Electrical stimulation of denervated muscle

Electrical stimulation can be used to stimulate so-called denervated muscle as well as innervated. This necessitates quite different stimulation approaches. In this article we discuss how denervation can result from damage to peripheral nerves and how and why electrical stimulation is used.

Electrical stimulation is much more in the public eye these days although the approach can be confusing as very different descriptive terms and techniques can be used. For example, we see electrical stimulation applied with so-called “drop foot splints” for use after a stroke or with MS and for exercise despite paralysis as with FES Cycling (FES being the commonly used Functional Electrical Stimulation). It’s also seen in use as a pain-relief modality as in “TENS”.

intercostal nerve damage

In these approaches and the most commonly seen applications, the electrical stimulation method relies on the muscle being innervated - with an intact nerve structure. When the nerve structure is intact relatively small amounts of electricity can cause a functionally useful effect such as a muscle contraction.

When the nerve structure is damaged resulting in denervation, any useful effect must be created by directly stimulating the muscle which is much harder to do. Until relatively recently it has also been controversial - partly due to the technical challenge, fears about tissue damage due to the nature of the stimulation required and doubts about the clinical value.

Damage to peripheral nerves

Peripheral nerve injuries are quite common and result in a loss of function the extent of which depends on the nature of the injury. The injuries were basically classified by Seddon many years ago according to the degree of damage -

• Neuropraxia -is segmental myelin damage with an intact axon, usually caused by compression. There is a temporary focal conduction block that resolves completely within 12 weeks once myelination is restored.

• Axonotmesis - from a crush mechanism is axonal injury where the connective tissue and nerve continuity remain intact. So called Wallerian degeneration ensues and slow axonal regeneration follows at a rate of 1 mm/day. Incomplete recovery is common, depending on the distance for regeneration between the injury and target tissue

• Neurotmesis - is complete physiological and anatomical transection of both axons and connective tissue. A neuroma may form but no spontaneous regeneration occurs without surgical intervention.

Sunderland in 1951 expanded the classification based on histology to include five injury grades, which broadly correspond to Seddon’s three-level classification but with more accurate prognosis of outcomes in axonotmesis injuries.

In practice the grading is perhaps not as useful clinically as one would hope as most nerve injuries are of mixed grades and not diagnostically separate. Damage can occur to the entire nerve, to part of a nerve, to axons within the endoneurial tube or to the adjacent connective tissues.

Muscle weakness or paralysis and changes in sensation occur immediately following trauma. Over time the muscles affected will lose bulk and change structure over time. At the tissue level, complete lesion injuries of all or part of a peripheral nerve result in destruction of the distal myelin section of the nerve. The extent of damage can be investigated by electroneurological testing or by using strength-duration testing.

The rationale for electrical stimulation

Neither voluntary or reflex activity is possible from degenerated muscle. An aim of electrical stimulation with peripheral nerve injuries is to reduce, prevent or even reverse the negative changes in muscle bulk and structure. Research shows that electrical stimulation may also be beneficial for nerve regrowth and repair. Even when functional recovery is not possible the use of electrical stimulation produces positive trophic benefits.

As the electrical stimulation must act directly on the muscle fibres significantly different stimulation parameters must be used compared with the case of innervated muscle.

The “classical” waveforms used for denervated muscle are triangular shaped. This form rises sufficiently slowly to activate muscle fibres directly.

We still use this form on occasion but more often now start by using a bipolar, rectangular waveform of perhaps 100 to 150 ms pulse width and approximately 1 Hz frequency. This pulsewidth is much longer than that used for example with innervated muscle.

This waveform creates a “muscle twitch” and over time appears to increase the excitability of the muscle. We use electrodes that cover as much of the muscle as possible as there is no longer a motor point. Typically once the muscle starts to respond to higher frequencies and a tetanic contraction is produced, we will use both the twitch and tetanic protocols. The RISE Stimulator (which replaced the Den2X) and the Stimulette Edition 5 S2X  units we use are both capable of providing the necessary waveforms and are proved both effective and safe when used as directed.

In working with denervated muscles we follow the pioneering work carried out in Austria and internationally. The work of Kern and others with the RISE study and subsequent international research has shown that electrotherapy for denervation has benefits and can indeed be carried out safely at home. We have many patients who use this technology effectively, despite our initial reservations that few would commit to the regular usage necessary for effective results.

If you have questions about stimulation of denervated muscles, or the devices and protocols we work with please Contact Us and we’ll be happy to chat the through with you.