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Basic Nerve Conduction
Studies Findings
Motor Responses
The motor response is obtained by stimulating a nerve and recording
from a muscle that it innervates. The muscle selected should have a fairly
well-defined motor point, and preferably be relatively isolated from other
muscles innervated by the nerve and from other nerves that may be stimulated
inadvertently during the test. The excitation of nearby muscles may alter
the response and make it difficult to determine the exact onset of the
desired motor response.
The motor response may be characterized by its amplitude, duration,
and wave form. The amplitude is measured from the baseline to the top of
the negative peak of the motor response and is expressed in millivolts.
The distal latency is measured from the onset of the stimulus artifact
to the point of takeoff from the baseline and is measured in milliseconds.
Extra care must be taken to use the corresponding takeoff points of both
the distal and proximal responses so that conduction velocities are measured
along the same fibers. The amplitude depends to a large extent on the number
and size of muscle fibers being activated, and supramaximal stimulation
of the nerve should ensure a maximal motor response. Any pathological process
that decreases the number of motor units or muscle fibers responding will
affect the amplitude. The normal motor response indicates a fairly synchronous
discharge of the motor units. If there is dispersion of the times when
the motor units discharge, then the amplitude will be lowered and the response
spread in time. This effect brings up the question of duration of the response.
In processes in which the nerve conduction slows differentially, the duration
of the response will be prolonged and thus its amplitude decreased.
The usual motor response has a fairly simple waveform. It may have one
or two initial negative (up) peaks (the latter usually indicating two muscle
being stimulated) and usually will be followed by a positive deflection
(down) toward the end. The response should have a clear initial negative
deflection as it takes off from the baseline. In some pathological processes,
the wave may have multiple phases, appearing extremely complex.
The motor response also changes in relationship to the point of nerve
stimulation. The more proximally the nerve is stimulated, the lower the
amplitude and the longer the duration of responses seen. These effects
are due to the temporal dispersion of the motor units activated because
of differential conduction velocities in the normal motor nerves.
Sensory-Nerve Action
Potentials
Sensory-nerve action potentials (NAP) are obtained by stimulating a
nerve and recording directly from it or one of its branches. The recording
site must be remote from muscles innervated by that same nerve because
muscle responses will obscure the much smaller NAP.
The NAP can also be characterized by it amplitude, duration, and wave
form. The amplitude of the NAP is measured from the peak of the positive
deflection the peak of the negative deflection and is measured in microvolts.
The sensory distal latency is traditionally measured from the stimulus
artifact to the takeoff or the peak of the negative deflection. When conduction
velocities are needed, distal latencies to the takeoff of the proximal
and distal responses should be used. The amplitude depends on the number
of axons being stimulated and the synchrony with which they transmit their
impulse. If the axons transmit impulses at comparable velocities, the response
duration will be short and amplitude high. However, if the axonal velocities
are widely dispersed, the NAP duration will be longer and its amplitude
lower.
Distal Latency
Defined as the time from the stimulus affecting the nerve to the response
(motor or sensory) being recorded, latency is usually measured in milliseconds (msec). Distal latency is that interval measured from the stimulation of
the distal-most accessible site on the nerve. This finding does not give
direct information on conduction velocities, because the distal segment
often follows a tortuous route that cannot be measured. The measurement
is useful, however, because it can be compared with normal data and indicate
the relative conductivity of the segment of the nerve. In measuring the
latency of the motor nerve, remember that a small portion of that time
is due to the delay in neuromuscular transmission, whereas no such delay
is present in sensory latencies.
Conduction Velocity
If a nerve can be stimulated at two points along its course, and a measurement
can be obtained of the distance between those points, conduction velocities
can be figured.
This is true for most motor nerves. In sensory studies however, only
one stimulation site is nromally used. Compute the velocity (V) by measuring
the distance (d) in millimeters (mm) between the two stimulation points
and dividing by the difference in latency (ms) between the proximal (tp)
and distal stimulation points (td), as indicated in this equation:
V=d/tp-td
The result is expressed as meters per second (m/sec.).
Because the proximal and distal latencies are measured to the takeoff
of the response, the conduction velocity obtained represents conductions
along the fastest conducting fibers, with those that first reach the muscle
causing the initial deflection.
Conduction velocities in the various nerves differ, depending on anatomical
considerations. However, several general principles apply to evaluating
nerve conduction studies:
 | The more proximal the segment of nerve being evaluated is, the faster
the velocity will be.
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| If the extremity being tested is cold, the velocity will be slowed
and the amplitude increased. This effect occurs especially in cold weather
and some provisions for warming the patient and for using a fairly constant
room temperature should be made.
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| At times anatomical considerations such as potential entrapment points
will also tend to slow the velocities.
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| The shorter the segment between the two stimulation points, the less
reliable the calculated velocities will be, due to a greater effect on
the margin of error by a shorter distance.
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Conduction velocities depend most on the integrity of the myelin sheath.
In segmental demyelinating diseases conduction velocities drop to below
50 percent of normal values. However, when axonal loss is severe, the velocity
will also be slowed due to a dropout of the fastest conducting fibers.
The drop in axonal loss is usually in the vicinity of 30 percent below
normal values.
Machine
Settings
In the study of sensory and motor responses, different filter, sweep
speed, and sensitivity settings are used. Sensory studies are performed
with the low frequency setting between 32 and 50 Hz and the high frequencies
between 1.6-2 and 3 KHz. The sweep speed is set to 2 ms/division and the
sensitivity at 10-20 µV/division. Motor studies are performed with
the low frequency set to 1.6-2 Hz and the high frequencies to 8-10 KHz.
Depending on the response's latency and duration, the sweep speed can be
set to anywhere between 2-5 ms/division and the sensitivity between 2-10 mv/division. Whatever the setting, the distal and proximal latencies should
be measured at the same setting, preferably using the faster sweep speed,
as the takeoff is easier to identify with faster sweeps.
Normal
Values
Normal values can be sorted according to age, sex, extremity length,
patient's height or a combination thereof. Unless otherwise specified,
we use the Cleveland Clinic Foundation's EMG Lab normal values which were
sorted according to patient's age. These normals were based on a sampling
of a minimum of forty patients for ages ten to nineteen, and seventy and
over, and at least ninety patients for the other age-groups. The ranges
(first two numbers) and averages (between parenthesis) are provided. These
values are based on the following standard distances: 13 cm for the median
sensory (wrist to active electrode), 11 cm for the ulnar sensory and 10
cm for the radial sensory. For the motor studies, a minimum of 4-6 cm is
used between the wrist and active electrodes (median and ulnar nerves).
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