ELECTRODIAGNOSIS
Modifications to electrical reactions
Changes in the way muscles or motor neurons react to electrical stimulation are likely to happen when these tissues are diseased or injured. The nature and extent of the lesion can be diagnosed with great help from the changed electrical reactions.A muscle's decreased or lost voluntary power could result from:
(a) an upper motor neuron lesion
The anterior horn cell and the higher motor neuron are not generally accessible for electrical stimulation, but the lower motor neuron below its exit from the spinal canal and the muscle itself are.
Lesions in upper motor neurons
There are no alterations in the lower motor neurone or muscle (i.e., in the accessible portion of the motor pathway) in response to an upper motor neurone lesion, which would result in changed electrical reactions. As a result, electrical stimulation produces a normal type of response; yet, occasionally, hyperexcitable nerves and muscles will react to a lesser level of current than is typically needed.Lesions to lower motor neurons
Anterior horn cells, nerve root fibers, or peripheral nerves can all be affected when a lower motor neuron is damaged. Three categories apply to lesions affecting the nerve fibers: axonotmesis, neurotmesis, and neurapraxia.Neurapraxia, also known as first-degree injury, is a situation in which the nerve is unable to conduct impulses past the site of the lesion due to pressure or bruise, but the damage is not severe enough to result in fiber degeneration. There is loss of response to a stimulus administered to the nerve trunk above the lesion, yet a typical type of response is achieved if the electrical reactions are rested on the damaged muscles. A more severe lesion may result in axonotmesis, or second-degree harm.
Neurotmesis: Severe Nerve Fiber Damage
The nerve sheath is unaffected even while the axons undergo degeneration. A radial nerve palsy linked to a broken humeral shaft serves as an illustration of this kind of injury. The electrical reactions change when the nerve fibers have deteriorated. The severing of the nerve fibers and sheath is known as third-degree damage, or neurotmesis. The same literations in the electrical reactions as axonormesis are caused by the fibers degenerating below the lesion site. However, the problem is more severe since nerve suturing is required before the nerve can regenerate satisfactorily. If a cut on the front of the wrist destroyed the ulnar nerve, such a lesion would be seen.Nerve Lesion Types and Reactions
Any
combination of two of these nerve lesion types, such as neurapraxia and
axonotmesis, is possible. All of these nerve lesion types can be partial or
total. The reactions indicative of complete denervation are seen when all of
the nerve fibers feeding a muscle degenerate, whereas partial denervation
reactions are seen when only a portion of the fibers degenerate.
Effects of Anterior Horn Cell Lesions
The degree of damage determines the responses seen in anterior horn cell lesions. The effects of denervation are seen if the injury is severe enough to cause degeneration of the nerve fibers. Partial denervation occurs when only a section of the nerve cells feeding a muscle are impaired, whereas complete denervation occurs when all of the cells are affected. Less severe lesions do not result in degeneration of the nerve fibers, and the reactions are normal.
The neuromuscular junction's shortcomings
Sometimes poor conduction at the neuromuscular junction causes a loss in voluntary power, as in the case of myasthenia gravis. The best tools for diagnosing these diseases are not electrical stimulation techniques.Muscle damage
When there is no degeneration of the motor neuron and the reduction in voluntary power is caused by muscular weakness or disease, the responses to electrical stimulation are normal in type but weaker. There won't be any reaction to electrical stimulation if the lesion is so bad that all muscle tissue is lost. This lack of reaction could be the result of muscle fibrosis brought on by chronic denervation, myopathies in their advanced stages, or disorders like ischemic contracture.Disorders of function
There may be no change in the electrical reactions if the loss of voluntary power is caused by hysterical paralysis.
Stages of denervation
Wallerian degeneration occurs when a nerve fiber is damaged, both above and
below the lesion site up to the first node of Ranvier. It could take up to
fourteen days for this deterioration to finish. An impulse is started and a
normal muscular response is produced if the nerve beneath the lesion location
is stimulated before degeneration has occurred. As a result, it might not be
feasible to fully evaluate the lesion until three weeks following a suspected
nerve injury, by which point any severed nerve fibers will have undergone
degeneration. However, tests performed prior to this date may yield valuable
data. Muscle Responses to Nerve Stimulation
All
of the muscles that a typical motor nerve trunk feeds will contract if it is
stimulated with a current strong enough to cause contraction beyond the
stimulation point. However, this response is diminished or eliminated if there
is degeneration of the nerve fibers, and the alterations show up three or four
days following the injury. Before the end of the first week, changes in the
muscles' responses to stimulation may be noticed; these changes suggest that
the nerve is degenerating, though it is still too early to determine the full
amount of the degeneration.
