The utility of magnetic stimulation for routine peripheral nerve conduction studies

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

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

Usefulness of cervical root magnetic stimulation in assessing proximal motor nerve conduction

OBJECTIVES

To evaluate the reliability and utility of cervical root magnetic stimulation in exploring proximal motor conduction.

METHODS

In 20 patients with demyelinating polyneuropathy (DPN), 20 patients with amyotrophic lateral sclerosis (ALS) and 25 healthy subjects, evoked compound muscle action potentials (CMAPs) were recorded from abductor digiti minimi muscle in response to electrical stimulation up to Erb's point and magnetic stimulation up to the cervical roots.

RESULTS

In all healthy and ALS subjects, magnetic root stimulation confirmed the absence of conduction abnormalities, including those in whom supramaximal responses at Erb's point were not achieved. In the DPN group, conduction block and/or temporal dispersion was revealed by magnetic root stimulation in 9 out of 20 patients (45%), 3 more than those detected at Erb's point.

CONCLUSIONS

Cervical root stimulation allowed clear distinction between motor neuronopathy and DPN. It is recommended as part of the routine evaluation of patients suspected of having DPN, especially when distal nerve studies are inconclusive.

Evaluation of Nerve Conduction Studies in Obese Children With Insulin Resistance or Impaired Glucose Tolerance

The aim of the study was to investigate nerve conduction studies in terms of neuropathic characteristics in obese patients who were in prediabetes stage and also to determine the abnormal findings. The study included 69 obese adolescent patients between April 2009 and December 2010. All patients and control group underwent motor (median, ulnar, tibial, and peroneal) and sensory (median, ulnar, sural, and medial plantar) nerve conduction studies and sympathetic skin response test. Sensory response amplitude of the medial plantar nerve was significantly lower in the patients with impaired glucose tolerance and insulin resistance. To our knowledge, the present study is the first study demonstrating the development of sensory and autonomic neuropathy due to metabolic complications of obesity in adolescent children even in the period without development of diabetes mellitus. We recommend that routine electrophysiological examinations be performed, using medial plantar nerve conduction studies and sympathetic skin response test.

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

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

Non-surgical therapies for peripheral nerve injury

BACKGROUND

Non-surgical approaches have been developed to enhance nerve recovery, which are complementary to surgery and are an adjunct to the reinnervation process.

SOURCES OF DATA

A search of PubMed, Medline, CINAHL, DH data and Embase databases was performed using the keywords 'peripheral nerve injury' and 'treatment'.

AREAS OF CONTROVERSY

Most of the conservative therapies are focused to control neuropathic pain after nerve tissue damage. Only physical therapy modalities have been studied in humans and their effectiveness is not proved.

GROWING POINTS

Many modalities have been experimented with to promote nerve healing and restore function in animal models and in vitro studies. Despite this, none have been actually translated into clinical practice.

AREAS TIMELY FOR DEVELOPING RESEARCH

The hypotheses proved in animals and in vitro should be translated to human clinical practice.

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.

Magnetic stimulation of peripheral nerves in dogs: a pilot study

A model for magnetic stimulation of the radial and sciatic nerves in dogs was evaluated. Onset-latencies and peak-to-peak amplitudes of magnetic and electrical stimulation of the sciatic nerve were compared, and the effect of the direction of the current in the magnetic coil on onset-latencies and peak-to-peak amplitude of the magnetic motor evoked potential was studied in both nerves. The results demonstrate that magnetic stimulation is a feasible method for stimulating the radial and sciatic nerves in dogs. No significant differences were observed in onset-latencies and peak-to-peak amplitudes during magnetic and electrical stimulation, indicating conformity between the techniques. Orthodromic or antidromic magnetic nerve stimulation resulted in no significant differences. This pilot study demonstrates the potential of magnetic stimulation of nerves in dogs.

