By Dr shereen algergawy
associate prof rheumatology and
rehabilitation
Benha faculty of medicine
The speed of conduction of both
sensory and motor fibers is
determined by the integrity of
heavily myelinated fibers and the
preservation of saltatory
conduction between individual
nodes of Ranvier.
. If the myelin is disrupted between the
stimulus and recording sites, the
recorded potential will be delayed in
onset. This measurement is called the
distal latencv and in sensory conductions
is the time from the stimulus onset to the
peak of the sensory nerve action potential
(SNAP
in sensory nerves is calculated by dividing the
measured distance between the stimulation and
recording sites by the distal latency. With a
supramaximal stimulus, all of the axons beneath
the stimulator should depolarize, resulting in a
waveform amplitude commensurate with the
number of underlying axons depolarized. The
amplitude is measured from the peak to the
trough of the SNAP .
Each EMG laboratory
should have normal
values with controls for
patient age and height.
Skin temperature
should be controlled,
which may require
warming the limb.
in sensory nerves is
calculated by dividing
the measured distance
between the stimulation
and recording sites by
the distal latency. With
a supramaximal
stimulus, all of the
axons beneath the
stimulator should
depolarize, resulting in
a waveform amplitude
commensurate with the
. Each EMG laboratory should have
normal values with controls for patient age
and height. Skin temperature should be
controlled, which may require warming
the limb.
Conduction velocity in sensory nerves is calculated by
dividing the measured distance between the
stimulation and recording sites by the distal latency.
With a supramaximal stimulus, all of the axons
beneath the stimulator should depolarize, resulting in
a waveform amplitude commensurate with the
number of underlying axons depolarized. The
amplitude is measured from the peak to the trough of
the SNAP (Fig-1B). Each EMG laboratory should have
normal values with controls for patient age and height.
Skin temperature should be controlled, which may
require warming the limb.
all of the axons beneath the stimulator should
depolarize, resulting in a waveform amplitude
commensurate with the number of underlying
axons depolarized. The amplitude is measured
from the peak to the trough of the SNAP (Fig-1B).
Each EMG laboratory should have normal values
with controls for patient age and height. Skin
temperature should be controlled, which may
require warming the limb.
Technical errors such as 1- not placing the
recording electrode directly over the nerve
being tested or
2-not achieving supramaximal stimulation
of the nerve can artificially lower the
amplitude of the SNAP. Factors such as
3- inaccurate distance measurements or
4-a cold limb can markedly alter distal
latency and conduction velocity
determinations

Motor NCSs can aid in the assessment of the etiology of weakness. A
recording surface electrode is placed over the belly of the muscle being
studied . Through orthodromic, supramaximal stimulation of the motor
nerve at a fixed distance, a waveform called the compound motor action
potential (CMAP) is obtained (Fig-2B). This waveform represents the
summation of the depolarization of muscle fibers beneath the recording
electrode. This distal latency is determined by recording the time from the
stimulus onset to the initial motor response. The motor nerve is then
stimulated from a second, more proximal site and a second CMAP is
obtained. The conduction velocity of the nerve segment between
stimulation sites is calculated by dividing the distance between
stimulation sites by the difference between distal latencies. In motor
nerves such as the ulnar or peroneal nerve that are commonly susceptible
to compression about fixed structures, a third stimulation site is used to
span the possible compression site. The conduction velocity can then be
calculated from each of the two proximal sites and compared. Focal
slowing of more than 10 m/s in a short segment is considered significant.
With supramaximal stimulation
of the motor nerve, all motor
fibers beneath the stimulus are
depolarized, resulting in a
maximal contraction of the
muscle being recorded. The
amplitude of the CMAP is thus
dependent on the state of the
motor axons. Amplitude is
measured from the baseline to
the peak of the CMAP.
However, other processes besides axonal
failure can result in a low CMAP
amplitude. If muscle mass is decreased
from any cause such as a previous central
nervous system injury or malnutrition, the
CMAP amplitude can be lowered. Also,
severe myopathy or neuromuscular
junction disease can result in a low CMAP
amplitude. EMG is thus needed to clarify
the cause of the low CMAP amplitude. The
area of the CMAP correlates with the
amplitude and may better reflect the
amount of muscle being depolarized
owever, other processes besides Evaluating
the amplitude and degree of dispersion of
the CMAP can greatly assist in
understanding the underlying
pathophysiology of the nerve lesion.
Neurapraxia refers to nerve conduction
failure without axonal loss and implies a
demyelinating lesion. If enough fibers fail
to conduct impulses because of conduction
block across a given segment, the CMAP
amplitude will decrease during nerve
stimulation proximal to the block; 25 to 30
percent is a significant degree of change in
most nerves. Focal slowing affecting fast
conducting fibers will delay the CMAP. If
there is differential slowing of slow
conducting fibers along a nerve segment,
the CMAP waveform will be dispersed,
thus demonstrating a desynchronization of
fiber firing.
Motor NCSs are
difficult technically and
errors may result from
improper placement of
electrodes, incorrect
measurements, or
submaximal
stimulation. Once again,
height, age, and skin
temperature are

Another parameter that can be measured to evaluate conduction
along a motor nerve is the F-wave, one type of late response. This
response is obtained with supramaximal stimulation while motor
conduction studies are being performed. When a nerve is
stimulated there is depolarization of that nerve in both directions.
The F-wave response is caused by recurrent firing of the anterior
horn cell after antidromic conduction. Therefore both the afferent
and efferent limbs of this response are motor. Because this
response evaluates proximal nerve conduction, it can be useful in
evaluating patients for root or plexus injury. It may be especially
useful in the acute stage before evidence of peripheral nerve
degeneration and denervation changes (as detected by EMG) has
developed. Because of the length of nerve traveled by the
impulses, normal values are different based on the subject's
height. F-wave latencies are determined by analyzing at least 10 Fwaves and recording the earliest latency.

