Evaluating denervated muscle with the RISE stimulator

How to we test for denervated muscle? As we will see in this article, a number of conditions can lead to denervation. Sometimes the extent of this denervation is hard to establish with equipment commonly available to the therapist. Perhaps the therapist tried “conventional” neuromuscular electrical stimulation (NMES) and could not produce a muscle contraction even with quite high intensity settings. Hence denervation was suspected, but without really being able to determine it’s extent.

The RISE stimulator provides a handy protocol that allows the presence and extent of denervation to be established and monitored over time. This article describes the process. First lets review some information on what exactly causes denervation, it’s consequences and the benefits of electrotherapy. We can then describe how the RISE unit can give is a denervation “benchmark”.

How does denervation occur?

Denervation of muscle is common and can arise due to a condition or injury that affect the nervous system's ability to communicate with the muscles. Here are some common causes of denervation:

• Direct nerve injury: Nerves can be damaged due to traumatic injury, compression, or stretching. This can disrupt the connection between the nerve and the muscle, leading to denervation. An example would be a brachial plexus injury.

• Spinal cord injury: This is the area that we are most commonly asked to work with. Damage to the spinal cord can interrupt nerve signals from the brain to the muscles. The so-called lower motor neurons are nerve cells located in the spinal cord and brainstem that transmit the natural electrical impulses from the central nervous system to the muscles. These neurons are responsible for controlling muscle movement and function, including muscle contraction, relaxation, and coordination. Lower motor neurons are classified into two types: alpha motor neurons and gamma motor neurons.
  
  Alpha motor neurons are responsible for activating the muscle fibers that produce movement, while gamma motor neurons are responsible for regulating the sensitivity of the muscle spindles, which are sensory receptors within the muscle that provide feedback to the central nervous system about the muscle's position and tension.

  Injuries to the spinal cord that occur at or below the level of the lumbar spinal cord, such as those that occur in the lower back, are more likely to result in damage to the lower motor neurons and denervation of the muscles that they control.

• Neuromuscular diseases: Various neuromuscular diseases, such as ALS (amyotrophic lateral sclerosis), muscular dystrophy, and myasthenia gravis,.

• Vascular disease: Reduced blood supply to a muscle can cause ischemic damage, which can lead to denervation.

• Aging: As we age, if we become inactive, the nerves that supply the muscles can deteriorate, leading to denervation.

• Infections: Certain viral or bacterial infections can affect the nervous system and cause denervation.

As we have described in previous articles, certain forms of electrical stimulation can help with muscle denervation by providing a source of artificial impulses to directly stimulate the muscle fibers. This can help to maintain muscle function, prevent muscle atrophy (shrinkage), enhance circulation and improve muscle strength.

Why denervation matters?

When a muscle becomes denervated the body adapts to this situation and without intervention the muscle and nerve tissue undergo structural changes. These changes have commonly been described as taking place in four distinct stages - the first within days of injury and the final stage after some years, representing the complete loss of the structure of the muscle which is gradually replaced with collagen and fat which is no longer capable of perfoming any contractile function.

When there is a possibility of re-innervation taking place with or without surgical intervention, it is always best to deal with denervation as soon as possible to ensure that the muscle retains its normal structure as far as possible. If reinnervation is not expected then the aim is to prevent long term health being compromised. As demonstrated by the original RISE study and related research, the “right kind” of electrical stimulation can preserve the trophic situation and reduce the risk of secondary complications.

Denervation test

A therapist using something like a RehaStim 2 or other NMES device, might suspect a muscle to be denervated because of the medical condition, or the level of injury to the spinal cord. The typical NMES device will output bipolar, rectangular pulses with a frequency of up to 50 Hz, pulse widths of 500 microseconds and maximum current of 130 mA. It may be that even with high settings no muscle contraction can be seen - but this does not really tell us to much about the severity of the apparent denervation.

To understand more about the status of a muscle we need to understand two descriptive terms and then how to measure them.

The terms are the Rheobase and Chronaxie

The Rheobase current, is the minimum current needed to produce a muscle contraction response when stimulation is applied for an infinitely long duration. It is a measure of the minimum amount of electrical energy needed to activate a muscle fiber.

Chronaxie refers to the minimum amount of time a specific electrical current needs to be applied to a muscle fiber in order to elicit a contraction response. It is the duration of an electrical pulse that is required to stimulate a muscle fiber with twice the intensity of the rheobase current.

This might sound a bit obtuse, but by understanding the chronaxie and rheobase of a particular muscle or group of muscles, healthcare professionals can more accurately and effectively use electrical stimulation.

Putting it into practice

Note that the Rheobase current definition above refers to a pulse of infinitely long duration. Of course we cant actually produce such a pulse but we can generate pulses with sufficiently long duration to be relevant. The RISE unit has a function to generate such long pulses we will refer to as ‘IMP’. This test can be used to evaluate the extent of muscle denervation and periodically determine the progress of training.

The IMP function allows short trains of pulses to be output from the RISE unit in either rectangular or triangular form and with a wide range of pulse widths - from very short pulses of 1ms to very long pulses of 400ms.

The Impulse Duration (ID), Impulse Pause (IP), maximum current and duration of the burst of pulses can be set. If there is any doubt about whether denervation exists, the ID value should be set to 1 ms to test. This should not allow a muscle contraction at any level of current if the muscle is denervated. The absence of an excitable nerve means that the muscle fibres accommodate to the stimulation at these low ID values and do not respond by contracting.

The IMP function allows a qualitative determination of the Rheobase and Chronaxie parameters of the response curve of the muscle. The User manual does not refer to these terms but we mention this here as the terms are often used in text books on electrotherapy.

The process is described here with reference to the image below

If the muscle is denervated you start with the maximum ID of 400 ms which is a very long pulse duration and increase the current intensity until a clear contraction is achieved.

Note the current intensity at which a contraction is achieved. This is the Rheobase Current value. (refer to the diagram - the waveform ‘A’ is representing a stylised denervated muscle’s so-called Strength Duration curve.)

Now set the ID value to 200 ms and again increase the current intensity until a muscle contraction is achieved once more.

You may find that a similar level of current is needed as with the 400 ms situation. Or you may find that a higher level of current is needed. Note this value. Typically at relatively long ID values the characteristic will remain relatively ‘flat’ - in other words as you reduce the ID value from 400, through 300 etc a similar level of current is needed to produce a contraction. Eventually though, you will reach a lower value of ID that no amount of current seems to produce a contraction. You are now on the ‘rising edge’ of the curve.

With denervated muscle, at the start of training, you may find that no response to any level of current is seen at ID values below 150 - 200 ms. As training progresses you may note a response at values down to 20 - 40 ms.

Formally the Chronaxie value is the ID value in ms which is sufficient to generate a contraction at twice the current level of the Rheobase. Imagine mapping out the current intensity versus ID values as shown in the diagram. This defines the characteristic level of denervation of the muscle.

Effectively Chronaxie measures the excitability of the muscle. The longer the Chronaxie value is, the less excitable is the muscle. Hence the second waveform ‘B’ alongside waveform ‘A’ is exhibiting greater levels of denervation. Note also the higher level of Rheobase typically associated with greater levels of denervation.

Conclusion

When denervation is suspected it may not be possible for the therapist to judge the extent of this or the impact of treatment. The Stimulator RISE, is designed as an effective electrotherapy tool to reverse the effects of denervation on skeletal muscle. It also has a useful ability to map out and monitor the extent of denervation and monitor the progress of treatment by allowing the Chronaxie and Rheobase to be easily determined.