Oestrogen status in relation to the early training responses in human thumb adductor muscles

AIMS

The aims of this study were to identify the mechanisms for the early response to training in women of different oestrogen status and to determine whether any oestrogen and exercise effects on these would be additive.

METHODS

We monitored training (ten 5-s contractions per day for 12 weeks)-induced changes in the size, strength, voluntary activation capacity and index of crossbridge force state (i.e. rapid stretch to isometric torque ratio), in the thumb adductor muscles of postmenopausal [eight who had never used, and 14 who were using, hormone replacement therapy (HRT)] and seven premenopausal eumenorrhoeic women. The contralateral untrained muscle was used as a control.

RESULTS

There was a significant effect of oestrogen status on the magnitude of training-induced strength increment, with the non-HRT postmenopausal group exhibiting the greatest benefits (28 +/- 6%, P = 0.024) from training. There were no significant or commensurate changes in either cross-sectional area or voluntary activation capacity. The index of crossbridge force state improved most in the no-HRT group (19 +/- 7%, P < 0.05).

CONCLUSIONS

Presence, rather than absence of oestrogen, is associated with relatively higher muscle function which limits the potential for any further training-induced increments in muscle performance, as would be expected if the muscle strengthening actions of training and oestrogen share a common, partially saturable physiological pathway. The mechanism that is involved in the early training-induced strength increment in the three differing oestrogen groups cannot be due to increased size or recruitment. It would appear instead that increased motor unit firing frequency is involved.

Basic mechanisms of TMS

Transcranial magnetic stimulation (TMS) is now established as an important noninvasive measure for neurophysiologic investigation of the central and peripheral nervous systems in humans. Magnetic stimulation can be used for stimulating peripheral nerves with a similar mechanism of activation as for electrical stimulation. When TMS is applied to the cerebral cortex, however, some features emerge that distinguish it from transcranial electrical stimulation. One of the most important features is designated the D and I wave hypothesis, which is now widely accepted as a mechanism of TMS of the motor cortex. Transcranial electrical stimulation excites the pyramidal tract axons directly, either at the initial segment of the neuron or at proximal internodes in the subcortical white matter, giving rise to D (direct) waves, whereas TMS excites the pyramidal neurons transsynaptically, giving rise to I (indirect) waves. There are still other phenomena with mechanisms that remain to be elucidated. First, not only excitatory effects but also inhibitory effects can be elicited by TMS of the cerebral cortex (e.g., the silent period and intracortical inhibition). The inhibitory effect may also be used to investigate cerebral functions other than the motor cortex, such as the visual, sensory cortices, and the frontal eye field, from which no overt response like the motor evoked potential can be elicited. Second, there is an abundance of intraregional functional connectivities among different cortical areas that can also be revealed by TMS, or TMS in combination with neuroimaging techniques. Last, repetitive transcranial stimulation exerts a lasting effect on brain function even after the stimulation has ceased. With further investigation of the neural mechanisms of TMS, these techniques will open up new possibilities for investigating the physiologic function of the brain as well as opportunities for clinical application.

Small-fiber neuropathy

Small-fiber neuropathy is a common disorder. It is often "idiopathic" and typically presents with painful feet in patients over the age of 60. Autoimmune mechanisms are often suspected, but rarely identified. Known causes of small-fiber neuropathy include diabetes mellitus, amyloidosis, toxins, and inherited sensory and autonomic neuropathies. Occasionally, small-fiber neuropathy is diffuse or multifocal. Depending on the type of small-fiber neuropathy, autonomic dysfunction can be significant or subclinical. Diagnosis is made on the basis of the clinical features, normal nerve conduction studies, and abnormal specialized tests of small-fiber function. These specialized studies include assessment of epidermal nerve fiber density as well as sudomotor, quantitative sensory, and cardiovagal testing. The sensitivities of these tests range from 59-88%. Each has certain advantages and disadvantages, and the tests may be complementary. Unless an underlying disease is identified, treatment is usually directed toward alleviation of neuropathic pain.

