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.
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 currentNormal reaction / Muscle
Nerve / Tetanic contraction / Brisk single contraction
Partial R.D. / Muscle
Nerve / Diminished response / Diminished response
Sluggish contraction
Full R.D. / Muscle
Nerve / No response / No response
Sluggish response
Absolute R.D. / Muscle
Nerve / No response / No response
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 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.
* 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 / hyper-polarization 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.
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.
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.
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.
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.
* 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.
* 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.
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.