The Role Of Electrical Stimulators In Contemporary Physical Therapy

INTRODUCTION The treatment of physical dysfunction or injury by the use of therapeutic exercise and the application of modalities intended to restore or facilitate normal function or development is called Physical Therapy. Physical Therapy provides services to develop, maintain and restore maximum movement and functional ability throughout life. This includes circumstances where movement and function are threatened by aging, injury, disease or environmental factors. Functional movement is a central element in what it means to be healthy. An exciting movement has taken place in the use of electricity to speed recovery from injuries and relieve pain by delivering very small amounts of electrical energy that facilitates the movement of ions in human soft tissue. The human body is basically made up of 67% salt water in which ions are transferred via electrochemical processes. When an injury or disease occurs, this normal process is disrupted. Low energy levels introduced effectively to the human body may facilitate the natural healing process.

Fig 1. An electrotherapy procedure in progress Electrotherapy is the treatment of patients by electrical means with the application of an electric current to stimulate a tissue in order to bring about physiological changes for therapeutic purposes for healing or restoring a lost function. Electrotherapy is used for three therapeutic purposes: (1) to relieve pain (2) to stimulate physiochemical changes and (3) to stimulate muscle contraction. The benefits of electrotherapy include pain relief, relaxation of muscle spasms, prevention of muscle wasting. Regardless, because electrotherapy has little risks associated with its use, it is definitely worth a try when addressing pain and recovery from an injury. It is a

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noninvasive, drug-free technique, it is non-addictive, and is completely safe when used appropriately and correctly.

ELECTRICAL STIMULATORS Electrical stimulators directly block transmission of pain signals along nerves. In addition, electrical stimulation promotes the release of endorphins, which are natural painkillers produced by the body. Electrical stimulators provide muscle contraction using electric impulses. The impulses are generated by a device and delivered through electrodes on the skin in direct proximity to the muscles to be stimulated. The impulses mimic the action potential coming from the central nervous system, causing the muscles to contract. The electrodes are generally pads that adhere to the skin. Bioelectric potentials are generated within individual cells, when these cells are stimulated. All cells are sensitive, to some degree to artificial electrical stimulation. When analyzing biomedical phenomena, it is often desirable to artificially stimulate a group of cells. Such artificial stimulation is achieved by using a pulse generator to pass a current through the cells concerned over a brief period of time. The low voltage is usually done on smaller, involuntary muscle groups, which cannot be stimulated in other ways. The low voltage also stimulates the brain, which starts sending impulses through the involuntary muscles, thus stimulating them as well.

Fig 2. Schematic of a basic Electrical Stimulation System. In Electrical Stimulation, electrodes are placed initially on the muscle that is to be stimulated. AC electrical stimulation is then applied at low levels (threshold levels for muscle movement). Based on muscle responses to the threshold stimulation, electrode positions are adjusted until the motor points of the muscle (optimal positions for generating muscle movement) are found. Electrodes are then secured at these optimal positions. After the electrodes have been placed, parameters for electrical stimulation are programmed into the electrical stimulator unit. The maximum pulse amplitude

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recommended for electrical stimulation is generally the maximum amplitude that can be tolerated by a given patient.

PRINCIPLE OF ELECTRICAL STIMULATION If a normal muscle or motor nerve is stimulated with a current of adequate intensity, it results in its contraction. When there is disease or injury of a motor nerve or muscle, alterations are liable to occur in their response to electrical stimulation. The changed electrical response may be of considerable help in the diagnosis of certain diseases affecting them. Quantitatively, these changes manifest themselves in that a higher or lower current intensity than normal is required to bring about a muscle contraction. It is, therefore, possible to determine the degeneration and regeneration processes in nerves and the muscle system by the use of stimulation current technique.

Brain

Spinal Cord Stimulator Limb

Fig 3. Application of electrical stimulation to a damaged CNS pathway. If the nerve to a muscle is damaged, then the motor axons in the muscle degenerate and disappear so that the muscle can no longer be excited by the nerve. Such a muscle is called a denervated muscle. Even though such a muscle can no longer be activated by its nerve it can be made to contract if the electrodes are placed directly onto the muscle and large currents are used to excite the muscle fibers themselves. When the adaptive potential of a denervated muscle was tested, it was found that just like a normal muscle its properties could be altered by electrical stimulation. If the denervated muscle is stimulated for long periods of time at low frequency it becomes a slow postural muscle, but when it is activated intermittently with burst at high frequencies, it becomes a fast contracting muscle. Thus the ability of the muscle to change its properties is inherent in

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the muscle itself and can occur without the nerve. However this situation arises only after injury to the nervous system or peripheral nerves.

ELECTROTHERAPY Electrotherapy is a technique that utilizes patterned electrical stimulation with the purpose of restoring or enhancing a lost or diminished function. Electrotherapy employs low-volt, low-frequency impulse currents. The biological reactions produced by low-volt currents have resulted in the adoption of this therapy in the management of many diseases affecting muscles and nerves.

Fig 4. A Simple Model of Electrotherapy. Electrotherapy modalities follow a very straightforward model that is presented above. In principle, the model (Fig 4) identifies that the delivery of energy from a machine or device is the start point of the intervention. The energy entry to the tissues results in a change in one or more physiological events. Some are very specific whilst others are multifaceted. The capacity of the energy to influence physiological events is the key to the processes. The physiological shift that results from the energy delivery is used in practice to generate what is commonly referred to as therapeutic effects. A simple, but effective clinical decision making model is utilized. All electrotherapy modalities involve the introduction of some physical energy into a biological system. In the clinical environment, select the most appropriate ‘dose’ of the therapy and then lastly, apply the treatment. The dose selection however is critical in that not only are the effects of the treatment modality dependent, but they appear to be dose dependent as well. In other 4

words, it is important to select the most appropriate modality based on the available evidence, but also to deliver it at the most effective known dose.

