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.     

Bony foramina facilitate magnetic stimulation: an experimental cat sciatic nerve model

We have established an experimental bony foramen model in vivo using a cat sciatic nerve, a section of skull bone and a column made from methylmethacrylate. In each model, a foramen-like slit or fissure was created. Motor responses of the cat right gastrocnemius were elicited with a figure-of-8-shaped magnetic coil. Very high intensities of magnetic stimulation were necessary to evoke motor responses with the coil placed on the thigh. However, when the bone section or methylmethacrylate column was placed under the thigh muscle layer with the sciatic nerve fitted into the foramen-like slit or fissure, motor responses could be elicited with a smaller intensity of magnetic stimulation. Despite changes in stimulation intensity or shifts in the magnetic coil, the latency of the motor responses remained constant. By comparing the latency with the electrical recording, the site of excitation was predicted to be at the exit of the foramen. Our studies have confirmed that bony structures, especially bony foramina, facilitate excitation of the nerve by magnetic stimulation and that the exit of the foramen could be the preferential site for magnetic stimulation.     

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

We elicited H-reflexes by magnetic and electrical stimulation of several different nerves in 10 healthy subjects and two patients with S-1 radiculopathy. The posterior tibial nerve at the popliteal fossa and the femoral nerve at the inguinal ligament were tested with both electrical and magnetic stimulation; the proximal sciatic nerve was tested only with magnetic stimulation. Muscle activity was recorded from the soleus muscle for posterior tibial and sciatic nerve stimulation and from the vastus medialis muscle for femoral nerve stimulation. No significant difference was found between the latency of H-reflexes evoked by magnetic or electrical stimulation. With magnetic stimulation, the mean (+/- SD) Ia sensory fiber conduction velocity in the proximal segment of the sciatic nerve was 72.4 +/- 3.3 m/s, while the motor nerve fiber conduction velocity in the same portion of the nerve was significantly slower, at 60.6 +/- 2.0 m/s. In two patients with unilateral S-1 radiculopathy, the latency of the H-reflex from the soleus muscle to both magnetic and electrical stimulation of the posterior tibial nerve was absent or prolonged on the affected side. Magnetic stimulation can be used to study the H-reflex and Ia fiber conduction velocity and is particularly advantageous when testing deeply located nerve trunks.     

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.     

Intracranial stimulation of facial nerve in humans with the magnetic coil

Using ourselves as subjects, maximal compound motor action potentials (CMAPs) were evoked in ipsilateral nasal and orbicularis oculi muscles (onset latency 4.9-5.4 msec) by a magnetic coil (MC) tangentially oriented over parieto-occipital scalp. The facial nerve was also electrically stimulated sequentially at the posterior tragus near the stylomastoid foramen, anterior tragus and 3 cm more distally. Onset latency of the CMAP elicited at posterior tragus ranged from 1.0 to 1.3 msec less than that elicited by the MC over scalp. Because the measured distal facial nerve motor conduction velocity was 50-60 m/sec, the locus of impulse generation induced by magnetic coil stimulation was estimated to be approximately 6.5 cm proximal to the site of electrical stimulation at the posterior tragus, i.e., closer to the exit of the facial nerve from the brain-stem than to its entrance into the internal auditory meatus. This non-invasive technique should be useful in evaluating patients with peripheral facial nerve disorders including Bell's palsy.     

Motor potentials of inferior orbicularis oculi muscle to transcranial magnetic stimulation. Comparison with responses to electrical peripheral stimulation of facial nerve.

Magnetic stimulation at the vertex evoked a motor potential (MP) in the inferior orbicularis oculi muscle of 10 healthy subjects with an onset latency of 8-13 msec. Its amplitude increased and its latency decreased when the muscle was contracted: the latency measured 9.5 +/- 1.3 msec with an intensity of stimulation 10-15% above threshold in the contracted muscle. This MP is secondary to excitation of the motor cortex. With the coil placed over the occipital scalp and the same stimulation intensity, an MP was recorded with an onset latency at 4.5 +/- 0.6 msec. This response reflects the activation of the facial nerve root. The peripheral electrical stimulation of the facial nerve at the mandible angle elicited an MP with an onset latency at 3.5 +/- 0.4 msec. Most records showed the presence of late components at about 30 msec for all types of stimulation.     

