Improved methodology for lumbosacral nerve root stimulation

The site of S1–S2 root activation following percutaneous high-voltage electrical (ES) and magnetic stimulation were located by analyzing the variations of the time interval from M to H soleus responses elicited by moving the stimulus point from lumbar to low thoracic levels. ES was effective in activating S1–S2 roots at their origin. However, supramaximal motor root stimulation required a dorsoventral montage, the anode being a large, circular surface electrode placed ventrally, midline between the apex of the xiphoid process and the umbilicus. Responses to magnetic stimuli always resulted from the activation of a fraction of the fiber pool, sometimes limited to the low-thresholds afferent component, near its exit from the intervertebral foramina, or even more distally. Normal values for conduction velocity in motor and 1a afferent fibers in the proximal nerve tract are provided. © 1996 John Wiley & Sons, Inc.     

Effects of coil characteristics for femoral nerve magnetic stimulation

The aim of this study was to compare the efficiency of two coils used for femoral nerve magnetic stimulation and to compare them with electrical stimulation in inducing maximal response of the quadriceps. The mechanical and electromyographic (EMG) responses were dependent on the coil used. The 45‐mm double coil showed greater efficiency to elicit a maximal quadriceps response, which was similar to electrical stimulation. Muscle Nerve, 2010     

Localization of nerve depolarization with magnetic stimulation.

The specific location on the magnetic stimulation (MS) coil that may correspond to the area of nerve depolarization has not been determined. In order to localize such an area, MS with 9-cm and 5-cm diameter coils was compared with conventional percutaneous electric stimulation (ES). On the 9-cm coil the distribution of points of nerve depolarization corresponded to that quarter of the coil which was placed over and parallel to the median nerve, whereas on the 5-cm coil, this area also extended outside the coil. The points of median nerve depolarization with MS were distributed over a distance of 7 cm on the stimulator head and was nearly identical for the 2 coil sizes at the wrist and elbow. Ulnar nerve costimulation was less frequent with the smaller coil at the wrist. A calculated reference point on the coil is suggested for more accurate NCV determinations.     

A comparison of magnetic and electrical stimulation of spinal nerves

The utility of the magnetic coil for stimulation of the cervical spinal nerves was compared to electrical stimulation by a monopolar needle cathode placed onto the spinal lamina in six volunteers. No statistical difference in average amplitudes or areas of evoked CMAPs was found although the size of the magnetic coil evoked potentials was less at C7-8 in several subjects. Electrical stimulation resulted in depolarization at a more proximal site. Electrical stimulation was associated with more discomfort, especially at C5-6. We conclude that electrical stimulation using a monopolar needle as the cathode is the superior technique for the clinical electrophysiologic study of the proximal brachial plexus and cervical spinal nerves.     

Intracranial stimulation of the facial nerve: normative values with magnetic coil in 240 nerves

This study was aimed at defining normative values of latency and amplitude of the compound muscle action potential (CMAP) from the orbicularis oculi muscle, obtained by transcranial magnetic stimulation. We evaluated the dependence of latency and amplitude on the age of the subjects, in order to calculate cut-off values for each age decade. A total of 120 healthy normal subjects, aged 15–78 years, were studied. CMAP from the orbicularis oculi muscle, pars inferior, was excited by means of transcranial magnetic stimulation of the homolateral scalp. A suitable mathematical model was developed to evaluate the mean variation of the latency and amplitude of CMAP for each age decade in the studied population. Mean values were, respectively, 4.62±0.40 ms for latency and 4.17±0.72 mV for amplitude. We found an increase in latency of about 0.12 ms for every ten years, that became 0.15 ms if referring only to subjects over 20 years, and a decrease in amplitude of 0.18 mV each ten years. Normality interval values for the latency and amplitude were calculated for each age decade. Our data, obtained in a representative population for range of age of evaluated subjects, provide normality values and variability coefficients useful for future confronting works. Received: 20 September 2002 / Accepted in revised form: 6 December 2002 Correspondence to D. Cocito     

Electrophysiological characterization of pre- and postoperative facial nerve function in patients with acoustic neuroma using electrical and magnetic stimulation techniques

