Hubert de Bruin

The effect of stimulus current pulse width on nerve fiber size recruitment patterns

There have been theoretical studies presented that postulate a change in the stimulus current amplitude required to recruit nerve fibers with different stimulus current pulse widths. Based on these theoretical predictions, it has been suggested that the stimulus pulse width parameter may be used to selectively recruit fibers of different sizes and that this selectivity should increase with increasing distance from the stimulus electrode. In this paper, a simulation study of the recruitment patterns of a population of motor nerve fibers with a histologically accurate fiber diameter distribution is presented. Nerve fiber excitation simulations coupled with a time varying field simulation suggest that, for surface stimulation, there is only a marginal selectivity achievable in the average nerve fiber diameter that is recruited across the range of commonly used stimulus pulse widths but this selectivity also increases with increased electrode distance. Experimental evidence consisting of estimates of nerve fiber diameter based on motor unit latency studies is also presented that is consistent with the predictions made by the electromagnetic field and nerve fiber excitation simulations.

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.

Using pre-treatment electroencephalography data to predict response to transcranial magnetic stimulation therapy for major depression

We investigate the use of machine learning methods based on the pre-treatment electroencephalograph (EEG) to predict response to repetitive transcranial magnetic stimulation (rTMS), which is a non-pharmacological form of therapy for treating major depressive disorder (MDD). The learning procedure involves the extraction of a large number of candidate features from EEG data, from which a very small subset of most statistically relevant features is selected for further processing. A statistical prediction model based on mixture of factor analysis (MFA) model is constructed from a training set that classifies the respective subject into responder and non-responder classes. A leave-2-out (L2O) cross-validation procedure is used to evaluate the prediction performance. This pilot study involves 27 subjects who received either left high-frequency (HF) active rTMS therapy or simultaneous left HF and right low-frequency active rTMS therapy. Our results indicate that it is possible to predict rTMS treatment efficacy of either treatment modality with a specificity of 83% and a sensitivity of 78%, for a combined accuracy of 80%.

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.

Lateral pterygoid muscle activity in mandibular retrognathism and response to mandibular advancement surgery.

Electromyographic patterns of muscle activity were recorded in 11 patients with mandibular retrognathism and compared with ten normal subjects. Categorization of facial morphology was based on standard cephalometric data. Seven patients in the retrognathic group have been studied 1 year after mandibular-lengthening surgery. A computer-based data acquisition and analysis system with a Selspot movement monitoring system was used to record and quantify simultaneously both mandibular movement patterns and associated electromyographic data. Of particular interest was the pattern of activity for the lateral pterygoid muscles of all patients in the retrognathic group compared with controls. Both the ipsilateral and contralateral lateral pterygoids contracted during either right or left lateral excursions for eight of the 11 patients in the retrognathic group compared to aphasic activity during this movement as expected in the control group. However, all seven of the patients tested 1 year after mandibular lengthening demonstrated normal aphasic firing patterns of the lateral pterygoid muscles (inferior belly) during right and left lateral excursions. The retrognathic group of patients also demonstrated abnormal recruitment patterns of the lateral pterygoid muscles during border movements of the mandible in the preoperative stage. Recruitment patterns approached normal levels after mandibular advancement surgery. The number of patients studied did not permit accurate statistical analysis. However, a trend is apparent that demonstrates previously unreported abnormal activity patterns of the lateral pterygoid muscles and an adaptive response of these muscles to orthognathic surgery.

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.

The effects of pulse configuration on magnetic stimulation.

Summary A study is presented in which the authors have examined the effects of pulse configuration, stimulation intensity, and coil current direction during magnetic stimulation. Using figure-8 and circular coils, the median nerve was stimulated at the cubital fossa and at the wrist of 10 healthy volunteers, and the response amplitude and site of stimulation were determined. The key findings of this study are in agreement with other researchers’ findings and confirm that biphasic stimulating pulses produce significantly higher M-wave amplitudes than monophasic stimulating pulses for the same stimulus intensity. Mean response amplitudes for biphasic stimuli applied by both coils at the elbow and wrist are consistently higher for the normal current direction. Mean response amplitudes for monophasic pulses are almost always higher for reversed currents. The site for effective stimulation (the position of the virtual cathode) cannot be defined within a fixed distance from the center of the coil (3 to 4 cm), as has been suggested by other researchers, but was found to vary depending on the coil current amplitude and direction as well as the degree of inhomogeneity of the tissues surrounding the nerve. There is a statistically significant relationship between virtual cathode shift and stimulus intensity for biphasic and monophasic pulses. Reversing the coil current direction has no statistically significant effect on the virtual cathode position. Virtual cathode shifts can be measured for biphasic and monophasic stimulations using a figure-8 coil at the wrist and the elbow. However, such a shift is difficult to determine with a circular coil.

TENDON REFLEXES ELICITED USING A COMPUTER CONTROLLED LINEAR MOTOR TENDON HAMMER

We present a novel instrumentation system for studying tendon and spinal reflexes using a commercial linear servo-motor as a precisely controlled tendon hammer. The system uses a LabVIEW-based program to both control electrical or mechanical stimuli and record and measure the resulting M and H waves. The hammer can deliver tendon taps with selected velocities, durations, frequencies and excursions. Preliminary results for both soleus and flexor carpi radialis muscles show that impact velocity is an important variable in eliciting tendon reflexes. As expected, the tendon reflex amplitude was also found to be dependent on excursion depth, but not as significantly as hammer velocity. Other stimulus paradigms are also presently being investigated

Application of wavelet based denoising techniques to rTMS evoked potentials

This paper presents a new method of removing noise from the EEG response signal recorded during repetitive transcranial magnetic stimulation (rTMS). This noise is principally composed of the residual stimulus artifact and mV amplitude compound muscle action potentials recorded from the scalp muscles and precludes analysis of the cortical evoked potentials, especially during the first 15ms post stimulus. The method uses the wavelet transform with a fourth order Daubechies mother wavelet and a novel coefficient reduction algorithm based on cortical amplitude thresholds. The approach has been tested and two methods of coefficient reduction compared using data recorded during a study of cortical sensitivity to rTMS at different scalp locations.