Adductor pollicis twitch tension assessed by magnetic stimulation of the ulnar nerve

Many critically ill patients develop significant skeletal muscle weakness in the Intensive Care Unit (ICU), which ultimately may cause difficulties in weaning from mechanical ventilation and a protracted, expensive ICU stay. Reliable monitoring of muscle strength in this environment is difficult. The purpose of this study was to develop a reproducible, nonvolitional method of measuring adductor pollicis (AP) muscle function by magnetic stimulation of the ulnar nerve (MSUN) that could be applied to patients in the ICU and operating theater (OT). Fifty subjects (32 healthy control subjects [12 of whom were elderly], 12 ICU patients with critical illness [mean APACHE II score 20], and six otherwise healthy patients requiring minor surgery in the OT) received MSUN. In 12 of the normal subjects electrical stimulation of the ulnar nerve (ESUN) and MSUN were compared and AP twitch tension (Tw AP) and surface electromyogram (EMG) were measured. Close agreement was found between supramaximal Tw AP (median [95% CI] for MSUN 6.3 N [5-7.2 N] and ESUN 6.9 N [5.2-7.8 N] [p = NS]). Median (95% CI) values with MSUN for the 20 young and 12 elderly control subjects were 6.9 N (5. 3-7.4 N) and 7.1 N (4.4-9.8 N). Median (95% CI) Tw AP for the ICU group was 4.2 (2.2-6.7 N) and for the OT group was 5.8 (4-9.1 N). Tw AP was significantly reduced in ICU patients compared with age-matched controls (p = 0.01). MSUN can be used to measure neuromuscular function in both the laboratory and clinical settings including the ICU.

Stimulation of peripheral nerves using a novel magnetic coil

Magnetic nerve stimulation (MNS) using a novel figure-8 magnetic coil was compared with conventional electric nerve stimulation (ENS) in normal subjects and in patients with disorders of the peripheral nervous system. In contrast to previously tested coils, the virtual cathode of the novel coil was independent of the geometrical or electric conditions of the stimulated tissue. Maximal compound muscle action potentials (CMAPs) were elicited by MNS in all motor nerves tested. The slopes of the recruitment curves of ENS were steeper than those of MNS, indicating a comparatively lower maximal stimulation intensity and a higher intensity resolution of the magnetic stimulator. In four patients with entrapment syndromes at the ulnar groove, motor conduction velocities and amplitudes were similar for MNS and ENS across the affected nerve segment. However, in two patients with chronic inflammatory demyelinating polyneuropathy (CIDP), CMAPs were slightly smaller following MNS. This new technique is a promising step toward the ultimate goal of replacing ENS with MNS.

Possible complications of magnetic coil stimulation in living tissue: assessment of changes in epiphyseal cartilage

To assess the influence of repeated magnetic coil stimulation (MCS) of the peripheral nervous system on epiphyseal cartilage, we evaluated histological and structural changes after repeated MCS applied to the knee joints of young rabbits. There was a slight but significant histological change when repeated MCS at 100% intensity (100% of the maximal output) exceeded 600 times per day. However, we found no gross structural effects on the bone. We conclude that, if these results can be extrapolated to humans, MCS should be safe for stimulating peripheral nervous system for clinical electrophysiologic examinations in children.

