Cervical magnetic stimulation

We stimulated the cervical region with a 9-cm-diameter magnetic coil on centered on the spinous processes in 21 normal subjects. We obtained maximal amplitudes with clockwise coil current in right-sided upper extremity muscles and counterclockwise coil current in left-sided upper extremity muscles. Optimal stimulation sites for biceps, triceps, and abductor digiti minimi were C-3 or C-4, C-4 or C-5, and C-4, C-5, or C-6, respectively. The latencies of the muscle responses varied little in the same subject in spite of marked amplitude changes due to suboptimal position of the coil or submaximal stimulator output. In abductor digiti minimi, the amplitude of the muscle response on cervical magnetic stimulation was 9 to 100% of the supramaximal amplitude on wrist electrical stimulation. We established normal values for latency, amplitude, and interside differences for the above 3 upper extremity muscles. The findings were reproducible, and the latencies obtained with large coils from different manufacturers in the same subjects were comparable. We found no advantage in bipolar recording over tendon-belly montage. Comparison of magnetic and electrical needle root stimulation in the same subjects showed that the magnetic stimulus was more proximal in biceps and triceps, and that the site of excitation was approximately the same in abductor digiti minimi. Indirect assessment of the longitudinal site of excitation based on F-wave minimal latency indicated that excitation occurred within millimeters of the emergence of axon of the peripheral motor neuron.     

Thoracic spinal nerve and root conduction: A magnetic stimulation study

We describe a technique of magnetic coil (MC) stimulation of the thoracic spinal nerves and roots in 12 normal subjects and a patient with diabetes mellitus. We kept the MC flat against the vertebral column in the midline over T-7, T-8, and T-9 spinous processes and obtained compound muscle action potentials from the upper rectus abdominis, external oblique, and intercostal muscles. We obtained mean latencies to these muscles after stimulation in the posterior axillary line. We noted that the onset latencies remained fixed despite increasing the intensity of stimulation from 30% to 100% and on moving the coil up to 3 cm lateral to the spinous processes suggesting that the stimulation of the fastest conducting fibers was occurring at a fixed site, most likely at the intervertebral foramina. Prolonged latencies in the diabetic patient confirmed the diagnosis of radiculoneuropathy. © 1995 John Wiley & Sons, Inc.     

Latency of motor evoked potentials to focal transcranial stimulation varies as a function of scalp positions stimulated.

We recorded motor evoked potentials (MEP) to transcranial magnetic stimulation from abductor pollicis brevis (APB), flexor carpi radialis (FCR), biceps brachii and deltoid muscles at rest and during slight voluntary activation. An 8-shaped coil connected to a Cadwell MES-10 magnetic stimulator was positioned over different scalp positions 1 cm apart. At least 24 stimuli were delivered at each location. Latencies of MEPs were compared with those obtained by electrical and magnetic stimulation during muscle activation. Progressively longer MEP latencies were obtained in 5 groups depending on the type and position of stimulation. The shortest latencies were obtained with (1) electrical stimulation during muscle contraction and (2) non-focal magnetic stimulation during muscle contraction; magnetic stimulation at rest produced longer latencies with stimulation of (3) an optimal position, (4) a suboptimal position, and (5) a non-optimal position. Mean latency differences between successive groups were 1.9, 2.0, 1.6, and 2.6 msec for APB. Similar latency differences were found for the other arm muscles. The results are compatible with the hypothesis that the different latencies evoked by stimulation at different scalp locations are determined by the summation at spinal motoneurons of excitatory postsynaptic potentials generated by successive numbers of I waves.     

A study of transcranial magnetic stimulation in older (>3 years) patients of malnutrition.