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THE STRENGTH DURATION CURVE The Strength Duration Curve is defined as the relationship between the strength of the stimulating current and the duration it must be applied to produce a response. The excitability of a tissue can be defined by the relationship between the stimulation amplitude and the stimulation duration. These curves are determined by means of rectangular and triangular pulses in such a manner that the threshold values are measured at progressively decreasing stimulation durations. The Strength Duration curves have characteristic shapes and deviations from the standard form lead to an indication of the state of the tissues.

Fig 5. The Strength Duration Curve The curve shows that decreasing excitability with progressive degeneration requires extended stimulation times and increased current strength for achieving successful stimulation. With degenerated muscle, the curve is shifted to the right and upwards. The Strength duration (SD) curve is a graphic representation of the integrity of the musclenerve complex. The value of the SD curve is a tool to access the amount of motor nerve dysfunction. Reliable results from the SD curve may be expected when plotted on the same muscle more than once. Denervation of skeletal muscles result in many changes in structure and function. Denervated muscles do not respond at all to short stimulating impulses. They require pulses of longer duration than those for innervated muscles. The use of electrical stimulation to alleviate the severe atrophy rising from denervation must take account of changes in muscle excitability.

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Stimuli of extremely short duration will not excite the tissue, no matter how intense they may be. With stimuli of longer duration, threshold intensity is inversely related to the duration of stimulus. Furthermore, with weak stimuli, a point is reached where no response occurs no matter how long the stimulus is applied There are two points on the curve that can be used to define the excitability, the ‘Rheobase’ and the ‘Chronaxie.’ RHEOBASE The minimum intensity of current that will produce a response if the stimulus is of infinite duration is called the Rheobase. The time for which it must be applied is the utilization time. A still weak current fails to stimulate the tissue no matter how long the duration of current flows. CHRONAXIE The minimum duration of impulse that will produce a response with a current of double the rheobase is called the Chronaxie. Within limits, the chronaxie of any given excitable tissue is constant for that tissue. Therefore, chronaxie values as a ‘unit of excitability’ have-been used to compare excitability of various tissues.

Fig 6. Determination of excitability of two nerves having the same rheobase. If two nerves have the same rheobase, the chronaxie can give an indication of their relative excitabilities. Fig 6. shows that the SD curve with smaller chronaxie, nerve B is more excitable. On comparing the strength duration curve for a set of slow fibers (not very excitable) to that for a set of quick fibers (very excitable), the curve for the slower fiber is shifted towards the right, indicating that for a given stimulus strength, a longer stimulation duration would be needed to bring the slower fibers to threshold. In order to plot SD curves, the tissue (muscle, nerve) to be examined is first stimulated with long impulses (usually of 1 s pulse duration and then with shorter and shorter impulses, (down to say, 0.05 ms). For each impulse duration, the current intensity is adjusted until the stimulation threshold has been exceeded and the effect of the stimulation detected. 7

TYPES OF STIMULATORS Stimulators are the devices which are used to stimulate innervated muscles, denervated muscles and nerves. Further, these are used for the treatment of paralysis with totally or partially denervated muscles, for the treatment of pain, muscular spasms and peripheral circulatory disturbances, and for several other applications. Although some of the principles upon which low-volt therapy depends have been known since the end of the last century, it is only in recent years that it has started being widely used with the availability of safe and simplified apparatus required for the purpose. The physiological action of stimulation depends on the passage of current across the cell membrane. Therefore, it may be expected that the magnitude of the current (as well as its waveform) is an important factor. It is often overlooked that for stimulation, the applied voltage and current are two distinct parameters. They are, no doubt related and the relationship depends on the source and load impedance. Different types of waveforms are used for carrying out specific treatments. The most commonly used pulse waveforms are discussed below: GALVANIC CURRENT Galvanic stimulation is most useful in acute injuries associated with major tissue trauma with bleeding or swelling. Galvanic stimulators apply direct current. Direct current creates an electrical field over the treated area that, theoretically, changes blood flow. When a steady flow of direct current is passed through a tissue, its effect is primarily chemical. It causes the movement of ions and their collection at the skin areas lying immediately beneath the electrodes. The effect is manifested most clearly on a bright red coloration which is an expression of hyperemia (increased blood flow). Galvanic current may be used for the preliminary treatment of atonic paralysis and for the treatment of disturbance in the blood flow. It is also used for iontophoresis, which means the introduction of drugs into the body through the skin by electrolytic means. In general, the intensity of the current passed through any part of the body does not exceed 0.3 to 0.5 ma/sq cm of electrode surface. The duration of the treatment is generally 10-20 minutes.

Fig 7. Stimulator waveform for galvanic current.

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INTERRUPTED GALVANIC CURRENT In an interrupted galvanic current, pulses are a series of negative, ongoing, rectangular pulses. The pulse duration is about 100 m/s with a repetition rate between 12/min and 70/min. Figure 8 (a) shows the unidirectional, interrupted galvanic pulses which create the ionization of the patient’s skin and produces discomfort and inflammation. The discomfort is reduced by the application of a positive current, Figure 8 (b), in between the negative pulses and proportional to the time interval. That is, a charge balance is obtained.

Fig

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Stimulator waveform for Interrupted Galvanic Current FARADIC CURRENT Faradic current is a sequence of pulses with a defined shape and current intensity. (Fig 9). The pulse duration is about 1 minute with a triangular waveform and an interval duration of about 20minutes. Faradic current acts upon muscle tissue and upon the motor nerves to produce muscle contractions. There is no ion transfer and consequently, no chemical effect. This may be used for the treatment of muscle weakness after lengthy immobilization and of disuse atrophy.

