The H-reflex to magnetic stimulation of lower-limb nerves

Transsynaptic activation of human lumbar spinal motoneurons by transvertebral magnetic stimulation

Noninvasive spinal stimulation has been increasingly used in research on motor control and neurorehabilitation. Despite advances in percutaneous electrical stimulation techniques, magnetic stimulation is not as commonly used as electrical stimulation. Therefore, it is still under discussion what neuronal elements are activated by magnetic stimulation of the human spinal cord. In this study, we demonstrated that transvertebral magnetic stimulation (TVMS) induced transsynaptic activation of spinal motoneuron pools in the lumbar cord. In healthy humans, paired-pulse TVMS was given over an intervertebral space between the L1-L2 vertebrae with an interpulse interval of 100 ms, and the stimulus-evoked electromyographic (EMG) responses were recorded in the lower limb muscles. The results show that the evoked EMG responses after the 2nd pulse were clearly suppressed compared with the widespread responses evoked after the 1st pulse in the muscles of the lower extremity, indicating that the transsynaptic activation of spinal motoneurons by the 2nd pulse was suppressed by the effects produced by the 1st pulse. The inconsistent modulation of response suppression to stimulus intensity across individuals suggests that the TVMS-evoked EMG responses are composed of the compound potentials mediated by the direct activation of motor axons and the transsynaptic activation of motoneuron pools through sensory afferents and that the recruitment order of those fibers by TVMS may be nonhomogeneous across individuals.

Activation of human spinal locomotor circuitry using transvertebral magnetic stimulation

Transvertebral magnetic stimulation (TVMS) of the human lumbar spinal cord can evoke bilateral rhythmic leg movements, as in walking, supposedly through the activation of spinal locomotor neural circuitry. However, an appropriate stimulus intensity that can effectively drive the human spinal locomotor circuitry to evoke walking-like movements has not been determined. To address this issue, TVMS was delivered over an intervertebral space of the lumbar cord (L1–L3) at different stimulus intensities (10–70% of maximum stimulator output) in healthy human adults. In a stimulus intensity-dependent manner, TVMS evoked two major patterns of rhythmic leg movements in which the left-right movement cycles were coordinated with different phase relationships: hopping-like movements, in which both legs moved in the same direction in phase, and walking-like movements, in which both legs moved alternatively in anti-phase; uncategorized movements were also observed which could not be categorized as either movement type. Even at the same stimulation site, the stimulus-evoked rhythmic movements changed from hopping-like movements to walking-like movements as stimulus intensity was increased. Different leg muscle activation patterns were engaged in the induction of the hopping- and walking-like movements. The magnitude of the evoked hopping- and walking-like movements was positively correlated with stimulus intensity. The human spinal neural circuitry required a higher intensity of magnetic stimulation to produce walking-like leg movements than to produce hopping-like movements. These results suggest that TVMS activates distinct neural modules in the human spinal cord to generate hopping- and walking-like movements.

M-wave and H-reflex recruitment curves in boys and men

The aim of the present study was to check whether the M-wave and H-reflex recruitment curves differ between prepubertal boys and men. Eleven boys (9-11 yr) and eleven men (18-35 yr) were magnetically stimulated at the tibial nerve in a prone position. M-wave and H-reflex maximal amplitudes (H_max; M_max ; H_max /M_max ), thresholds, regression slopes (H_slp ; M_slp ; H_slp /M_slp ) were extracted from M-wave and H-reflex recruitment curves and compared between the two age groups. Overall, no significant difference in M-wave and H-reflex recruitment curve parameters was found between the two populations. Nevertheless, the size of the M-wave associated with maximal H-reflex amplitude was lower in boys as compared to men when expressed relative to maximal M-wave amplitude (M_Hmax /M_max : 0.18 ± 0.06 vs. 0.31 ± 0.13; p < .05). This result suggests that the development of peripheral nerve was completed in 9 to 11-year-old boys and did not affect the M-wave and H-reflex recruitment curves parameters. In neuromuscular function studies, it implies that H_max /M_max and H_slp /M_slp could be used indifferently to compare spinal motoneuron excitability between 9-11-year-old boys and men. Conversely, evoking H-reflexes at a given percentage of M_max may bias the comparison between boys and men.

Potential advantages of the H-reflex of the biceps femoris-long head in documenting S1 radiculopathy

PURPOSE

A novel H-reflex method using the biceps femoris-long head (BF-LH) was investigated to collect the normative data for this reflex arc and assess its clinical utility for S1 radiculopathy evaluation. Comparability with conventional tibial and Sol H-reflex findings was also determined.