Transcranial magnetic stimulation of the facial nerve]

It was the object of the present study to determine whether transcranial facial nerve stimulation using a magnetic coil can be clinically applicable, and to find the site where the facial nerve is best stimulated. A magnetic coil was placed over the parieto-occipital skull of the subjects for stimulation, and the facial nerve was electrically stimulated in its intracranial and peripheral courses. Then an electromyogram was recorded from the nasalis muscle of the face on the stimulated side. In 9 healthy volunteers, 18 facial nerves received magnetic and electric stimuli in the peripheral region, and the actual site of stimulation was estimated from the conduction velocity of the nerve. The conduction velocity was 56.6 +/- 4.8 m/s, and the latency between CMAPs for electric at the magnetic stimuli to the posterior tragus was 1.23 +/- 0.21 ms. Therefore, the position stimulated by magnetic coil was estimated to be 70.0 +/- 11.4 mm central to the posterior tragus, i.e., near the root exit zone. In two patients undergoing surgery in the cerebellopontine angle, transcranial magnetic stimulation and electrical stimulation of the intracranial facial nerve were compared intraoperatively. The CMAP produced by transcranial magnetic stimulation coincided closely with that produced by direct electrical stimulation of the root exit zone. Thus, the facial nerve was stimulated at the root exit zone, and this method could be expected to be useful for evaluation of disorders of the intracranial facial nerve.     

Magnetic stimulation of corticospinal pathways at the foramen magnum level in humans

Magnetic stimulation done with a double cone coil placed over the back of the head activated descending motor pathways and produced electromyographic responses in muscles of the arms and legs. The latencies of these responses were the same as those of responses to electrical brainstem stimulation. The threshold was lowest when the coil was placed over the inion or below it on the median line. Placement of the coil on the side ipsilateral to the muscle was more effective than placement on the contralateral side. These results indicate that activation occurs at the foramen magnum level (just below the pyramidal decussation). Collision experiments that used cortical and magnetic brainstem stimulation indicated that the major part of the responses to the latter stimulation were conducted via the large diameter component of the corticospinal tract. Collision experiments done with the peripheral nerve and magnetic brainstem stimulation showed that this stimulation produced a single descending volley in the descending tract. We conclude that magnetic brainstem stimulation produces a single descending volley in the corticospinal tract at the foramen magnum level with less discomfort.     

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.     

A comparison of magnetic and electrical stimulation of facial nerve at the cerebello-pontine angle in the dog.

Magnetic stimulation is a painless, non-invasive technique which allows an alternative method for testing cranial nerves which were previously inaccessible. We compared the latency of muscle responses obtained by electrical stimulation of the facial nerve at the cerebello-pontine angle (CPA) to high intensity transcranial magnetic stimulation (TMS) in 6 dogs. Evoked muscle response from the levator nasolabialis during electrical stimulation had a mean latency of 6.24 +/- 0.42 msec, compared with a mean of latency of 6.13 +/- 0.50 msec obtained by magnetic stimulation. Orbicularis oculi had a mean latency of 3.65 +/- 0.34 msec compared with a mean latency of 3.53 +/- 0.36 msec for magnetic stimulation. This suggests that high intensity TMS results in direct activation of the facial nerve as it exits the brain-stem in dogs. This observation is in accord with previous clinical studies that magnetic stimulation results in activation of the intracranial segment of the facial nerve in man.     

High voltage electrical stimulation of the proximal hypoglossal nerve in normal subjects.

OBJECTIVES: It is often difficult to stimulate the proximal hypoglossal nerve by magnetic occipital stimulation, even in normal subjects. Therefore, we tested an improved method of stimulating the proximal hypoglossal nerve, using high voltage electrical stimulation. METHODS: The proximal hypoglossal nerve was activated by high voltage electrical stimulation using surface electrodes over the occipital skull. The compound muscle action potential (CMAP) was recorded from the lingual muscles using surface electrodes in 10 normal subjects. CMAP and F waves produced by distal hypoglossal nerve stimulation and motor evoked potentials produced by transcranial magnetic stimulation were also recorded. RESULTS: When the anode electrode was placed at the mastoid process and the cathode below the inion, the unilateral proximal hypoglossal nerve was readily stimulated supramaximally in all the subjects. The CMAP latency was the same as that obtained with magnetic occipital stimulation. The central motor conduction time (CMCT) calculated from the proximal CMAP was 4.1+/-0.4 ms in the contralateral corticobulbar tract and 4.4+/-0.4 ms in the ipsilateral. The CMCT calculated from the minimal F wave latency was 3.3+/-0.2 ms. CONCLUSIONS: The high voltage electrical stimulation is a useful method for stimulating the proximal hypoglossal nerve to estimate the CMCT of the corticobulbar tract.     