Transcranial magnetic stimulation was performed in 40 subjects. Twenty patients in the age group of 3 to 8 years and having different grades of malnutrition were included in the 'study group' whereas 20 normal children having no complaints comprised the 'control group'. The coil of the magnetic stimulator was applied tangentially over the vertex to stimulate the cortex. The motor evoked potential (MEP) was obtained using root stimulation by applying the coil at the cervical and lumbosacral spines. Recordings were made from the abductor pollicis brevis (APB) and extensor digitorum brevis (EDB) muscles of both sides. Cortical threshold, latency and amplitude of motor evoked potential and central conduction time were recorded. Malnourished children showed significantly increased cortical threshold, prolonged cortical latency and central conduction time and reduction in amplitude of MEP. Observed delay in central motor conduction in malnourished children suggests asymptomatic involvement of corticospinal pathways.     

Percutaneous magnetic coil stimulation of the phrenic nerve roots and trunk

We describe a technique of percutaneous magnetic coil NO stimulation of the phrenic nerve trunk on one side of the neck and phrenic roots over the upper cervical vertebral column in 10 normal subjects and 2 patients. We were able to obtain compound muscle action potentials (CMAPs) from the diaphragm at two sites (xiphoid process and 7th intercostal space) after stimulation of the phrenic nerve trunk and roots. We noted that the onset latencies after phrenic root stimulation remained fixed despite increasing the stimulus intensity from 50% to 100% and on moving the MC vertically or laterally, suggesting that stimulation of the fastest conducting fibers was occurring at a fixed site, most likely at the intervertebral foramina. Absent responses unilaterally in one and prolonged latencies to diaphragmatic CMAPs in another patient confirmed phrenic neuropathy in these patients.     

Electrical stimulation over the human vertebral column: which neural elements are excited?

Supramaximal percutaneous electrical stimuli applied over the human cervical vertebral column produce maximal compound muscle action potentials (CMAPs) in abductor digiti minimi. It is important to know which neural elements are excited by these stimuli and experiments were performed to answer this question. With stimulating electrodes placed progressively lateral to the midline, submaximal CMAPs with the same latency are produced. With shocks over the cervical vertebrae in the midline, the threshold for excitation of arm muscles is much lower than for excitation of leg muscles. Comparison of conduction time from the cervical column to more distal sites on the ulnar nerve by direct measurement and by F wave latency determination shows that the latter exceeds the former by 1.6 msec. Collision experiments in which paired shocks were given at the wrist and Erb's point or the wrist and cervical column showed that recovery from blocking as interstimulus interval lengthened was similar for the two sites, and that it was possible to detect F waves from the proximal stimulus. The latency of CMAPs evoked from midline surface stimuli was identical to that from a needle stimulus near the C8 root. It is concluded that electrical stimuli applied over the cervical vertebrae in the midline excite the motor roots at their exit from the spinal canal. This finding has implications for clinical studies of pyramidal tract and proximal peripheral nerve conduction.     

Sensory potentials evoked by magnetic stimulation of the cervical spine

Magnetic stimulation of cervical spinal roots was shown to elicit sensory potentials (MESP) which could easily be recorded at the fingers with ring electrodes. The latency of the MESP recorded at digit I was significantly shorter and the amplitude higher than of digits III and V. The latencies were largely independent of stimulus strength. In an attempt to localize the place of depolarization, the latencies of these potentials were compared with the N11 of the SEP (reflecting the arrival in the spinal cord) and with F-wave latencies and motor evoked potentials (MEP) to abductor pollicis brevis. The MESP latencies showed a very constant difference with the N11, being 0.6 ms faster. The mean difference between F latency and MEP was 1.2 ms. It is concluded that the origin of these MESPs is very near the spinal foramina, possibly in the sensory ganglia. © 1993 John Wiley & Sons, Inc.     

Lumbosacral nerve root stimulation comparing electrical with surface magnetic coil techniques.

