Electrotherapy II Lecture Notes PDF

Electrotherapy II 4TH Lecture Stimulation of denervated muscle (Long pulse duration stimulation) Dr. ahmed aboulfotouh  Denervated muscle ◦ When a muscle becomes denervated by nerve injury or disease, it no longer contracts physiologically, nor can a...

Electrotherapy II 4TH Lecture Stimulation of denervated muscle (Long pulse duration stimulation) Dr. ahmed aboulfotouh  Denervated muscle ◦ When a muscle becomes denervated by nerve injury or disease, it no longer contracts physiologically, nor can a contraction be produced by the usual electrical stimulus used for NMES. Neuromuscular Electrical Stimulation ◦ Denervated muscle is different in many respects from innervated muscle, including its response to electrical stimuli. ◦ Without a functional nerve supply muscle can only be caused to contract by direct stimulation of the muscle fiber. ◦ However, if the electrical current lasts longer than 10 milliseconds, the denervated muscle will contract. This is called electrical muscle stimulation (EMS).  Differences between innervated and denervated contraction ◦ In innervated muscle contraction, ◦ When a bundle of motor nerves is stimulated at the motor point it causes the simultaneous stimulation of many motor units, each of which activates many muscle fibers thus causing the synchronous contraction of a large part of the muscle. ◦ This is evident as a brisk twitch, or a series of twitches, or a tetanic contraction if the frequency is high enough. ◦ In denervated muscle contraction, ◦ If there is no nerve the individual muscle fibers are stimulated when the current density across them reaches sufficient intensity so that the contraction spreads slowly through the muscle. ◦ Furthermore, the rate of contraction and relaxation of denervated muscle fibers is slower than that of normal muscle. Both these effects contribute to the different quality of contraction which is sometimes called a ‗worm-like‘ contraction. ◦ Usually, a continuous direct current (DC) is applied for a number of seconds to produce contractions in denervated muscle.  structural and functional changes in denervated muscle ◦ 1- There is an immediate loss of all voluntary and reflex activity. ◦ 2- There is atrophy. ◦ 3- degeneration and fibrosis progressing over weeks and months. ◦ 3- There is a rapid loss of muscle weight during the first few months which slows to become almost steady later. rapid > slow > steady ◦ 4- fibrillation occurs.  Does electrical stimulation have a role in nerve regeneration? ◦ There is conflicting evidence regarding the effects of electrical stimulation on motor nerve regeneration. ◦ Although DC electrical stimulation has traditionally been used for treatment of Bell’s palsy (facial paralysis resulting from damage to the seventh cranial nerve), evidence indicates that this treatment is no more effective than placebo, although some studies have shown improved clinical recovery in patients with chronic facial palsy in response to long-term sensory level electrical stimulation.  Effect of electrical stimulation for denervated muscle ◦ It has been suggested that ongoing electrical stimulation of denervated muscles may retard, or even reverse, this atrophy and fibrosis. ◦ A recent study used a biphasic waveform with a 120 to 150 millisecond pulse duration to contract denervated lower extremity muscles of individuals with complete lower extremity lower motor neuron denervation due to cauda equina injury. Subjects who completed the 2 year program had an 1187% increase in quadriceps muscle force output, a 35% increase in cross-sectional area, and a 75% increase in mean muscle fiber diameter in the stimulated muscles. ◦ Of these patients, 25% were able to perform Functional Electrical Stimulation -assisted stand-up exercises, and all had improved cosmetic appearance of the lower extremities.  Examination, Evaluation, and Prognosis of denervation ◦ A review of the patient’s medical history should include the cause and length of time of denervation. ◦ Electromyography tests should be conducted by a trained electromyographer to evaluate the degree of denervation. ◦ Patient prognosis for improvement with EMS appears to depend upon length of time since denervation and the number of remaining motor units. ◦ Overall, the prognosis for strengthening denervated muscle with EMS is not as good as it is for innervated muscle.  