Curves of strength-duration
The most effective technique now in use for routine testing of electrical reactions in peripheral nerve lesions is the charting of strength-duration curves, which show the intensity of impulses of different durations necessary to cause contraction in a muscle. This approach to measuring electrical reactions has the advantages of being straightforward, dependable, and able to reveal changes in the state through a series of tests in addition to providing the percentage of denervation. Its drawbacks include the fact that in large muscles, just a fraction of the fibers may react, making the entire picture difficult to see, and that it is unable to pinpoint the exact location of the nerve lesion. Nonetheless, nerve conduction tests can be used to identify the location.Devices
Rectangular impulses with varying durations are supplied by the device that generates strength-duration curves. A stimulator made specifically for muscle testing is required to ensure that the impulses are accurate in both shape and duration. The device should also undergo routine maintenance to guarantee optimal performance. It is necessary to use impulses with durations of 0.01, 0.03, 0.1, 0.3, 1, 3, 10, 30 and 100 ms.Comparison of Muscle Stimulator Types
Either constant-voltage or constant-current stimulators are possible. Although the differences between these two types of stimulators are outside the purview of this book, the former records the voltage applied, and the latter the intensity of current employed. According to recent research, the distinctions between the two stimulator types' effects were previously overstated. Although the constant-voltage stimulator is more patient-friendly, the constant-current simulator was supposed to yield more accurate findings. By making sure that the skin resistance is as low as possible, the discomfort associated with both kinds of stimulators can be reduced.
Technique
Washing and soaking in warm water reduces the skin's resistance before the current is administered, and any abrasions are covered. The patient needs to be well-lit, warm, and supported. The active electrode should be placed over the muscle's fleshy portion, and the indifferent electrode should be put to a convenient location, usually the body's midline or the muscle group's origin. An alternative would be to place one tiny electrode over each end of the muscle belly. In either scenario, the active electrodes should be somewhat tiny so as to allow for the isolation of the muscles from one another.Precision in Strength-Duration Testing
The shortest stimulus is used to apply current, which is then raised until the smallest detectable contraction is achieved. Depending on the muscle being tested, this can be evaluated visually or by palpating the tendon. The impulse is shortened and the current (or voltage) magnitude is recorded. This process is iterated for every stimulus length sequentially, increasing the current magnitude as needed. For the results to be correct, the highest level of precision is required. The active electrode must be maintained on the same spot over the muscle for the duration of the test. The minimal observable contraction is employed since it facilitates the detection of any changes in strength. Plotting the strength-duration curve is done using the test findings. The crucial aspect is the curve's shape, even though the constant-voltage stimulator will cause it to be farther to the left than the constant-current stimulator.
Typical strength duration curves
typical innervation Strength-duration curves with intact nerve fibers feeding the muscle have a form typical of normally innervated muscle. The reason the curve has this common form is that all longer-duration impulses require the same stimulus strength to elicit a response, whereas shorter-duration impulses require an increase in stimulus strength for each reduction in duration. The curve's starting point for ascent varies, although it typically occurs at 1 ms for constant-current stimulators and 0.1 ms for constant-voltage stimulators.
Total deprivation
The strength-duration curve that results from the total denervation of a muscle
is indicative of the degeneration of all nerve fibers supplying that muscle.
The curve rises rapidly and is further to the right than that of a normally
innervated muscle for all impulses with a duration of 100 ms or less. This is
because no response is elicited for impulses of very short duration, and the
strength of the stimulus must be raised each time the duration is lowered. Partial starvation
It
is evident from the characteristic curve obtained that partial denervation
occurs when some of the nerve fibers feeding a muscle have deteriorated while
others remain intact. Longer length impulses cause both innervated and
denervated muscle fibers to contract, allowing for the application of a mild
stimulus. Denervated fibers respond less quickly to shorter impulses, requiring
a stronger stimulus to cause a noticeable contraction. As a result, the curve
rises steeply, resembling that of denervated muscle. This portion of the curve
resembles that of innervated muscle because denervated fibers cannot contract
when impulses have shorter durations than innervated fibers do. As a result,
denervated fibers respond to a weaker stimulus than innervated fibers can.
As
a result, the curve's right portion resembles denervated muscle, while its left
portion resembles innervated muscle. A kink can be seen where the two sections
converge.