Importance of soft tissue inhomogeneity in magnetic peripheral nerve stimulation

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

Stimulation of neurite outgrowth using an electrically conducting polymer

Damage to peripheral nerves often cannot be repaired by the juxtaposition of the severed nerve ends. Surgeons have typically used autologous nerve grafts, which have several drawbacks including the need for multiple surgical procedures and loss of function at the donor site. As an alternative, the use of nerve guidance channels to bridge the gap between severed nerve ends is being explored. In this paper, the electrically conductive polymer--oxidized polypyrrole (PP)--has been evaluated for use as a substrate to enhance nerve cell interactions in culture as a first step toward potentially using such polymers to stimulate in vivo nerve regeneration. Image analysis demonstrates that PC-12 cells and primary chicken sciatic nerve explants attached and extended neurites equally well on both PP films and tissue culture polystyrene in the absence of electrical stimulation. In contrast, PC-12 cells interacted poorly with indium tin oxide (ITO), poly(L-lactic acid) (PLA), and poly(lactic acid-co-glycolic acid) surfaces. However, PC-12 cells cultured on PP films and subjected to an electrical stimulus through the film showed a significant increase in neurite lengths compared with ones that were not subjected to electrical stimulation through the film and tissue culture polystyrene controls. The median neurite length for PC-12 cells grown on PP and subjected to an electrical stimulus was 18.14 micron (n = 5643) compared with 9.5 micron (n = 4440) for controls. Furthermore, animal implantation studies reveal that PP invokes little adverse tissue response compared with poly(lactic acid-co-glycolic acid).

AAEM minimonograph #48: autonomic nervous system testing

The autonomic nervous system maintains internal homeostasis by regulating cardiovascular, thermoregulatory, gastrointestinal, genitourinary, exocrine, and pupillary function. Testing and quantifying autonomic nervous system function is an important but difficult area of clinical neurophysiology. Tests of parasympathetic cardiovagal regulation include heart rate analysis during standing (the 30:15 ratio), heart rate variation with deep breathing, and the Valsalva ratio. Tests of sympathetic adrenergic vascular regulation include blood pressure analysis while standing, the Valsalva maneuver, sustained handgrip, mental stress, and cold water immersion. Tests of sympathetic cholinergic sudomotor function include the sympathetic skin response, quantitative sudomotor axon reflex test, sweat box testing, and quantification of sweat imprints. Pupil function is tested pharmacologically and with pupiilographic techniques. Tests of gastrointestinal and genitourinary function do not satisfactorily isolate autonomic regulation from their other functions. The available tests have various sensitivities and ease of administration. They are typically administered in a battery of multiple tests, which improves sensitivity and reliability, and allows probing of various autonomic functions.

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.

A volume-conduction analysis of magnetic stimulation of peripheral nerves

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

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.

Comparison of various coils used for magnetic stimulation of peripheral motor nerves: physiological considerations and consequences for diagnostic use

We compared the ability of 4 magnetic coils to activate peripheral nerves in healthy subjects. No differences in motor threshold intensities were found between the coils, but the intensities needed to elicit maximum compound muscle action potential (CMAP) amplitudes were different. For superficial nerves maximum CMAPs in comparison with electrical stimulation were usually but not always found. CMAPs were at their maximum only when the direction of induced current flowed from proximal to distal and when a certain part of the coil was over the nerve. Distal nerve stimulation was time consuming. Due to artifacts many stimuli were necessary and sometimes no maximum CMAP could be elicited. CMAPs were much less sensitive to position changes of the coil than to changes in an electrical stimulator. Small circular coils were superior to larger coils in terms of the lower intensities necessary to elicit maximum CMAPs, better focusing of the stimulus, and less artifacts. For deep nerves amplitudes were always submaximal. Coactivation of nearby nerves and underlying muscles was another main drawback especially at proximal sites and for coils of large diameter. Despite better focusing, double coils are less useful due to their great diameter. Magnetic stimulation cannot replace electrical neurography at the moment, even if different coils are used at different sites of stimulation.

