Strength-duration curve Definition .
Strength-duration curve is an electrodiagnostic procedure characterized by plotting a graph of, amount of intensity required against various durations of impulse; so as to determine the status of innervations of muscles. It is also called intensity-time curve.
Apparatus and Other Requirements .
A diagnostic muscle stimulator is used for plotting strength-duration curve. For this purpose, apparatus should provide different durations of interrupted direct current.
In addition to this, electrodes, leads, velcro straps, cotton swabs, water or normal saline, graph paper, pencil and eraser are required for strength-duration curve plotting.
Procedure .
Procedure for plotting the strength-duration curve can be described in a stepwise manner.
1. Preparation of Patient .
Explain the procedure in brief to the patient. Position the patient in a comfortable position, which allows you an easy access to the body part where you would like to perform the strength-duration curve assessment. Request your patient to expose the area to be examined and ensure adequate draping. Make sure that there is adequate light to see the visible contractions of muscle without straining your eyes.
2. Application of Electrodes .
Secure the indifferent electrode at a convenient area, usually over midline of the body or origin of muscle and active electrode over motor point. Neuromuscular Electrical Stimulation
3. Application of Current and Recording .
Switch on the stimulator then select the longest impulse, say 300 m/s and increase the intensity so as to get minimal contraction. This minimal contraction may be assessed visually or by palpation of the tendon. Note, the intensity of current required to produce minimal contraction and then decrease the intensity to zero.
Shorten the impulse, say now 100 m/s, again increase the intensity and note the current at which minimum accessible contraction is produced. Repeat this procedure by shortening the impulse duration say 30,10,3,1 and so on.
4. Drawing the Curve .
After noting the current for various durations a graph is plotted with strength of current on Y-axis and impulses or pulse durations on X-axis.
Characteristics of Curve .
It is the shape of curve, which is an important feature in determining the status of muscle innervations. So, when all the nerve fibers supplying the muscles are intact, then strength-duration curve is typical. Faradic Current .
1. Normal Innervation .
Strength-duration curve with normal status of innervation of muscle will have various characteristics such as; initial portion of the graph is a straight line, which is parallel to the X-axis , there is no kink, the graph is complete and in other words, it is plotted from highest duration such as 300 millisecond to 0.01. Rise in graph line occurs around one millisecond duration. The graph looks smooth.
The curve of this typical shape is there because equal strength of current is required to get minimum assessable response with longer durations, while shorter pulses need a slight increase in the strength of stimulus each time when duration is shortened. The point at which this curve begins to rise is variable, but usually it is one millisecond with constant current.
2. Complete Denervation .
The shape obtained on plotting the strength-duration curve of a muscle that is, completely denervated is typical in appearance. Initial part of the curve is not parallel. Plotted graph line looks somewhat vertical, starting from the X-axis. There is no kink in the curve. The graph line is incomplete because denervated muscle does not respond to smaller durations like 1, 0.3, 0.1 and so on.
This typical appearance is due to required increase in the strength of stimulus for all the impulses with a duration less than 100millisecond. Response may not be obtained to very small durations and hence the curve rises steeply and is further to the right than that of a normally innervated muscle.
3. Partial Denervation .
The graph of a muscle that is partially innervated is typical in appearance. The plotted graph line shows kink. The kink appears because the graph shows the features of innervated as well as denervated muscle. It happens due to the response of the muscle to various impulses. Impulses with higher pulse durations stimulate denervated as well as innervated muscle fibers. But the response to the smaller pulse durations is obtained by stimulating only innervated muscle fibers and hence to get minimal palpable or visible contractions from these innervated muscle fibers more intensity is required.
Appearance of kink may give an approximate idea about the extent or proportion of innervation and denervation. If a large number of fibers are denervated then a greater part of the curve resembles that of denervated muscles and vice versa. If the kink is located exactly at the center, then one may conclude that innervation and denervation may be 50 percent. But if three-fourth of the graph appears like innervated and one-fourth appears like denervated then innervation could be 75 percent and denervation could be 25 percent.
Appearance of kink can have two interpretations. If initially, muscle showed strength-duration curve of complete denervation and later there is the appearance of kink, that means that muscle is getting reinnervated. On the other hand, a muscle initially showed normal strength-duration curve and now kink appears, that means this muscle is getting denervated. In progressive innervation or denervation, the graph will move from the kink towards left side or right side, respectively.
