Strength-Duration Curve in Physiotherapy

The document details the use of SD Curve during nerve injury. It describes the types of nerve injury and how we can diagnose it using galvanic current and how many types of SD curve are there. Additionally the document outlines the Advantages, disadvantages, factors affecting SD curve graph and its practical uses in physiotherapy treatment.

1. Strength-Duration Curve DR. MOHIT KUMAR CHATTERJEE (PT) MPT (NEUROLOGY) ASSISTANT PROFESSOR DR. APJ ABDUL KALAM COLLEGE OF PHYSIOTHERAPY

2. NERVE Nerves are bundle of fibers in the body that transmits impulses of sensation to brain or spinal cord and impulses from these to muscles and organs.

3. LAYERS OF NERVE Endoneurium Perineurium Epineurium Endoneurium – is outer most connective tissue covering of a neuron. Perineurium - is middle connective tissue covering of neuron. Epineurium - is inner most connective tissue covering of whole nerve.

4. Reduction or loss of voluntary power of muscle may be due to Upper Motor Neuron (UMN) Lesion Lower Motor neuron (LMN) Lesion Damage to the muscle Fault at the motor point or neuro-muscular junction Functional disorder The parts of the motor pathway which are normally accessible for electrical stimulation are the lower motor neuron below its exit from the vertebral canal, but not the upper motor neuron

5. Wallerian degeneration When a nerve is cut / severed, Wallerian degeneration takes place below the site of the lesion and above it as far as the first node of Ranvier. Debris is cleared by macrophagic activity. Process takes up to 21 days to complete and is a preparation for regeneration.

6. Types of Injury Seddon’s classification of injury 1. Neuropraxia: (First degree injury) In this there is a block in the conduction of impulse down the nerve fiber and recovery takes place without Wallerian degeneration. This lesion is caused by compression or stretch. Eg Saturday night palsy, peroneal palsy due to prolonged crossed legged sitting Nerve conduction possible below lesion Sensory part frequently least affected is motor. The prognosis of this type of nerve injury is very good as there is no degeneration of the axon

7. 2. Axonotmesis: (Second degree injury) In axonotmesis there is loss of relative continuity of axon with its myelin sheath but with intact nerve sheath, hence Wallerian degeneration is must to occur. Recovery following axonotmesis strictly depends upon the rate and extent of regeneration. Proximal lesion grow little faster than distal lesion. If the lesion is proximal then it takes place at a rate of 2-3mm per day but if the lesion is distal then it takes place at a rate of 1-1.5mm per day. Recovery is always better in axonotmesis as compared to neurotmesis mainly due to the intact connective tissue framework.

8. 3. Neurotmesis: (Third degree injury) Injuries which cause severe stretch or laceration produce neurotmesis in which not only the axon but the connective tissue framework gets disrupted and lose their continuity. The fibres degenerate below the site of lesion, causing same alterations in the electrical reaction as axonotmesis. This condition is however more serious as suture of the nerve is necessary before the nerve regeneration takes place. Eg – Ulnar nerve is cut on the front of the wrist

9. STRENGTH-DURATION CURVE The strength duration curve is a graphical representation of the relationship between the intensity of an electrical stimulus at the motor point of a muscle and the length of time taken to elicit a minimal contraction in that muscle. Strength refers to the stimulus intensity on the vertical axis. Duration refers to the pulse duration on the horizontal axis. The purpose of SD curve plotting is to know whether the stimulated muscle is innervated, denervated or partially denervated There are also other method for this purpose like EMG and NCV.

10. Optimum timing for SD Curve SD curve should be plotted after 20 th day of injury/lesion as the Wallerian degeneration will be completed After 21st/22nd day, regeneration of nerve will start, generally it take about 270 days to regenerate It is best done weekly under the same conditions until there is recovery. Once recovery takes place there may be found changes in the curve once every two weeks.

11. Advantages & Disadvantages of SD Curve Plotting Advantages I t is simple, reliable and cheaper. It is quick & easy to perform Indicates the proportion of denervation. Less time consuming It requires minimal training Disadvantages Only a proportion of fibers may respond in few muscles, therefore a full picture is not obtained. It also does not tell about the site of lesion.

12. Method of SD Curve Plotting Select a neuromuscular stimulator that provides rectangular impulses of various durations such as 0.01, 0.03, 0.1, 0.3, 1, 3, 10, 30, 100 & 300 ms , which may be the constant current or constant voltage type . Constant Current Stimulators produce a contant current independent of resistance encountered. The voltage adjusts to maintain constant current flow. The advantage of this type of of stimulator is to ensure a consistency physiologic response during the treatment. The negative is potential pain when the voltage increases to overcome resistance. Constant Voltage Stimulators, conversely, produce a constant voltage. The current adjusts to depending on changes in resistance. This unit is advantageous in preventing discomfort with changes in resistance, such as an electrode losing full contact, but quality of response can be decreased with these automatic resistance changes. Prepare the skin of the patient as described for the motor point stimulation and position the patient in such a way that the part over which the muscle contracts, remain fully supported.

13. Put the passive electrode either over the midline of the body or over the origin of the muscle to be tested. Put the active electrode over the fleshy part of the muscle. First apply the current using the stimulus having the longest duration and look for minimum perceptible contraction that is either observed visually or by palpating the tendon of the muscle. Note the magnitude of the current or voltage required for contraction. Gradually shorten the impulse durations up to 0.01ms and increase the intensity for the particular pulse duration. Note the corresponding current / voltage requirements to get the same type of muscle contraction. Keep the active electrode over the same point over the muscle throughout the test. Plot the strength duration curve from the results of this test by putting the durations (in ms ) along the x-axis and the corresponding strength (in milliampere or volts) along the y-axis. Note the name, age, gender, date of injury and date of performing the test along with diagnosis and interpretation.