Fig 9. Stimulator waveform with faradic current. SURGED FARADIC CURRENT 9

If the peak current intensity applied to the patient increases and decreases rhythmically, Figure 10, and the rate of increase and decrease of the peak amplitude is slow, the resulting shape of the current waveform is called a surging current. The main field of application of the faradic surge current is in the treatment of functional paralysis. The surge rate is usually 6-60 surges per minute in most of the instruments. The ratio of interval to the duration of the surging is also adjustable so that graded exercise may be administered. This type of current is usually required for the treatment of spasm and pain.

Fig 10. Stimulator waveform for Surged Faradic Current. EXPONENTIALLY PROGRESSIVE CURRENT Figure 11 shows the exponential pulse used for the treatment of severe paralysis. The main advantage of this method lies in the possibility of providing selective stimulation for the treatment of the paralyzed muscles. With these kinds of pulses, the surrounding healthy muscles, even in the immediate neighbourhood of the diseased muscles, are not stimulated. The slope of the exponential pulse is kept variable.

Fig 11. Stimulator waveform with Exponentially Progressive Current.

AN ELECTRODIAGNOSTIC THERAPEUTIC STIMULATOR 10

Several types of commercial units are available which give specific output waveforms for specific applications. However, the trend is in favour of having a versatile apparatus which gives output current waveforms to cover the whole range of electrodiagnostic and therapeutic possibilities. Another important consideration is that the apparatus must be either of constant voltage or constant current type. For clinical practice, the maximum tolerance permitted in the pulse parameters is ±15%. The instrument generally has a floating output and incorporates an isolation transformer in the output.

Fig 12. Schematic diagram of a diagnostic/ therapeutic stimulating unit TYPICAL SPECIFICATIONS OF ELECTRODIAGNOSTIC THERAPY UNIT The typical specifications of an electrodiagnostic therapy unit are as follows: Galvanic current up to 80 mA, ripple less than 0.5% as constant current or surging current with adjustable surge frequency from 6 to 30 surges per minute.  Exponentially progressive current pulse sequences with continuously variable pulse duration from and independently adjustable interval duration.  Faradic surging current with 25 surges per minute, up to 80 mA. Precision and constancy of the values set better than ±10%; peak current measurement facility. Constant current circuit, both poles earth- free. FUNCTIONAL BLOCK DIAGRAM DESCRIPTION 

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A versatile electrodiagnostic therapeutic stimulator shown in Figure 12 makes use of a variable rate multi-vibrator (Ml) to set the basic stimulus frequency. The output from the free running multi-vibrator triggers a monostable multi-vibrator (M2) circuit which sets the pulse width. The output pulse from the monostable provides an interrupted galvanic output whose rate as well as duration can be independently controlled. Another astable multi-vibrator produces short duration pulses called faradic currents. Faradic currents are usually modulated at the frequency set by the multi-vibrator Ml, in a mixer circuit (M4). Since the modulation of faradic pulses takes place with a slow rate of increase and decrease, the output of M4 is surged faradic currents. By integrating the output of M2, the interrupted galvanic pulses can be modified to have an exponential rise and fall. The shape of these pulses is similar to a triangular waveform. Galvanic current is also made available by suitably tapping the DC supply. Finally, any one of the waveforms can be selected through a selector switch and fed either to an emitter-follower stage in order to provide a low output impedance constant voltage output or to a high output impedance constant current stage. The output of a diagnostic/therapeutic stimulator is kept floating, i.e. it is isolated from earth. The usual method is to have an isolation transformer at the output of the stimulator which has floating terminals and is fitted with an electrostatic shield to reduce capacitive coupling with the earth. Another method of isolation of the output from earth is by the use of a radio-frequency output stage. The two methods have been widely used for providing isolation of the stimulator output, but they have some drawbacks. The simple transformer cannot transmit square waves without distorting the waveform and the method of radio-frequency is rather complex. Isolation can also be provided through the opto-isolation technique. The advantages of constant current therapy are detailed below: • The current flow is largely constant irrespective of the patient’s resistance. The selected current intensity remains constant, even if the resistance in the tissue between the electrodes should vary, as a result of, say, changes in the blood circulation during treatment, or after previous therapy. • The current waveform is applied, and distortion-free, since micro-voltages between the electrode and the skin have no influence. • Current therapy avoids accompanying symptoms such as irritating stimulatory sensations between electrodes by applying electrodes firmly to the skin. In case of constant voltage, the current flow is dependent on the resistance of the patient, that is, if the electrical resistance of the tissue increases, less current will flow, and vice versa. Irritating stimulatory sensations do not occur, even beneath electrodes that are not firmly applied or are moved over the skin, for example, during the search for a trigger point. This operating mode is recommended for the combined use of stimulation current and ultrasound. Modem electrodiagnosis/therapy units are microprocessor-controlled which make possible a number of automatic sequences in selecting the type and quality of waveform.

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TYPES OF ELECTRODES FOR ELECTRODIAGNOSTIC THERAPEUTIC APPLICATIONS Two methods of electrode systems are in common use for electrodiagnostic therapeutic applications: the mono-polar technique and the bipolar technique. The mono-polar technique makes use of small active stimulation electrode. The indifferent or dispersive electrode is of larger area and is placed near to the active electrode. This technique is used for testing of the galvanic and faradic excitability and for determining the chronaxie. For diagnostic purposes, a ball or plate electrode which is provided with a small thick muslin strip is mounted on a special handle. The handle carries a finger-tip switch to facilitate convenient control of output. Similarly, a small metal electrode can be secured on the motor point, particularly for therapeutic applications.