Stimulation of lumbosacral nerve roots using a monopolar needle electrode was compared with magnetic stimulation using a 7-cm diameter surface coil. Compound muscle action potentials were recorded from the tibialis anterior (TA) and flexor hallucis brevis (FHB) muscles. Although the mean latency of CMAPs did not differ using the two techniques, amplitudes were considerably larger using a needle. Mean amplitudes were 66% (TA) and 64% (FHB) of the direct M response obtained by distal, supramaximal stimulation compared with mean values using maximal magnetic coil stimulation of 36% (TA) and 25% (FHB). Minimum F-wave latencies from FHB were used to estimate the site of nerve root stimulation using both techniques. Although there was a large amount of variability in the data from individual subjects, the results suggested that, on the average, both forms of stimulation act proximal to the intervertebral foramen. We conclude that a needle electrode is a more suitable technique for stimulating lumbosacral nerve roots.     

Facilitation and reciprocal inhibition by imagining thumb abduction.

It is well known that motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) of the motor cortex are facilitated by voluntary muscle contraction. We evaluated the effects of imagination of movements on MEP latencies of agonist and antagonist muscles in the hand using TMS. Twenty-two healthy volunteers were studied. TMS delivered at rest and while imagining tonic abduction of the right thumb. MEPs were recorded in response to magnetic stimulation over the scalp and cervical spine (C7-T1), and central motor conduction times (CMCT) were calculated. MEPs were recorded from right abductor pollicis brevis muscle (APB) and adductor pollicis muscle (AP) simultaneously. Imagination of abduction resulted in a shortened latency of MEPs in the APB muscle, and a prolonged latency in the AP muscle. But the imagination caused no significant change in the latency of MEPs elicited by stimulation over the cervical spine. The changes of the CMCT may account for these latency changes with imagination of movement. These findings indicate that imagination of thumb abduction facilitates motoneurons of agonist muscle and has an inhibitory effect on those of antagonist muscle (reciprocal inhibition).     

Magnetic stimulation over the spinal enlargements.

Magnetic stimulation over the cervical and lumbar spinal enlargements was performed in 10 normal volunteers using a 9 cm diameter coil. Although the threshold and the amplitude of responses depended on the position of the coil and the direction of current flow within it, the latency was constant. The latencies obtained by magnetic stimulation were compatible with those obtained using high voltage electrical stimulation of the spinal nerve roots and always were shorter than the peripheral motor conduction time estimated by F-wave techniques. The site of activation by magnetic stimulation appears to be very similar to that stimulated by the high-voltage electrical method. Stimulation of descending motor tracts within the cord was not possible using the magnetic stimulator.     

Determining the site of stimulation during magnetic stimulation of a peripheral nerve.

Magnetic stimulation has not been routinely used for studies of peripheral nerve conduction primarily because of uncertainty about the location of the stimulation site. We performed several experiments to locate the site of nerve stimulation. Uniform latency shifts, similar to those that can be obtained during electrical stimulation, were observed when a magnetic coil was moved along the median nerve in the region of the elbow, thereby ensuring that the properties of the nerve and surrounding volume conductor were uniform. By evoking muscle responses both electrically and magnetically and matching their latencies, amplitudes and shapes, the site of stimulation was determined to be 3.0 +/- 0.5 cm from the center of an 8-shaped coil toward the coil handle. When the polarity of the current was reversed by rotating the coil, the latency of the evoked response shifted by 0.65 +/- 0.05 msec, which implies that the site of stimulation was displaced 4.1 +/- 0.5 cm. Additional evidence of cathode- and anode-like behavior during magnetic stimulation comes from observations of preferential activation of motor responses over H-reflexes with stimulation of a distal site, and of preferential activation of H-reflexes over motor responses with stimulation of a proximal site. Analogous behavior is observed with electrical stimulation. These experiments were motivated by, and are qualitatively consistent with, a mathematical model of magnetic stimulation of an axon.     