METHODS

BF-LH H-reflexes were recorded using surface electrodes to S1-root stimulation in 43 unilateral S1 radiculopathy patients (radiculopathy group) and 34 normal subjects (control group) from March 2009 to December 2011. H-M interval and peak-to-peak amplitudes were measured. The BF-LH H-reflex and the H-reflex from the soleus muscle (Sol H-reflex) to both tibial nerve stimulation (tibial H-reflex) and S1-root stimulation were used and compared for application in S1 radiculopathy evaluation.

RESULTS

BF-LH H-reflexes were reliably recorded for all control group subjects. Abnormal BF-LH H-reflexes were recorded for 40 (93.0%) radiculopathy group patients in the involved extremity, and abnormal involved side tibial H-reflexes and Sol H-reflexes were recorded in 31 (72.1%) and 41 (95.3%) radiculopathy group patients, respectively. The BF-LH H-reflex exhibited significantly higher sensitivity for evaluation of S1 radiculopathy, accurate in 40 (93.0%) radiculopathy group patients, than that provided by the conventional tibial H-reflex of only 31 (72.1%) (P < 0.05).

CONCLUSIONS

The BF-LH H-reflex is a highly sensitive and reliable clinical tool for evaluation of the S1 spinal reflex pathway in radiculopathy that is distinct from the Sol H-reflex and conventional tibial H-reflex arcs.

Proximally evoked soleus H-reflex to S1 nerve root stimulation in sensory neuronopathies (ganglionopathies)

INTRODUCTION

Sensory neuronopathy (SNN) mimics distal sensory axonopathy. The conventional H-reflex elicited by tibial nerve stimulation (tibial H-reflex) is usually abnormal in both conditions. We evaluated the proximally evoked soleus H-reflex in response to S1 nerve root stimulation (S1 foramen H-reflex) in SNN.

METHODS

Eleven patients with SNN and 6 with distal sensory axonopathy were studied. Tibial and S1 foramen H-reflexes were performed bilaterally in each patient.

RESULTS

Tibial and S1 foramen H-reflexes were absent bilaterally in all patients with SNN. In the patients with distal sensory axonopathy, tibial H-reflexes were absent in 4 and demonstrated prolonged latencies in 2, but S1 foramen H-reflexes were normal.

CONCLUSIONS

Characteristic absence of the H-reflex after both proximal and distal stimulation reflects primary loss of dorsal root ganglion (DRG) neurons and the distinct non-length-dependent impairment of sensory nerve fibers in SNN.

Diagnostic advantage of S1 foramen-evoked H-reflex for S1 radiculopathy in patients with diabetes mellitus

Hoffmann reflex to tibial nerve stimulation at the popliteal fossa (tibial H-reflex) is routinely used to evaluate S1 radiculopathy. However, it lacks sensitivity because other lesions along this reflex circuit affect the H-reflex bilaterally. This study was undertaken to determine whether the H-reflex evoked by stimulating proximally at the S1 foramen (S1 foramen H-reflex) could improve S1 root lesion evaluation sensitivity in patients with diabetes mellitus. A randomized paired study was designed to evaluate tibial and S1 foramen H-reflexes; bilateral H-M interval (HMI) and H-reflex latency were compared in 22 diabetic patients with unilateral S1 radiculopathy. Other electrophysiological evaluations included standard tibial conduction studies, sural conduction studies and needle electromyography (EMG). The S1 foramen H-reflex had a significantly higher sensitivity (91.0%, 20 of 22) in evaluating S1 radiculopathy than the conventional tibial H-reflex (63.6%, 14 of 22, p < 0.05). Bilateral tibial compound muscle action potential amplitudes were reduced in 3 patients, and sural sensory nerve action potential amplitudes decreased in 7 patients. Needle EMG revealed denervation restricted to the paraspinal muscle and myotomes supplied by the S1 nerve root on the ipsilateral side in 18 patients, and multiple lumbosacral nerve roots were involved bilaterally in the other 4 patients. Our results demonstrate that the S1 foramen H-reflex is a more sensitive assessment of S1 compressive radiculopathy in patients with diabetes mellitus.