14. Characteristics of SD Curve Normal Innervations: When all the nerve fibers supplying the muscles are intact, the curve obtained has a characteristic shape The curve is of this typical shape because the same strength of stimulus is required to produce a response with all the impulses of longer duration. While those with a shorter duration require an increase in the strength of stimulus each time the duration is reduced. The point at which the curve begin to rise is variable, but usually with a constant current stimulator is around 1 ms , whereas that with the constant voltage stimulator is at 0.1 ms.

15. Complete Denervations : When all the nerve fibers supplying a muscle have degenerated, the SDC produced, is characteristic of complete denervation. In this curve as the response is from the muscles which lacks nerve continuity, only the long duration pulses will elicit the response & there will be a need to increase the current intensity from about 10 ms duration. The curve is no longer horizontal on the right-hand side but is converted into a distinctive steeply rising parabola, which is displaced towards the right.

16. Partial Denervation: When some of the nerve fibers supplying a muscle have degenerated, while the others are intact, the characteristic curve obtained clearly indicates partial denervation. As the impulses are shortened, the denervated fibers respond less readily, so that a stronger stimulus is required to a produce a perceptible contraction & the curve rises steeply, like that of denervated muscle. With the impulses of shorter durations, the innervated fibers respond to a weaker stimulus than that required for the denervated fibers, so a contraction of the denervated fibers is not obtained & this part of the curve is similar to that of innervated muscles. Thus, the right-hand part of the curve resembles that of denervated muscle & left-hand part that of the innervated muscle & a “kink” is seen at the point where the two sections meet. The appearance of a kink or discontinuity in the curve is a reliable early sign of denervation & in progressive involvement of nerve lesions.

17. The extent of denervation can be picked up by the shape of the curve and kinks. If a larger part of the muscle is denervated, the greater part of the curve resembles that of denervated curve with the kink but if only a part of the fibers are denervated & the majority innervated, then the curve resembles a normal curve with a kink in it.

18. Rheobase Rheo = current Base = foundation It is the intensity of current required to produce a minimal perceptible & palpable contraction using a pulse of infinite duration. Generally, pulses of 100 or 300ms duration are used to record rheobase. It is measured in milliampere or volt, depending upon whether a constant current or constant voltage stimulator is used. Rheobase is usually measured by placing the cathode on the motor point of the nerve. Normal values: 2-10 mA or 10-20 volts.

19. Deltoid 14 volts, 5mA Triceps 18 volts, 5 mA Abductor digiti minimi 30 volts, 8mA Frontalis 14 volts, 4mA Normal values of Rheobase of different muscle

20. Chronaxie: Chrona = time Axie = axis (time over axis) At the double intensity of rheobase, the minimal pulse width required to produce the twitch is called chronaxie of the muscle. It is the index of excitability & is the time in milliseconds, that is necessary to induce minimal visible contraction, with a stimulus of twice the strength of rheobase. Normal values: < 1ms ( 0.05-0.5ms). In denervation, the value of chronaxie is more than that of innervated muscles. The denervated muscle values for chronaxie are around 10ms or more.

21. Muscle Constant voltage Constant current Deltoid 0.01ms 0.1ms Abductor digiti minimi 0.04ms 0.2ms Tibialis anterior 0.04ms 0.1ms Normal values of chronaxie of different muscle

22. Factors that Affect the Accuracy (validity and reliability) of the S-D Curves Skin temperature : This alters the value of rheobase (minimum current required by the long duration impulse). An increase in temperature, lowers values while a decrease in temperature will raise the value of rheobase. It is important to ensure a consistent temperature of the skin prior to testing. Humidity : High humidity decreases the value of rheobase and vice versa. Location of muscles : Deeply placed muscles can not accurately be located, therefore, the result may not be valid. Edema : Due to spread of current, the value of rheobase may be altered. Ischemia: a greater intensity may be required, increasing the value of rheobase. Superficial fat: If large superficial fat lies over the muscle, it may increase the resistance to the current flow, thereby gives incorrect result. Electrode position : The electrode, if not positioned properly may cover some of the fibers of the adjacent muscle, therefore, giving inaccurate result. Pressure variations : The variation of pressure of the hand-held electrode during the entire testing procedure may give faulty

23. Practical Uses of S-D Curve in Diagnosis To detect the presence or absence of the excitable nerve fibers in the muscle. It also assesses the extent of the denervation/innervations. It detects the signs of reinnervations in a muscle, as in most cases, the onset of regeneration is evident well in advance of the clinical return of the voluntary contraction. The values of rheobase and chronaxie which indicate the status of innervations is also measured from the S-D curve. It monitors the progress of the lesion and denotes whether the lesion is recovering/ progressing. It permits the identification of the utilization time. It is the point at which the curve begins to flatten horizontally. From this point, a line is taken vertically down to the x-axis and the pulse duration is noted. This will signify the probable pulse duration which will be suitable for stimulation, if the muscle is to be treated therapeutically with low frequency currents.

24. References John Low and Ann Reed. Electrotherapy Explained. 4th edition. Elsevier publications. BK Nanda. Electrotherapy simplified. 1st edition. Jaypee publications. N Vyas. Principles & practice of rehabilitation. 1st edition. Jaypee publications. Jagmohan Singh. Textbook of Electrotherapy. 3 rd edition. Jaypee publications