Another type of late response, the H-reflex, is
different from the F-wave in that the afferent
limb of the H-reflex is sensory and the efferent
limb is motor. The H-reflex is tested by
stimulating the tibial nerve in the popliteal
fossa and recording from the gastrocnemius
muscle. The H-reflex afferent limb is through
the S1 root. Responses are determined with
submaximal stimulation and are compared to
the responses on the contralateral side. An
asymmetry of 2 ms is considered significant.

Normal patients may have bilaterally absent Hreflexes so that bilateral absence of response is not
necessarily pathologic. Both F-wave latency and Hreflexes are most useful when peripheral
conduction studies are normal; abnormal
responses suggest a proximal lesion. However,
when routine motor conduction is abnormal,
abnormality of these late responses may not
necessarily be indicative of a proximal lesion. After
nerve injury, such as with a remote history of a
radiculopathy, late responses may remain
abnormal indefinitely. Therefore the interpretation
of an abnormality would benefit from comparison
with a previous study.
To summarize, distal latency and
conduction velocity measurements are
particularly helpful in evaluating the speed
of conduction along distal and midportions
of
a
peripheral
nerve,
respectively. The F-wave latency is
particularly
useful
in
evaluating
conduction along proximal segments of a
motor nerve if the distal segments are
normal. When the electromyographer uses
the term demyelinating features. reference
is made to prolonged distal latency, slow
conduction velocity, prolonged F-wave
latency, or dispersed waveforms. The
amplitude of the CMAP is altered by
failure of conduction to the muscle and the
waveform
may
be
helpful
in
understanding the reason for the altered
conduction. Axonal features usually imply
low amplitudes. However, an EMG study
of the muscle is needed to clarify the
reasons for a low CMAP amplitude
Both sensory and motor conduction
studies are highly reproducible, although
there is better intra-examiner reliability
than inter-examiner reliability. Conduction
studies are focused on an area of clinical
abnormality; distant areas are studied also,
to classify the abnormality as focal,
multifocal, or diffuse. In studies in which a
focal conduction block is suspected but not
definitely proven by the routine studies, a
technique called inching can be used. The
region of the suspected block is studied by
nerve stimulation above and below the
presumed site of the block at 1-cm
intervals searching for a focal dramatic
change in distal latency. These studies are
frequently useful in the evaluation of a
suspected carpal tunnel syndrome, ulnar
neuropathy at the elbow, and peroneal
neuropathy at the knee.
excess of acetylcholine
packets and receptors are
present, which ensures
successful neuromuscular
junction transmission.
However, in patients with
neuromuscular junction
disorders this safety factor
is diminished and
repetitive stimulation,
usually at 1 to 3 Hz, causes
failure of neuromuscular
junction transmission,
resulting in a
excess of acetylcholine
packets and receptors are
present, which ensures
successful neuromuscular
junction transmission.
However, in patients with
neuromuscular junction
disorders this safety factor
is diminished and
repetitive stimulation,
usually at 1 to 3 Hz, causes
failure of neuromuscular
junction transmission,
resulting in a
decremental response in
CMAP amplitude or area.
Standard guidelines
include comparing the
response produced by the
first stimulus to the
response produced by the
fourth stimulus; an
abnormality is defined as a
decrement of at least 10
percent (Fig-3). Decrements
on repetitive stimulation
are not specific for primary
neuromuscular
junction disease and can be
seen in any circumstance in
which neuromuscular
junction transmission is
faulty. Such circumstances
include motor neuron
disease and patients
receiving drugs that are
active at the neuromuscular
junction. The sensitivity of
repetitive nerve stimulation
is higher when a clinically
weak muscle is being
tested. A
normal test in a clinically
normal muscle does not
rule out the presence of
neuromuscular junction
disease, and additional
muscles should be studied
to increase the yield.
Commonly studied
muscles include the
abductor pollicis brevis,
abductor digiti quinti,
extensor digitorum brevis,
trapezius, and facial
muscles
A repetitive nerve stimulation study demonstrating
a 61 percent d to the fourth stimulation.
ecrement in area and a 54 percent decrement in
amplitude from the first

Blink responses, like the corneal reflex, allow
evaluation of trigeminal sensory and facial
motor conduction. Surface electrodes are
placed on the orbicularis oculi muscles
bilaterally, along with surface reference
electrodes and a ground. Stimulation of the
supraorbital nerve or a glabellar tap results in
an ipsilateral response via a pontine pathway
through the main sensory trigeminal nucleus
and the facial nucleus. The response is
designated R1. Thus,
, this R1 response evaluates
trigeminal and facial nerve
conduction. Subsequent to
the R1 response is a second
bilateral response,
designated R2, that is
polysynaptic and more
diffuse in brain stem
localization. The RI
response is best used for
evaluating conduction
velocity along the
trigeminal and facial

nerves because it is a
shorter reflex. The R2
response is best used in
localizing the lesion to
right or left trigeminal
or facial nerves. These
studies along with
routine motor
conduction studies of
the facial nerve and
EMG of the facial
muscles may be useful
in analyzing several
disorders affecting the
facial and trigeminal
nerves.