Variations of Strength-duration curve .
Strength-duration curve can be used to find out the status of sensory as well as motor nerve fibers. If we take into consideration only sensory threshold or current perception threshold (just perception of current), then it can be helpful in evaluating the status of A-beta fibers. If we take motor-level intensity then motor nerve fibers or A-alpha can be evaluated.
If we take suprasensory painful threshold, then it can tell us the status of C-fibers and A-delta type of fibers. But, hardly these variations are used clinically since there are sophisticated methods such as motor and sensory nerve conduction velocity tests which can be performed with electromyography. However, most commonly strength-duration curve is used to find out motor nerve fiber innervations of muscle.
Advantages of Strength-duration curve .
Strength-duration curve is simple, reliable and cheaper. It indicates proportion of denervation. It may be less time consuming as compared to electromyography. It can be used as hand-side method.
Disadvantages of Strength-duration curve .
In large muscles only proportion of fibers may respond, hence picture is not clearly shown. It is a qualitative rather than quantitative method of testing innervation. It will not point out the site of lesion. However, the site of lesion may be determined by nerve conduction test. There are certain other electrodiagnostic tests which are being widely used for various diagnostic purposes.
Rheobase .
Rheobase is the smallest amount of current required to produce a muscle contraction with a stimulus of infinite duration. Usually impulse of 100 ms duration is used to find out the rheobase for practical purpose. Denervation of muscles reduces rheobase and hence it may be less than that of innervated muscles. However, it may increase if reinnervation commences.
Chronaxie .
Chronaxie is the shortest duration of electrical impulse that will produce a response with current, double that of rheobase. The chronaxie of the denervated muscles is higher than innervated muscles. The innervated muscle’s chronaxie is 1 m/s, if constant voltage stimulator is used.
Chronaxie is not a satisfactory method in case of partial denervation. In such case, chronaxie will be of predominant fibers that are innervated or denervated. Say, in case of 75 percent denervation, chronaxie will be same as that of completely denervated muscle. (Here is a memory jogger to avoid confusion between rheobase and chronaxie.
Pulse Ratio .
Pulse ratio is the ratio of current needed to produce a muscle contraction with an impulse of one millisecond to that of 100 milliseconds. In case of innervated muscle, very small or no increase in current is required when impulse is reduced from 100 m/s to 1m/s. So the ratio is small and it is around one, but it may vary up to 2.2 for innervated muscles (1:2.2).
In case of denervated muscles, amount of current required to produce a contraction is more and consequently the ratio is more than 2.5. The advantage of pulse ratio is, it can be performed swiftly. But the disadvantage is that the picture of innervation is not clear if a muscle is partially innervated.
Faradic Interrupted direct current (IDC) Test .
Faradic IDC test is also vaguely termed as faradic-galvanic test rather than calling it as faradic interrupted direct current test (since galvanic current term indicates direct current and not modified direct or interrupted direct current). It was widely used in the past to rule out whether a muscle is innervated or denervated. Pertaining to characteristics of faradic current (pulse duration 0.1 to 1 m/s and frequency 50 to 100 Hz), it will stimulate only innervated muscles and not denervated muscles.
Interrupted direct current with pulse duration such as 100 m/s will stimulate both innervated and denervated muscles. If a muscle responds to interrupted direct current with 100 m/s duration or higher duration but not to faradic current, then it may be a denervated muscle.
Reaction of Degeneration .
Reaction of degeneration may occur following peripheral nerve injuries. Reaction of degeneration is of three types, such as, complete reaction of degeneration (CRD), partial reaction of degeneration (PRD) and absolute reaction of degeneration (ARD).
Complete Reaction .
Complete reaction of degeneration is also known as full or total reaction of degeneration. In complete reaction of degeneration, nerve does not respond to either faradic or interrupted galvanic current. However, muscle may respond to long-durated pulse of interrupted galvanic current, muscle contraction is slow or sluggish, motor point may get shifted and polar formula may get reversed.
Partial Reaction .
In partial reaction of degeneration, there is decreased response by nerve to faradic and interrupted galvanic current. However, muscle may have decreased response to faradic but better response to interrupted galvanic current. The muscle contraction may be slow but it is not pronounced as in CRD.