Single measurement reliability and reproducibility of volitional and magnetically-evoked indices of neuromuscular performance in adults

This study documents intra-session and inter-day reproducibility (coefficient of variation [V%]) and single measurement reliability (intra-class correlations [R(I)]; standard error of a single measurement [SEM%] [95% confidence limits]) of indices of neuromuscular performance elicited during peripheral nerve magnetic stimulation. Twelve adults (five men and seven women) completed 3 assessment sessions on 3 days, during which multiple assessments of knee flexor volitional and magnetically-evoked indices of electromechanical delay (EMD(V); EMD(E)), rate of force development (RFD(V); RFD(E)), peak force (PF(V); P(T)F(E)), and compound muscle action potential latency (LAT(E)) and amplitude (AMP(E)) were obtained. Results showed that magnetically-evoked indices of neuromuscular performance offered statistically equivalent levels of measurement reproducibility (V%: 4.3-31.2%) and reliability (R(I): 0.98-0.51) compared to volitional indices (V%: 3.7-25.2%; R(I): 0.98-0.64), which support the efficacy of both approaches to assessment and the indices PF(V), EMD(V), EMD(E) and LAT(E) offer the greatest practical utility for assessing neuromuscular performance.

A new approach to estimation of the number of central synapse(s) included in the H-reflex

BACKGROUND

Among the main clinical applications of the H-reflex are the evaluation of the S1 nerve root conductivity such as radiculopathy and measurement of the excitability of the spinal motoneurons in neurological conditions. An attempt has been made to reduce the pathway over which H-reflex can be obtained in a hope to localize a lesion to the S1 nerve root, so the S1 central loop has been suggested. The main goal of this study is the estimation of the H-reflex number of synapse(s) for better understanding of the physiology of this practical reflex.

METHODS

Forty healthy adult volunteers (22 males, 18 females) with the mean age of (37.7 +/- 10.2) years participated in this study. They were positioned comfortably in the prone position, with their feet off the edge of the plinth. Recording electrodes were positioned at the mid point of a line connecting the mid popliteal crease to the proximal flare of the medial malleolus. Stimulation was applied at the tibial nerve in the popliteal fossa and H, F and M waves were recorded. Without any change in the location of the recording electrodes, a monopolar needle was inserted as cathode at a point 1 cm medial to the posterior superior iliac spine, perpendicular to the frontal plane. The anode electrode was placed over the anterior superior iliac spine, and then M and H waves of the central loop were recorded. After processing the data, sacral cord conduction delay was determined by this formula: sacral cord conduction delay = central loop of H-reflex - (delays of the proximal motor and sensory fibers in the central loop).

RESULTS

The central loop of H-reflex was (6.77 +/- 0.28) msec and the sacral cord conduction delay was (1.09 +/- 0.06) msec.

CONCLUSION

The sacral cord conduction time was estimated to be about 1.09 msec in this study and because at least 1 msec is required to transmit the signal across the synapse between the sensory ending and the motor cell, so this estimated time was sufficient for only one central synapse in this reflex.

Magnetically evoked EMGs in rats

Magnetic stimulation of the brain and spinal cord was carried out in rats to record electromyogram (EMGs) from the gastrocnemius. A figure-eight coil was set over the middle of the dorsum, and shifted from the cervical vertebrae to the sacrum. The motor evoked potentials (MEPs) with 4.8 msec latency by transcranial magnetic stimulation and the descending wave with 4.7 msec latency by C3-C4 stimulation were recorded. In evoked EMGs by magnetic stimulation over T9-T10, L4-L5, S2-S3 and Ca2-Ca3 spinal cord levels, the causes of these two evoked components with short (1.5 msec) and long (4.1 msec) latencies were estimated to be the eddy current generated from the rostral to the caudal portion of the spinal cord. With the increase in magnetic stimuli, the relative sizes and disappearance of H- and M-like responses were comparable with the ordinary M- and H-responses in electrically evoked EMGs. The magnetic stimulation of the spinal cord activated the sciatic nerve at their vertebral exit, because the latencies of the H- and M-responses were constant despite the changing stimulus sites. Although magnetic stimulation with the figure-eight coil can be focused on the target, it is necessary to take into consideration the influence of the eddy current flowing in the body.

Proximal and distal motor nerve conduction in obturator and femoral nerves

OBJECTIVE

To study the proximal and distal motor conduction properties of obturator and femoral nerves.

DESIGN

For evaluation of distal motor conduction properties, obturator and femoral nerves were stimulated at the inguinal ligament, and M responses were recorded with needle electrodes from gracilis and rectus femoris muscles. Upper lumbar roots were stimulated with needle electrodes inserted between L1-L2 vertebral laminae.

