Focal stimulation of human peripheral nerve with the magnetic coil: a comparison with electrical stimulation

Measurement of Knee Extensor Torque During Repetitive Peripheral Magnetic Stimulation: Comparison of the Forces Induced by Different Stimulators

OBJECTIVE

To investigate the factors that induce strong contractions during repetitive peripheral magnetic stimulation (rPMS) and compare the muscle torque induced by two stimulators (Stim A and Stim B) with different coil properties.

METHODS

rPMS was applied to the right vastus lateralis of 30 healthy young adults. Stim A contained a 10.1 cm2 rectangular iron core coil, while Stim B contained a 191 cm2 round coil. The knee extensor torque (KET) induced by rPMS at 30 Hz was measured isometrically and divided by the maximum voluntary contraction (MVC) to obtain a relative value of MVC (%MVC). KET at 100% intensity of Stim A (A100%, 1.08 T) was compared to those at 100% or 70% intensity of Stim B (B100%, 1.47 T vs. B70%, 1.07 T). Additionally, we conducted a comprehensive literature search for studies that measured the KET during rPMS.

RESULTS

Both the mean values of %MVC using B100% and B70% were significantly greater than that using A100%. Furthermore, the KET induced by Stim B was found to be larger than that described in previous reports, unless booster units were used to directly stimulate the main trunk of the femoral nerve.

CONCLUSION

Stim B induced a stronger muscle contraction force than Stim A did. This may be because the larger the coil area, the wider the area that can be stimulated. Additionally, a circular coil allows for deeper stimulation.

Factors Involved in Higher Knee Extension Torque Induced by Repetitive Peripheral Magnetic Stimulation

OBJECTIVE

The study aimed to determine the relationship between knee extension torque induced by repetitive peripheral magnetic stimulation and the characteristics of the participants.

DESIGN

This was a basic study with noninvasive intervention. Knee extension torque induced by repetitive peripheral magnetic stimulation (repetitive peripheral magnetic stimulation-induced torque) and maximum voluntary contraction were measured. Stepwise method of multiple regression was performed to determine the factors affecting repetitive peripheral magnetic stimulation-induced torque at 100% intensity and repetitive peripheral magnetic stimulation-induced torque divided by maximum voluntary contraction (percent maximum voluntary contraction). Subcutaneous fat thickness, vastus lateralis muscle thickness measured by ultrasound, maximum voluntary contraction, and mean power frequency of electromyography during maximum voluntary contraction were selected as independent variables.

RESULTS

Repetitive peripheral magnetic stimulation was applied to the right vastus lateralis of 30 young healthy adults (average age, 21.1 ± 0.3 yrs). In the multiple regression analysis, repetitive peripheral magnetic stimulation-induced torque ( P < 0.001) was shown to be independently and significantly associated with maximum voluntary contraction (β = 0.510), subcutaneous fat thickness (β = -0.358), and vastus lateralis muscle thickness (β = 0.208), while percent maximum voluntary contraction value ( P < 0.05) was independently and significantly associated with vastus lateralis muscle thickness (β = 1.059).

CONCLUSIONS

Repetitive peripheral magnetic stimulation-induced torque decreases with thicker subcutaneous fat and increases with stronger maximum voluntary contraction or with thicker muscle.

Noninvasive Brain Stimulation: Multiple Effects on Cognition

Noninvasive brain stimulation (NIBS) techniques are widely used tools for the study and rehabilitation of cognitive functions. Different NIBS approaches aim to enhance or impair different cognitive processes. The methodological focus for achieving this has been on stimulation protocols that are considered either inhibitory or facilitatory. However, despite more than three decades of use, their application is based on incomplete and overly simplistic conceptualizations of mechanisms of action. Such misconception limits the usefulness of these approaches in the basic science and clinical domains. In this review, we challenge this view by arguing that stimulation protocols themselves are neither inhibitory nor facilitatory. Instead, we suggest that all induced effects reflect complex interactions of internal and external factors. Given these considerations, we present a novel model in which we conceptualize NIBS effects as an interaction between brain activity and the characteristics of the external stimulus. This interactive model can explain various phenomena in the brain stimulation literature that have been considered unexpected or paradoxical. We argue that these effects no longer seem paradoxical when considered from the viewpoint of state dependency.

Short-pulsed micro-magnetic stimulation of the vagus nerve

Vagus nerve stimulation (VNS) is commonly used to treat drug-resistant epilepsy and depression. The therapeutic effect of VNS depends on stimulating the afferent vagal fibers. However, the vagus is a mixed nerve containing afferent and efferent fibers, and the stimulation of cardiac efferent fibers during VNS may produce a rare but severe risk of bradyarrhythmia. This side effect is challenging to mitigate since VNS, via electrical stimulation technology used in clinical practice, requires unique electrode design and pulse optimization for selective stimulation of only the afferent fibers. Here we describe a method of VNS using micro-magnetic stimulation (µMS), which may be an alternative technique to induce a focal stimulation, enabling a selective fiber stimulation. Micro-coils were implanted into the cervical vagus nerve in adult male Wistar rats. For comparison, the physiological responses were recorded continuously before, during, and after stimulation with arterial blood pressure (ABP), respiration rate (RR), and heart rate (HR). The electrical VNS caused a decrease in ABP, RR, and HR, whereas µM-VNS only caused a transient reduction in RR. The absence of an HR modulation indicated that µM-VNS might provide an alternative technology to VNS with fewer heart-related side effects, such as bradyarrhythmia. Numerical electromagnetic simulations helped estimate the optimal coil orientation with respect to the nerve to provide information on the electric field’s spatial distribution and strength. Furthermore, a transmission emission microscope provided very high-resolution images of the cervical vagus nerve in rats, which identified two different populations of nerve fibers categorized as large and small myelinated fibers.