Fig 13. Intramuscular stimulating electrodes. For recording i-t curves, the bipolar electrode technique is usually preferred. Both the electrodes are fixed to the body so that the hands of the operator are free to operate the apparatus. The active electrode in this method need not be a small as we deal with higher current intensities and small area electrode may cause unpleasant heat sensations. Suitably sized metal sheets are used as electrodes in this system. The electrodes are fastened to a moistened pad of about 1 cm thickness and 1 cm wider than the electrode sheet on all sides. The material used for pads is of good absorbency and ordinary water can be used to moisten the electrodes. The electrodes are held in position by rubber straps. The stimulation current therapy can also be administered with suction electrodes. Successful muscle stimulation can only be achieved if the activating currents are properly applied. 13

ORGANIZATION OF THE NERVOUS SYSTEM The nervous system is a network of specialized cells that communicate information about an animal's surroundings and itself. It processes this information and causes reactions in other parts of the body. The nervous system is divided broadly into two categories: the peripheral nervous system and the central nervous system.

Fig 14. Divisions of the nervous system CENTRAL NERVOUS SYSTEM The Central Nervous System (CNS) consists of the brain and spinal cord. The CNS processes the information received from the peripheral nervous system. It is responsible for receiving and interpreting signals from the peripheral nervous system and also sends out signals to it, either consciously or unconsciously. PERIPHERAL NERVOUS SYSTEM The Peripheral Nervous System (PNS) comprises sensory receptors and the nerves which provide communication lines to and from the CNS. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. The main function of the PNS is to connect the CNS to the limbs and organs. The PNS is not protected by bone or by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. 14

SOMATIC NERVOUS SYSTEM The Somatic Nervous System is made up of efferent nerves or motor nerves transporting information from the CNS to parts of the body concerned with voluntary functions such as walking, talking, manipulating with the fingers etc. The somatic nervous system is responsible for coordinating the body movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control.

Fig 15. Diagrammatic representation of the somatic nervous system AUTONOMIC NERVOUS SYSTEM The Autonomic Nervous System (ANS) is made up of efferent nerves and concerned with the regulation of those movements which do not normally require conscious thought such as heart beat, breathing, peristalsis, release of digestive juices etc. The autonomic nervous system is then split into the sympathetic division, parasympathetic division, and enteric division. The sympathetic nervous system responds to impending danger or stress, and is responsible for the increase of one's heartbeat and blood pressure, among other physiological changes, along with the sense of excitement one feels due to the increase of adrenaline in the system. The parasympathetic nervous system, on the other hand, is evident when a person is resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of the heart and the dilation of the blood vessels.

Fig 16. Diagrammatic representation of the autonomic nervous system 15

SYMPATHETIC NERVOUS SYSTEM Stimulation of the sympathetic branch of the autonomic nervous system prepares the body for emergencies for "fight or flight" and enhances the memory of the event that triggered the response. The sympathetic nervous system responds to impending danger or stress, and is responsible for the increase of one's heartbeat and blood pressure, among other physiological changes, along with the sense of excitement one feels due to the increase of adrenaline in the system.

Fig 17. Representation of the sympathetic and parasympathetic nervous system PARASYMPATHETIC NERVOUS SYSTEM The parasympathetic system returns the body functions to normal after they have been altered by sympathetic stimulation. In times of danger, the sympathetic system prepares the body for violent activity. The parasympathetic system reverses these changes when the danger is over. The sympathetic and parasympathetic system work in tandem to create a synergistic stimulation that is not merely on or off, but can be described as a continuum depending upon how vigorously each division is attempting to carry out its actions. The parasympathetic nervous system promotes digestion, synthesizes glycogen, and allows for normal function and behavior. The parasympathetic division functions with actions that do not require immediate reaction.

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NERVE MUSCLE STIMULATOR Nerve Muscle Electrical Stimulation is the application of electrical stimuli to a group of muscles, most often for the purpose of muscle rehabilitation. Nerve Muscle Electrical Stimulation is primarily used by physical therapists as a form of rehabilitation after injury, stroke, or other incident that results in loss of muscle function. Nerve Muscle Stimulation is achieved by passing an electrical impulse from a device through electrodes placed on the skin over the targeted muscle or muscles. For the purpose of rehabilitation, Neuromuscular Electrical Stimulation is typically used in conjunction with other methods of physical therapy. The intent is to stimulate the nerves in the muscle with electrical impulses. Electrical impulses are a natural part of the normal communication between the brain and the muscular system in an uninjured or unaffected body. With Neuromuscular Electrical Stimulation, these natural impulses are simulated and can help the muscles to function again. Even with the use of Neuromuscular Electrical Stimulation, most rehabilitation patients must also undergo physical therapy to prevent muscles from atrophying, or dying. In some cases, depending on the cause and extent of injury, other forms of electrical stimulation therapy may also be used. Similarly, the same electrical technology is used to measure the performance of nerves and muscles for diagnostic purposes and measuring improvement. While the use of Nerve Muscle Stimulation is most often used in rehabilitation of injured muscles or stroke, Nerve Muscle Stimulation has also been used to improve the health of damaged tissue. Nerve Muscle Stimulation is also sometimes used as a way to manage chronic pain relief. Not all individuals are candidates for Nerve Muscle Stimulation Therapy either for rehabilitation or pain management. Patients who have suffered heart attacks, have a pacemaker, and those with certain other medical conditions are not candidates for Nerve Muscle Stimulation. In the event of muscle failure from injury or stroke, a physical therapist will work in conjunction with the patient’s doctor to determine the right therapy and may adjust the therapy as needed. A Nerve Muscle Stimulator sends weak electrical signals into the body. Such devices apply extremely small (less than 1000 microampere) electrical currents to nerves using electrodes placed on the skin. Nerve Muscle Stimulation uses include treatments for age-related macular degeneration, wound healing, tendon repair, and ruptured ligament recovery. Since most of the treatments concentrate on speeding healing and recovery, the largest current use is for professional athletes. Electrical Stimulation is a powerful tool that can exploit the full adaptive potential of muscles. Using particular patterns of electrical stimulation, muscles can be made more efficient, fatigue resistant and stronger. Electrical Stimulation can replace activity in cases of injury to the nervous system. Muscle fibers have a tremendous potential to adapt to different tasks. Such adaptive changes in muscle can be induced by altering moto-neurone activity, or by externally applied electrical stimulation of the muscle. Different patterns of electrical stimulation produce specific changes of muscle properties.