A preliminary clinical evaluation of magnetic stimulation of the ulnar nerve for monitoring neuromuscular transmission

The evoked motor responses to magnetic and electrical peripheral nerve stimuli were quantitatively assessed after vecuronium in 15 women undergoing gynaecological surgery. Anaesthesia was induced with thiopentone and fentanyl and maintained with intermittent doses of fentanyl and 66% nitrous oxide in oxygen. After immobilisation of both forearms in splints, the ulnar nerves were stimulated supramaximally every 10 s with a magnetic stimulator (Magstim Model 200) and an electric stimulator (Myotest) on opposite sides. The adduction forces of both thumbs to magnetic and electric stimuli were measured simultaneously before and after 0.06 mg.kg-1 of vecuronium. After vecuronium administration the twitch responses to magnetic stimulation decreased more slowly than those to electric stimulation. The difference in the evoked responses between the two types of stimulation was approximately 20% overall and was significant 2 min after vecuronium administration (p < 0.05). The rate of recovery of the evoked twitch responses was more rapid with magnetic than electric stimulation. It is concluded that magnetic stimulation of peripheral nerve is a useful technique for evaluating residual neuromuscular blockade.

Focal magnetic stimulation of an axon

The induced electric field produced by a circular coil during magnetic stimulation of an axon is derived from Maxwell's equations. The foci and virtual cathodal and anodal regions are predicted as a function of coil radius and orientation. Two virtual anode and cathode pairs are predicted, one lying outside the coil's perimeter and predominant in the far field, and one lying within the perimeter of the coil which may stimulate the axon when the coil and nerve are in close proximity. When the coil is positioned tangent to the nerve, an orientation commonly used in clinical magnetic stimulation, the foci of the predominant cathode and anode pair are extremely sensitive to changes in coil placement. In addition, the radius of curvature of the activating function, a measure of the size of the virtual cathode at threshold, is predicted to decrease with decreasing coil diameter and distance to the nerve. These predictions may help explain observed variability in measurements of conduction velocity and latency during magnetic stimulation of peripheral axons.

Repetitive magnetic nerve stimulation: technical considerations and clinical use in the assessment of neuromuscular transmission

We assessed to what extent repetitive magnetic stimulation can replace the electrical method. Fifteen healthy subjects and 3 patients with myasthenia gravis were investigated using stimulation of the median, ulnar, axillary and accessory nerves. Single, as well as 3/sec repetitive magnetic and electrical stimuli were applied. When comparing the results of magnetic vs. electrical stimulation, amplitudes, areas and shapes of compound muscle action potentials were not significantly different. Although single magnetic stimuli were much less uncomfortable than the electrical stimuli, differences in comfort were much smaller in the repetitive protocol, because muscular contractions under the magnetic stimulation coil caused unpleasant movements of, for example, the neck. Additional problems arose from technical limitations of the prototype magnetic stimulator used: stimulation intensity was significantly limited, resulting in an inability to elicit supramaximal responses in 11 of the 154 investigations. Overheating of the stimulator coil forced us to give the coil extra time to cool down. These problems might be solved in the future by more focused stimulus geometry and introduction of cooling devices. It is concluded that magnetic stimulation can elicit responses which are equivalent to the electrical method in repetitive nerve stimulation. At present due to some shortcomings it cannot replace electrical stimulation in routine repetitive nerve stimulation.

Somatosensory evoked potentials (SEPs) elicited by magnetic nerve stimulation

Magnetic stimulation of peripheral nerves at distal and proximal sites of the upper and lower extremities and at the midlumbar level were used to elicit cortical somatosensory evoked potentials. Evidence is provided that peripheral nerve trunks, rather than distal receptor afferents, are the anatomical structures stimulated by the electromagnetic fields. Magnetic stimulation of peripheral nerves is considered to be useful for an evaluation of the integrity of proximal nerves, nerve roots and central conduction along sensory pathways. In contrast to electrical nerve stimulation, magnetic stimulation is painless and can be applied to proximal nerves and plexus. By means of proximal nerve stimulation central sensory conduction can be tested even in patients with peripheral nerve lesions or polyneuropathy.

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 patient'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.

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.