Finding the Location of Axonal Activation by a Miniature Magnetic Coil

Magnetic stimulation for neural activation is widely used in clinical and lab research. In comparison to electric stimulation using an implanted electrode, stimulation with a large magnetic coil is associated with poor spatial specificity and incapability to stimulate deep brain structures. Recent developments in micromagnetic stimulation (μMS) technology mitigates some of these shortcomings. The sub-millimeter coils can be covered with soft, biocompatible material, and chronically implanted. They can provide highly specific neural stimulation in the deep neural structure. Although the μMS technology is expected to provide a precise location of neural stimulation, the exact site of neural activation is difficult to determine. Furthermore, factors that could cause the shifting of the activation site during μMS have not been fully investigated. To estimate the location of axon activation in μMS, we first derived an analytical expression of the activating function, which predicts the location of membrane depolarization in an unmyelinated axon. Then, we developed a multi-compartment, Hodgkin-Huxley (H-H) type of NEURON model of an unmyelinated axon to test the impact of several important coil parameters on the location of axonal activation. The location of axonal activation was dependent on both the parameters of the stimulus and the biophysics properties of the targeted axon during μMS. The activating function analysis predicted that the location of membrane depolarization and activation could shift due to the reversal of the coil current and the change in the coil-axon distance. The NEURON modeling confirmed these predictions. Interestingly, the NEURON simulation further revealed that the intensity of stimulation played a significant role in the activation location. Moderate or strong coil currents activated the axon at different locations, mediated by two distinct ion channel mechanisms. This study reports several experimental factors that could cause a potential shift in the location of neural activation during μMS, which is essential for further development of this novel technology.

Coupling Magnetically Induced Electric Fields to Neurons: Longitudinal and Transverse Activation

We present a theory and computational models to couple the electric field induced by magnetic stimulation to neuronal membranes. Based on the characteristics of magnetically induced electric fields and the modified cable equation that we developed previously, quasipotentials are derived as a simple and accurate approximation for coupling of the electric fields to neurons. The conventional and modified cable equations are used to simulate magnetic stimulation of long peripheral nerves by circular and figure-8 coils. Activation thresholds are obtained over a range of lateral and vertical coil positions for two nonlinear membrane models representing unmyelinated and myelinated straight axons and also for undulating myelinated axons. For unmyelinated straight axons, the thresholds obtained with the modified cable equation are significantly lower due to transverse polarization, and the spatial distributions of thresholds as a function of coil position differ significantly from predictions by the activating function. However, the activation thresholds of unmyelinated axons obtained with either cable equation are very high and beyond the output capabilities of conventional magnetic stimulators. For myelinated axons, threshold values are similar for both cable equations and within the range of magnetic stimulators. Whereas the transverse field contributes negligibly to the activation thresholds of myelinated fibers, axonal undulation can significantly increase or decrease thresholds depending on coil position. The analysis provides a rigorous theoretical foundation and implementation methods for the use of the cable equation to model neuronal response to magnetically induced electric fields. Experimentally observed stimulation with the electric fields perpendicular to the nerve trunk cannot be explained by transverse polarization and is likely due to nerve fiber undulation and other geometrical inhomogeneities.

Shielding effects of myelin sheath on axolemma depolarization under transverse electric field stimulation

Axonal stimulation with electric currents is an effective method for controlling neural activity. An electric field parallel to the axon is widely accepted as the predominant component in the activation of an axon. However, recent studies indicate that the transverse component to the axolemma is also effective in depolarizing the axon. To quantitatively investigate the amount of axolemma polarization induced by a transverse electric field, we computed the transmembrane potential (Vm) for a conductive body that represents an unmyelinated axon (or the bare axon between the myelin sheath in a myelinated axon). We also computed the transmembrane potential of the sheath-covered axonal segment in a myelinated axon. We then systematically analyzed the biophysical factors that affect axonal polarization under transverse electric stimulation for both the bare and sheath-covered axons. Geometrical patterns of polarization of both axon types were dependent on field properties (magnitude and field orientation to the axon). Polarization of both axons was also dependent on their axolemma radii and electrical conductivities. The myelin provided a significant “shielding effect” against the transverse electric fields, preventing excessive axolemma depolarization. Demyelination could allow for prominent axolemma depolarization in the transverse electric field, via a significant increase in myelin conductivity. This shifts the voltage drop of the myelin sheath to the axolemma. Pathological changes at a cellular level should be considered when electric fields are used for the treatment of demyelination diseases. The calculated term for membrane polarization (Vm) could be used to modify the current cable equation that describes axon excitation by an external electric field to account for the activating effects of both parallel and transverse fields surrounding the target axon.

Effects of Repetitive Peripheral Magnetic Stimulation Over Vastus Lateralis in Patients After Hip Replacement Surgery

Objective

To investigate the effects of repetitive peripheral magnetic stimulation (rPMS) on the vastus lateralis (VL) in the early stage after hip replacement surgery.

Methods

Twenty-two patients who underwent hip replacement after proximal femur fracture were included in this study. After hip surgery, the experimental group was applied with 15 sessions of 10 Hz rPMS over the VL 5 times per week for 3 weeks, while the control group took sham stimulation. All patients were also given conventional physical therapy. The VL strength was measured with the root mean square (RMS) value of the VL with surface electromyography technique. The ratio of RMS values between fractured and unfractured legs and tandem stand test were used to assess standing balance. Usual gait speed was measured to evaluate gait function. Pain in two groups was assessed with visual analog scale (VAS).

Results

Both RMS value of the VL and the ratio of RMS values after rPMS were significantly improved (p<0.05). Also, tandem standing time and usual gait speed in rPMS group were dramatically increased (p<0.05). However, no significant difference in VAS was found between the two groups after 3 weeks.

Conclusion

rPMS on the VL improved muscle strength, standing balance and gait function in the early stage after hip surgery. Therefore, rPMS could be applied to patients who cannot take electrical stimulation due to pain and an unhealed wound.

