The application of an electric current in the environment of a muscle deprived of its peripheral nerve control remains probably the most delicate area of electrotherapy.
The reason for this is first and foremost historical, since for many years the proponents of the “pros”, as numerous as the adepts of the “cons”, have clashed, sometimes vigorously, in
rather sterile debates, lacking any real scientific evidence. It is also due to a lack of understanding both of the pathophysiology of peripheral nerve damage, and often of the very basis of electrotherapy. Before presenting the different ways of treating denervated muscle with electrotherapy, it is essential to examine how stimulation of a denervated muscle can be beneficial.
About the author: PASCAL ADAM, Masseur Kinésithérapeute D.E. Electrotherapy teacher IFMK Paris
1. Physiological processes of traumatic denervation and nerve regeneration
Seddon proposes the simplest classification of traumatic nerve damage, distinguishing between :
– Neurapraxis:
Most often corresponds to a simple nerve compression that does not result in a break in continuity of the axon or its sheaths. This is a highly localized demyelination between 2 or more nodes of Ranvier, resulting in a conduction block with a good prognosis, provided the compression is lifted within a reasonable time. The usual recovery time is 6 to 8 weeks, corresponding to the repair time of the myelin sheath.
Saturday-night palsy is a good example of radial nerve neurapraxia, in this case caused by prolonged pressure of the partner’s head on one arm.
– Axonotmesis:
The axon is broken or severed, but the axonal sheath and endoneural tubes are intact. It can also be the result of prolonged compression.
The distal part of the axon degenerates rapidly within a few days: this is Wallerian degeneration. Axonal regeneration begins almost immediately from the proximal bud, at an average rate of 1 mm per day. The prognosis is generally good, as the risk of a false route is eliminated by the persistence of functional endoneural tubes. Recovery times depend mainly on the level of the initial lesion and the distance of regrowth.
– Neurotmesis:
For Seddon, there is rupture or sectioning of all the constituent elements of the nerve, sometimes with associated loss of substance. The distal end of the nerve degenerates (Wallerian degeneration) and axonal regeneration takes place, but usually without good functional results, since in the absence of its sheath, the axon takes false routes through neighboring sheaths or tangles like a ball of wool to form a neuroma.
The aim of surgical repair is to transform neurotmesis into axonotmesis, with a much better functional prognosis. There are other classifications, such as Sunderland’s, which proposes intermediate stages depending on the extent of lesions in the various sheaths (endoneurium, perineurium, epineurium), but above all, all authors recognize the near-constancy of mosaic attacks on the same nerve, i.e. attacks in which various anatomical lesions coexist to varying degrees.
2 Influence of electricity on nerve regeneration
2-1 What do the studies say?
Numerous studies have sought to determine whether the application of a stimulation current is beneficial in promoting nerve regrowth, or harmful in inhibiting or slowing down the regeneration process.
However, the stimulation parameters used in these studies are very heterogeneous. Indeed, some studies involved stimulating muscle fibers with long-duration pulses and very low frequencies, while others used nerve stimulation with very short pulses and tetanizing frequencies.
What’s more, the human populations studied rarely, if ever, present homogeneous nerve lesions (axonotmesis, neurapraxia, etc.). Results are therefore necessarily heterogeneous, with some studies tending to improve hair regrowth, while others conclude that stimulation is ineffective or even has a harmful effect on regrowth mechanisms.
A recent review of the literature carried out in 2009 by T. Gordon, however, points in the direction of a positive effect of electrotherapy… But what are we really talking about?
2-2 The right questions to ask!
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– Stimulation of muscle fibers :
When muscle fibers are stimulated directly, the electrical impulse triggers an action potential that travels through the muscle fibers as far as the T tubules, but never reaches the motor neurons, which in any case do not reach the motor plate in the case of denervated motor units. Under these conditions, it’s hard to see how stimulation of this kind could influence nerve regeneration in any way (figure 1a).
