Electrodiagnosis
Electrodiagnosis deals with the reaction of muscles and motor
nerves to electrical stimuli. The altered electrical reactions may aid in
diagnosis, prognosis or therapy in pathological conditions of the motor
tract including the brain, spinal cord, peripheral nerves and the muscles.
Using the electrical reactions of nerves and muscles as aids in
diagnosis, one must always put in mind that these tests are only part of
the evidence. The other part is being supplied by the clinical examination
of the muscular strength, reflexes and sensory condition of the affected
region. A knowledge of the clinical pathology of the central and
peripheral nervous systems is, therefore, equally essential for the proper
interpretation of the findings.
The motor point of a normal muscle is usually located near the
origin of the muscle belly, where the motor nerve enters the muscle; this
area known as the “end plate zone”. In the nerve trunk, the motor point
can be found where the position of the nerve is nearest to the skin. There
may be several points of maximal irritability in the course of a long
nerve. In pathological conditions leading to a reaction of degeneration,
the motor point of a muscle is displaced distally and the motor point of a
nerve disappears entirely.
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1. Alternating Current / Direct Current
Electrical stimulation generally refers to surface electrode
techniques. It stimulates the superficial nerve fibers, which in turn leads
to nerve trunk and motor point in the muscles. Testing with faradic
stimulation and interrupted direct currents was widely used in the past but
it is not very accurate. The faradic-type current provides impulses with a
duration of 0.1-1.0 msec and a frequency of 50 - 100 Hz. Interrupted
direct current was used in impulses with a duration of 100 msec, repeated
30 times / minute. Factors such as temperature, swelling, dry skin, pain
and infection may inhibit excitation. This test distinguishes between
normal and denervated muscles. It should be put into consideration that:
- Normal musculature responds to electrical stimulation by either
alternating current (AC) or direct current (DC).
- A poor or absent response to AC stimulation indicates the presence of
neural damage “reaction of degeneration” (RD). If this occurs, DC will
be preferable.
- Sluggish response with DC indicates moderate to severe damage to the
nerve.
- Absent response with DC indicates nerve section (absolute RD).
Electrical reactions of muscles and nerves:
RD
Muscle / Nerve
Faradic current
Interrupted direct current
Normal reaction
Muscle
Nerve
Muscle
Nerve
Muscle
Nerve
Muscle
Nerve
Tetanic contraction
Brisk single contraction
Diminished response
Diminished response
Sluggish contraction
No response
Sluggish response
No response
Partial R.D.
Full R.D.
Absolute R.D.
No response
No response
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Course of RD:
Whether it is partial or full, the course can be divided into:
1) The initial stage: Lasts from 10 days to 2 weeks. After the first week,
the nerve looses all response to tetanic stimulation; the muscle responds
feebly or does not respond at all to the tetanic current for galvanic “only
slow response”.
2) The stage of full RD: Lasts from a few weeks to a year or more,
according to the severity of the lesion.
3) The final stage: There is a gradual return of the voluntary function; the
electrical response or absolute RD develops and the outcome is definitely
unfavorable.
Loss of voluntary muscle power:
The part of the pathway, normally accessible for electrical
stimulation, is the lower motor neuron, below its course from the
vertebral canal and the muscle itself but not the anterior horn cell or the
upper motor neuron. Loss of voluntary power of a muscle may be due to:
1) Upper motor neuron lesions (UMNL):
There are no changes in the LMN or muscle, which would lead to
altered electrical reactions. Normal type of response with electrical
stimulation is obtained. Sometimes, nerve and muscle are hyper-excitable
and react to lower intensities.
2) Lower Motor Neuron Lesions (LMNL):
Damage may involve the anterior horn cell, the fibers of the nerve
roots or the peripheral nerves. Lesions involving the nerve fibers can be
classified into:
* Neuropraxia (first-degree injury): It occurs for example as a result of
wrist drop after sleeping with the arm under the head. Bruising or
pressure causes the nerve incapable of conducting impulses, past the site
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of lesion but the damage is not severe enough to cause degeneration of
the fibers. Normal response to electrical current for the muscle below the
site of lesion occurs but there is loss of response to stimulus applied to the
nerve trunk above the lesion. Prognosis takes 3 to 4 weeks for early
recovery, if there is no R.D.
* Axonotemesis (second-degree injury): Degeneration of the axons takes
place but the sheath of the nerve remain intact (radial nerve palsy). There
is alternation in the electrical reactions. Prognosis takes 3 to 12 months, if
there is full RD and 6 to 12 weeks, if there is partial RD.
* Neurotemesis (third-degree injury): The nerve sheath and fiber are
severed. The fibers degenerate below the site of the lesion, causing
alternation of electrical current as axonotemesis. The condition is more
serious as the nerve needs suture before regeneration can take place.
3) Muscle lesions:
* If reduction of voluntary power is due to weakness or disease of the
muscle, there will be no RD (Normal response but reduced in strength).