Expanded Safety and Efficacy Data for a New Method of Performing Electroconvulsive Therapy: Focal Electrically Administered Seizure Therapy

OBJECTIVE

Electroconvulsive therapy (ECT) is the most rapid and effective antidepressant treatment but with concerns about cognitive adverse effects. A new form of ECT, focal electrically administered seizure therapy (FEAST), was designed to increase the focality of stimulation and better match stimulus parameters with neurophysiology. We recently reported on the safety and feasibility of FEAST in a cohort (n = 17) of depressed patients. We now report on the safety, feasibility, preliminary efficacy, and cognitive effects of FEAST in a new cohort.

METHODS

Open-label FEAST was administered to 20 depressed adults (6 men; 3 with bipolar disorder; age 49.1 ± 10.6 years). Clinical and cognitive assessments were obtained at baseline and end of course. Time to orientation recovery was assessed at each treatment. Nonresponders switched to conventional ECT.

RESULTS

Participants tolerated the treatment well with no dropouts. Five patients (25%) transitioned from FEAST to conventional ECT due to inadequate response. After FEAST (mean, 9.3 ± 3.5 sessions; range, 4-14), there was a 58.1% ± 36.0% improvement in Hamilton Rating Scale for Depression scores compared with that in the baseline (P < 0.0001); 13 (65%) of 20 patients met response criteria, and 11 (55%) of 20 met remission criteria. Patients achieved reorientation (4 of 5 items) in 4.4 ± 3.0 minutes (median, 4.5 minutes), timed from eyes opening. There was no deterioration in neuropsychological measures.

CONCLUSIONS

These findings provide further support for the safety and efficacy of FEAST. The remission and response rates were in the range found using conventional ECT, and the time to reorientation may be quicker. However, without a randomized comparison group, conclusions are tentative.

The spontaneous fluctuation of the excitability of a single node modulates the internodes connectivity: a TMS-EEG study

Brain effective connectivity can be tracked by cerebral recruitments evoked by transcranial magnetic stimulation (TMS), as measured by simultaneous electroencephalography (TMS-EEG). When TMS is targeting the primary motor area, motor evoked potentials (MEPs) can be collected from the "target" muscles. The aim of this study was to measure whether or not effective brain connectivity changes with the excitability level of the corticospinal motor pathway (CSMP) as parameterized by MEP amplitude. After averaging two subgroups of EEG-evoked responses corresponding to high and low MEP amplitudes, we calculated the individual differences between them and submitted the grand average to sLORETA algorithm obtaining localized regions of interest (RoIs). Statistical differences of RoI recruitment strength between low and high CSMP excitation was assessed in single subjects. Preceding the feedback arrival, neural recruitment for stronger CSMP activation were weaker at 6-10 ms of homotopic sensorimotor areas BA3/4/5 of the right nonstimulated hemisphere (trend), weaker at 18-25 ms of left parietal BA2/3/40, and stronger at 26-32 ms of bilateral frontal motor areas BA6/8. The proposed method enables the tracking of brain network connectivity during stimulation of one node by measuring the strength of the connected recruited node activations. Spontaneous increases of the excitation of the node originating the transmission within the hand control network gave rise to dynamic recruitment patterns with opposite behaviors, weaker in homotopic and parietal circuits, stronger in frontal ones. The effective connectivity within bilateral circuits orchestrating hand control appeared dynamically modulated in time even in resting state as probed by TMS.

Reference values and clinical application of magnetic peripheral nerve stimulation in cats

Magnetic stimulation of radial (RN) and sciatic (SN) nerves was performed bilaterally in 40 healthy cats. Reference values for onset latency and peak-to-peak amplitude of magnetic motor evoked potentials (MMEPs) were obtained and compared with values of electric motor evoked potentials (EMEPs) in 10/40 cats. Onset latencies and peak-to-peak amplitudes of the MMEPs of three cats with polyneuropathy (PNP) were compared to the reference values. Magnetic motor evoked responses were easily recorded in all normal cats. Significant differences were found in onset latencies between MMEPs and EMEPs, but peak-to-peak amplitudes were equal. The MMEPs of three cats with PNP can be seen as outliers in comparison to the reference values. MMEPs from the RN and SN were easily obtained and reproducible in normal cats. The technique could represent a useful adjunct in the assessment of peripheral nerve disorders.

Transmembrane potential generated by a magnetically induced transverse electric field in a cylindrical axonal model

During the electrical stimulation of a uniform, long, and straight nerve axon, the electric field oriented parallel to the axon has been widely accepted as the major field component that activates the axon. Recent experimental evidence has shown that the electric field oriented transverse to the axon is also sufficient to activate the axon, by inducing a transmembrane potential within the axon. The transverse field can be generated by a time-varying magnetic field via electromagnetic induction. The aim of this study was to investigate the factors that influence the transmembrane potential induced by a transverse field during magnetic stimulation. Using an unmyelinated axon model, we have provided an analytic expression for the transmembrane potential under spatially uniform, time-varying magnetic stimulation. Polarization of the axon was dependent on the properties of the magnetic field (i.e., orientation to the axon, magnitude, and frequency). Polarization of the axon was also dependent on its own geometrical (i.e., radius of the axon and thickness of the membrane) and electrical properties (i.e., conductivities and dielectric permittivities). Therefore, this article provides evidence that aside from optimal coil design, tissue properties may also play an important role in determining the efficacy of axonal activation under magnetic stimulation. The mathematical basis of this conclusion was discussed. The analytic solution can potentially be used to modify the activation function in current cable equations describing magnetic stimulation.