PARTICIPANTS

Sixteen healthy adults, eight of each gender, age 22 to 52 years (mean 37.5).

MAIN OUTCOME MEASURES

Description of a method for assessing motor conduction along the obturator nerve and evaluating proximal motor conduction measurements obtained with stimulation of obturator and femoral nerves.

RESULTS

Distal motor conduction latencies were 3.9 +/- 0.7 msec for gracilis and 4.6 +/- 0.5msec for rectus femoris after stimulation of obturator and femoral nerves, respectively. Proximal conduction times from lumbar vertebral level were 10.4 +/- 0.3msec for the obturator nerve and 10.8 +/- 0.4msec for the femoral nerve. Conduction velocities of proximal segments of both nerves were similar, 62m/sec for the obturator nerve and 65m/sec for the femoral nerve. Submaximal stimulation of both nerves evoked H-reflex responses from their associated muscles.

CONCLUSIONS

Motor conduction properties of the obturator nerve can be assessed by the method described, particularly to differentiate between peripheral, plexus, or radicular lesions that involve the obturator nerve.

Soleus H-reflex to S1 nerve root stimulation

H-reflexes in normals were elicited by percutaneous electrical and magnetic stimulation of proximal nerve roots at the cauda equina. H-M interval to S1 nerve root stimulation at the level of the S1 foramen was 6.8 +/- 0.33 ms, with side to side difference of 0.16 +/- 0.13 ms. Compression/ischemia of the sciatic nerve in the mid-thigh abolished the H-reflex to stimulation of the tibial nerve at the popliteal fossa when the H-reflex to S1 nerve root stimulation was preserved. The length of the S1 nerve root in human cadavers was measured to be 17.5 +/- 03 cm, providing an estimated dorsal root conduction velocity of 67.3 m/s and a ventral root conduction velocity of 54 m/s. We conclude that the H-M interval to S1 root stimulation can provide reliable measures of conduction within the spinal canal including proximal afferents, anterior horn cells and ventral roots.

Magnetic stimulation of muscle evokes cerebral potentials by direct activation of nerve afferents: a study during muscle paralysis

We tested the hypothesis that magnetic stimulation of muscle evokes cerebral potentials by causing a muscle contraction that then activates muscle receptors. We measured cerebral evoked potentials accompanying magnetic stimulation of muscle in 3 patients during surgery both before and after muscle paralysis with succinylcholine, a depolarizing agent. The magnetic stimulation was at low intensity (30%) and at a 2/s rate. The administration of succinylcholine sufficient to produce muscle paralysis did not alter cerebral potentials evoked by either low-intensity magnetic stimulation of muscle (gastrocnemius/soleus) or electrical stimulation of peripheral nerve (tibial nerve). In 1 normal subject, the S1 nerve root action potentials conducting at rapid velocity (> 60 m/s) were detected at the S1 foramen with a needle electrode using electrical stimulation of the tibial nerve. However, no S1 nerve root potentials could be identified to magnetic stimulation of muscle that evoked a cerebral potential. We conclude that magnetic stimulation of muscle activates terminal afferents in the muscle to provide the afferent drive for the cerebral potentials independent of muscle contraction. The failure to detect the afferent volley in S1 nerve root to magnetic stimulation suggests that only a few afferents are activated or that the activation of afferents is temporally dispersed.

Proximally evoked soleus H reflexes in the evaluation of axonal neuropathy

It is sometimes difficult to differentiate physiologically distal axonopathy from proximal root or nerve disease. Standard soleus H reflexes evoked by distal tibial nerve stimulation at the popliteal fossa are often absent in both syndromes. In this study we distinguish the two by the presence or absence of soleus H reflexes evoked by proximal sciatic nerve stimulation at the gluteal fold. In 12 normal subjects maximum H reflex amplitudes evoked by distal tibial and proximal sciatic stimulation were essentially equal. In 12 patients with suspected distal polyneuropathy, proximal but not distal stimulation evoked H reflexes. By contrast, in 10 patients with lumbosacral root or proximal nerve disease, both proximal and distal stimulation failed to elicit H reflexes. In 2 patients with lower motor neuronopathy, low but comparable amplitude H reflexes were evoked at both sites. We conclude that the presence of a proximally evoked soleus H reflex may provide specific evidence of distal axonopathy when standard soleus H reflexes are absent.