Facial nerve function was examined in patients who underwent posterior fossa surgery for unilateral acoustic neuroma. Examinations took place prior to surgery (n = 47 patients), early after surgery (0–12 days, n = 16 of 47 patients), and late after surgery (187–1505 days, n = 29 of 47 patients). Clinical signs of facial palsy were present to a variable extent in 13 of 47 patients before, in 12 of 16 patients early, and in 18 of 29 patients later after surgery. Electrophysiologically, the facial nerve was stimulated electrically at the stylomastoid fossa and magnetically at its proximal intracanalicular segment. In addition, the faceassociated motor cortex was stimulated magnetically. In patients with facial palsy, any of these stimulation methods resulted in a decreased amplitude of the response in the nasalis muscle. The decrease showed a linear relationship to the clinical grade of palsy, pre- and postoperatively. Corticomuscular latencies remained unchanged. We conclude that: (i) the electrophysiological characteristics of facial nerve lesions due to compression by acoustic neuromas or due to a complication of neuroma removal are those of a purely axonal neuropathy; (ii) the three stimulation techniques have a similar diagnostic yield, thus making the use of all three of them redundant; and (iii) the electrophysiological techniques allowed no prediction of the final facial nerve function. © 1994 John Wiley & Sons, Inc.     

Transcranial magnetic stimulation of the trigeminal nerve: Intraoperative study on stimulation characteristics in man

We studied responses from the masseter and nasalis muscles following magnetic stimulation (magStim) and compared these responses with those obtained by direct electrical stimulation of the trigeminal (NV) and facial (NVII) nerve near the root exit zone during microvascular decompression operations of NVII. We found that (1) magStim threshold to excite the nerve is high for NV and low for NVII; (2) excitation of all motor fibers is impossible for NV, and easy for NVII; (3) optimal coil placement is critical for NV, but not critical for NVII; and (4) between and within subjects, the excitation site is variable on NV, but stable on NVII. We estimated that the anatomical location of magStim to be either within or outside the cerebrospinal fluid for NV, and to be in the labyrinthine segment of the facial canal for NVII. Physical models explain and clinical lesion models support these differences found between NV and NVII. © 1995 John Wiley & Sons, Inc.     

Voluntary contraction and responses to submaximal cervical nerve root stimulation

Voluntary contraction of hand muscles increases compound muscle action potential (CMAP) amplitudes evoked by submaximal electrical percutaneous cervical stimulation (EPCS). This has been reported to be due to an intraspinal, presynaptic mechanism. We studied the effects of voluntary contraction on hypothenar CMAP amplitudes in 5 volunteers following electrical peripheral nerve stimulation at the wrist, EPCS, magnetic stimulation at the neck and the effects of a conditioning subthreshold cortical magnetic stimulus on CMAPs evoked by EPCS at rest. CMAP amplitudes increased with voluntary contraction of the target muscle, regardless of type or location of stimulus (P < 0.001). No increase in CMAP amplitude occurred when a conditioning transcranial stimulus was employed with EPCS (P = 0.35). Our findings indicate a peripheral rather than central mechanism underlies this effect of voluntary contraction. It is probably related to the recruitment order of motor axons, comparing voluntary activation with electrical or magnetic stimulation. © 1994 John Wiley & Sons, Inc.     

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.     

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.     

Significance of magnetic coil position in peripheral motor nerve stimulation

The influences of coil position and coil-nerve distance on compound muscle action potentials (CMAPs), recorded from the first dorsal interosseus muscle during magnetic stimulation of the brachial segment of the ulnar nerve, were studied in 10 healthy volunteers. A 14-cm coil was held tangentially to the skin with the center overlying the nerve. Mapping of the CMAP latencies and amplitudes was made as the coil was displaced laterally in steps of 1 cm and in planes 0-3 cm from the skin surface. Stimulation with the coil center positioned 3 cm laterally to the nerve with the coil current directed proximally yielded the largest amplitudes with minimal variability and the most constant relationship to electrically evoked CMAPs. In this position the interindividual and intraindividual reproducibility of the magnetically evoked latencies were at least as good as those of electric stimulation when coil-skin distance was less than or equal to 2 cm.     

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.     

Treatment of myoclonic dystonia with transcutaneous electrical nerve stimulation

The use of a transcutaneous nerve stimulator yielded positive results in a condition marked by involuntary movements (tremors and myoclonias) and dystonia. In the absence of clues to the pathophysiology of the myoclonus and dystonia, it is argued that transcutaneous stimulation induces an action on the neurotransmitters that inhibits neuromuscular function.

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Sommario L'uso dello stimolatore nervoso transcutaneo ha dato positivi risultati in una forma caratterizzata da movimenti involontari (tremori e mioclonie) e da distonia. Data la nostra mancanza di elementi interpretativi sulla patofisiologia del mioclono e della distonia gli AA. ritengono che la stimolazione transcutanea determina una azione sui neurotrasmettitori tale da inibire la disfunzione neuromuscolare.