– Nerve stimulation downstream of the lesion:
If it’s now the motor nerve that’s being stimulated, remember that the nerve does not transmit the electrical impulse to the muscle, but that the electrical impulse simply triggers an action potential identical in every way to those initiated by a voluntary command (+30mV). As a result, the electrical impulse does not propagate beyond its point of application.
If stimulation takes place close to the motor point (as should be the rule for neurostimulation), the electrical impulse acts downstream of the nerve stump, and here too it is difficult to imagine any effect on regrowth (figure 1b).
– Nerve stimulation upstream of the lesion:
If motor neurons are stimulated in the nerve trunk upstream of the lesion, healthy motor neurons can transmit the electroinduced action potentials to the muscle fibers they control. The motor response obtained can only come from the innervated part of the muscle.
Quant to the “amputated” motor neurons, the electrical impulse triggers the action potential which propagates to the nerve stump where it therefore ends up in a dead end. Here, it would be legitimate to wonder about the possible effect of these nerve impulses on the process of nerve regrowth: do they promote it? Does it prevent or slow it down? We don’t know! It should be noted, however, that the same question may arise for voluntary contractions, since in this case the patient seeking to contract his denervated muscle activates his nerve command, whose action potentials arrive in the same way at the end of the amputated axon. However, it does not seem to have been established that rehabilitation exercises involving the voluntary stimulation of a denervated muscle should be banned because they would inhibit nerve regeneration (figure 1c)!
2-3 So what’s the point?
Given the current lack of scientific evidence demonstrating a favorable or unfavorable effect of muscle stimulation on the quality and speed of motor recovery following peripheral nerve damage, it seems wise to conclude that this technique is not capable of influencing nerve regrowth.
On the other hand, stimulation of denervated muscle fibers is the only way to impart mechanical activity to muscles deprived of their peripheral control.
On the one hand, this electro-induced muscular activity will help maintain an acceptable trophic state in combination with other rehabilitation techniques (passive mobilization, massage, heat, etc.), but above all it will limit amyotrophy and maintain muscle fiber contractility.
Muscle sclerosis, which corresponds to the irreversible disappearance of contractile units (sarcomeres), appears on average between 12 and 18 months when a muscle is no longer used. This is always a catastrophic situation that compromises the functional future of a muscle, even when nerve regeneration is favorable but delayed.
The aim of electrotherapy of denervated muscle is therefore to reduce amyotrophy and maintain contractility to promote functional restoration in the event of favorable nerve regeneration.
3 Reminders of nerve and muscle electrophysiology
As the only excitable structures, i.e. those with the ability to invert the electrical potential of their membrane and then propagate this signal, or action potential, along their structure, nerves and muscles differ greatly in excitability (Figure 2).
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Thus, the stimulus required to trigger an action potential on a muscle fiber is considerably greater than that required to trigger the same phenomenon on a nerve fiber.
For an electrical impulse, this means that the quantity of electrical charges that must be applied to excite a muscle fiber is several hundred times greater than that required to excite a nerve fiber. This requirement translates into the need to use much longer pulse durations for muscle stimulation than for nerve stimulation.
3-1 The neurostimulation pulse is unable to excite muscle fibers
For nerve stimulation, average pulse durations of around 200 μs (0.2 ms) are commonly used for analgesic electrotherapy and for neuromuscular stimulation (between 30 and 400 μs).
Such short durations are ideal for stimulating the various nerve fibers, guaranteeing optimum comfort of use and optimal efficiency, but under no circumstances can they be used to stimulate muscle fibers directly (figure 3).
Anesthetists are well aware of this, and use nerve stimulation to assess the efficacy of curarization or decurarization. Curare, commonly used in anesthesia, causes “reversible therapeutic paralysis” by transient blockage of the synapse. An absence of muscular response to the nerve stimulation test indicates effective curarization, while a muscular response indicates that curarization is not or no longer effective.
The I/t curves for nerve and muscle also clearly show that the use of a long-duration rectangular pulse (several tens of ms) can stimulate the muscle fiber, but that this is inevitably accompanied by stimulation of the motor neurons.