Focal electrically administered seizure therapy: a novel form of ECT illustrates the roles of current directionality, polarity, and electrode configuration in seizure induction

Electroconvulsive therapy (ECT) is a mainstay in the treatment of severe, medication-resistant depression. The antidepressant efficacy and cognitive side effects of ECT are influenced by the position of the electrodes on the head and by the degree to which the electrical stimulus exceeds the threshold for seizure induction. However, surprisingly little is known about the effects of other key electrical parameters such as current directionality, polarity, and electrode configuration. Understanding these relationships may inform the optimization of therapeutic interventions to improve their risk/benefit ratio. To elucidate these relationships, we evaluated a novel form of ECT (focal electrically administered seizure therapy, FEAST) that combines unidirectional stimulation, control of polarity, and an asymmetrical electrode configuration, and contrasted it with conventional ECT in a nonhuman primate model. Rhesus monkeys had their seizure thresholds determined on separate days with ECT conditions that crossed the factors of current directionality (unidirectional or bidirectional), electrode configuration (standard bilateral or FEAST (small anterior and large posterior electrode)), and polarity (assignment of anode and cathode in unidirectional stimulation). Ictal expression and post-ictal suppression were quantified through scalp EEG. Findings were replicated and extended in a second experiment with the same subjects. Seizures were induced in each of the 75 trials, including 42 FEAST procedures. Seizure thresholds were lower with unidirectional than with bidirectional stimulation (p<0.0001), and lower in FEAST than in bilateral ECS (p=0.0294). Ictal power was greatest in posterior-anode unidirectional FEAST, and post-ictal suppression was strongest in anterior-anode FEAST (p=0.0008 and p=0.0024, respectively). EEG power was higher in the stimulated hemisphere in posterior-anode FEAST (p=0.0246), consistent with the anode being the site of strongest activation. These findings suggest that current directionality, polarity, and electrode configuration influence the efficiency of seizure induction with ECT. Unidirectional stimulation and novel electrode configurations such as FEAST are two approaches to lowering seizure threshold. Furthermore, the impact of FEAST on ictal and post-ictal expression appeared to be polarity dependent. Future studies may examine whether these differences in seizure threshold and expression have clinical significance for patients receiving ECT.

A comparison of functional electrical and magnetic stimulation for propelled cycling of paretic patients

OBJECTIVE

To compare isometric torque and cycling power, smoothness and symmetry using repetitive functional magnetic stimulation (FMS) and functional electrical stimulation (FES) in patients with paretic legs with preserved sensibility and in patients without sensibility.

DESIGN

Repeated-measures design.

SETTING

Laboratory setting.

PARTICIPANTS

Eleven subjects with complete spinal cord injury (SCI) and 29 subjects with chronic hemiparesis (16.6+/-5.5mo poststroke) volunteered.

INTERVENTIONS

Using a tricycle testbed, participants were exposed to isometric measurements and ergometric cycling experiments, performed during both 20Hz FMS and FES stimulation. Subjects with hemiparesis and with complete SCI were stimulated at maximally tolerable level and maximal intensity, respectively.

MAIN OUTCOME MEASURES

Maximal isometric pedaling torque and mean ergometric power, smoothness, and symmetry were recorded for voluntary, FES, and FMS conditions.

RESULTS

Two different patterns of the efficacy of FMS were identified. (1) Patients with complete SCI did not benefit (less torque and power was evoked with FMS than with FES, P<.003 and 10(-4) respectively). (2) Patients with hemiplegia and preserved sensibility could improve their torque output (P<.05), smoothness, and symmetry of pedaling (P<.05) with FMS more than with FES.

CONCLUSIONS

FMS is a potential alternative to surface FES of the large thigh musculature in stimulation-supported cycling of patients with partially or completely preserved sensibility.

An initial transient-state and reliable measures of corticospinal excitability in TMS studies

OBJECTIVE

The objective of this study was to determine if an initial transient state influences the acquisition of reliable estimates of corticospinal excitability in transcranial magnetic stimulation (TMS) studies. Whereas muscle evoked potential (MEP) amplitudes are an important index of cortical excitability, these are severely limited by sweep-to-sweep variability. Interesting in this context is the experimental observation that the first MEP amplitudes might be much larger than subsequent responses [Brasil-Neto JP, Cohen LG, Hallet M. Central fatigue as revealed by postexercise decrement of motor evoked potentials. Muscle Nerve 1994;17:713-9]. This led to the hypothesis that an initial transient-state of increased excitability affects MEP amplitude derived estimates of corticospinal excitability.

METHODS

To address this issue we acquired repeated measures of single pulse MEP amplitudes over the primary motor cortex with and without navigated brain stimulation (NBS) and with various TMS-coils. Importantly, NBS allows for the sweep-to-sweep differentiation of physical and physiological variability.

RESULTS

We found a significant decline in estimates of corticospinal excitability and a transition from log-Normal to Normal distributed state, after which reliable measures (British Standards Institute) could be acquired.

CONCLUSIONS

We argue that an initial transient state of physiological origin influences measures of corticospinal excitability.

SIGNIFICANCE

This has important implications for investigations of cortical excitability. For example, it could reduce variability over studies and within small group comparisons.

Magnetic Stimulation of the Quadriceps Femoris Muscle

OBJECTIVE

The objective of this study was to compare pain induced by magnetic stimulation of the quadriceps femoris (QF) muscle with that induced by transcutaneous neuromuscular electrical stimulation (NMES).

DESIGN

Magnetic stimulation and transcutaneous NMES were applied to QF muscles of 17 normal volunteers. The intensity of each mode of stimulation was increased in a stepwise manner. Peak torque values of isometric contractions of QF muscles and visual analog scale (VAS) scores were recorded at each intensity level. The VAS scores of the two stimulating modalities were compared at the intensity-generating same peak torque values.

RESULTS

The median VAS scores for electrical and magnetic stimulation were 5.7 and 0.3, respectively. The median difference between the VAS scores for electrical and magnetic stimulation was 3.7 (range, 1.7-8.5). The mean of the maximum peak torque obtained from each subject was higher in magnetic stimulation than in electrical stimulation (9.5 +/- 4.8 vs. 4.4 +/- 2.9 Nm).

CONCLUSIONS

Magnetic stimulation of the QF muscle produced less pain at the same level of isometric peak torque than did transcutaneous NMES. Magnetic stimulation is a potential alternative to transcutaneous NMES, especially for persons with intact or residual sensory function.