     

Magnetic stimulation: C-8 root standardization based on arm length and arm position

Normal values for C-8 root distal latencies (DL) by magnetic stimulation (Magstim) have been reported; however, the methods of standardization have failed to consider the variables of arm length and arm position. Consequently, an artificial widening of the “normal” range occurs due to a false elevation of the standard deviation. In this study, Magstim of the C-8 nerve root was performed on 30 normal volunteers. Gender, hand temperature, age, arm length, and DL with the arm resting at the patien's side and fully abducted were recorded. DL was 13.3 ± 1.1 ms with a side-to-side variation of 0.3 ± 0.3 ms. The difference in DL between the arm at rest and in full abduction was 0.2 ± 0.3 ms. Data analysis also demonstrated a direct correlation between arm length and DL. Utilizing this information a formula has been derived to more accurately describe the normal distribution of the C-8 root DL by Magstim. © 1993 John Wiley & Sons, Inc.     

Preferential activation of muscle fibers with peripheral magnetic stimulation of the limb

Magnetic and electric activation of limb nerve and muscle were compared in normal subjects of different age, gender, and habitus. Direct stimulation of nerve and muscle showed that activation of intramuscular nerve fibers in the arm and leg occurs at a lower threshold for magnetic stimulation than for electric stimulation. Sensory nerve fibers had a lower threshold with electric stimulation. Muscle activation and stimulus artifact with magnetic stimulation precluded reliable recording of distal sensory nerve action potential in all subjects.     

Magnetic nerve root stimulation in two types of brachial plexus injury: Segmental demyelination and axonal degeneration

Magnetic cervical nerve root stimulation was performed in 9 patients with plexopathies secondary to suspension (SP) and in 12 cases with neurogenic thoracic outlet syndrome (NTOS). The findings were compared with those of the previously reported case groups: n-hexane polyneuropathy (HPNP), inflammatory demyelinating polyneuropathy (IDP), and motor neuron disease (MND). Muscle responses elicited by magnetic stimulation had very high rates of amplitude and area loss in the neck–axilla segments of the 6 SP patients. This, along with the other electrophysiological findings, suggested the presence of segmentally demyelinating plexus lesions. In NTOS patients, magnetic stimulation findings were not significantly different from those of the controls. Neck–axilla segment amplitude and area reduction rates in SP and IDP patients were significantly higher than those found in NTOS, HPNP, and MND groups, implying that magnetic nerve root stimulation may have a role in the demonstration of segmentally demyelinating lesions involving proximal nerve segments. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 823–832, 1997     

Comparison of electric and magnetic coil stimulation in the supraclavicular region.

We compared the compound motor action potentials (CMAPs) evoked in the biceps, triceps, and abductor digiti minimi (ADM) muscles by conventional electrical stimulation at Erb's point (EP), and by magnetic coil stimulation of the supraclavicular region in 11 normal subjects. We found that magnetic coil stimulation was less effective than conventional stimulation in activating motor fibers in the brachial plexus in 45% of the recordings analyzed. CMAP amplitudes greater than those obtained with EP electrical stimulation were seen in 16% of recordings with supraclavicular magnetic stimulation, and in 33% of recordings with cervical magnetic stimulation, indicating that EP electrical stimulation is submaximal in a large proportion of cases.     

Rapid rate magnetic stimulation of human sacral nerve roots alters excitability within the cortico‐anal pathway

Abstract Sacral nerve root stimulation (SNS) can produce dramatic symptomatic improvement in faecal incontinence (FI). However, the physiological mechanism behind this improvement remains unknown. One hypothesis is that SNS may modulate cortico‐anal pathways and drive compensatory changes within the spinal cord or cerebral cortex that beneficially alter sphincter function. Our aim was to assess whether short‐term experimental SNS can induce changes in the human cortico‐anal pathway. Eight healthy volunteers (mean age 30 years) were studied. Subjects were investigated on three separate occasions and randomized to either active (5 and 15 Hz) or sham rapid‐rate lumbosacral magnetic stimulation (rLSMS). Anal sphincter electromyograms (EMG) were recorded from an anal probe following single‐pulse transcranial magnetic stimulation, at baseline, immediately, 30 and 60 min following rLSMS at either (i) 5 Hz for 15 min, (ii) 15 Hz for 15 min or (iii) sham stimulation for 15 min. In addition, manometry and anal sphincter sensation was measured in a subset of subjects. Interventions were compared to sham using anova . Fifteen hertz rLSMS increased cortico‐anal EMG response amplitude in the 1 h postintervention (F_{4, 28} = 3.2, P = 0.027), without a shift in response latency. This effect was not demonstrated with either 5 Hz or sham stimulation. rLSMS had no short‐term effect on sensation or physiology. Short‐term magnetic stimulation of the sacral nerve roots induces changes in cortico‐anal excitability which is frequency specific. These data support the hypothesis that SNS produces some of its beneficial effect in patients with FI by altering the excitability of the cortico‐anal pathway.