In such a case, it is never possible to determine the respective contribution of nerve and muscle stimulation to the mechanical response obtained (Figure 4).
3-2 Which impulses stimulate muscle fibers directly?
3.2.1 – Total deprivation
When denervation is complete, the denervated muscle fibers are stimulated with a long-duration rectangular pulse.
Preferably, the pulse duration will be equal to or close to the chronaxy, which can be evaluated using an electrotherapy device with a “manual” mode that enables pulse durations of between 5 ms and 1 second to be selected. This maximum pulse duration is then selected and applied to the fleshy part of the muscle, with a gradual increase in intensity. The first muscular response is obtained when rheobase is reached. Simply select a pulse duration of a few tens of ms and increase the intensity to twice the rheobase.
If a response is obtained, repeat the maneuver, reducing the pulse duration by 5 or 10ms. If there is no response, try again with a pulse of longer duration. Another way of proceeding is to use a pulse duration of 100 ms, which is reputed to be the “average” duration of the chronaxy of denervated muscle fibers.
3.2.2 – Partial denervé
While a long-duration rectangular pulse is perfectly satisfactory for stimulating the muscle fibers of a completely denervated muscle, this is not the case when denervation is only partial. Indeed, as we saw in Chapter 3.1, a rectangular impulse also stimulates, and even primarily stimulates, healthy motor units.
It is therefore interesting to exploit the physiological phenomenon of accommodation (the term climalysis is no longer used today), which occurs when an electric current is applied progressively rather than instantaneously, as with a rectangular pulse. This event consists of a leak or a rise in the excitation threshold, i.e. the rheobase. This phenomenon appears rapidly for nerve fibers (20 to 30 ms) and more slowly for muscle fibers (between 100 and 300 ms).
electrotherapy_4Figure 5 shows how a triangular-shaped pulse with an appropriate slope can stimulate denervated muscle fibers without first exciting intact motor neurons, or muscle fibers that are still innervated. Determining the right slope is an essential point, since an insufficient slope will not allow any stimulation, whereas a slope that is too steep will reach the innervated structures first.
Some devices offer an automatic slope detection mode, which is obtained by automatically incrementing the intensity (+ 0.5mA with each pulse), while the pulse duration is fixed (100ms). The physiotherapist must then monitor the occurrence of the first motor response, which occurs when the denervated part of the muscle is stimulated, and record this data by pressing a button on the device.
4 Characteristics of stimulation currents for denervated muscles
– Balanced pulses :
To avoid the accumulation of electrical particles in tissues (the phenomenon of polarization), it is customary to reverse the direction of the current: after a first phase of a given polarity, a second phase immediately follows, perfectly symmetrical but of opposite polarity.
This is what is done with modern neurostimulation currents (symmetrical compensated biphasic impulse).
Because of the very long duration (≈ 100ms) of the rectangular or triangular pulses used to stimulate denervated muscle fibers, and in order not to double this duration and thus further increase the discomfort of these treatments, we prefer to use monophasic but alternating pulses so as to obtain a current with zero electrical mean and thus be able to use these currents close to metallic implants (figure 6).
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– Very low frequency :
A common feature of denervated muscle is its high fatigability, reflected in the progressive weakening of its mechanical response to electrical stimulation.
A very low frequency, such as 0.5 Hz, i.e. a pulse every 2 seconds, limits this fatigue. If, despite this very low frequency, the motor response becomes exhausted (as is sometimes the case with very old denervations), it may be a good idea to space the pulses further apart, for example by choosing a stimulation mode with 1 pulse every 4 or 5 seconds, or even longer, as is possible with quality devices.
– Session duration :
Undoubtedly a consequence of the high fatigability of the denervated muscle, the failure to exceed a certain number of motor responses, which should not exceed 4, 6 or 10 depending on the opinion, was for a long time a dogma at a time, it is true, when the opposite of fatigue, i.e. rest, was the cornerstone of many therapeutic means! Today, we prefer longer treatments, of the order of 8 minutes but with very low frequencies, to impose a very modest amount of work, but nevertheless sufficient to achieve the desired effects.