Quadriceps strength and fatigue assessed by magnetic stimulation of the femoral nerve in man

There is no nonvolitional method of assessing quadriceps strength which both supramaximally activates the muscle and is acceptable to subjects. In 10 normal subjects and 10 patients with suspected muscle weakness we used magnetic stimulation of the femoral nerve to elicit an isometric twitch and measured twitch tension (TwQ), surface electromyogram in addition to the maximum voluntary contraction force (MVC). Supramaximality was achieved in all subjects at a mean of 83% of maximum stimulator output. When supramaximal, TwQ was reproducible (mean coefficient of variation 3.6%, range 0.7-10.9) and correlated well with MVC (r2 = 0.83, P<0.001). In 7 normal subjects we measured TwQ before and after a fatiguing protocol; after 20 min TwQ was a mean of 55% (range 29-77%) of baseline and remained substantially reduced at 90 min. Magnetic femoral nerve stimulation is a painless, supramaximal method of assessing quadriceps strength and fatigue which is likely to be of value in clinical and physiological studies.

A new method using neuromagnetic stimulation to measure conduction time within the cauda equina

Using principles derived from electric field measurements and studies of phrenic nerve in vitro, neuromagnetic stimuli in humans were predicted to excite selective low threshold sites in proximal and distal cauda equina. Physical models, in which induced electric fields were recorded in a segment of human lumbosacral spine immersed in a saline filled tank, supported this prediction. Conclusions from the model were tested and confirmed in normal human subjects. Ipsilateral motor evoked potentials were elicited in lower limb muscles and striated sphincters by magnetic coil (MC) stimulation of both proximal and distal cauda equina. Over proximal cauda equina a vertically oriented MC junction and cranially directed induced current elicited a newly identified compound muscle action potential (CMAP). The F response latency and lack of attenuation when the target muscle was vibrated suggest that the proximal response is a directly elicited M response arising near or at the rootlet exit zone of the conus medullaris. Over distal cauda equina, lumbar roots were optimally excited by a horizontally oriented MC junction, and sacral roots by an approximately vertically oriented MC junction, eliciting CMAPs with similar appearance but shorter latency consistent with the known intrathecal lengths of the lower lumbar and sacral nerve roots. The induced current was usually most effective when directed towards the spinal fluid filled thecal sac. Normal subjects showed stable CMAP onset latencies elicited at proximal and distal cauda equina despite wide variation in amplitude. Thus, cauda equina conduction time can be directly calculated. This new method may improve the detection and classification of peripheral neuropathies affecting lower limbs and striated sphincters.

Comparison of magnetic coil stimulation and needle electrical stimulation in the diagnosis of lumbosacral radiculopathy

Electrical stimulation (ES) of lumbosacral nerve roots using a needle electrode inserted to the laminar level at the midline of Th12-L1 or L1-2 intervertebral interspace, was compared with magnetic stimulation using a 9-cm diameter coil (MCS) at the L3-4 or L4-5 spine levels, Compound muscle action potentials (CMAP) were superficially recorded from homologous muscles in both sides in 15 normal control subjects and in 20 patients with lumbosacral radiculopathy. Soleus muscles were used for S1, tibialis anterior (TA) for L5, and rectus femoris (RF) muscles for L4 roots. According to the clinical or radiological diagnosis (CAT, MRI and/or myelography) conventional needle EMG was capable to localise the root lesion in 16 of 20 patients (80%) and ES localised the root involvement in 18 of 20 patients (90%); the diagnostic value of MCS was lower, about 65% (13 of 20 patients). Although ES is uncomfortable and invasive, it is superior to needle EMG in localising unilateral or multiple lumbosacral root involvement. At present, MCS is not suitable for the diagnosis of lumbar radiculopathy.

Magnetic stimulation in the determination of lumbosacral motor radiculopathy

Magnetic stimulation was utilised to diagnose lumbosacral motor radiculopathy non-invasively. Magnetic coil stimulation estimated peripheral motor nerve conduction time (MNCT) which, used in combination with F response, allowed calculation of "motor root conduction time (MRCT)," response being recorded from abductor hallucis. Twenty-five normal controls and 26 patients with lumbar spondylosis were studied. The mean interside difference (left minus right) of MRCT in the control was +0.06 msec (range: -0.88 to +0.74 msec). On clinical and radiological grounds, patients with spondylosis were grouped into those with: (I) no lumbosacral root compression, (II) root compression without motor sign, and (III) root compression with motor deficit. All patients in group III and 36% of cases of group II had MRCT significantly prolonged on the affected side.