Neural stimulation with magnetic fields: analysis of induced electric fields

Spatial distributions of the derivative of the electric field induced in a planar semi-infinite tissue model by various current-carrying coils and their utility in neural stimulation are evaluated. Analytical expressions are obtained for the electric field and its spatial derivatives produced by an infinitely short current element. Fields and their derivatives for an arbitrarily shaped coil are then obtained by numerical summation of contributions from all the elements forming the coil. The simplicity of the solution and a very short computation time make this method particularly attractive for gaining a physical insight into the spatial behavior of the stimulating parameter and for the optimization of coils. Such analysis is useful as the first step before undertaking a more complex numerical analysis of a model more closely representing the tissue geometry and heterogeneity.

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.

Stimulation of a myelinated nerve axon by electromagnetic induction

A model of electromagnetic stimulation predicts the transmembrane potential distribution along a myelinated nerve axon and the volume of stimulated tissue within a limb. Threshold stimulus strength is shown to be inversely proportional to the square of the axon diameter. It is inversely proportional to pulse duration for short pulses and independent of pulse duration for long ones. These results are also predicted by dimensional analysis. Two dimensionless numbers, Sem, the ratio of the induced transmembrane potential to the axon's threshold potential, and Tc/T, the ratio of the pulse duration to the membrane time constant, summarise the dependence of threshold stimulus strength on pulse duration and axon diameter.

Spatial dispersion of magnetic stimulation in peripheral nerves

To assess the longitudinal dispersion of the stimulus induced by the magnetic coil, collision experiments were performed in seven normal ulnar nerves. A supramaximal electrical stimulus S1 was delivered at the wrist, and followed by a supramaximal stimulus S2 in the upper arm, which was either electrical (electrical collision studies), or magnetic (magnetic collision studies). The interstimulus interval was varied by 0.2 msec increments from the time of complete cancellation of the S2 evoked motor response onwards, to include the entire span of recovery of that compound motor action potential. Collision curves were obtained for both magnetic and electrical stimuli by plotting the amplitude of the motor response elicited by S2 as a function of the interstimulus interval. In all seven normal ulnar nerves, comparison of the collision curves showed that the S2 evoked motor response is restored significantly more slowly when magnetic stimulation is used. This finding is best explained by longitudinal dispersion of the stimulus induced by the magnetic coil relative to conventional electrical stimulation, the large fibers being stimulated further away from the coil than the small ones. This interpretation is confirmed by the findings obtained with the same method in two cases of ulnar neuropathy, and by comparison of different intensities of magnetic stimulation.

AAEM minimonograph #35: Clinical experience with transcranial magnetic stimulation

We elicited motor evoked potentials (MEPs) using transcortical magnetic stimulation in 150 control subjects aged 14 to 85 years and 275 patients with a variety of diseases. There were no significant side effects. Cortex-to-target muscle latencies measured 20.2 +/- 1.6 ms (thenar), 14.2 +/- 1.7 ms (extensor digitorum communis), 9.4 +/- 1.7 ms (biceps), and 27.2 +/- 2.9 ms (tibialis anterior). Central motor delay between the cortex and the C-7 and L-5 measured 6.7 +/- 1.2 ms and 13.1 +/- 3.8 ms, respectively. Mean spinal cord motor conduction velocity measured 65.4 m/s. MEP amplitude expressed as a percentage of the maximum M wave was never less than 20% of the M wave. A value of less than 10% is considered abnormal. MEP latency increases linearly with age and central motor delay is longer in older subjects. Compound muscle action potentials and absolute MEP amplitudes decreased linearly with age. In multiple sclerosis (MS), MEP latency and central delay were often very prolonged. The MEP was more sensitive than the SEP in MS. In amyotrophic lateral sclerosis, MEP latencies were only modestly prolonged; the characteristic abnormality was reduced amplitude. When pseudobulbar features predominated MEPs were often absent. The MEP was of normal latency in Parkinson's disease, but age-related amplitude was often increased. MEP latency and amplitude were normal in Huntington's disease. Abnormal MEPs persisted several months after stroke despite good functional recovery. The MEP could be used to advantage to demonstrate proximal conduction slowing and block in demyelinating neuropathies. In plexopathy, ability to elicit an MEP several days after onset of paresis was good evidence of neuronal continuity in motor fibers.

