Michael P. Willand

Daily Electrical Muscle Stimulation Enhances Functional Recovery Following Nerve Transection and Repair in Rats

Background. Incomplete recovery following surgical reconstruction of damaged peripheral nerves is common. Electrical muscle stimulation (EMS) to improve functional outcomes has not been effective in previous studies. Objective. To evaluate the efficacy of a new, clinically translatable EMS paradigm over a 3-month period following nerve transection and immediate repair. Methods. Rats were divided into 6 groups based on treatment (EMS or no treatment) and duration (1, 2, or 3 months). A tibial nerve transection injury was immediately repaired with 2 epineurial sutures. The right gastrocnemius muscle in all rats was implanted with intramuscular electrodes. In the EMS group, the muscle was electrically stimulated with 600 contractions per day, 5 days a week. Terminal measurements were made after 1, 2, or 3 months. Rats in the 3-month group were assessed weekly using skilled and overground locomotion tests. Neuromuscular junction reinnervation patterns were also examined. Results. Muscles that received daily EMS had significantly greater numbers of reinnervated motor units with smaller average motor unit sizes. The majority of muscle endplates were reinnervated by a single axon arising from a nerve trunk with significantly fewer numbers of terminal sprouts in the EMS group, the numbers being small. Muscle mass and force were unchanged but EMS improved behavioral outcomes. Conclusions. Our results demonstrated that EMS using a moderate stimulation paradigm immediately following nerve transection and repair enhances electrophysiological and behavioral recovery.

ELECTRICAL MUSCLE STIMULATION AFTER IMMEDIATE NERVE REPAIR REDUCES MUSCLE ATROPHY WITHOUT AFFECTING REINNERVATION

Electrical stimulation of denervated muscle has been shown to minimize atrophy and fibrosis and increase force in animal and human models. However, electrical stimulation after nerve repair is controversial due to questions of efficacy.

Electrical muscle stimulation elevates intramuscular BDNF and GDNF mRNA following peripheral nerve injury and repair in rats.

Despite advances in surgery, patients with nerve injuries frequently have functional deficits. We previously demonstrated in a rat model that daily electrical muscle stimulation (EMS) following peripheral nerve injury and repair enhances reinnervation, detectable as early as two weeks post-injury. In this study, we explain the enhanced early reinnervation observed with electrical stimulation. In two groups of rats, the tibial nerve was transected and immediately repaired. Gastrocnemius muscles were implanted with intramuscular electrodes for sham or muscle stimulation. Muscles were stimulated daily, eliciting 600 contractions for one hour/day, repeated five days per week. Sixteen days following nerve injury, muscles were assessed for functional reinnervation by motor unit number estimation methods using electromyographic recording. In a separate cohort of rats, surgical and electrical stimulation procedures were identical but muscles and distal nerve stumps were harvested for molecular analysis. We observed that stimulated muscles had significantly higher motor unit number counts. Intramuscular levels of brain-derived and glial cell line-derived neurotrophic factor (BDNF and GDNF) mRNA were significantly upregulated in muscles that underwent daily electrical stimulation compared to those without stimulation. The corresponding levels of trophic factor mRNA within the distal stump were not different from one another, indicating that the intramuscular electrical stimulus does not modulate Schwann cell-derived trophic factor transcription. Stimulation over a three-month period maintained elevated muscle-derived GDNF but not BDNF mRNA. In conclusion, EMS elevates intramuscular trophic factor mRNA levels which may explain how EMS enhances neural regeneration following nerve injury.

Determining the effects of electrical stimulation on functional recovery of denervated rat gastrocnemius muscle using motor unit number estimation

The use of electrical muscle stimulation to treat denervated muscle prior to delayed reinnervation has been widely debated. There is evidence showing both positive and negative results following different protocols of electrical stimulation. In this study we investigated the role electrical stimulation has on muscle reinnervation following immediate and delayed nerve repair using motor unit estimation techniques. Rat gastrocnemius muscle was denervated and repaired using the peroneal nerve either immediately or following three-months with and without electrical stimulation. Motor unit counts, average motor unit sizes, and maximum compound action potentials were measured three-months following per-oneal nerve repair. Motor unit counts in animals that were denervated and stimulated were significantly higher than those that were denervated and not stimulated. Both average motor unit sizes and maximum compound action potentials showed no significant differences between denervated and denervated-stimulated animals. These results provide evidence that electrical stimulation prior to delayed nerve repair increases muscle receptivity to regenerating axons and may be a worthwhile treatment for peripheral nerve injuries.

Sensory nerve cross-anastomosis and electrical muscle stimulation synergistically enhance functional recovery of chronically denervated muscle.