Diagnosis of thoracic outlet syndrome by magnetic stimulation of the brachial plexus

The abductor pollicis brevis (APB) and abductor digiti minimi (ADM) compound muscle action potential (CMAP) latencies, and median and ulnar motor conduction velocities (MCVs), obtained by magnetic stimulation of the brachial plexus, were evaluated for the diagnosis of thoracic outlet syndrome (TOS). These measurements were compared in three groups of limbs: (1) the symptomatic limbs of patients with TOS (symptomatic group), (2) the asymptomatic con-tralateral limbs of these patients (asymptomatic group), and (3) the limbs of healthy control subjects (control group). Although no significant differences were observed in MCVs among the three groups, the APB CMAP latency in the sym-ptomatic group (12.0 +/- 1.2 ms) was significantly prolonged compared with that in the control group (10.4 +/- 0.64 ms; P < 0.01), and the ADM CMAP latency in the symptomatic group (11.0 +/- 0.82 ms) was also significantly prolonged compared with that in the control group (10.1 +/- 0.59 ms; P < 0.01). The possibility is suggested that the evaluation of APB and ADM CMAP latencies by magnetic stimulation of the brachial plexus may be helpful for the diagnosis of TOS.

Functional magnetic stimulation of the colon in persons with spinal cord injury

OBJECTIVE

To evaluate the usefulness of functional magnetic stimulation (FMS) as a noninvasive method to stimulate the colon in individuals with spinal cord injury (SCI).

DESIGN

A prospective before-after trial consisting of 2 protocols.

SETTING

FMS laboratories of 2 SCI centers.

PARTICIPANTS

Two able-bodied men and 13 men with SCI levels ranging from C3 to L1. Protocol 1 consisted of 9 subjects, 2 of whom were excluded from the analysis. Protocol 2 consisted of 4 subjects.

INTERVENTION

Commercially available magnetic stimulators with round magnetic coils (MCs) were used. Protocol 1 measured the effects of FMS on rectal pressure by placing the MC on the transabdominal and lumbosacral regions. Protocol 2 consisted of a 5-week stimulation period to investigate the effects of FMS on total and segmental colonic transit times (CTTs).

MAIN OUTCOME MEASURE

An increase in rectal pressure and a decrease in CTT by magnetic stimulation.

RESULTS

Data were averaged and the standard error of the mean was calculated. Statistically significant changes in rectal pressure and CTT were also measured. Rectal pressures increased from 26.7 +/- 7.44cmH(2)O to 48.0 +/- 9.91cmH(2)O, p =.0037, with lumbosacral stimulation, and from 30.0 +/- 6.35cmH(2)O to 42.7 +/- 7.95cmH(2)O, p =.0015, with transabdominal stimulation. With FMS, the mean CTT decreased from 105.2 to 89.4 hours, p =.02.

CONCLUSION

FMS is able to stimulate the colon and reduce CTT. FMS is a noninvasive, technological advancement for managing neurogenic bowel in patients with SCI.

Calculation of electric fields in a multiple cylindrical volume conductor induced by magnetic coils

A method is presented for calculating the electric field, that is induced in a cylindrical volume conductor by an alternating electrical current through a magnetic coil of arbitrary shape and position. The volume conductor is modeled as a set of concentric, infinitely long, homogeneous cylinders embedded in an outer space that extends to infinity. An analytic expression of the primary electric field induced by the magnetic coil, assuming quasi-static conditions, is combined with the analytic solution of the induced electric scalar potential due to the inhomogeneities of the volume conductor at the cylindrical interfaces. The latter is obtained by the method of separation of variables based on expansion with modified Bessel functions. Numerical results are presented for the case of two cylinders representing a nerve bundle with perineurium. An active cable model of a myelinated nerve fiber is included, and the effect of the nerve fiber's undulation is shown.

Functional magnetic stimulation facilitates gastrointestinal transit of liquids in rats

The purpose of this study was to investigate the effect of a relatively novel technology, functional magnetic stimulation (FMS), on gastrointestinal transit of liquids in rats. Orogastric gavage with technetium-99 solution was used to assess gastric emptying and gastrointestinal transit time in 92 rats. FMS was performed over the anterior cervical and/or dorsal thoracolumbar regions using a figure-8 coil. Stimulation protocols were 1, 2, or 4 h in length. FMS accelerated gastric emptying and decreased gastrointestinal transit time. The acceleration was dependent on the stimulation parameters used as well as on the duration of the protocol; high levels of FMS produced a quicker effect, whereas lower levels were effective at later times. This study provides evidence that FMS could be an alternative or adjunct therapy to treat disorders in gastrointestinal motility.

Magnetic and electrical stimulation of undulating nerve fibres: a simulation study

Mathematical models of myelinated nerve fibres are highly stylized abstractions of real nerve fibres. For example, nerve fibres are usually assumed to be perfectly straight. Such idealizations can cause discrepancies between theoretical predictions and experimental results. One well-known discrepancy is that the currently used models predict (contradictory to experimental findings) that an activation of nerve fibres is not possible with a pure transverse electric field. This situation occurs when a magnetic coil is placed symmetrically above a straight nerve fibre for magnetic nerve stimulation, or when an anode and a cathode are placed equidistantly on a line perpendicular to the fibre in the case of electrical stimulation. It is shown that this discrepancy does not occur if the physiological undulation of peripheral nerve fibres is included in the models. Even for small undulation amplitudes (e.g. 0.02 mm), it is possible to activate the fibre in these positions. For physiological undulations, as found in the literature, and favourable (off-centre) positions, the typical reduction of the thresholds is in a range between one and five, compared with perfectly straight fibres.

The effect of magnetic coil orientation on the excitation of the median nerve

OBJECTIVE

We investigated the effect of magnetic coil orientation on the excitation of the median nerve in healthy subjects.

METHODS

An 8-shaped coil, 3.2 cm in outer diameter, was used. The median nerve was stimulated at the elbow while the compound muscle action potentials (CMAPs) of abductor pollicis brevis muscle were recorded at 4 different directions of the induced current: orthodromic, antidromic, medio-lateral and latero-medial.

