Electrical Stimulation for Lower Motor Neuron Lesions - Why it matters
Introduction
NOTE: The Den2x has been superceded by a new model called the Stimulator RISE - but the content of this article is still valid.
Recent experimental and clinical work now provides strong evidence that (certain types of) FES can be a powerful tool for the regeneration, functional restoration and maintenance of denervated muscle. This is a fact that for various reasons was either not recognised, was considered controversial, or not technically advisable until recently. We have a growing number of home based clients using a product from Austria - the Stimulette Den2x (Now the RISE stimulator) to combat the consequences of denervation that result from lesions of the lower motor neurons.
The consequences of denervation
Causes of lower motor neuron lesions that result in denervation include
Peripheral nerve lesions (cut, compression injury)
Traumatic brachial plexus injury (for example)
Spinal cord injury (conus and conus caudal lesion)
Such injuries are serious. In the absence of innervation, skeletal muscle undergoes very rapid atrophy, leading finally to a state in which the muscle tissue is largely replaced by fibrous connective tissue and fat, along with a scattering of extremely thinned muscle fibres. We can expect skin to become fragile, wound healing to be impaired and tissue perfusion too be poor alongside other consequences.
If these trophic changes are not addressed even restoring innervation at some point will not produce functional recovery because the tissue mass that remains is no longer “muscle” and cannot contract as normal.
Clinically, if reinnervation is expected, then FES can preserve the structure of the muscle to maximise the prospect for functional recovery. If reinnervation is not expected then the long trophic changes can be minimised
Pioneering work
Up until the late 1980’s, pretty much 100 years of experimentation had not answered the primary question of whether FES could have value in preventing or reducing muscle atrophy in these cases. The second controversial issue has been whether using FES could prevent reinnervation.
One of the pioneers in this field since the 1990’s has been Helmut Kern of Vienna. Kern convinced Engineers in Vienna and then myologists in Italy (Carraro’s team in Padua and Feliciano Protasi in Chieti) to implement two pilot trials supported by the EU Project RISE.
The first was a cross-sectional study and then this was followed by a longitudinal-study to demonstrate that a home-based strategy of FES could recover muscle mass and function of permanently denervated human muscle. RISE was a 10 year multi-centre, multinational study using home-based FES to attempt to restore muscle bulk and tissue quality in complete denervation.
One of the outcomes of this project was the product Stimulette Den2x manufactured in Austria. Since 2010 we have used this product and its successor, The Stimulator RISE with many individuals who use it at home.
The technical demands for excitation of denervated muscle are in fact very different from applications where the lower motor neurons are intact. A new stimulator design and electrodes were needed in order to enable effective FES application with some safety.
Technical issues
Most FES units that physiotherapists see will rely on activation of the muscle indirectly through the nerve structure. Technically, the stimulation parameters are typically based on bilateral, rectangular pulses in the ranges as follow:
The RISE stimulator with electrodes
current - 0 to 150 mA
frequency - 20 to 50 Hz
pulse width - 0 to 500 microseconds
With denervated muscle the approach taken is direct stimulation of the muscle fibres and in order to do this current levels can be up to 300 mA. ( The new Stimulator RISE generates up to 250 mA)
We commonly still use bilateral, rectangular pulses but the parameters are very different from “conventional” FES too. The frequency of the energy is much lower and the pulse widths are much longer for denervated muscle. The parameters are expressed rather differently as follows
Impulse duration (range 40ms to 200ms)
Impulse pause (range 10ms to 400ms)
Burst duration (range 3 to 10 seconds)
Burst pause (1 or 2 seconds)
Impulse frequency (less than 1 Hz up to 20 Hz)
Protocols
We most often use two types of protocols - one that causes the muscle to twitch and one that induces a tetanic contraction. When an isolated pulse of energy is delivered to a muscle it is well known that you can induce it to “twitch”. When that pulse is followed by others relatively frequently the result is a tetanic contraction of the muscle. Our adoption of these protocols is based on published work from Austria and our contact with clinicians in Austria (in particular Michaela Mödlin)
Research shows that twitch contractions help with the reconstruction of the muscle tissue architecture over time. Tetanic contractions may not be possible initially but over time are used to develop strength in the muscle that has recovered its structure.
We commonly use a wet sponge / carbon rubber electrode
The challenge with this approach is that the application of FES is demanding - 5 or 6 days per week and the ideal situation is to commence using FES as soon as possible after injury and certainly within 18 months of injury. The RISE study showed positive effects when commencing FES many years post-injury but it takes much longer to produce effects in these cases. In the long run, clients can reduce their training load to preserve the tissue they have gained or can continue with more strength oriented training with greater effects on bone density.
The electrode designs we use are either carbon rubber/wet sponge or a specially designed “safety electrode”/gel combination. Electrodes commonly used for conventional FES cannot be used as the current density would likely cause burns.
Sometimes with conventional FES it is necessary to choose an electrode size based on isolating an individual muscle. In this application we are aiming to cover as much of the muscle group as possible because this is important for muscle regeneration. This can be challenging with incomplete denervation when for example in the shoulder girdle we have muscles with preserved innervation close to denervated muscle.
The protocol we give to clients is day by day for a year and is very easy to follow. Keys on the stimulator are programmed with the protocol so all clients need to do is push a button and select a current level. We can set a maximum current level so that we minimise any risk of skin irritation or localised burns. As stated above, the twitch protocol is generally the dominant one used in the early days and we would indeed expect to see the muscle twitch in response to the programme. The tetanic contraction protocol will generally not produce a visible and strong contraction for 6 months or more.
Related Reading
Bruce M. Carlson (2007) “The Denervated Muscle” – 45 Years Later
Basic Applied Myology 17 (3&4): 113-117, 2007
Bruce M. Carlson (2014) "The biology of long-term denervated skeletal muscle"
Eur J Trans Myol - Basic Appl Myol 2014; 24 (1): 5-11
Ugo Carraro (2006) "Regeneration of the long term denervated human muscle"
Basic Appl Myol 16 (3&4): 102-104, 2006
Michaela Mödlin et al (2005) "Electrical Stimulation of Denervated Muscles: First Results of a Clinical Study"
Artificial Organs 29(3):203–206,
Helmut Kern et al (2010) "Home-Based Functional Electrical Stimulation Rescues Permanently Denervated Muscles in Paraplegic Patients With Complete Lower Motor Neuron Lesion"
Neurorehabilitation and Neural Repair. 24(8) 709–721. DOI: 10.1177/1545968310366129
Helmut Kern et al (2005) "Muscle biopsies show that FES of denervated muscles reverses human muscle degeneration from permanent spinal motoneuron lesion"
Journal of Rehab R&D, Volume 42, Number 3, Pages 43–54. May/June 2005, Supplement 1
Helmut Kern et al (2005)
Recovery Of Long-Term Denervated Human Muscles Induced By Electrical Stimulation
Muscle Nerve 31: 98–101, 2005