Strength-Duration Curve
The percentage of denervation is indicated by the curve's form. A curve that
mimics denervation for the most part rises sharply when a high number of fibers
are denervated. The curve is flatter and lower, more closely resembling that of
complete innervation, if most of the fibers are innervated. A shift in the
strength-duration curve's form could be a precursor to the recovery of a
muscle's nerve supply. The curve has a kink that develops, and as
re-innervation continues, the curve descends and Tests for nerve conduction
Conductivity of nerves
The muscles that are supplied contract when a nerve trunk is stimulated. Plotting strength-duration curves and nerve conduction testing together are frequently done by that nerve distal to the place of stimulation. Any contraction of the muscles supplied below this point shows that at least some of the nerve fibers are intact. An impulse lasting 0.1–0.3 ms is applied at a position where the nerve trunk is superficial. Assessing the intensity of the lesion by comparing it to the stimulus needed to elicit a comparable response on the unaffected side of the body provides some insight into the extent of the damage, but it does not reveal whether the muscle or nerve is at fault.Dispersion of nerves
Individual differences exist in the distribution of the various nerves, which can be deceptive when evaluating nerve lesions. By stimulating the nerve trunk and monitoring the ensuing contractions of the muscles, one can ascertain the distribution of a nerve.Conduction velocity
With the right tools, one may measure how quickly an impulse travels through a nerve fiber; for further information, go to the section on electromyography.Additional electrical reaction tests
Although they are no longer in use, there are still a number of alternative techniques for assessing electrical responses that may occasionally be employed. The guiding ideas behind a few of them are listed below, along with the explanations for why the outcomes are currently seen acceptable.Rheobase
In practical terms, an impulse of 100 ms (0.1 sec) is employed as the rheobase, which is the smallest current that, in the event that the stimulus is of indefinite length, will cause a muscle contraction. The rheobase during denervation may be lower than that of innervated muscle, and it frequently increases as re-innervation takes place. However, these changes are not predictable enough to serve as trustworthy guidelines. A rise in the rheobase may be the result of muscular fibrosis, however it varies greatly throughout muscles and depends on the skin resistance and temperature of the affected area.Chronaxie
The shortest impulse duration that will result in a response with a current
double that of the rheobase is known as the chronaxie. When a constant-voltage
stimulator is utilized, the chronaxie of the innervated muscle is significantly
smaller than that of the denervated muscle—the former being less and the latter
more than 1 ms. The numbers are higher when using the constant-current
stimulator, but they still have a comparable association. Chronaxie is not a
good way to test electrical reactions because it does not clearly show partial
denervation; instead, it measures the predominant state of the fibers. For
instance, a muscle with 25% of its fibers innervated would have the same
chronaxie as a muscle with complete denervation.
Faradic and interrupted direct current tests
In the past, testing with interrupted
direct currents and faradic types was common, but it is incredibly incorrect.
Impulses with a frequency of 50-100 Hz and a duration of 0.1-1 ms are produced
by the faradic-type current. These generate a tetanic contraction in innervated
muscle; however, because the stimuli are brief in duration, it is difficult or
impossible to elicit a response from denervated muscle when using a faradic
coil. Today's stimulators, however, typically produce a response with impulses
of this length from denervated muscle because of their higher output and more
bearable current forms than those found in the outdated apparatus. Errors
resulting from changes in the shape and length of the impulses have also been removed.
Thirty times a minute, impulses lasting roughly 100 ms were applied using interrupted direct current. These often result in a slow contraction of denervated muscle fibers but a rapid contraction of innervated muscle fibers. However, in certain situations, such as myxoedema, innervated muscles may react slowly when their temperature drops, but denervated muscles contract more forcefully when their temperature rises.
The electromyography
An overview of the electromyography features that directly affect physiotherapists is provided below. A more comprehensive examination of the topic is recommended for the student in Walton's Disorders of Voluntary Muscle.
Surface electrodes cannot be used for
diagnostic electromyography (EMG); instead, a coaxial needle electrode is used
for recording. Initially, electrical activity is measured while the muscle is
at rest, and subsequently, during voluntary contraction. An anterior horn cell,
the nerve and its branches that emerge from it, and the muscle fibers that
these branches supply make up a motor unit. In external ocular muscles, there
are thirty muscle fibers per motor unit, whereas in big muscles, there are
fifteen thousand. A unit's voluntary control is more exact the fewer fibers it
has.
0 Comments