P422 Stimulation of peripheral nerves using a novel magnetic coil

Poster Session: Motor Evoked Potentials tremor in comparison to that of non-parkinsonian tremors. In this connection the author come to the conclusion that the obtained results can help the nosometricion of the PD (p < 0.05), PS (p < 0.05) and ET (p < 0.02).

Motor Evoked Potentials [P~

MATHEMATICALMODELLING OF EDDY CURRENTS IN HUMAN BRAIN DURING TMS

A. Krawczyk, S. Wiak, T. Zyss. Institute of Electrical Engineering,

Warsaw, Poland The paper deals with the problem of how to model eddy currents in neural cells in human brain while the transcranial magnetic stimulation (TMS) is considered. The magnetic stimulation uses low frequency pulse magnetic field in order to induce eddy currents in brain space. The value of current density which is required to occur in neural cells is somehow known, thus one needs to predict the magnetic field causes such a current. It can be done experimentally but then a lot of experiments should be carried out on physical models as well as on human subjects. This is very cumbersome way and we suggest that mathematical modelling should be employed here. The human brain can be modelled as a multilayer sphere and it is subjected to time-varying magnetic field that is homogenous. Prescribing values of electrical parameters to each particular layer, one establishes the mathematical model which is described by set of partial differential equations. In the paper such a model has been described and this is the first step. The next step is to solve this model and in order to do it the one of numerical methods should be applied. For the purpose considered, the finite element method has been used and the method is oriented towards some specific features of the brain. The numerical method as well as the results of calculations are presented.

DESIGN AND INSTRUMENTATION OF A REPETITIVE RAPID RATE MAGNETIC STIMULATOR A. Domino, A. Krawczyk, P. Kwasnowski, J. Waszczyszyn, T. Zyss.

Institute of Electrotechnics, Warsaw, Poland The repetitive rapid-rate transcranial magnetic stimulation (rr-TMS) opens totally new areas of interests in neurophysiology. This method is capable of stimulating at rates similar to those used for direct electrical stimulation. The currently brand and serially made magnetic stimulators (Dantec Mag Pro and Cadwell Rapid-Rate Magnetic Stimulator) used for the technique of rr-TMS have two major pitfalls. The devices may operate with frequencies of up to 30 (60) Hz, but at rates above 15-25 Hz, the power supply is incapable of charging the capacitors bank fully, and so the amplitude of the output drops. The second problem is that stimulation induces a rapid overheating of coil, which limits repetition rates and the total stimulation time. At our laboratory, we have begun a program to develop magnetic stimulator for rr-TMS with following parameters: induction of magnetic field B = 1-2 T, frequency up to 100 Hz, total stimulation time up to 1-5 minutes. Stimulators of similar characteristics are unavailable - for technological reasons they are not manufactured. Three main activities are described: a) fabrication of prototype stimulator coils with water/oil cooling in closed system, b) developing a high power supply with a high voltage, high current switching element, c) developing an electronics controller.

~

REPETITIVE MAGNETIC STIMULATIONS OF THE RAT

Poul Jennum, Henrik Klitgaard. Department of Clinical Neurophysiology,

UniversiO, of Copenhagen, Denmark As repetitive transcranial magnetic stimulation (RTMS) has been reported to induce epileptic seizures we studied the effect of acute or chronic stimulations with RTMS on the induction of pentylenetetrazol (PTZ) induced clonic seizures in the rat. Male Wistar rats were stimulated with a stimulus frequency of 50 hz. The motor threshold (Tm) was determined by single transcranial stimuli. Acute stimulation was performed with a stimulus

373

intensity of 0.9 x Tm and 1.5 × Tm using a duration of the train of stimuli of five seconds. Chronic stimulation was performed with a duration of the train of stimuli of one and five seconds using a stimulusintensity of 1.8 × Tm. The stimulations was performed every day in 30 days. Time to onset of PTZ induced clonic seizure was determined after the acute or the last stimulation in chronic RTMS. Some rats showed facial contractions, chewing or head movements during or immediately after the stimulations, but none of the rats developed tonic or clonic seizures. Time to onset of PTZ clonic seizures was reduced in both groups receiving chronic RTMS with a stimulus duration of one and five seconds compared control rats, whereas no differences were observed in the rats receiving acute RTMS. These results suggest that acute RTMS does not affect the induction of clonic PTZ seizures, whereas chronic stimulations possess a facilatory effect. This indicates that chronic stimulation with RTMS especially if the stimulus intensity is further increased - may induce a kindling process in the rat.