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PERIPHERAL NERVE STIMULATOR Peripheral Nerve Stimulation is a form of neurostimulation which is generally used for the treatment of unresponsive pain that originates from peripheral nerves. There are many different nerves that can be stimulated using this form of therapy, examples of some are the median, ulnar and radial nerves of the arm. Peripheral nerve stimulation (PNS) is a neuromodulation technique in which electrical current is applied to the peripheral nerves to ameliorate chronic pain. Peripheral Nerve Stimulation is indicated for the treatment of chronic pain, localized to a peripheral nerve distribution that is not amenable to less invasive measures. It is extremely useful for treating pain in a distribution that is not accessible by spinal cord or spinal nerve root stimulation. Examples include trigeminal branch stimulation, occipital nerve stimulation, and subcutaneous peripheral nerve stimulation.

Fig 18. A Peripheral Nerve Stimulator Unit In a peripheral nerve stimulator, a pulse generator (astable multivibrator) determines the pulse repetition rate and generates repetitive negative rectangular pulses, which trigger a monostable vibrator, which determines the pulse width. The output of the monostable vibrator drives an emitter-follower and a transconductance amplifier which increases the maximum output voltage. Finally the transformer is used to couple the stimulator and the skin with suitable energy to stimulate the nerve trunk. For superficial peripheral nerves, such as the trigeminal and occipital nerves, individual leads are placed just under the skin, overlying the nerves. Fluoroscopic guidance may be helpful in some cases. Intraoperative testing confirms that stimulation paresthesias are in the appropriate place.

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STIMULATORS FOR PAIN AND RELIEF Pain is felt as a result of the brain's response to electrical (neural) and chemical (hormonal) changes in the body as a result of damage. Signals from damage or injury are picked up by sensory receptors in nerve endings. The nerves then transmit the signal via the nerves to spinal cord and brain (Fig 19). Pain can be managed in the short term using analgesics, but long-term use can be detrimental to the patient's health. Side effects of the long use of analgesics may affect on liver, kidney or stomach. Electrical Stimulation can reduce pain by blocking the pain signals (nociceptive) transmission at the spinal cord and by increasing production of endorphins (body’s natural pain-killer).

Fig 19. Mechanism of Pain Electrotherapy is used to manage both acute and chronic pain. In the gate model of pain, the neural fibers that carry the signal for pain and those that carry the signal for proprioception (body position) are mediated through the same central junction. Because signal transmission along pain fibers is slower than transmission along proprioception fibers, the gate model suggests that intense stimulation of proprioception fibers can block the slower-moving pain signals. Some forms of electrotherapy attempt to stimulate these proprioception fibers to reduce the sensation of pain. Other forms of electrotherapy alleviate pain by introducing analgesics and anti-inflammatory medications via electric current to the pain.

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Fig 20. Summary of basic pain pathways 20

TRANSCUTANEOUS ELECTRICAL NERVE STIMULATION Transcutaneous Electrical Nerve Stimulation (TENS) is a method of electrical stimulation which primarily aims to provide a degree of symptomatic pain relief by exciting sensory nerves, and thereby stimulating the pain gate mechanism and the opioid system. The two primary pain relief mechanisms which can be activated are: the Pain Gate Mechanism and the Endorphin Release Theory.

Fig 21. Dual channel TENS unit. GATE TENS The gate-control theory suggests that there is a neural mechanism in the spinal cord that acts as a kind of gate, shutting down or opening up the flow of signals from the periphery to the brain. Whether the gate is open, closed or partially closed depends on what sort of signal it receives from the brain to change the perception of pain in the user’s body. These frequencies interfere with the transmission of pain messages at the spinal cord level, and help block their transmission to the brain. ENDORPHIN TENS Another theory is called ‘Endorphin Release’, which suggests that electrical impulses stimulate the production of endorphins and enkephalins in the body, the natural morphine-like substances that block pain messages from reaching the brain. An endorphin release TENS has a slow acting effect. Sustained stimulation at low levels of pulse intensity has the strongest effect on managing chronic nagging pain. 21

TENS MACHINES TENS as a treatment technique is has few side effects when compared with drug therapy. The most common complaint is an allergic type skin reaction (about 2-3% of patients) and this is almost always due to the material of the electrodes, the conductive gel or the tape employed to hold the electrodes in place. Most TENS applications are now made using self adhesive, pre gelled electrodes which have several advantages including reduced cross infection risk, ease of application and lower allergy incidence rates. Digital TENS machines are becoming more widely available and extra features (like automated frequency sweeps and more complex stimulation patterns) are emerging. Some of these devices do offer pre-programmed and automated treatment settings.