* If there is complete loss of muscle action, there will be no response to
electrical stimulation (fibrosis and ischemic contracture).
4) Functional disorder:
Loss of voluntary power may be due to hysterical paralysis, in
which there will be no alteration in electrical reactions.
Criteria for response of RD:
* Response to AC is absent.
* Response to DC is sluggish.
* Displacement of the motor point indicates RD. With damage, the motor
point becomes desensitized and the greatest response is usually found at a
point somewhat distal to the true motor point, where the bulk of fibers are
found.
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* Loss of the ability to accommodate to slow-rising current. With
damage, the chronaxie (length of time, needed by an intensity equals
double the rheobase to excite a tissue) is increased.
* There is greater response at the anode more than at the normally
hypersensitive cathode, probably due to the polarization / hyperpolarization phenomena at the membrane surface of the nerve fibers.
* Higher intensities of current are required to excite.
Testing technique:
A low-voltage generator, with both alternating and direct current
circuits is required, along with an interrupter-key stimulating electrode.
1. Test the normal side first.
2. With AC:
Place large indifferent pad moistened with warm tap water at a
distance from the testing site; not on an antagonistic muscle group. With
the interrupter-key electrode, stimulate motor point of the target muscle
using rheobase (least current intensity needed to excite the tissue).
- The presence of a brisk response lessens the possibility of RD.
- Normal response cannot be relied upon within 10 - 14 days after injury,
as RD may take this period to develop and manifest itself clinically.
- If there is no response with AC, DC will be used.
3. With DC:
Repeat the previous procedure with the stimulating electrode as the
negative pole. Less current will be needed since the negative electrode is
usually more irritating because of the alkaline chemical effects produced.
- The presence of a brisk response indicates only partial or slight damage
of the nerve.
- Sluggish contraction indicates moderate to full RD.
- Lack of response may indicate nerve section.
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Signs of regeneration:
- Negative response is greater than positive response.
- Reduced intensities are required.
- Brisker contractions are elicited with normal current.
- Return of the motor point to its traditional site.
- Possible response to alternating current.
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2. Strength-Duration Curve
Strength/duration curve test plots, in graphic terms, the strength of
the stimulating currents against the duration of the stimuli. The traditional
rule is “the stronger the current, the shorter the duration necessary to
produce a contraction”. Selected durations are utilized to stimulate
muscles through the nerve at the motor points. The intensity required at
each duration, is then plotted on a graph, developing a mathematical
curve, indicating normal or abnormal conditions as compared with norms.
Subsequent testing usually shows the curve “shifting” to the left or
right, depending on whether improvement or deterioration is occurring. A
shift to the left indicates improvement, since shorter durations with more
currents is noted, while a shift to the right indicates deterioration as there
is an increase in duration with less current.
Results:
The plotted curve must be compared with subsequent test graphs to
determine progress or regression. With chronaxie test results, a simple
check on results is possible. The duration plotted at intensity equals
double the rheobase should fall on the curve or close to it. Re-evaluation
at weekly intervals is suggested to determine the presence of shifting.
Curves falling within normal limits indicate no damage, whereas
deviations from normal limits indicate relative severity of damage to the
neuromuscular system tested. Plotting of strength duration curves indicate
the strength of impulses of various duration required to produce
contraction in a muscle. It is the most satisfactory method in peripheral
nerve lesions.
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Testing technique:
- Set all dials to zero.
- Place indifferent electrode distant from the target muscle and not on an
antagonistic group.
- Set strength duration-testing apparatus to chronaxie mode.
- Select several durations, greater and smaller than known norms, on the
millisecond dial.
- Stimulate the target motor point at each of these durations, noting in the
graph the intensity required to elicit a minimal contraction in each time.
- Plot at least four points to establish a minimal curve,
- Connect points to form a visible curve.
N.B. Optimum timing for strength duration curves is 10 to 14 days after
the onset of the lesion, when the motor end plate is no longer functioning
and Wallarian degeneration would have occurred.
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3. Rheobase and Chronaxie
The rheobase (intensity) is the smallest current that will produce a
muscle contraction, if the stimulus is of definite duration (in practice, an
impulse of 300 msec is used).
* In denervation, the rheobase may be less that of innervated muscle and
it increases as re-innervation commences.
* The intensity producing the minimal contraction at 300 msec, is termed
rheobase. The rheobase is doubled to reduce the subjective testing.
The chronaxie test measures the duration of the stimulation
necessary to produce a minimal contraction at specific intensities of
current. So, it is primarily a measure of time not intensity. Normal
muscles respond to short-duration stimuli, usually less than 1 msec. In
nerve damage, extended duration is necessary to elicit a contraction. The
greater the deviation from the known norms, the more extensive the
damage and the poorer the prognosis for rapid recovery is. Subsequent
testing at periodic intervals will give indications of progress or
regression.