~-2-~

METABOLICCHANGES PRODUCED BY REPETITIVE MAGNETIC BRAIN STIMULATION

A. Oliviero, V. Di Lazzaro, D. Restuccia, L. Ferrara, D. Iacono, F. Della Corte i, M.A. Pennisi i O. Piazza I p. Profice, P. Tonali. Institute of

Neurology, Catholic University, Rome; I Institute of Anesthesiology, Catholic University, Rome, Italy The hemodynamic effects of repetitive magnetic stimulation (RMS) of the human brain were investigated by several authors that reported an increase of the cerebral blood flow (CBF). CBF is closely coupled with brain metabolism. To evaluate if CBF changes induced by RMS are associated with metabolic activation we measured the brain oxygen availability and consumption before and after RMS using NIRS (Near Infra-Red Spectroscopy; CRITIKON Cerebral Redox Research Monitor 2001). NIRS is a non invasive technique based on the ability of near infrared light (650-1000 nm) to pass through tissues. We studied 10 healthy volunteers and evaluated cerebral variation in HHb, HbO2, and Cit aa3 correlated to magnetic stimulation. The M R S probe was placed on motor and premotor areas. Magnetic stimulation was performed using a Magstim 200 (Novamelrix, UK). Thirty stimuli (100% of the maximal stimulator output; 0.25 Hz) were delivered through a figure of 8 coil over the premotor and motor cortex. The data averaging of 10 minutes recording before and of 5 minutes after the magnetic stimulation were recorded. Immediately after stimulation a significant increase in HbO2 and a decrease in Cit aa3 was observed (p < 0.05). Our data suggest that RMS induces metabolic activation of the cerebral cortex together with an increase of CBE

[ - P ' 4 ~ STIMULATIONOF PERIPHERAL NERVES USING A NOVEL MAGNETIC COIL F. Binkofski l, j. Classen 2, R. Benecke 3. ~Dept. of Neurology,

Heinrich-Heine-University, Dusseldorf, Germany; "National Institutes of Health, Bethesda, USA; 3 University of Rostock, Germany The clinical use of magnetic peripheral nerve stimulation has been limited because of its inability to produce maximal compound action potentials (CMAP) and of the difficulty to assess the exact site of nerve excitation. We characterized the electrophysiological properties of a novel figure-8 magnetic coil (Magstim Co.) differing from conventional coils in size (O of one wing 25 mm) and peak magnetic field (4.6 T). Stimulation of motor nerves of upper and lower extremities (N. ulnaris, N. medianus, N, radialis, N. tibialis, N. peroneus, N. femoralis) was compared with conventional electric stimulation in 4 healthy volunteers and in 4 patients with conduction block at the ulnar groove. Single magnetic pulses were delivered through the coil connected with a magnetic stimulator (Magstim 200, Novametrix). Maximal CMAP could be elicited by magnetic stimulation in each nerve of the upper and lower extremity stimulated. Amplitudes and shapes of CMAPs, as well as motor conduction velocities were comparable with magnetic and electrical stimulation within 5% range. The virtual cathode of the small butterfly coil was empirically located at the anterior meeting point of the windings. Variability of latencies was minimal (< 0.1%) in maximal CMAPs. The differences in the latencies between the magnetic and electric stimulation did not exceed 0.1 ms. The conduction velocities obtained over shorter distances at the ulnar groove and over the caput fibulae were also similar with magnetic and electric stimulation in