RESULTS

We found that the amplitude of the CMAP was the greatest in a medio-lateral (ML) direction. We also measured the induced electric field in the saline tank that mimicked the forearm. The induced electric field and its spatial gradient were the greatest in the ML direction among 4 directions.

CONCLUSION

The fact that the forearm is a restrictive volume conductor may result in the different effects of coil orientation on the excitation of the median nerve at the elbow.

Importance of soft tissue inhomogeneity in magnetic peripheral nerve stimulation

In magnetic peripheral nerve stimulation with a figure-of-eight coil, a 'tangential-edge' coil orientation (the nerve is beneath the coil intersection and perpendicular to the coil wings) is ideal theoretically. However, some experimental results show that strong muscle responses are elicited with a 'symmetrical-tangential' coil orientation (the nerve is beneath the coil intersection and parallel to the coil wings), which is inconsistent with the cable theory. We hypothesized that the 10:1 conductivity difference between muscle and fat would cause inconsistent results during magnetic median nerve stimulation in the elbow, which was verified using an inhomogeneous volume conductor model. The induced electric fields were measured in a model composed of saline solutions of different concentrations divided by a cellophane sheet. A nerve was imagined along the boundary between the two solutions, and the coil was held in a 'symmetrical-tangential' position. Virtual cathodes, which were off the nerve in the homogeneous model, were on the nerve in the inhomogeneous model. The previous inconsistent results were explained by considering soft tissue inhomogeneity without any modification of the assumption in the cable theory that only the induced electric field component parallel to the nerve is responsible for nerve excitation.

A volume-conduction analysis of magnetic stimulation of peripheral nerves

Magnetic stimulation is a method to study several nervous disorders as well as the intact nervous system in humans. Interest in magnetic stimulation of peripheral nerves has grown rapidly, but difficulties in locating the site of excitation have prevented it from becoming a routine clinical tool. It has been reasoned that the activating function of long and straight nerves is the first spatial derivative of the electric field component parallel to the nerves. Therefore, to predict the site of activation, one has to compute this field feature. We describe here an analytical mathematical model and investigate the influence of volume-conductor shape on the induced field. Predictions of the site of activation are given for typical stimulation coil arrangements and these results are compared with experimental and literature data. Comparisons suggest that the activating function is not simply the spatial gradient of the induced electric field, but that other mechanisms are also involved. The model can be easily utilized in the search for more efficient coil constructions and improved placements with respect to the target nerves.

An analytical model to predict the electric field and excitation zones due to magnetic stimulation of peripheral nerves

The main unknown factor in understanding magnetic stimulation of peripheral nerves is the distribution of the induced electric field. We have applied the so-called reciprocity theorem and developed an analytical model to compute the electric field and its spatial derivatives inside pseudocylindrical structures. The results can be used to predict the site of excitation in magnetic stimulation of peripheral nerves.

The polarity of the induced electric field influences magnetic coil inhibition of human visual cortex: implications for the site of excitation

Human perception of 3 briefly flashed letters in a horizontal array that subtends a visual angle of 3 degrees or less is reduced by a magnetic coil (MC) pulse given, e.g., 90 msec later. Either a round or a double square MC is effective when the lower windings or central junction region, respectively, are tangential to the skull overlying calcarine cortex and symmetrical across the midline. The modeled, induced electric field has peak amplitude at the midline, but the peak spatial derivatives lie many centimeters laterally. Thus, the foveal representation near the midline is closer to the peak electric field than to its peak spatial derivatives, i.e., excitation of calcarine cortex differs from excitation of a straight nerve. With an MC pulse that induces an electric field which is substantially monophasic in amplitude, the lateral-most letter (usually the right-hand letter) in the trigram is preferentially suppressed when the electric field in the contralateral occipital lobe is directed towards the midline. Inferences from using peripheral nerve models imply that medially located bends in geniculo-calcarine or corticofugal fibers are the relevant sites of excitation in visual suppression; end excitation of fiber arborizations or apical dendrites is considered less likely. This conclusion is supported by the fact that the induced electric field polarity in paracentral lobule for optimally eliciting foot movements is opposite to that for visual suppression, the major bends occurring at different portions of the fiber trajectories in the two systems.

Limitations of electromyography and magnetic stimulation for assessing laryngeal muscle control

The development of new phonosurgical techniques has increased the level of interest in the field of neurolaryngology. This field requires valid techniques for determining if muscle activation is normal. Laryngeal electromyography is being used more frequently to assess muscle innervation and synkinesis. Further, magnetic stimulation has been introduced as a noninvasive technique for nerve stimulation. Technical limitations that affect the clinical utility of both these techniques are reviewed: 1) difficulties obtaining selective and accurate electromyographic laryngeal muscle recordings, 2) normal variation in movement and muscle activation patterns within and between normal individuals when producing the same speech syllables, and 3) variation in laryngeal muscle response latencies between and within normal subjects during peripheral magnetic stimulation. Given the normal variation in laryngeal electromyography and magnetic stimulation response latencies, these techniques may not yet be reliable or accurate for assessing reinnervation or synkinesis following recurrent laryngeal nerve injury.

Magnetic mapping of human cervical nerve roots: variation in normal subjects

Cervical magnetic stimulation occurs preferentially at the neural foramen and is more effective with the induced electric field oriented transversely to the spine rather than vertically. This geometry suggests the possibility of mapping innervation from specific nerve roots. A "butterfly" stimulus coil, positioned horizontally, was used to determine the morphologies and amplitudes of compound motor action potentials from multiple muscles in the upper extremities of 15 normal adults. Averages of 5 responses were taken at 1 cm intervals along the cervical-thoracic spine. Needle electrodes were used to verify atypical patterns of innervation. Cervical spine X-rays allowed anatomic correlation. Rarely abductor digiti minimi was difficult to stimulate in the horizontal orientation; satisfactory responses were then obtained with the stimulator angled upward towards the side of stimulation. Averaged compound motor action potentials were reproducible, and as the stimulus coil was moved rostro-caudally they often changed morphology in well-defined patterns, suggesting an origin in different cervical nerve roots. The sequence of muscle activation in most subjects was consistent with conventional descriptions of segmental innervation, but 5 appeared to show substantial variation in the nerve roots supplying one or more muscles. Magnetic mapping may be a useful method for studying segmental innervation in human subjects.