Tongue motor responses following transcranial magnetic stimulation of the motor cortex and proximal hypoglossal nerve in man

Surface recordings of EMG responses were performed bilaterally from the tongue following transcranial magnetic cortex (TCS) and nerve stimulation (TNS) to characterize the activated corticonuclear pathways and to obtain normative data for a diagnostic use. TCS over the face-associated motor cortex with 1.3 times the response threshold for relaxed muscles produced bilateral tongue responses with similar latencies and amplitudes for ipsi- (8.3±1.1 ms, 1.3±0.7 mV) and contralateral responses (8.5±1.0 ms, 1.7±0.8 mV, n=20, 10 subjects). In individual subjects maximal ipsilateral and contralateral responses were elicited by stimulation over about the same cortex area which lay 2–4 cm lateral and 0–2 cm anterior to the center of the hand motor representation area. Magnetic stimulation of the hypoglossal nerve with 70% of the maximal stimulator output and a circular coil placed over the posterior lateral skull produced a more proximal nerve excitation than electrical stimulation at the mandible, as reflected by the response latencies (3.4±0.9 ms vs. 2.1±0.7 ms). The effect of magnetic TNS was independent of the direction of the coil currents. Central motor latencies as calculated by subtracting the response latencies after TNS from the overall latency after TCS were 4.8±1.2 ms and 5.0±1.1 ms for ipsi- and contralateral responses, respectively. The findings suggest the existence of a direct and fast conducting connection between motor cortex and brainstem tongue motor nuclei on both sides in man.     

Voluntary contraction shortens peripheral conduction time in response to transcranial magnetic stimulation of the brain

We investigated the effects of voluntary contraction on peripheral conduction time in response to transcranial magnetic stimulation (TMS) of the brain in 10 normal subjects. We obtained surface recordings of compound muscle action potentials (CMAP) from the abductor digiti minimi muscle (ADM) and nerve action potentials (NAP) from the ulnar nerve, at rest and during contraction (10% of maximal voluntary contraction) in response to TMS delivered at 100% output using a coil shaped like a figure 8. The distance between the two recording electrodes was 10 cm. The distal latency in response to TMS was calculated by subtracting the NAP latency from the CMAP latency. Distal latency was also measured by recording ADM responses to supramaximal electrical stimulation (ES) 10 cm proximal to the recording electrode. TMS-induced distal latency was significantly shorter during voluntary contraction than at rest (P < 0.00001). There was no significant difference between TMS-induced distal latency during contraction and ES-induced distal latency. TMS-induced distal latencies at rest and during contraction were correlated with the ES-induced distal latencies (r² = 0.468, P = 0.028 and r² = 0.769, P = 0.0009, respectively). Our results showed that the peripheral conduction time in response to TMS was related to the activity of the target muscle and to the fastest conduction velocity of the target nerve. Voluntary contraction reduced the peripheral conduction time in response to TMS.     

Transcortical and cervical magnetic stimulation with recording of the diaphragm

Transcortical and cervical magnetic stimulation is a potential method of examining the central inspiratory pathway to phrenic motor neurons. The reliability and accuracy of this technique were studied. We performed magnetic stimulations of the cortex and cervical spinal cord with recording from both hemidiaphragms in 35 normal subjects using two different stimulation coils (90-mm circular coil and 70-mm figure-eight coil). Needle electrode recordings and ultrasound real-time documentation in 2 subjects excluded volume-conducted contaminations from adjacent chest wall and abdominal muscles. The effect of diaphragmatic facilitation (stimulation at the end of a deep breath) on latency, and amplitude were compared to the effect of hypothenar muscle facilitation. Normal ranges were established for: latency; central motor conduction time; amplitude; amplitude ratio between peripheral and both cortical and cervical amplitude; and excitability threshold. The latencies were similar for both coils. The amplitudes were significantly higher, and excitability thresholds significantly lower for the 90-mm circular coil, indicating that this coil is preferable for transcortical diaphragmatic stimulations. The effect of facilitation was greater for hypothenar than diaphragmatic recordings. There was excellent right-left agreement for all measurements. Transcortical and cervical magnetic stimulation with recording from the diaphragm can be used routinely to diagnose and monitor patients with impaired central respiratory drive. © 1996 John Wiley & Sons, Inc.     