Parameters ◦ Waveform ◦ Monophasic or DC ◦ Pulse duration ◦ 1–450 msec (long) ◦ Frequency ◦ 1–500 pps ◦ Amplitude ◦ To obtain contraction but low to prevent burns ◦ Treatment time ◦ 20 min , 5–7 days per week ◦ duration ◦ From days to years  Electrodes ◦ Electrodes used for denervated muscles tend to be either very large to cover the entire bulk of the muscle. ◦ A probe type of electrode can also be used for stimulating small muscles. ◦ Risk of denervated stimulation ◦ One potential risk associated with this is burning the patient. Careful attention must be given to the skin during treatment, especially because these patients are also likely to have impairments in sensation.  Strength duration curve ◦ The amount of electricity required to produce an AP depends on the type of nerve and can be represented by the nerve’s strength-duration curve. ◦ The strength duration curve for a nerve is a graphic representation of the minimum combination of current strength (amplitude) and pulse duration needed to depolarize that nerve. ◦ This interplay of amplitude and pulse duration forms the basis for the specificity of the effect of electrical stimulation. ◦ In general, ◦ lower current amplitudes and shorter pulse durations can depolarize sensory nerves, ◦ higher amplitude or longer pulses are needed to depolarize motor nerves. ◦ higher amplitudes and longer pulses are needed to depolarize pain-transmitting C fibers. ◦ Short pulses, generally less than 80 micro second in duration, are used to produce sensory stimulation only, whereas longer pulses, 150 to 350 micro second in duration, are used to produce muscle contractions. ◦ When stimulating contractions of smaller muscles and muscles in younger children or in the frail elderly, shorter pulses of 125 to 250 micro sec pulse duration range may be effective, more comfortable, and better tolerated than longer duration pulses.  but sharp pain depolarized ◦ By keeping pulse durations well below 1 ms, pain is minimized because C fibers are not depolarized. However, much longer duration pulses—longer than 10 ms—are required to produce contractions of denervated muscle. ◦ Where the stimulus directly depolarizes the muscle cell rather than the motor nerve, This type of stimulation is generally uncomfortable because it also stimulates pain transmitting A-delta and C fibers if they are present. ◦ The minimum current amplitude with very long pulse duration, as required to produce an action potential, is called rheobase. ◦ The minimum duration it takes to stimulate that tissue at twice rheobase intensity is known as chronaxie. ◦ Rheobase is a measure of current amplitude, and chronaxie is a measure of time (duration). left right > weak left > mean improvement normal ◦ Moving from left to right on the curve, as the intensity of the stimulus is decreased, the tissue may still be excited by increasing the duration. in healthy tissue ◦ In contrast, as the duration of a pulse is decreased, the amplitude must increase to excite the tissue. ◦ Chronaxie can be used to assess the integrity of the tissue because healthy innervated tissue should have a chronaxie less than 1 msec. ◦ Prolonged chronaxie, often 10- to 20-fold longer, is indicative of denervation or other pathology involving the excitability of the tissue. ◦ The S-D curve is based on the stimulus parameters capable of eliciting a minimally detectable motor response, but different nerve types have their own combinations of strength and duration needed for excitation.  Factors affecting nerve response ◦ When a nerve is excited, the excitatory response is dependent on the stimulus parameters but also on two other key factors: nerve size and location of the electrodes. ◦ Most applications of clinical electrical stimulation involve stimulation of a peripheral nerve. ◦ Peripheral nerves are commonly termed mixed peripheral nerves to reflect the various nerve subtypes contained within a peripheral nerve. ◦ Nerve fibers within a peripheral nerve differ in diameter and in their resistance to excitation. ◦ In the presence of a stimulus of sufficient strength and duration, nerve fibers with the greatest diameter and lowest resistance will depolarize first ◦ The largest of the nerve fibers within a mixed peripheral nerve are the A-alpha (A-á) carrying motor and proprioceptive signals; thus, these are first to be depolarized. ◦ To excite the smaller-diameter A-beta fibers (touch and pressure sensation) and even smaller-diameter A-delta (pain and temperature) and C fibers (pain), stimuli of progressively greater amplitude and duration are required. ◦ Although larger-diameter nerve fibers are more easily excited, the location of the fibers to the electrodes will affect the order of recruitment. The fibers closest to the electrode will be excited before those fibers farther away. ◦ Thus, A- sensory nerves of the superficial dermis are activated before the larger-diameter A-, which lie deeper. This largely explains why you feel the stimulus before a motor response is noted.