5 rules of thumb for denervated muscle stimulation
5-1 Electrode selection and placement
An old custom, specific to the treatment of denervated muscle (still sometimes practiced today), was to use a stylet-type accessory to perform ponctiform stimulation. This is strange, to say the least, because either we’re looking for a localized area that produces a mechanical muscular response: this is the motor point that corresponds to the motor plate, or we’re stimulating individual muscle fibers, and of course on the “sine qua non” condition that the duration of the impulse is sufficiently long. However, the motor plate (motor point) no longer exists for denervated motor units! If there is a response, it can only come from the innervated part of the muscle, and this indicates partial damage. Individual stimulation of muscle fibers is of little interest either, unless you want to prolong an unpleasant treatment for a considerable time!
Electrotherapy_6 Today, we recommend placing 2 soft silicone electrodes coated with conductive gel on the fleshy part of the muscle, so that the electrodes cover as much of the muscle as possible (figure 7).
The advantage of silicone electrodes is that they are usually sold by the meter, which means they can be cut to size, but also that they conduct the current better if a sufficient thickness of gel covers them. This is important, as the trophic disorders of the skin that accompany old denervations modify the skin’s electrical resistance, which can reach high values and thus exceed the tolerance thresholds of the usual adhesive electrodes.
Silicone electrodes are held in place with medical adhesive or lightweight tape.
5-2 Intensity settings
For the same reasons as for stimulation of a normally innervated muscle, the intensity that directly determines spatial recruitment will be gradually increased throughout the session, while remaining bearable for the patient.
This is essential in order to recruit the greatest number of muscle fibres to the depth of the muscle.
It should be remembered that the large quantity of electrical charges administered with each pulse (and necessary to reach the excitation threshold of the muscle fiber) is responsible for the discomfort of the treatment, which is often perceived as painful… by patients who do not have associated hypoesthesia.
For patients with severe sensitivity disorders, we recommend first stimulating the healthy side to the limit of the tolerable threshold, and then gradually applying the same level of intensity to the pathological side.
For the stimulation of partially denervated muscles, where triangular pulses are used, we have seen (chapter 3.2.2) that it is essential to determine the appropriate slope to avoid stimulating healthy motor units.
If the intensity is now increased beyond that required to obtain the correct slope, the slope will straighten out and the impulse may reach the innervated structures. It is therefore necessary to have a device capable of memorizing the appropriate slope, and maintaining it by lengthening the pulse duration each time the intensity is increased.
5-3 Frequency of sessions
The aim of electrotherapy sessions is to preserve muscle trophicity and the contractility of denervated motor units. This can only be achieved with very regular use, which should be daily whenever possible.
In a neighboring country like Switzerland, where the patient can rent an electrotherapy device to treat denervated muscles, after being educated by his or her physiotherapist, 2 sessions a day are prescribed, ideally with one session in the morning and another in the evening.
6 Clinical attitudes in everyday practice
Before determining the type of stimulation a patient with peripheral neurological damage could benefit from, it is essential to be able to “classify” his lesion or pathology into one of the following four situations:
– total denervation with hope of recovery,
– partial denervation with hope of recovery,
– total denervation beyond recovery time,
– partial denervation outside the recovery period.
There is no real consensus on how long recovery is possible, even if the theoretical time is fairly easy to estimate. All you need to do is assess the distance of regrowth, i.e. the distance between the lesion and the motor point of the muscle, and divide this distance in centimetres by 3, which corresponds to the average monthly speed of nerve regrowth (1mm per day or 3 cm per month).
By way of example, a radial nerve lesion following a fracture of the middle of the humeral shaft reaches the nerve at a distance of around 20 centimetres from the motor points of the epicondyle muscles.