Fig 22. Variables on modern TENS machines MACHINE PARAMETERS The main treatment variables which are available on modern machines and the location of these controls on a typical (analogue) TENS machine is illustrated in Fig 22.The CURRENT INTENSITY (A) (strength) will typically be in the range of 0–80 mA, though some machines may provide outputs up to 100mA. Although this is a small current, it is sufficient because the primary target for the therapy is the sensory nerves, and so long as sufficient current is passed through the tissues to depolarize these nerves, the modality can be effective. The machine will deliver discrete ‘pulses’ of electrical energy, and the rate of delivery of these pulses which is the PULSE RATE or FREQUENCY (B) will normally be variable from about 1 or 2 pulses per second (pps) up to 200 or 250 pps. To be clinically effective, the TENS machine should cover a range from about 2 – 150 pps (or Hz). In addition to the stimulation rate, the DURATION (OR WIDTH) OF EACH PULSE (C) may be varied from about 40 to 250 micro seconds (µs). The most effective setting in the clinical environment is probably around 200µs. MECHANISM OF ACTION 22

The type of stimulation delivered by the TENS unit aims to excite (stimulate) the sensory nerves, and by doing so, activates specific natural pain relief mechanisms. Pain relief by means of the pain gate mechanism involves activation (excitation) of the A beta (Aβ) sensory fibers, and by doing so, reduces the transmission of the noxious stimulus from the ‘c’ fibers, through the spinal cord and hence on to the higher centers. The Aβ fibers appear to appreciate being stimulated at a relatively high rate (in the order of 90-130 Hz or pps). Clinically it is important to enable the patient to find their optimal treatment frequency which will almost certainly vary between individuals. Setting the machine and telling the patient that this is the ‘right’ setting is almost certainly not going to be the maximally effective treatment, though of course, some pain relief may well be achieved. An alternative approach is to stimulate the A delta (Aδ) fibers which respond preferentially to a much lower rate of stimulation (in the order of 2-5 Hz), which will activate the opioid mechanisms, and provide pain relief by causing the release of an endogenous opiate (encephalin) in the spinal cord which will reduce the activation of the noxious sensory pathways. NORMAL TENS OR HIGH TENS Normal TENS usually uses stimulation at a relatively high frequency (90-130Hz) and employ a relatively narrow (short duration) pulses (starts at about 100µs). The stimulation is delivered at ‘normal’ intensity - definitely there but not uncomfortable. 30 minutes is probably the minimal effective time, but it can be delivered for as long as needed. The main pain relief is achieved during the stimulation with a limited ‘carry over’ effect i.e. pain relief after the machine has been switched off.

Fig 23. Normal TENS or High TENS using stimulation at a high frequency ACUPUNCTURE TENS OR LOW TENS

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Acupuncture TENS or low TENS uses a lower frequency stimulation (2-5Hz) with wider (longer) pulses (200-250µs). The intensity employed will usually need to be greater than with the normal TENS, still not at the patient’s threshold, but quite a definite, strong sensation. As previously, something like 30 minutes will need to be delivered as a minimally effective dose. It takes some time for the opioid levels to build up with this type of TENS and hence the onset of pain relief may be slower than with the traditional mode. Once sufficient opioid has been released however, it will keep on working after cessation of the stimulation. Many patients find that stimulation at this low frequency at intervals throughout the day is an effective strategy. The ‘carry over’ effect may last for several hours, though the duration of this carry over will vary between patients.

Fig 24. Acupuncture TENS using stimulation at a low frequency HIGH TENS

Appears to be more effective with acute pains and most effective during the stimulation period with limited carry over period and can be used for as long as needed. LOW TENS

Appears to be more effective with chronic pains and effective during the stimulation period but has a significant carry over period and can be used for as long as needed.

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SPINAL CORD STIMULATOR A Spinal Cord Stimulator (SCS), also known as a dorsal column stimulator (DCS) is an implantable medical device used to treat chronic neurological pain. An electric impulse generated by the device produces a tingling sensation that alters the perception. The device is implanted into the epidural space either by percutaneous approach or by surgical laminectomy or laminotomy. A pulse generator or RF receiver is implanted in the abdomen or buttocks. A wire harness connects the lead to the pulse generator.

Fig 25. Spinal cord stimulator leads in thoracic spine.

MAGNETIC STIMULATION A problem with electric stimulation is that it is painful. The pain is not very different from that induced by the stimulation of peripheral nerves, but it is sufficient to limit its clinical acceptability. It is possible to stimulate both the nerve and brain magnetically. A magnetic pulse is generated by passing a brief, high- current pulse through a coil of wire. The technique has an advantage in that the stimulation is almost painless. Magnetic stimulation is simply the application of the principle of induction to get electrical current across the insulating tissues without discomfort. When the coil is energized by the rapid discharge, a rapidly changing current flows in its windings. This produces a magnetic field oriented orthogonally to the plane of the coil. The magnetic field passes unimpeded, inducing an oppositely directed current. The current induced activates nearby nerve cells in much the same way as currents applied directly to the cortical surface.

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INTERFERENTIAL CURRENT THERAPY Interferential electrical stimulation is a unique way of effectively delivering therapeutic currents to tissue. Interferential therapy utilizes two medium frequency currents, passed through the tissues simultaneously, where they are set up so that their paths cross and they literally interfere with each other. This interaction gives rise to an interference current (or beat frequency) which has the characteristics of low frequency stimulation, in effect the interference mimics low frequency stimulation. The exact frequency of the resultant beat frequency can be controlled by the input frequencies. If for example, one current was at 4000Hz and its companion current at 3900Hz, the resultant beat frequency would be at 100Hz, carried on a medium frequency 3950Hz amplitude modulated current. By careful manipulation of the input currents it is possible to achieve any beat frequency.

Fig 26. Basic principle of interference currents The basic principle of Interferential Therapy (IFT) is to utilize the significant physiological effects of low frequency electrical stimulation of nerves without the associated painful and somewhat unpleasant side effects sometimes associated with low frequency stimulation. To produce low frequency effects at sufficient intensity and at sufficient depth, patients can experience considerable discomfort in the superficial tissues (i.e. the skin). This is due to the impedance of the skin being inversely proportional to the frequency of the stimulation. The lower the stimulation frequency, the greater the impedance to the passage of the current and so, more discomfort is experienced as the current is ‘pushed’ into the tissues against this barrier. The result of applying a higher frequency is that it will pass more easily through the skin, requiring less electrical energy input to reach the deeper tissues & giving rise to less discomfort.