Testing procedures:
1. Rheobase:
Establishment of a base intensity to produce a minimal contraction
is necessary. Most chronaxie units will offer 300-msec duration for this
procedure, since this duration is sufficient to stimulate even severely
damaged nerve-muscle targets. The intensity producing the minimal
contraction at 300 msec is termed the rheobase. To reduce the human
error in measurement, the rheobase is doubled.
* Establish the rheobase by setting the selector dial to rheobase.
* Place an indifferent electrode at a distance from the test site.
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* The interrupter-key electrode may be used to stimulate the muscle to be
tested at 0.5- or 1-sec intervals. With modern chronaxie equipment, it is
possible to select the interval and place an electrode on the target site,
allowing the machine to stimulate at the selected frequency mechanically.
* When a contraction is noted, throw the selector switch to chronaxie.
This will automatically double the rheobase and reduces the duration
from 300 msec to zero.
2. Chronaxie:
* Turn the main duration control slowly in the direction of increased
milliseconds, starting from zero, until a minimal contraction is observed.
* Turn the main dial back one level and turn the vernier dial up from zero
to obtain a third decimal value when the minimal contraction is reached.
* Read the main dial and the vernier dial to obtain the three-place decimal
value for the duration established.
* Compare this figure with the norm charts to determine comparative
status of the tested musculature.
Results:
Deviations from the norms (increased duration values) are
indicative of nerve damage. Subsequent testing at weekly or monthly
intervals is recommended for treatment management and prognosis
determination. A convenient method of checking accuracy with chronaxie
testing results is to follow a strength duration curve, performed prior to or
subsequent to the chronaxie test, to locate the chronaxie duration at the
doubled rheobase intensity on, or close to the line of the curve.
* Chronaxie test measures the duration of the stimulus necessary to
produce a minimal contraction at specific intensities of current. It is
primarily a measure of time not intensity.
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* Specialized apparatus is required for these tests (Chronaxie and
Rheobase).
* Normal muscle responds to short-duration stimuli, usually less than 1
msec.
* In RD or nerve damage, extended duration are necessary to elicit a
contraction.
* The greater the deviation from known normal, the more extensive the
damage, and the poorer the prognosis for rapid recovery.
* There are charts with normal values for comparison studies.
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4. Electromyography
Electrocardiograph deals with recording the charges of potentials
(electric currents) produced during contractions of the heart muscle. Any
voluntary movement produces changes of potential i.e. a small electric
current which can be recorded by a special apparatus, this is called
electromyography. It is the most recent method of testing the muscles for
the presence of degeneration. Special electrodes are used or inserted in
the muscle and these are connected to a special apparatus with amplifiers
which amplify the current produced. The electric impulses can be seen on
a screen or recorded on a special tape or heard by a loudspeaker. The
electrodes may be applied over the skin of the affected muscle when the
bulk of muscle is great.
a) Normal muscle with intact supply:
* At rest, there is no electric discharge.
* Voluntary movements are recorded as multiple mono-phasic and diphasic or tri-phasic, with less than 10 % poly-phasic spikes, varying in
amplitude from 1 to 100 mv and in duration from 5 to 10 msec. The
waveform depends on the position of the electrode tip, with respect to the
wave of excitation.
Each motor unit produces an action potential (an electric current)
when set to activity. Each motor unit (a nerve cell and a nerve fiber (a
lower motor neuron), called the axon. When the axon reaches the muscle,
it branches to several branches and terminates in several motor end plates.
Each end plate supplies a muscle fiber. When a muscle contracts,
the motor unit do not work synchronously and therefore the waves of
current produced interfere with each other. In the loudspeaker, they are
heard as low-pitched sounds, sometimes as a loud.
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b) Denervated muscles:
In denervated muscles, fibrillary twitches are seen when the muscle
begins to degenerate. This begins from 10 to 14 days after the lesion
causing denervation and may go on for many years.
* At rest, the fibrillation action potentials are recorded. They are seen as
mono-phasic spikes, of lower potential and of very short duration
(amplitude 1 to 50 mv and time 1 to 2 msec).
* During voluntary effort, there is no change.
c) Partial reaction of denervation:
A partial nerve lesion will show both phases i.e. some fibrillation
potentials when the muscle is at rest and some normal active potentials
when attempting active muscular contraction.
d) Regeneration:
When regeneration occurs and the growing fibers of the nerve
reach the muscle end plates (and before the faradic reaction appears),
there is gradual return of the normal findings. The fibrillation potentials
become less in number and few small action potentials occur on
attempting active contraction of the muscles. Electromyography is
valuable in detecting minimal degrees of lower motor neuron lesions and
in detecting recovery.
Electromyography is also used for measuring the conduction
velocity of different motor nerves, for testing for myasthenia, myotonia
and other neuromuscular dysfunctions.
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