR)
|
|
Stage 8
|
|
|
|
–
|
Statistical significance level set at .001 because of multiple testing.
Discussion
From a clinical standpoint, it is critical to understand how different types and severity
of nerve injury affect the CMAP so that the CMAP can be used to predict when there
is significant injury to a nerve. Many criteria have been used to interpret intra-operative
neurophysiologic studies [[13]] and these depend on the specifics of the surgical procedure, the structures at
risk and the specific testing modality [[14],[15],[16],[17],[18]]. Despite this, the most commonly used criteria for deciding when there is a significant
change in somatosensory evoked potentials is either a 10% reduction in velocity (or
10% increase in latency) or a 50% reduction in amplitude. For transcranial motor evoked
potentials the criteria are often taken as complete disappearance of the potential
rather than a 50% decrease in amplitude.
One difficulty with clinical studies to assess the best warning criteria is that it
is often impossible to know the exact timing and magnitude of the forces applied to
a monitored nerve during a surgical procedure. The other difficulty is that the clinical
outcome of the surgical procedure is not known until the procedure is over. Thus,
if the surgeon is provided a warning based upon the one set of criteria and corrective
action is taken, it is impossible to decide whether the criteria used to provide the
warning yielded a false positive warning or accurately identified a true impending
injury to the nerve that was corrected. Hence, experimental studies on animals can
provide useful complementary information. In studies of stretch related to limb lengthening,
Jou [[19]] suggests that a 50% change in a somatosensory evoked potential amplitude is associated
with a clinical deficit due to stretching of the peripheral nerve. Wall [[9]] found that stretching a nerve to a strain of 6% longitudinally in rabbit tibial
nerve produced a 70% reduction in the nerve action potential and at 12% strain conduction
was blocked and never recovered fully. In the current study, strain was not longitudinal
(in fact it was primarily perpendicular to the axis of the nerve) as in other studies
but had a magnitude up to 35%. The result of Wall were confirmed by studies of Brown
[[8]] on the CMAP showing that 15% strain produced a 99% reduction in amplitude and Li
[[10]] showing severe conduction block in nerve action potential at strains of 20%. The
current study did not include outcome measures but the study of Fowler [[11]] in rat sciatic nerve indicated that those nerves could tolerate 50 gm of stretch
for 2 minutes before permanent injury ensued. The hamster sciatic nerve is much smaller
than the rat and is likely more susceptible to injury. This provides evidence that
the highest stretch levels used in this study would likely have been associated with
a clinical deficit in a survival study.
In terms of interpretation criteria, for stretch forces < 40 gm, the main effect is
an increase in latency and decrease in the standard velocity measure during nerve
stretch, with velocity changes as low as 5% being significant At stretch forces >
40 gm, the changes in amplitude and area under the curve are more significant and
better able to classify the changes in the CMAP than the velocity. This is different
from the case of a purely compressive injury where the amplitude of the CMAP is always
the best variable for classifying signals as being from baseline or one of the compression
stages even at low compression force. This is the expected result since in the pure
localized compression model, conduction abnormalities develop in a segment of the
nerve that is small in comparison to the distance between the stimulating and recording
electrodes. Thus, even if there were a severe reduction in conduction velocity in
this small length, the overall conduction velocity would change little. In this particular
model, at low stretch forces, the degree of compression at the point where the vascular
loop transfers force to the nerve is too small to cause conduction block and so the
amplitude does not decrease significantly. However at high stretch forces, there is
significant compression at the point where the vascular loops transfer force to the
nerve and the amplitude declines. For the low stretch forces, the increase in conduction
velocity is unlikely to be related to a change in the passive properties of the axon
since the diameter of the axon must decline as its length increases in order to maintain
a constant volume and axons with smaller diameters have reduced conduction velocities.
It also cannot be related to a change in the distribution of conducting axons since
the conduction velocity is computed from the onset latency and so reflects the velocity
of only the most rapidly conducting axons. Also, because of the very short stimulus
durations used, only the largest and most rapidly conducting axons are tested in this
paradigm. The most probable explanation is that stretch affects some of the properties
of ion channels and hence excitability of the axonal membrane [[20],[21],[22],[23]]. This might also be a likely explanation for the fact that the CMAP amplitude often
becomes larger after mild degrees of compression than at baseline especially if the
small degree of stretch depolarizes the membrane slightly and increases excitability.
An analogous phenomenon is seen after an axon is exposed to low doses of 4-aminopyridine
which at low doses blocks potassium channels and increases excitability but at high
doses reduces excitability [[24],[25]]. However, in order to verify this hypothesis, additional experiments studying the
membrane properties of the stretched axons would be needed.
Returning to the clinical question regarding CMAP based decision criteria, it is clear
that is important to look at many different characteristics of the CMAP. Even 5% reductions
in the conduction velocity can signal that a nerve has been subjected to a significant
stretch. Although changes of this magnitude in only the velocity are associated with
good recovery after the stretch is released, they still would provide a valuable early
warning to a surgeon. The peak to peak amplitude is more variable during the nerve
stretch experiments but both velocity and amplitude are abnormal when there is a high
level of stretch. However, a high level of sensitivity of the CMAP for predicting
high stretch levels cannot be achieved unless as demonstrated in [Figure 9], an abnormality in either velocity or amplitude is considered significant. The presence
of both increases specificity.
It is important to be cautious in generalizing this information to human recordings
for at least 2 reasons. First, in most clinical situations, the nerve is not exposed
to atmospheric oxygen as in this experiment and so is much more sensitive to the effects
of change in blood flow [[26]] than the nerves in this experiment. Second, the composition of and the amount of
connective tissue are different in human and hamster nerves [[27]]. Despite these limitation, there are some possible clinical implications that may
be helpful for intra-operative neurophysiologic monitoring. First, spontaneous EMG
activity may not be the first sign of injury to a nerve and its presence or absence
may be strongly influenced by anesthesia. Second, the type of change to be expected
in the CMAP depends on the mechanism of injury. Early changes in the velocity occur
with stretch while with compression over small areas, the first changes are in amplitude.
However, when there is significant injury, there is a decline in amplitude no matter
what the mechanism.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MS participated in study design, data collection, data analysis, and writing of the
paper. KB and MS participated in data collection, data analysis and in writing of
the paper. JW participated in the data analysis and the data collection. All authors
have read and approved the final version of the manuscript.
Cite this article as: Stecker et al.: Acute nerve stretch and the compound motor action potential. Journal of Brachial Plexus and Peripheral Nerve Injury 2011 6:4.