High-voltage stimulation over the human spinal cord: sources of latency variation.

Percutaneous electrical stimuli (up to 600 V) were applied over the cervical spinal cord to evoke responses in the biceps brachii and thenar muscles. Cathodal stimulation over the C7 spinous process was more effective than anodal stimulation or stimulation over the C5 or C3 spinous process. As the stimulus intensity was increased, the response amplitude increased and the latency decreased. When progressively higher levels of supramaximal stimuli were delivered the latency often decreased further. The shortest latencies evoked by stimulation over the C7 spinous process were close to the latencies of the responses evoked by supramaximal stimulation near Erb's point. Thus, with this type of stimulation, the site of nerve activation changes with different stimulus intensities. The variability in latency introduced by distal spread of the site of activation will affect measurements of central motor conduction time and should be considered in the diagnostic use of this technique.     

Transcranial magnetic stimulation of the central and peripheral motor pathways to the lingual muscles in cats.

OBJECTIVE: We have assessed a technique to stimulate the intracranial hypoglossal nerve with ease and reproducibility by using magnetic coils (MCs) and to detect a reliable site of excitation in animal experiments in order to establish a method to evaluate the motor pathway to lingual muscles. METHODS: We recorded the motor responses from the lingual muscles of 5 adult cats under general anesthesia by magnetic and electrical stimulation of the intracranial hypoglossal nerves. Figure of 8 and round MCs were used to investigate the optimal position and direction to evoke the motor responses. RESULTS: The round MC was useful for cortical stimulation. The figure of 8 coil, positioned in the back of the head of the examined side, parallel to the cervical spine, was essential for stimulation of the intracranial hypoglossal nerve. Analysis of the latencies, and the observation that the motor responses disappeared after transection of the nerves at the exit of the hypoglossal canal, demonstrated that the site of the excitation is at the exit of the hypoglossal canal. CONCLUSION: Magnetic stimulation using a figure of 8 coil can elicit tongue motor responses with ease and reliable reproducibility, stimulating the hypoglossal nerve at the exit of the hypoglossal canal.     

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.     

Withdrawal reflexes in adductor muscles elicited by electrical and magnetic stimulation of the obturator nerve

The withdrawal reflex in the short head of the biceps femoris muscle after electrical stimulation of the sural nerve at the ankle has been investigated in numerous studies. These studies have described two distinct responses: early (R-II) and late (R-III). However, withdrawal reflex activity of the adductor muscles in the legs has not been studied systematically. Adductor muscle reflex activity is important because it can produce serious clinical problems, such as adductor spasticity and spasms, during bladder surgery. The present study examined withdrawal reflex features of adductor muscles obtained by electrical and magnetic stimulation of the obturator nerve (ON) in 34 normal healthy subjects. Early adductor muscle withdrawal reflex responses were elicited by ipsilateral ON electrical stimulation with a mean latency of 45.7+/-2.0 ms (responses in 94% of subjects). Reflex responses were also obtained using magnetic stimulation at a similar incidence rate. Contralateral ON electrical stimulation resulted in a similar reflex, but with a lower incidence. ON and femoral nerve electrical and magnetic coil stimulation produced similar low-incidence responses in the vastus medialis. These findings indicate that short latency adductor withdrawal reflexes are easily obtained on both sides following electrical or magnetic stimulation of the ON, and they can be elicited by both nociceptive and nonnociceptive stimuli. These reflexes prepare the body for a proper response to incoming signals and likely serve to protect the pelvic floor and pelvic organs.     

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.     