Cortical control of voluntary blinking: a transcranial magnetic stimulation study.

OBJECTIVE: To investigate cortical regions related to voluntary blinking. METHODS: Transcranial magnetic stimulation (TMS) was applied to the facial motor cortex (M1) and the midline frontal region (Fz) in 10 healthy subjects with eyes opened and closed. Motor-evoked potentials were recorded from the orbicularis oculi (OOC), orbicularis oris (OOR), abductor digiti minimi and tibialis anterior using surface and needle electromyography electrodes. Facial M waves and blink reflex were measured using supramaximal electrical stimulation of the facial and supraorbital nerves. RESULTS: TMS at Fz elicited 3 waves in OOC with no response in other tested muscles except for the early wave in OOR. Facial M1 stimulation produced only early and late waves. Because of their latencies, shapes, and relationship to coil position and stimulation intensity, early and late waves appeared to be analogous to the facial M wave and R1 component of the blink reflex. The intermediate wave at 6-8 ms latency was elicited in OOC by Fz stimulation with eyes closed. CONCLUSIONS: Since its latency matches the central conduction time of other cranial muscles and it has characteristic of muscle activation-related facilitation, the intermediate wave is presumably related to cortical stimulation. This result provides evidence that the cortical center for the upper facial movements, including blinking, is not principally located in the facial M1, but rather in the mesial frontal region.     

Nonspecific facilitation of responses to transcranial magnetic stimulation.

We examined the effect of facial muscle contraction and eye movements on motor evoked potentials (MEPs) from the abductor pollicis brevis muscle (APB) evoked by transcranial magnetic stimulation (TMS). The hypothesis was that activity of large cortical regions (face) influences the excitability of spinal motoneurons via cortical or subcortical pathways. MEPs were recorded in 12 healthy subjects during the following conditions: (1) rest; (2) facial muscle contraction; (3) eye movements; (4) 10% precontraction of the target muscle; and (5) simultaneous target muscle precontraction and facial muscle contraction. In 9 subjects, spinal motoneuron excitability was assessed by measurements of F waves during the same facilitation maneuvers. Activation of eye and facial muscles clearly facilitated MEPs from the APB. The facilitation of MEP size during nonspecific maneuvers was almost similar to that obtained by target muscle precontraction, whereas shortening of latencies was significantly smaller. The occurrence and amplitude of F waves increased in parallel with MEP size during specific and nonspecific facilitation, pointing to spinal motoneuronal threshold changes as a potential facilitatory mechanism by facial and eye muscle activation. The different MEP latencies during specific and nonspecific facilitation were not explained by different spinal motoneuron excitability, but raise the possibility that supraspinal mechanisms contributed to nonspecific facilitation.     

Transcranial magnetic stimulation of deep brain regions: evidence for efficacy of the H-coil.

OBJECTIVE: Standard coils used in research and the clinic for noninvasive magnetic stimulation of the human brain are not capable of stimulating deep brain regions directly. As the fields induced by these coils decrease rapidly as a function of depth, only very high intensities would allow functional stimulation of deep brain regions and such intensities would lead to undesirable side effects. We have designed a coil based on numerical simulations and phantom brain measurements that allows stimulation of deeper brain regions, termed the Hesed coil (H-coil). In the present study we tested the efficacy and some safety aspects of the H-coil on healthy volunteers. METHODS: The H-coil was compared to a regular figure-8 coil in 6 healthy volunteers by measuring thresholds for activation of the abductor pollicis brevis (APB) representation in the motor cortex as a function of distance from each of the coils. RESULTS: The rate of decrease in the coil intensity as a function of distance is markedly slower for the H-coil. The motor cortex could be activated by the H-coil at a distance of 5.5 cm compared to 2 cm with the figure-8 coil. CONCLUSIONS: The present study indicate that the H-coil is likely to have the ability of deep brain stimulation and without the need of increasing the intensity to extreme levels that would cause a much greater stimulation in cortical regions. SIGNIFICANCE: The ability of non-invasive deep brain stimulation potentially opens a wide range of both research and therapeutic applications.     