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Interference stimulators use a fixed carrier frequency of 4000 Hz and also a second adjustable frequency of 4000-4400 Hz. When the fixed and adjustable frequencies combine (heterodyne), they produce the desired beat frequency or interference frequency as shown in Fig 27. Interferential stimulation is concentrated at the point of intersection between the electrodes. This concentration occurs deep in the tissues as well as at the surface of the skin. With interferential stimulators, the current perfuses to greater depths and over larger volume of tissue than other forms of electrical therapy.

Fig 27. Principle of generation of interference currents Interferential current therapy is a treatment to aid the relief of pain and the promotion of soft-tissue healing. Tiny electrical impulses are induced into the tissues in the area of the pain. Where these waves intersect below the surface of the skin, the low-frequency stimulation induces the body to secrete endorphins, which are the body's natural painkillers. Most patients find interferential therapy to be very beneficial and describe the treatment as being relaxing and having a 'pins and needles' sensation. Ligament sprains, muscle strains and spasms often respond well to interferential current therapy. Modern machines usually offer frequencies of 1-150Hz, though some offer a choice of up to 250Hz or more. The magnitude of the low frequency interference current is (in theory) approximately equivalent to the sum of the input amplitudes. The resultant current will be stronger than either of the two input currents. 27

ULTRASONIC STIMULATORS Ultrasonic Stimulation is a deep heating modality that is most effective in heating tissues of deep joints. It is helpful in improving the distensibility of connective tissue, which facilitates stretching. It is perhaps best used to improve limitations in segmental spinal range of motion following recurrent or chronic low back pain as an adjunct in facilitating soft tissue mobilization and prolonged stretching by a skilled manual therapist. The effect of ultrasonic stimulation on the tissues is a high speed vibration of micro-massage.

Fig 28. Block diagram of an ultrasonic therapy unit CIRCUIT DESCRIPTION The equipment required for ultrasonic stimulation is electronically very simple. Fig 28. shows the block diagram. The heart of the system is a timed oscillator which produces the electrical oscillations of the required frequency. The oscillator output is given to a power amplifier which drives the piezoelectric crystal to generate ultrasound waves. Power amplification is achieved by replacing the transistor in typical LC tuned Colpitt oscillator by four power transistors placed in bridge configuration. The delivery of ultrasound power to the patient is to be done for a given time. This is controlled by incorporating a timer to switch on the circuit. The timer can be a mechanical spring–loaded type or an electronic one, allowing time settings from 0 to 30 minutes. The power output in case of triac controlled machines can be continuously varied from 0 to 3 watts/ cm². The machine can be operated in either continuous or pulsed mode. A full wave rectifier comes in the circuit for continuous operation. The mains supply is given to the oscillator without any filtering. The supply voltage is therefore at 100 Hz which causes the output 1 MHz to be amplitude modulated by this 100 Hz. 28

In pulsed mode, the oscillator supply is provided by the half wave rectifier and the oscillator gets the supply only for a half cycle. Thus the output 1 MHZ is produced only for one half of the cycle and is pulsed. The output of the oscillator can be controlled by either of the following two methods: • Using a transformer with a primary winding having multi–tapped windings and switching the same as per requirement: • Controlling the firing angle of a triac placed in the primary circuit of the transformer, and thereby varying the output of the transformer. The transducer may be barium titanate or lead zirconate titanate crystal, having 5-6 cm2 effective radiating area. The length of the cable connecting the transducer with the oscillator is of critical dimension and should not be altered. In front of the crystal lies a metal face plate which is made to vibrate by the oscillations of the crystal. Ultrasonic waves are emitted from this plate. The crystal has a metal electrode pressed against its back surface by a coiled spring. Voltage is applied to the crystal via this electrode. The front diaphragm is grounded and provides a return path for the excitation voltage. DOSAGE CONTROL The dosage can be controlled by varying any one of the following variables: • Frequency of the ultrasound. • Intensity of ultrasound. • Duration of the exposure. It has been established that a frequency of approximately 1 MHz is the most useful. Below a frequency of 1 MHz, the beam of ultrasonic energy tends to diffuse and no efficient treatment can be expected. A frequency in the range of 800 kHz to 1 MHz is, therefore, most widely adopted. In order to achieve maximum therapeutic efficiency, it is necessary to ascertain the correct ultrasonic intensity and duration of application for a given indication. Some instruments are equipped with a ‘dose tabulator’ from which the data concerning dosing can be taken at a glance. In this table, a dose mark is given for every (disease) and all that is required is to set a pointer appropriately to ensure that the apparatus is providing the correct output intensity. APPLICATION TECHNIQUE There are several ways for applying ultrasonics to the body. The probe can be put in direct contact with the body through a couplant provided the part to be treated is sufficiently smooth and uninjured. In case a long area is to be treated, the probe is moved up and down, and for small areas it is given a circular motion to obtain a uniform distribution of ultrasonic energy. If there is a wound or an uneven part (joints etc.), the treatment may be carried out in a water bath. This is to avoid mechanical contact with the tissues which may damage an already injured surface. It should be ensured that air bubbles are not present either on the probe or the skin. For this treatment any vessel with warm water would be suitable. The part of the body to be treated is rubbed with alcohol

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or soaped. The probe is moved over the area to be treated but held at a distance of about 1-2 cm from the area under the treatment. THERAPEUTIC BENEFITS One of the therapeutic effects for which ultrasound has been used is in relation to tissue healing. It is suggested that the application of ultrasound to injured tissues will, amongst other things, speed the rate of healing & enhance the quality of the repair. In thermal mode, ultrasound will be most effective in heating the dense collageneous tissues and will require a relatively high intensity, preferably in continuous mode to achieve this effect. The non-thermal effects of US are now attributed primarily to a combination of cavitation and acoustic streaming.