Poster Session: Motor Evoked Potentials

374

controls and patients. Small deviations of the coil from the optimal position led to a rapid decrease of CMAP. The conduction velocities in 4 patients with entrapment syndromes at the ulnar groove were comparable to those of the electric stimulation. However, in 2 patients CMAP were considerably smaller after magnetic stimulation, probably due to suboptimal placement of the coil. We conclude that electrodiagnostic results similar to standard electric stimulation can be produced with the novel magnetic coil. We suggest that in certain patient populations (e.g. children) when painlessness is crucial to the test setting magnetic peripheral stimulation be employed first. Pathological results, however, need to be confirmed by electric stimulation.

~ - ~

THE COMBINED ELECTRICAL AND MAGNETIC STIMULATION IN DIAGNOSIS OF LUMBOSACRAL RADICULOPATHY

M. Rakowicz, D. Wochnik-Dyjas, D. Sieklicka. Institute of Psychiat~

and Neurology, Warsaw, Poland Twenty-two patients aged from 17 to 65 years with radiculopathy due to lumbosacral disc herniation were investigated by combined paravertebral magnetic and peripheral nerves electrical stimulation. The functional abnormalities of individual spinal motor roots L3, L4, L5 and SI supplying Vastus lateralis, Tibialis anterior, Extensor digitorum brevis and Soleus muscles were correlated with clinical and MRI findings. Following bilateral electrical stimulation of femoral, peroneal and tibial nerves, the CMAPs and F waves were recorded from the appropriate muscles. The motor root evoked potential (Rx-MEP) from homologous muscles were obtained using paravertebral magnetic stimulation. The measured latencies of CMAPs, F wave and Rx-MEP allow calculation of the motor root conduction time (RxCT), F wave conduction time and proximal motor conduction velocity. In 92 of 136 (68%) examined muscles the complex of analysed parameters disclosed abnormality of motor root innervation. The RxCT, a main parameter were prolonged in 32% of muscles indicating slowing in the cauda equina segment or at the exit zone. A shortened RxCT was obtained in 30% of muscles reflecting pathology of main root, blocking of its F wave or axonal loss, depending on normal or reduced peripheral CMAP. In 52 of 136 (38%) roots a normal RxCT was observed suggesting the origin of F wave from healthy or from preserved neighbouring roots coinnervating involved muscles. The functional deterioration of motor root innervation was found in all 22 patients, including nine with sensory deficit only. The motor root dysfunction was topographically wider than can expected from clinical syndromes or MRI findings. The hypothesis of a vascular pathogenesis of spinal nerve root impairment will be discussed.

[--~--~

TRANSIENT SCOTOMAS AND PHOSPHENES INDUCED BY TRANSCRANIAL MAGNETIC STIMULATION

S. Kastner 2, I. Demmer t , U. Ziemann i. i Dept. of Clinical Neurophysiol,

University of Goettingen, Germany; 2 Dept. of Psychiatry, University of Goettingen, Germany Transcranial magnetic stimulation (TMS) was used to investigate transiently induced scotomas and phosphenes in 16 volunteers using a Dantec stimulator with a maximum output of 1.4 Tesla and a round coil of 9 cm diameter. The volunteers had to detect small, bright dots presented randomly for 20 ms in one of 60 locations on a computer screen resulting in a plot of the central 9 ° of the visual field, while TMS was applied 100 ms after stimulus presentation over the occipital pole (midline; 2-4 cm above inion). In 10/16 volunteers transient scotomas were inducible at TMS intensities of 85-100%. Scotomas varied in shape. They included large pans of the lower hemifield and never extended beyond the horizontal meridian (HM) except for the very foveal portion (1-3 °) which sometimes exceeded the HM resulting in a complete scotoma of the central portion of the visual field. Phosphenes were induced at clearly lower TMS-intensities of 50-70% in the dark. Most subjects described hyperbola-like phosphenes along the HM fading to the periphery. Some volunteers perceived chromatophosphenes. The known topography of the human visual cortex suggests that transient scotomas of the central portion of the visual field (up to 3°) are most likely due to stimulation of striate cortex (V 1) while scotomas of the lower

hemifield probably relate to stimulation of extrastriate areas (V2, V3). The lower intensity needed to induce phosphenes as well as their specific configuration suggest a different stimulation site which probably is that part of the optic radiation directly adjacent to the lateral ventricles.