Subtypes of carpal tunnel syndrome: median nerve F wave parameters

The carpal tunnel syndrome (CTS) provides a model for analyzing the effects of focal nerve injury on F waves. We studied 127 patients (164 CTS) with clinical and electrophysiological CTS and 35 healthy controls in order to determine the alteration of F wave parameters in different types of CTS and to evaluate the most predictive F wave abnormality for each type. Minimal, maximal and mean F wave latencies, F wave persistence and chronodispersion recorded from abductor pollicis brevis (APB) muscle with wrist stimulation were compared. Twenty-three patients (29 CTS) had prominent demyelinating type CTS, 37 patients (45 CTS) had prominent axonal type CTS and 60 patients (90 CTS) slight demyelinating CTS according to electrophysiological parameters. The amplitude of APB muscle and F wave persistence were correlated significantly (r: 0.36, P<0.001). Minimal F wave latency was more prolonged in demyelinating group than in the axonal and slight demyelinating groups (P=0.001). In conclusion, F wave determination, as a simple and valuable method, allows the discrimination between demyelinating injury and axonal degeneration and increases the diagnostic yield in CTS.     

Hand preference and transcranial magnetic stimulation asymmetry of cortical motor representation

Human handedness may be associated with asymmetry in the corticospinal motor system. Previous studies measuring the threshold for eliciting motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) have provided evidence consistent with this hypothesis. However, TMS asymmetry observed in previous studies may have reflected cortical or spinal differences. We therefore undertook this investigation to test the hypothesis that handedness is associated with asymmetry in cortical motor representations. We used TMS to map contralateral cortical motor representations of the right and left abductor pollicis brevis (APB) and flexor carpi radialis (FCR) muscles in nine normal subjects (three left-handed). Using focal stimulation with a figure-of-8 shaped magnetic coil, we found no differences in MEP threshold or MEP size between the preferred and the nonpreferred hand. However, we observed that the number of scalp stimulation sites eliciting MEPs was statistically greater for APB and FCR muscles of the preferred limb. We found significant asymmetry between right-handed and left-handed subjects, such that in right-handers, the representation of the right APB was larger than that of the left APB, but in left-handers the representation of right APB was smaller than that of the left APB. These results suggest that handedness is associated with asymmetry in cortical motor representation.     

Riche-Cannieu anastomosis as an inherited trait.

OBJECTIVE: To explore the basis for the Riche-Cannieu anastomosis (RCA) and specifically whether this anomaly is an hereditary characteristic. METHODS: Three individuals from the same family were evaluated after initial studies in the index case indicated an RCA. Nerve conduction, needle electromyography (EMG), and axonal excitability studies of the median and ulnar nerves were undertaken in each case. RESULTS: In all subjects onset thresholds for CMAPs from abductor pollicis brevis (APB) were lower with ulnar nerve stimulation, but of similar latencies when compared with median nerve stimulation. Larger CMAP amplitudes were obtained with ulnar nerve stimulation, at lower stimulus intensities. No sensory anomalies were detected. Needle EMG confirmed dual innervation of APB by both median and ulnar nerves. Nerve excitability studies recorded from APB following ulnar nerve stimulation were within previously established normative limits for the median nerve. In the index case, no innervation anomaly was visible on magnetic resonance imaging from the forearm to hand. CONCLUSIONS: Dual innervation of APB by the median and ulnar nerves consistent with RCA was demonstrated in all 3 family members without co-existent sensory anomalies. SIGNIFICANCE: These findings infer an hereditary basis for RCA, consistent with an autosomal dominant pattern of inheritance.