The theoretical reinnervation time in this example is therefore 20 ÷ 3 = 6 to 7 months.
All this, of course, for straight-line regrowth, i.e. without meandering, and at speeds recognized as average! It is therefore always wise to extend this theoretical period, especially as every therapist remembers cases where recovery was sometimes very late and well beyond the theoretical timeframe!
The nature of the initial injury or pathology is also a factor to be taken into account when assessing the time frame during which there is reasonable hope of recovery.
6-1 Total denervation with hope of recovery
In this case, the denervated muscle(s) should be stimulated with long rectangular pulses. The aim is to maintain the best possible trophicity and contractile properties of the denervated muscles while awaiting a favorable evolution of the situation.
It will be necessary to regularly reassess motor possibilities, since in the event of a favorable evolution, some motor units may have recovered, and the patient will then find himself in a situation of only partial denervation.
6-2 Partial denervation with hope of recovery
Analytical stimulation of denervated fibers requires the use of triangular pulses whose slope must be determined and kept fixed throughout each session.
The goals are identical to those of complete denervation: maintenance of trophicity and contractility until recovery is as complete as possible.
The innervated part of the muscle can also benefit from electrostimulation, using a classic neurostimulation program such as the one used to treat muscular atrophy.
6-3 Total denervation outside the recovery period
This situation, which is not the best for the patient, is the simplest for the therapist, since here we can wisely recommend abstention from all excitomotor electrotherapy.
Indeed, the race to maintain an acceptable and, above all, lasting trophicity is lost in advance for a muscle that will not regain its innervation.
Other techniques can sometimes be implemented, in particular to try and develop substitutes, but we’re getting off the subject here.
6-4 Partial denervation outside the recovery period
The muscle here is made up of functional motor units: the innervated part, but also another irreversibly non-functional part: the denervated part.
Obviously, the larger the denervated area, the more serious the functional damage.
In this situation, an interesting strategy may be to try to develop the healthy part of the muscle as much as possible, in order to create what some call compensatory hypertrophy. This will be achieved by means of the classic amyotrophy treatment and strengthening programs.
For this strategy to be accompanied by significant gains, however, the innervated part that we will be seeking to develop must not be reduced to a few rare motor units. It is generally considered that testing at 2 is the minimum threshold at which this type of treatment can reasonably be introduced.
Conclusion
Electrotherapy of denervated muscle has no clearly demonstrated influence on improving nerve regeneration mechanisms. Nevertheless, it is the only technique capable of maintaining the contractility of muscle fibers deprived of their control, and thus preserving the muscle’s functional capital during the often very long time required for axonal regeneration.
References
1. CHAMMAS H., COULET B., THAURY M.N. – 2007,Peripheral nerve injuries: classifications, etiologies and management principles. Elsevier Masson
2.LOWJ. -1979, A review of the uses and reliability of strenght-duration curves. NZ Journal of Physiotherapy, November : 16-20
3. STEPHENSW G.S. – 1973, The use of Triangular Pulses in Electrotherapy.Physiotherapy, 59 (9) : 292-294
4. PETROFSkY J.S. – 1991, The training effects of wide pulse width Electrical Stimulation on Denervated Muscle J. Neuro. Rehabil,5 (3), 161-68
5. EbERSTHEIn A., EbERSTHEIn S. – 1995 Electrical Stimulation of DenervatedMuscle: is it worthwhile? Medicine and Science in Sports and Exercise, 28 (12), 1463-69
6. BRUSHART T.M, HOFFMAN P.N., ROyALL R.M – 2002, Electrical Stimulation Promotes motoneuron Regeneration without increasing its speed or conditioning the neuron. The Journal of Neuroscience, 22 (15):6631 – 6638
7. GORDON T, UDINE E, VERGE V.M, DE CHAVES E.L – 2009, Brief electrical stimulation accelerates axon regeneration in the peripheral nervous system and promotes sensory axon regeneration in the central nervous system Motor Control, Oct; 13 (4) : 412 – 441 review