Magnetic coil stimulation of straight and bent amphibian and mammalian peripheral nerve in vitro: locus of excitation

1. According to classical cable theory, a magnetic coil (MC) should excite a linear nerve fibre in a homogeneous medium at the negative-going first spatial derivative of the induced electric field. This prediction was tested by MC stimulation of mammalian phrenic and amphibian sciatic nerve and branches in vitro, immersed in Ringer solution within a trough, and identifying the sites of excitation by recording responses of similar latency to local electrical stimulation. Subsequently, the identified sites of excitation were compared with measurements of the induced electric field and its calculated first spatial derivative. A special hardware device was used to selectively reverse MC current direction and to generate predominantly monophasic- or polyphasic-induced pulse profiles whose initial phases were identical in polarity, shape and amplitude. When using the amphibian nerve preparation, a complication was excitation at low threshold points related to cut branches. 2. Reversal of monophasic current resulted in latency shifts corresponding approximately to the distance between induced cathode and anode. The location of each site of excitation was at, or very near, the negative-going first spatial derivative peaks of the induced electric field measured parallel to the straight nerve. Significantly, excitation of the nerve did not occur at the peak of the induced electric field above the centre of the 'figure of eight' MC junction. 3. A polyphasic pulse excited the nerve at both sites, by the negative-going first phase at one location, and approximately 150 microseconds later, by the reversed negative-going second phase at the other location. Polyphasic and monophasic pulses elicited responses with similar latency when the induced current flowed towards the recording electrode. 4. Straddling a nerve with non-coding solid lucite cylinders created a localized spatial narrowing and increase in the induced electric field, resulting in a lowered threshold of excitation. The corresponding closer spacing between first spatial derivative peaks was exhibited by a significant reduction in latency shift when MC current direction was reversed. 5. When a nerve is bent and the induced current is directed along the nerve towards the bend, the threshold of excitation is reduced there. Increasing the angle of the bend from 0 deg to more than 90 deg graded the decrease in threshold. 6. In a straight nerve the threshold was lowest when current was directed towards the cut end.(ABSTRACT TRUNCATED AT 400 WORDS)

Modelling magnetic coil excitation of human cerebral cortex with a peripheral nerve immersed in a brain-shaped volume conductor: the significance of fiber bending in excitation

To help elucidate some basic principles of magnetic coil (MC) excitation of cerebral cortex, a model system was devised in which mammalian phrenic nerve, or amphibian sciatic nerve with its branches was suspended in appropriate Ringer's solution in a human brain-shaped volume conductor, an inverted plastic skull. The nerve was recorded monophasically out of the volume conductor. The site of nerve excitation by the MC was identified by finding where along the nerve a bipolar electrical stimulus yielded a similar action potential latency. MC excitation of hand-related corticospinal (CT) neurons was modelled by giving the distal end of nerve attached to the lateral skull an initial radial (perpendicular) trajectory, with subsequent bends towards the base and posterior part of the skull; this nerve was optimally excited by a laterally placed figure 8 or round MC when the induced electric field led to outward membrane current at the initial bend. By contrast, nerve given a trajectory modelling CT neurons related to the foot was optimally excited when the coil windings were across the midline, but again when membrane current flowed outward at the first bend. Corticocortical fibers were modelled by placing the nerve in the anteroposterior axis lateral to the midline; with the round MC vertex-tangentially orientated, optimal excitation occurred at the bend nearest the interaural line, i.e., near the peak electric field. The findings emphasize the importance of orientation and direction of current in the MC and fiber bends in determining nerve excitation. The findings in the peripheral nerve-skull model help explain (1) why lateral and vertex-tangentially orientated MCs preferentially excite arm-related CT neurons directly and indirectly (through corticocortical fibers), respectively, and (2) why the MC orientations for optimally exciting directly arm and leg-related CT neurons differ.

Nerve, spinal cord and brain somatosensory evoked responses: a comparative study during electrical and magnetic peripheral nerve stimulation

Somatosensory evoked potentials (SEPs) and compound nerve action potentials (cNAPs) have been recorded in 15 subjects during electrical and magnetic nerve stimulation. Peripheral records were gathered at Erb's point and on nerve trunks at the elbow during median and ulnar nerve stimulation at the wrist. Erb responses to electrical stimulation were larger in amplitude and shorter in duration than the magnetic ones when 'electrical' and 'magnetic' compound muscle action potentials (cMAPs) of comparable amplitudes were elicited. SEPs were recorded respectively at Cv7 and on the somatosensory scalp areas contra- and ipsilateral to the stimulated side. SEPs showed a statistically significant difference in amplitude only for the brachial plexus response and for the 'cortical' N20-P25 complex; differences were not found between the magnetic and electrical central conduction times (CCTs) or for the peripheral nerve response latencies. Magnetic stimulation preferentially excited the motor and proprioceptive fibres when the nerve trunks were stimulated at motor threshold intensities.