Absolute Reaction .
There is absence of response to any current in muscle as well as nerve. It represents the final stage of complete reaction of degeneration with unfavorable outcome. You should be sure of this before informing it to the patient.
Nerve Conductivity Test .
It is preferable to call it as nerve transmission test in order to avoid the possibility of the confusion between nerve conductivity, nerve distribution and nerve conduction velocity (NCV) tests. Normally, a stimulus to a nerve trunk can produce the contraction of muscles supplied by it. But if the nerve fibers are degenerating, then, conductivity distal to the lesion is lost.
For nerve conductivity test interrupted direct current with pulse duration of 0.1 or 0.3 millisecond is used. After applying this stimulus to a superficial nerve trunk, contractions of muscles supplied below this point is noted. If it produces the contractions then it suggests that at least few or all the nerve fibers are intact and functioning. If this test is performed along the course of a nerve, then it may give us a clue about the possible site of the lesion.
Nerve Distribution Test .
Stimulating the nerve trunk and observing the resultant muscle contractions can determine the distribution of nerve and thereby it is easy to find out if there is any individual variation in it. This can help us to find out if there is any variation in the distribution of the nerve.
Neurotization Time .
Neurotization time is a useful index that represents the ratio of the duration of neuropathy to the theoretical time necessary for reinnervation to take place. In order to calculate the neurotization time, measure the distance from the probable site of the lesion up to the distal most muscle supplied by the affected nerve, in millimeter. Since, the regeneration of nerve can occur at a rate of approximately one millimeter per day, calculate the anticipated number of days accordingly (remember that rate of nerve growth can vary from 1–5 mm per day).
For instance, if the distance of lesion is 105 mm then anticipated time is 105 days. Now find out the elapsed time in days from the patient. Less than 100 percent neurotization time indicates that a minimal time is elapsed for reinnervation. When neurotization time is 250 percent or more and it is accompanied with no electrodiagnostic evidence of regeneration, then the prognosis is poor and surgical intervention may be considered.
Galvanic-Tetanus Ratio .
Galvanic-tetanus ratio (GTR) is also called tetanic frequency ratio or the tetanus-twitch ratio. Normally frequency of 20 to 50 cycles per second is required to get tetanic response with pulse duration equal to chronaxie. But in case of denervated muscle, it gets reduced to 5 to 10 cycles per second. This happens because of loss of accommodation to long durated impulses and slow type of contraction in denervated muscle.
Galvanic-tetanus ratio requires the use of a stimulator with variable impulse frequency output of 1 to 50 per second and calibrated duration of impulses. For testing denervated muscle, this duration must correspond to the chronaxie. It is convenient to stimulate the muscle with a single active electrode and with a current intensity sufficient to cause minimal contraction. Gradually vary the frequency, starting at a lower range like one per second until tetanus is produced.
Dermo-ohmometry .
The study of human skin resistance is known as dermo ohmometry. It may also be called neurodermometry or galvanic skin response. It can be studied with GSR device that is at present rarely used for this purpose. It is nowadays used for relaxation purpose by incorporating it with biofeedback kind of devices.
Normal skin resistance may vary from 10,000 to 20 million ohms, depending on the distribution of the sweat glands. In complete lesion of a mixed nerve, there is anhydrosis due to reduced activity of the sweat glands. As a result of anhydrosis, there is increase in the skin resistance due to reduced sweat that permits easy passage of the current.
Polar Formula .
It is also known as Erb’s polar formula. Normal response obtained to cathode and anode is CCC > ACC > AOC > COC. It means normally a better contraction is obtained with cathodal closing current than anodal closing current. Closing and opening terminologies are used because when this experiment must have been tried a simple key switch which is “on” by closing and “off” by opening must have been used. In denervation, reversal of this formula may be noted. In denervated muscle it may be ACC > CCC > AOC > COC.
Myotonic Reaction .
In myotonic cases, typical response to the faradic stimulation occurs. The muscles remain in tetanic contraction for sometime as long as twenty seconds even after the stimulus has ceased. This response can be obtained initially and later on, due to the exhaustion of the muscle, contraction response ceases altogether. But the same response once again can be obtained after a period of rest.
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