Background: Long-term muscle denervation leads to severe and irreversible atrophy coupled with loss of force and motor function. These factors contribute to poor functional recovery following delayed reinnervation. The authors’ previous work demonstrated that temporarily suturing a sensory nerve to the distal motor stump (called sensory protection) significantly reduces muscle atrophy and improves function following reinnervation. The authors have also shown that 1 month of electrical stimulation of denervated muscle significantly improves function and reduces atrophy. In this study, the authors tested whether a combination of sensory protection and electrical stimulation would enhance functional recovery more than either treatment alone. Methods: Rat gastrocnemius muscles were denervated by cutting the tibial nerve. The peroneal nerve was then sutured to the distal tibial stump following 3 months of treatment (i.e., electrical stimulation, sensory protection, or both). Three months after peroneal repair, functional and histologic measurements were taken. Results: All treatment groups had significantly higher muscle weight (p < 0.05) and twitch force (p < 0.001) compared with the untreated group (denervated), but fiber type composition did not differ between groups. Importantly, muscle weight and force were significantly greater in the combined treatment group (p < 0.05) compared with stimulation or sensory protection alone. The combined treatment also produced motor unit counts significantly greater than sensory protection alone (p < 0.05). Conclusions: The combination treatment synergistically reduces atrophy and improves reinnervation and functional measures following delayed nerve repair, suggesting that these approaches work through different mechanisms. The authors’ research supports the clinical use of both modalities together following peripheral nerve injury.

Design and testing of an instrumentation system to reduce stimulus pulse amplitude requirements during FES

Functional Electrical Stimulation (FES) has been used for decades to restore muscle function following neural trauma. Another promising use has been to maintain or increase muscle strength following injury. Unfortunately in the latter case there is considerable stimulation pain for the sensorially intact subject during effective levels of stimulation using surface electrodes. Recent research [1][2] has suggested using a constant long (up to 10 ms) low amplitude or ramped conditioning pulse just prior to the high amplitude stimulus pulse (100 – 200 μsec) to reduce the excitably of sensory nerve fibers. However, commercial muscle stimulators cannot be easily modified to provide such complex pulse patterns and flexible pulse train control. We have designed and implemented a novel, very flexible LabVIEW based monophasic constant current muscle stimulator that provides pulse trains with long duration prepulses and high voltage stimulus pulses with selectable shapes, amplitudes, durations and frequencies. As well, the stimulator system includes an isolated EMG amplifier to record the evoked M-waves, which are used to estimate the fraction of muscle motor units being stimulated. We are presently testing this system, by stimulating the median nerve of human subjects and measuring the evoked M-waves for a range of stimulus pulse amplitudes, preceded by either ramped or rectangular low amplitude prepulses. The results indicate that, rather than inhibiting the activation of motor axons, as has been suggested [4], the prepulses at very low amplitudes excite these axons to subthreshold levels. The amplitudes of the stimulus pulses can then be significantly reduced while still achieving high levels of muscle activation.

Electrical muscle stimulation increases early reinnervation following nerve injury and immediate repair

Treating partially or completely denervated muscle following nerve injury using electrical muscle stimulation has been met with much controversy. Previous studies have shown that chronic electrical muscle stimulation or neuromuscular activity leads to impaired reinnervation of muscle end plates. In this study we investigated the use of a moderate stimulation paradigm delivered daily over a 2 week period and the effects on functional recovery and reinnervation. Rat gastrocnemius muscle was denervated by complete tibial nerve transection and immediately repaired using epineurial sutures. Electrical muscle stimulation was carried out 5 days per week in 1 hour sessions. Our results show that numbers of motoneurons reinnervating muscle and reinnervated endplates were significantly higher in animals that received daily muscle stimulation compared to those without stimulation. Other functional measurements such as muscle force, weight, and contractile properties were no different between groups. Our results provide evidence that the improved reinnervation may be due to antidromic depolarization of axons proximal to the repair site.

Serial estimation of motor unit numbers using an implantable system following nerve injury and repair in rats

Motor unit number estimation (MUNE) is an established technique to assess recovery following peripheral nerve injury. In rats, where the vast majority of peripheral nerve research is conducted, assessing motor units at various time points requires a terminal procedure due to the invasive nature of current techniques. Here, we present an implanted system that was used to serially assess MUNE after peripheral nerve injury and repair in rats. This system significantly increases the efficiency of peripheral nerve research by negating the need for terminal procedures, allowing for serial MUNE assessment over time in the same rat. Our system utilizes a commercial implantable stimulator, custom designed cuff electrode, and corresponding custom software with automatic M-wave classification to quickly assess functional reinnervation up to 8 weeks following nerve injury and repair. The concepts presented in this paper are applicable to any implanted device with a transcutaneous radio frequency or inductive link that can be used to trigger nerve stimulation. The methodology is also applicable to researchers without access to implantable devices.Motor unit number estimation (MUNE) is an established technique to assess recovery following peripheral nerve injury. In rats, where the vast majority of peripheral nerve research is conducted, assessing motor units at various time points requires a terminal procedure due to the invasive nature of current techniques. Here, we present an implanted system that was used to serially assess MUNE after peripheral nerve injury and repair in rats. This system significantly increases the efficiency of peripheral nerve research by negating the need for terminal procedures, allowing for serial MUNE assessment over time in the same rat. Our system utilizes a commercial implantable stimulator, custom designed cuff electrode, and corresponding custom software with automatic M-wave classification to quickly assess functional reinnervation up to 8 weeks following nerve injury and repair. The concepts presented in this paper are applicable to any implanted device with a transcutaneous radio frequency or inductive link that can be used to trigger nerve stimulation. The methodology is also applicable to researchers without access to implantable devices.