Measurement of the electric field induced into inhomogeneous volume conductors by magnetic coils: application to human spinal neurogeometry

We measured the electric fields induced by round and figure "8" magnetic coils (MCs) in homogeneous and inhomogeneous volume conductors. In homogeneous media, the round MC held tangential (i.e., flat) to the volume conductor induced an annular electric field. When the round MC was held on-edge (i.e., orthogonal) to the volume conductor, the induced electric field consisted of two loops mainly parallel to the surface of the volume conductor and which approximated each other directly under the contacting edge of the MC. The tangentially oriented figure "8" MC similarly induced two electric field loops which approximated one another maximally under the region of the junction in its long axis. In a complex inhomogeneous volume conductor, such as a segment of human cervical-thoracic vertebral spine located eccentrically within a large cylindrical tank and submerged in isotonic saline, the direction of electric fields within the spinal canal and across the intervertebral neuroforamina was similar to that observed in the homogeneous volume conductor. However, in and near a single neuroforamen, the electric field and especially its first spatial derivative were markedly elevated compared to that recorded within the long central axis of the vertebral canal. Motor unit and compound muscle action potentials elicited in limb muscles by MC stimulation of human cervical spine confirmed predictions derived from the physical model. The predictions included: (1) absence of spinal cord stimulation compared to relative ease of nerve root stimulation by current that is most likely concentrated at the neuroforamina. When stimulating current is directed towards the periphery, the most likely low threshold site of stimulation is inferred to be just distal to the neuroforamina. It is emphasized that with supramaximal stimulation, more distal sites of excitation may occur; (2) invariant latency shifts at threshold intensities when moving the MC along the rostrocaudal axis of the cervical vertebral column; (3) significant effect (on motor unit activation thresholds) of the direction of induced current flow across the neuroforamina; (4) reduced stimulation when the targeted nerve roots are close to the null point of the electric field, i.e., between locations of high electric field intensity, of opposite polarity; and (5) relatively focal nerve root stimulation by the junction of a transversely orientated figure "8" MC, i.e., parallel to the nerve roots.

A comparison of magnetic and electrical stimulation of peripheral nerves

We compared magnetic stimulation using different coil designs (2 rounded coils and a butterfly-prototype coil) with electrical stimulation of the median and ulnar nerves in 5 normal subjects. Using magnetic stimulation we were able to record technically satisfactory maximal sensory and motor responses only with the butterfly coil. Submaximal electrical stimuli preferentially activated sensory rather than motor axons, but submaximal magnetic stimuli did not. The onset latency, amplitude, area and duration of responses elicited electrically or magnetically with the butterfly coil during routine sensory and motor nerve conduction studies were similar, and motor and sensory conduction velocities were comparable when studied over long segments of nerve. However, the motor conduction velocities with magnetic and electrical stimulation differed by as much as 18 m/sec in the across-elbow segment of ulnar nerve. Thus, recent developments in magnetic stimulator design have improved the focality of the stimulus, but the present butterfly coil design cannot replace electrical stimulation for the detection of focal changes in nerve conduction velocity at common entrapment sites, such as in the across-elbow segment of the ulnar nerve.

A comparison of corticospinal activation by magnetic coil and electrical stimulation of monkey motor cortex

The effects of different orientations of a Cadwell round magnetic coil (MC) were compared with each other and with surface electrical stimulation of motor cortex in monkeys anesthetized with pentobarbital or urethane. Recordings were made from within the lateral corticospinal tract, either from axonal populations or with a microelectrode from individual axons. A lateral-sagittally orientated MC directly excited corticospinal neurons at lower stimulus intensity than was required for indirect, i.e., transsynaptic excitation via inputs to corticospinal neurons. By contrast, in 2 out of 3 macaques tested, a vertex-tangential orientation could excite corticospinal neurons indirectly at lower intensities than were required for direct excitation; at higher intensities, direct excitation also occurred. The site of direct corticospinal excitation by a lateral-sagittally orientated MC was inferred by comparing the response variability and latency to MC and surface electrical stimuli. Cathodal stimuli elicited more variable corticospinal population responses and later individual axonal responses than were obtained with anodal stimuli. The variability in response is attributed to interaction between nearby, on-going synaptic bombardment and the stimulus, implying that surface cathodal stimuli directly activate corticospinal neurons at the spike trigger zone (presumably the initial segment). By contrast, the consistency and reduced latency of the corticospinal responses to surface anodal stimuli are attributed to the direct excitation of corticospinal fibers within the white matter. When the stimulus intensity is clearly above threshold, surface anodal and cathodal stimuli can activate corticospinal neurons both directly and indirectly. Direct corticospinal excitation by the MC can resemble the effects of either surface anodal or surface cathodal stimuli. We conclude that the MC can activate corticospinal neurons at the spike trigger zone or their fibers deeper in white matter. The findings in the monkey are used to interpret the effects of different MC orientations in the human.

Spatial distribution of the electric field induced in volume by round and figure '8' magnetic coils: relevance to activation of sensory nerve fibers

The electric fields induced in finite homogeneous volume conductors by a round and a figure '8' magnetic coil (MC) were measured and related to MC stimulation of the median nerve. The volume conductors, filled with isotonic saline, consisted of a large rectangular trough ('unrestricted') and a smaller trough, whose dimensions approximated human forearm ('restricted'). Various MC orientations were applied to the volume conductor. Bipolar recordings were obtained with a coaxial electrode, which measured the voltage gradient between the exposed edge of the cable shield and the central wire at its tip, 1 cm distant (a linear probe). The probe was moved in 3 dimensions, allowing computer reconstruction of the electric field as a function of the 3 spatial axes. When the probe was parallel to the plane of the round MC and tangential to the direction of current in its windings, the induced electric field was maximal; it tended towards zero when the probe was over the center of the MC, or when the probe, remaining parallel to the plane of the MC, was radial (i.e., perpendicular) to the direction of the current in the windings. For a variety of MC orientations, the electric field was consistently increased when the probe was adjacent and parallel to the edge of the trough, indicating the important effect of boundaries. The electric field was greatly increased focally when the round MC was applied orthogonally to the volume conductor, or when the figure '8' MC was applied tangentially (i.e., flat) to the volume conductor. With the figure '8' MC, a sharp central peak parallel to the long axis was bounded on each side by smaller (less than half amplitude) peaks. The findings from physical modeling led to correct predictions as to the most effective orientations of round and figure '8' MCs for eliciting sensory nerve action potentials (SNAPs) from the median nerve.