Transcranial stimulation of the leg area of the motor cortex in humans

We used transcranial magnetic stimulation on nine normal volunteers to establish an effective way to stimulate the leg area of the motor cortex. Three types of coils: a large figure-eight coil, small figure-eight coil, and a round coil were used. Surface electromyographic activities were recorded from the left tibialis anterior muscle, and the latencies and amplitudes compared with those obtained by anodal electrical stimulation. The most stable responses were obtained when the large figure-eight coil was centered over the vertex and backward current was run through it or when the round coil was centered two to three centimeters anterior to the vertex with left-flowing current in it at the posterior widening. The latencies obtained under these stimulation conditions were the same as those obtained by electrical stimulation. We conclude that direct activation of the pyramidal cells occurs in the leg area of the motor cortex in all forms of magnetic and electrical stimulation.     

Pseudo-bilateral hand motor responses evoked by transcranial magnetic stimulation in patients with deep brain stimulators.

OBJECTIVES: In 3 of 5 patients with dystonia and bilaterally implanted deep brain stimulating electrodes, focal transcranial magnetic stimulation (TMS) of one motor cortex elicited bilateral hand motor responses. The aim of this study was to clarify the origin of these ipsilateral responses. METHODS: TMS and electrical stimulation of corticospinal fibres by the implanted electrodes were performed and the evoked hand motor potentials were analysed. RESULTS: In comparison with responses elicited by contralateral motor cortex stimulation, ipsilateral responses were smaller in amplitude (3.0+/-1.4 versus 5.8+/-1.5 mV), had shorter peak latencies (first negative peak: 20.9+/-0.8 versus 25.1+/-0.4 ms) and were followed by a shorter-lasting silent period (46+/-4 versus 195+/-35 ms). Ipsilateral responses following TMS had similar peak latencies to responses elicited subcortically by deep brain stimulation (DBS) (20.4+/-0.9 ms). CONCLUSIONS: Hand motor responses ipsilateral to TMS result from a subcortical activation of corticospinal fibres, via the implanted electrode in the other hemisphere, secondary to currents induced by TMS in subcutaneous wire loops that underlie the magnetic coil. Studies of TMS in patients with DBS have to take this potential source of confounding into account.     

Study of central and peripheral motor conduction in normal subjects

Motor potentials to transcranial and cervical magnetic stimulation and F-wave were recorded in 37 arms of 25 normal subjects. Clockwise and anticlockwise cervical stimulation were performed over C5, C7 and T2 spinous processes. A significant correlation was found between height and measurements of central and peripheral motor conduction. Peripheral motor conduction measured by F-wave derived techniques (Kimura formula) was 0.5 msec higher as compared with responses to cervical magnetic stimulation. The site and current flow direction of cervical magnetic stimulation influenced the amplitude of responses but not their latency: responses were larger in the right arm when the centre of the coil was placed over the C5 spinous process and clockwise stimuli were used. The same results were obtained in the left arm when the coil was reversed.     

Origin of muscle action potentials evoked by transcranial magnetic stimulation in cats

We studied the effects of transcranial magnetic stimulation on ipsilateral and contralateral forelimb extensor muscles in anesthetized cats. A magnetic stimulator, operating at 100% intensity; was used through a circular coil which was placed tangentially over the midline scalp. Bilateral activation of extensor muscles was readily obtained in all animals. The onset latencies were 7.3 ± 1.1 and 7.07 + 0.8 msec for the contralateral and ipsilateral muscles, respectively. The amplitude of muscle response was unstable in magnitude, nevertheless, it did not show any significant difference between the two sides. The latency of response for ipsilateral and contralateral muscles was similar, which suggests simultaneous activation of motor pathways serving forelimb muscles. Lesioning or ablation of the motor cortex and decerebration at mid-colliculi level did not abolish the evoked responses elicited at high intensity magnetic stimulation. Stereotactic electrical stimulation of the vestibular nuclei complex was performed[ and satisfactory ipsilateral motor responses were obtained. Subsequently; a stereotactic radiofrequency lesion was made at the vestibular nuclei complex, with morphological confirmation. After this lesion, the motor evoked potentials (MEPs) were significantly diminished in amplitude. This finding strongly suggests that the generator of the MEPs resides in the brainstemi, mainly at the vestibular nuclei complex. [Neurol Res 1995; 17: 469-473]