Can you stimulate a denervated muscle?

The short answer is ‘Yes you can and probably you should’. A related question of course might be why would you want to; we will get to that shortly. Denervation of skeletal muscle can occur following a spinal cord injury affecting the low lumbar and sacral regions or with peripheral nerve injuries.

The many nerves that fan out at the end of the spinal cord are known as the cauda equina, due to its appearance being likened to a horse's tail. The spinal cord actually ends at the upper portion of the lumbar (lower back) spine but the individual nerve roots at the end of the spinal cord that provide motor and sensory function to the legs and the bladder still continue further along the spinal canal. Damage to these by trauma, tumour or other cause can cause many "red flag' problems including a flaccid type of paralysis.

Peripheral nerve injuries are commonly seen and can be difficult and frustrating to manage. These result from contusion, crushing, stretching or cutting of nerves such as the axillary nerve at the shoulder, the radial nerve in the arm, the ulnar nerve at the elbow and so on.

What happens when a muscle is denervated?

Following an injury a denervated muscle alters it's structure in many ways and over time the normally excitable and contractable muscle fibre tissue is replaced with collagen and fat. This tissue becomes less capable of contracting as this process takes shape. The muscle atrophy and changes that rapidly follow denervation can make any intended rehabilitation process an extremely difficult task. If reinnervation is a possibility, this opportunity is lost without effective intervention. Reinnervation cannot complete if the former muscle's structure is lost and no longer supports contraction.

As these effects progress, the loss of muscle bulk and tissue quality lead to a poor general trophic situation.

The "traditional" approach to dealing with peripheral nerve injuries has tended to focus on limb positioning and the prevention of contractures through passive or stretching exercises.

How do you deal with muscle atrophy?

Electrical stimulation can now be used to stimulate denervated muscles and to cause contractions that prevent or reverse atrophy as well as possibly supporting the regeneration of peripheral nerves - if this is to be expected. Once upon a time there was concern that using electrical stimulation could actually deter reinnervation but now evidence suggests that this should not be an issue.

The whole topic of using electrotherapy for denervation has been controversial over the years. It is a fact that forms of electrical stimulation have been used with denervated muscle since the mid-1800's.

In 1962, "The Denervated Muscle", edited by Ernest Gutmann was published and this work is still not a bad place to start when you wish to learn about this phenomenon. Gutmann and his collaborators were conducting research on long-term denervation of skeletal muscle beginning in the 1940's and his work is still widely cited even today. Of course, this research was largely descriptive and what was possible was limited by the technology available at the time.

Even in the 1990's though, using electrical stimulation with cases of denervation has been controversial with fears that the level of intensity needed would lead to the potential for skin burns. In our experience, providing good methods, are followed the risks are very low.

In Europe the multinational European RISE study and the work of Helmut Kern in Austria and many others has helped to change the minds of many on the safety and effectiveness of electrotherapy for denervation. The RISE study used specially developed electrical stimulation equipment and showed that it could be used safely by clients at home who had complete denervation. The equipment could be used to rescue the muscle tissue bulk and quality even with clients many years after their injury. The researchers also developed protocols for application which guide our own practical work today. This study utilised the equipment which has evolved into the RISE stimulation unit we use today.

What stimulation parameters are needed?

With spinal cord injuries resulting in a flaccid paralysis of the legs we would today use large electrodes (either specially designed safety electrodes or wet sponge bags with carbon rubber) that cover as much muscle tissue as possible. This is because the aim is to make the muscle fibres contract directly rather than rely on the (absent) nerve structure that would typically be used by commonly available electrical stimulation units. Muscle stimulators that rely on an intact nerve structure will typically deliver bipolar rectangular pulses with waveforms described by three parameters - frequency, pulsewidth and current. You would expect to see these deliver pulses in the ranges 20 - 50 Hz (frequency), 0 - 130 mA (current) and 100 - 500 microseconds pulsewidth. When the nerve structure is intact these parameters are sufficient to excite the muscle fibres to contract.

In contrast, the RISE Stimulator we use can still deliver a bipolar waveform but the stimulation parameters are very different. We may use frequencies as low as 1 or 2 Hz, pulsewidths of up to 200 milliseconds and currents as high as 200 mA.

RISE Stimulator

Two protocols are typically used - one that is low frequency and seems to induce a twitch in the muscle and a second protocol that produces a tetanic contraction. The first protocol encourages a normalisation of the muscle structure and the second protocol strengthens what has been recovered.

Quadriceps stimulation

As therapy progresses to use tetanic contractions these can produce leg extensions and utilise ankle weights for resistance if necessary

With peripheral nerve injuries we can quite often find that we have muscles with preserved innervation alongside denervated ones. We then do not want to overload the normally innervated muscles.

The fact that the paralysed muscle has lost its normal accommodation ability allows for selective stimulation of denervated muscles. We do this by changing the waveform shape to a long exponential (triangular) pulse rather than a rectangular one. We can therefore achieve selective contraction of the paralyzed muscles.

Specialist equipment?

There are a number of hand-held units on the market that in general terms can generate the parameters suitable to elicit a denervated muscle to contract. However, these rarely generate the current intensity or range of parameters necessary to cover all applications. Typically these units are Ok with smaller upper limb muscles but not adequate when dealing with quadriceps, gluteal and other larger muscles.

The RISE unit we use has two channels and is very simple to use. The unit can have the settings fine tuned via an easy to use interface and then the unit can be placed in a "home user" mode which prevents accidental changes to stimulation parameters. The user need only then understand how to secure the electrodes and rotate a control knob to set the stimulation intensity. In starting to use this equipment it is possible that a twtich contraction can be illicited but not a tetanic one. Period testing can be used to verify when a tetanic contraction can be generated and a feature is built into the RISE unit to allow this. Clients will generally have to use the unit 5 or 6 days a week at the beginning.

Conclusion

One of the real drivers for our clients to use electrical stimulation has been the resulting appearance of their legs. They easily add 3 to 5 cm of muscle girth to their thighs this way. Stimulation builds muscle and this has aesthetic benefits that users care about. It is also important to know that developing muscle bulk and quality reduces pressure ulcer risk and improves local circulation.