Fig 29. Cellular activity process for the therapeutic benefits of Ultrasonic Stimulation. The result of the combined effects of stable cavitation and acoustic streaming is that the cell membrane becomes ‘excited’ (up regulates), thus increasing the activity levels of the whole cell. The ultrasonic energy acts as a trigger for this process, but it is the increased cellular activity which is in effect responsible for the therapeutic benefits of the modality. Ultrasonic Stimulators are constructed on the piezo-electric effect. A high frequency alternating current is applied to a crystal whose acoustic vibration causes the mechanical vibration of a transducer head, which itself is located directly in front of the crystal. These mechanical vibrations then pass through a metal cap into the body tissue through a coupling medium. The larger the diameter of the applicator, the smaller would be the angle of divergence of the beam and less the degree of penetration. Employed at an appropriate treatment dose, with optimal treatment parameters intensity, pulsing and time, the benefit of Ultrasonic Stimulation is to make as efficient as possible to earliest repair phase, and thus have a promotional effect on the whole healing cascade. For tissues in which there is an inflammatory reaction, but in which there is no ‘repair’ to

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be achieved, the benefit of ultrasound is to promote the normal resolution of the inflammatory events, and hence resolve the ‘problem’.

FUTURE CHALLENGES AND IMPLEMENTATIONS Although each modality of electrotherapy has its specific set of contraindications, cardiac pacemakers are a general contraindication for electrotherapy. Electrical signals from electrotherapy devices can interact with the electrical signals from pacemakers and interfere with pacemaker functioning. Pacemaker disruption is particularly acute with modalities as electrical nerve stimulation. Therapists ought to work to improve these areas of dysfunction and treat the patients accordingly. Patient comfort always remains a challenge to the physical therapist. It ought to be monitored throughout the therapeutic session. After treatment, the patient should be queried about excessive levels of discomfort. Excessive discomfort should serve as a guide for modulating electrotherapy parameter settings in the future .In the case of such direct electrical stimulation, the skin under the probes should be massaged with a neutral cream after treatment. After treatment, the patient should be checked for burns from electrodes or poorly placed wires. Treatment should be discontinued immediately or alternative modalities should be applied in such cases. For individuals with neurological impairments, functional electrical stimulation (FES) remains a challenge to help make real what was once only imagined: the restoration of movement to paralyzed arms and legs. Depending on the location and severity of the disability, FES can significantly improve quality of life by enabling the individual to regain capabilities such as walking, grasping objects, or maintaining bladder control. This is one area which requires accurate implementation so as to bring about significant positive changes in the lives of tetraplegic patients. Moreover, numerous types of electrical stimulation devices and methods, as well as a variety of therapeutic applications, are available for clinical use. But, selection of the proper device can be challenging for all clinicians with regard to a thorough understanding and correct application of electrical stimulation principles, theories, and operational procedures. Rehabilitation and treatment goals incorporating the use of electrotherapy in the clinical setting can often be ineffective or unwarranted unless one has a working knowledge of the purposes, effects and benefits, and contraindications of individual treatments. The physical therapist should take into consideration these factors during the treatment procedure. Accuracy and reliability of a particular modality in electrotherapy remains a challenge and may vary from patient to patient depending on the injury or dysfunctionality as well as the knowledge of the physical therapist. Depending on the diagnosis and symptoms, the therapist should correctly evaluate the flexibility, strength, co-ordination and monitor them. With the right dose administration, the therapist faces the challenge to restore functionality in patients with acute neurological impairments.

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CONCLUSION Electrotherapy is, overall, a safe treatment when used appropriately. However, there are some precautions. Because electrotherapeutic devices may interfere with pacemaker function, patients with pacemakers should avoid the application of electrotherapy during physical therapy treatments. Patients should also avoid any electrotherapy modality directly over a malignant tumor. Caution should be taken when pregnant women are receiving physical therapy treatment. Although it has its benefits, electrotherapy alone is not a cure, it is only a tool that is used in physical therapy to address pain, swelling, spasm, and weakness, and should be a small piece of comprehensive rehabilitation program to allow for a fast, safe, and complete recovery. Electrophysical agents are utilized to bring about physiological effects, and it is these changes which bring about the therapeutic benefit rather than the modality itself. Clinical decision protocols employing the available evidence should enable the most appropriate modality to be employed for a particular patient. Indiscriminate use of electrotherapy is unlikely to yield significant benefit, however used at the right time, it has the potential to achieve beneficial effect. The patient management programme which combines manual therapy, exercise therapy and electrotherapy, based on current evidence, should enable the most efficacious management of a patients' dysfunction. Setting goals is the best way to achieve a successful rehabilitation outcome. When starting physical therapy, we must think what is it to be accomplished at the end of the program. The goals set should be important. However, they must also be realistic and attainable. The physical therapist will then work to devise an appropriate treatment program to help achieve the rehabilitation goals. Modern electrotherapy practice needs to be evidence based and used appropriately. Used at the right place, and at the right time for the right reason, it has phenomenal capacity to do good. Used unwisely, it will either do no good at all, or worse still, make matters worse. Hence, it is essential to select the appropriate modality and deliver the appropriate therapy after consultation with the physical therapist.

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[1]

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Vrbova Gerta, Hudlicka Olga and Centofanti Kristin Dchaefer “Application of Muscle/ Nerve Stimulation in Health and Disease”, Springer.

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Watkins, Arthur Lancaster, “A manual of electrotherapy”, Lea & Febiger.

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Singh Jagmohan, “Textbook of Electrotherapy”, JAYPEE Brothers Medical Publishers, New Delhi.

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