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.

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

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

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.

A theoretical calculation of the electric field induced by magnetic stimulation of a peripheral nerve

A mathematical model is presented that predicts the electric field induced in the arm during magnetic stimulation of a peripheral nerve. The arm is represented as a homogeneous, cylindrical volume conductor. The electric field arises from two sources: the time-varying magnetic field and the accumulation of charge on the arm surface. In magnetic stimulation both of these contributions are significant. The magnitude of the electric field is greatest near the surface of the arm, and is well localized. Various coil orientations are examined; the smallest electric fields are induced when the coil is perpendicular to the arm surface, the largest when the coil is parallel. These results are consistent with many experimental observations in the literature, and aid in the basic understanding of magnetic stimulation of the peripheral nervous system.

Clinical use of the magnetic stimulator in the investigation of peripheral conduction time

The application of rapidly changing magnetic fields (magnetic stimulation) over the neck or lower back elicits EMG responses in the muscles of the arm or leg respectively. Such responses have stable onset latencies but their amplitudes vary depending on the position of the coil over the neck or lower back. Supramaximal responses could not be obtained. Comparison of onset latencies with estimates of peripheral conduction time using a conventional F-wave technique suggest that the site of excitation of the motor axons is about 1.3 msec conduction time distal to the cervical motoneurons and 3 msec distal to the lumbosacral motoneurons. Response configuration after paravertebral magnetic stimulation was similar to that of the standard electrically evoked M-wave in the small hand muscles but not in lower limb muscles. Responses in lower limb muscles after paravertebral magnetic stimulation may consist of additional F-wave and H-reflex components. The possible clinical role of paravertebral magnetic stimulation in the investigation of peripheral and central motor pathways is discussed in the light of these findings.

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.

The influence of stimulus type on the magnetic excitation of nerve structures

The voltage measured was that induced in a measuring coil from 3 different commercially available magnetic stimulators. The strongest stimulus was from the Cadwell, followed by Novametrix and then Digitimer. The Digitimer and Novametrix produced a monophasic pulse, whilst the Cadwell stimulator produced a polyphasic pulse, all measured by an induction coil. This is thought to be the reason why reversed coil polarity does not influence the position of peripheral nerve excitation with a Cadwell stimulator; this is, however, the case with the two other magnetic stimulators. Nevertheless, electrical stimulation was found to be the most useful method for exciting peripheral nerves. The lack of influence of Cadwell coil polarity on the excitation of spinal roots and motor cortex is also thought to be due to the bipolar stimulus effect mentioned above. The stimuli induced by Digitimer and Novametrix are monophasic, exciting one hemisphere first, depending on the direction of the current impulse. The stimulus generated by Cadwell excites both hemispheres by reversal of current direction.

Cortical magnetic stimulation in amyotrophic lateral sclerosis

Forty patients with ALS underwent cortical magnetic stimulation. Twelve had marked pseudobulbar signs; in these motor evoked potentials (MEPs) could not be elicited. Mean MEP latencies in the others, who had predominantly lower motor neuron signs, measured 23.3 +/- 2.1 msec (thenar), 18.7 +/- 5.3 msec (EDC), and 13.4 +/- 2.9 msec (biceps), respectively. These values were significantly longer (P greater than 0.001) compared with normal values (n = 35), which measured 20.2 +/- 1.6, 14.2 +/- 1.7, and 9.4 +/- 1.7 msec, respectively. MEP amplitude was often markedly reduced (less than 15% of the M wave) compared with a normal mean of 39.5 +/- 13.0%. Overall abnormal MEPs (delayed, absent, or reduced in amplitude) approached 100%. It is argued that measuring central motor delay, which was not significantly different in the patients compared with normals, is subject to error in ALS.