MODULATION OF MOTOR EVOKED POTENTIAL TO MAGNETIC TRANSCRANIAL STIMULATION BY CONTRALATERAL AND IPSILATERAL DIGITAL ELECTRICAL STIMULATION P. Manganotti, G. Zanette, C. Bonato, M. Tinazzi, A. Polo, A. Fiaschi.

Dipartimento di Scienze Neurologiche e della Visione, Sezione di Neurologia, Universita" di Verona, Italy We evaluated the influence of contralateral and ipsilateral cutaneous digital nerve stimulation on the motor evoked potential (MEP) elicited in hand muscles by transcranial magnetic stimulation (TMS). Ten normal subjects were examined. Circular and focal magnetic coils were used. The MEP was recorded from right thenar eminence muscle. Electrical stimulus to the right and left index finger (D2) preceded the cortical magnetic stimulus. TMS was at 10%-15% above threshold. The interstimuhis time intervals (ISI) between the D2 and magnetic shock were set between 0 to 200 msec with steps of 10 msec. The inhibition of MEP was inversely related to the magnetic stimulus intensity; the smaller TMS stimulus intensity produced the larger decrement of MEP. We reproduced the pattern of facilitation (ISI: 10-20 msec), inhibition (ISI: 30--60 msec) and late mild facilitation (ISI: 70-100 msec) of MEP following digital nerve stimulation ipsilateral to the target muscle. The ipsilateral MEP inhibition ranges from 50% to 70% of control MEP. Main finding of the study was MEP inhibition following contralateral digital stimulation (ISI 40-90 msec) ranging from 30% to 50%. The different onset of inhibitory effect between contralateral and ipsilateral stimulation ranged from 10 to 20 msec. Spinal and cortical mechanisms should be considered to explain direct and crossed inhibitory effects.

~2-6~

TOPOGRAPHIC CHANGES OF CORTICAL MOTOR AREAS IN PATIENTS WITH FACIAL PALSY

M. Tegenthoff, J. Liepen, M. Rijntjes 1, S. Kotterba, C. Weiller i, J.-P. Malin. Department of Neurology, BG-Kliniken Bergrnannsheil,

Ruhr-University Bochura, Germany; i Department of Neurology, University Essen, Germany Background: The ability of the cortex to adapt to functional impairments is called "cortical plasticity". Changes of motor cortex areas can be investigated by focal transcranial magnetic stimulation (TMS). We studied the cortical motor area of a small hand muscle in patients with slight and severe facial palsies. Methods: Eight patients with a peripheral facial palsy (duration: 29 days to 36 ys) were studied. Five patients exhibited a severe, three patients a slight palsy. Motor representation areas of the abductor pollicis brevis muscle (APB) were mapped within a 1 × 1 cm 2 coordinate system using focal TMS with a stimulus intensity of 120% motor threshold. Results: The calculation of the center of gravity (CoG) showed a lateralisation over the hemisphere contralaterally to the affected side in patients with a clinical severe palsy, but not in patients with a slight palsy. Significant changes in a fronto-occipital direction could not be confirmed. The representation area of the APB was significantly enlarged over the hemisphere contralaterally to the facial palsy. Conclusions: The results demonstrate that the cortical motor area of a hand muscle shows a lateral extension into the cortical face muscle representation dependent on the clinical severity of a facial palsy. A reduction of GABAergic intracortical inhibition or axonal sprouting must be discussed as possible underlying mechanisms.