Jump to ContentJump to Main Navigation
Laboratory Reference for Clinical Neurophysiology$

Jay A. Liveson and Dong M. Ma

Print publication date: 1999

Print ISBN-13: 9780195129243

Published to Oxford Scholarship Online: March 2012

DOI: 10.1093/acprof:oso/9780195129243.001.0001

Show Summary Details
Page of

PRINTED FROM OXFORD SCHOLARSHIP ONLINE (www.oxfordscholarship.com). (c) Copyright Oxford University Press, 2018. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a monograph in OSO for personal use (for details see www.oxfordscholarship.com/page/privacy-policy). Subscriber: null; date: 16 January 2019

Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Chapter:
(p.357) Chapter 17 Evoked Motor Responses of the Brain, Spinal Cord, and Roots
Source:
Laboratory Reference for Clinical Neurophysiology
Author(s):

Jay A. Liveson

Dong M. Ma

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780195129243.003.0017

Abstract and Keywords

Since 1980, techniques have been developed to stimulate the motor cortex using surface stimulation. P. A. Merton and H. B. Morton utilized brief, high-voltage stimuli. Subsequently, A. T. Barker and associates developed magnetic stimulation. These methods have been used to stimulate cortical structures and also the spinal cord and root regions. The safety, side effects, reliability, and clinical applicability of these procedures are still being evaluated. Potentially, they present a measure of descending motor pathways, from the cortex, through the cord, to the periphery. Special stimulators are required. High-voltage methodology necessitates an output of 750 volts and a low-output impedance. Magnetic stimulation is via a flat helical coil. A large current (maximally 5,000 amperes) produces a magnetic field of two tesla. The former procedure is better localized. The latter is painless and can be delivered without skin contact, but it is difficult to localize the point of stimulation precisely.

Keywords:   motor cortex, surface stimulation, high-voltage stimuli, magnetic stimulation, spinal cord, motor pathways, magnetic field

Since 1980, techniques have been developed to stimulate the motor cortex using surface stimulation. Merton and Morton (1980)474 utilized brief, high-voltage stimuli. Subsequently, Barker and associates (1985)34,35 developed magnetic stimulation. These methods have been used to stimulate cortical structures and also the spinal cord and root regions. The safety, side effects, reliability, and clinical applicability of these procedures are still being evaluated. Potentially, they present a measure of descending motor pathways, from the cortex, through the cord, to the periphery.

Special stimulators are required. High-voltage methodology necessitates an output of 750 V, and a low-output impedance. Magnetic stimulation is via a flat helical coil. A large current (maximally 5000 A) produces a magnetic field of 2 tesla. The former procedure is better localized. The latter is painless and can be delivered without skin contact, but it is difficult to localize the point of stimulation precisely.

Stimulation Procedure (Fig. 103)

Setup

Stimulation

Magnetic coil is placed over the vertex of the skull (Sv). Cervical stimulation is over the lower cervical cord, approximately C5/6(Sc). Lower lumbar stimulation is approximately over L4/5.

(p.358)

                      Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Figure 103. Evoked motor response studies, transcranial (Sv) and cervical (Sc).

Ground

This is placed near the recording electrode.

Recording

Surface electrodes over the biceps brachii (BB), abductor pollicis brevis (APB), tibialis anterior (TA). Other muscles used include the abductor digiti minimi (ADM) and abductor hallucis (AH).

Values

Barker et al. (1987)34

s = 27 (21−61 y.o., m = 39)                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

(p.359) Ingram, Thompson, and Swash (1988)296

s = 10 (14−53 y.o., m = 37)                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Technical Comments

The stimulator can be directed to the opposite side by turning the coil over. The evoked response can be increased in amplitude and decreased in latency by minimal voluntary contraction of the muscle being monitored. Contralateral potentiation also occurs but with a slightly greater contraction. Central conduction is calculated by subtracting the latency on segmental stimulation from the cortically evoked latency.

Studies

Hess et al. (1987)276

n = 46 s = 32 (21−78 y.o., m = 36.8)                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Magnetic stimulation of cortex and cervical spine. Recording from abductor digiti minimi (ADM). Central time is calculated by subtracting the cervical latency from the cortical latency.

(p.360) Chokroverty et al. (1989)109

s = 7                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Cadwell magnetic stimulator; coil placed in midline and moved vertically; stimulation performed at 100% of output.

Schmid, Hess, and Ludin (1989)588

s = 31                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Stimulation over C7/T1 or C6/7; onset latency.

Schmid et al. (1990)587

s = 48 (23 – 39 y.o., m = 28.21)                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Transcutaneous stimulation over the prominens, using magnetic field of 2.3 tesla; onset latency and negative peak amplitude taken.

(p.361) Dvorak, Herdmann, and Theiler (1990)170

n = 103 s = 53 (18−75 y.o., m = 42)                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Stimulation via a Dantec or Novametric ME 200 magnetic stimulator yielding a field of 1.5 to 2 tesla; transcranial stimulation of upper limb at the vertex; cervical stimulation over C7; neck in flexion and muscle facilitation achieved by having patient keep arms outstretched; latency taken to three muscles; in addition, latency difference between central and cervical stimulation measured — central motor latency.

Seki et al. (1990)600

n = 20 s = 10 (28−42 y.o., m = 36.3)                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Cadwell MES-10 stimulator; stimulation point maximal over T5 (EEG localization), and supramaximal stimulation; surface recording from nasalis muscles (contralateral functioning as active electrode referred to ipsilateral side); peak-to-peak amplitude; onset latency.

Britton et al. (1990)68

s = 5 (25−44 y.o.)                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Largest first dorsal interosseous (FDI) response occurs with stimulation over C3, while biceps response is maximal with C5 stimulation; optimal position for extensor digitorum brevis (EDB) response over S3, while quadriceps response maximal over L1.

Eisen and Shtybel (1990)183

s = 150 (14−85 y.o.) (p.362)                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Correlation with arm length:

Latency = 0.19 × arm length + 6.9

If amplitude of response is compared to the maximum M wave, it is never less than 20% in normals; therefore 10% considered abnormal; in addition, side-to-side amplitude difference is never greater than 25%; therefore difference of 〉50% is taken as abnormal.

Central delay in motor pathway (CMD)

CMD = [magnetic latency] – ([F latency – M latency] – 1)/2                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Dantec stimulator used, delivering magnetic field of 2.3 tesla; stimulation performed at 120% of threshold for motor response; cortical stimulation of the hand or arm area performed by placing coil tangential to scalp, just anterior to vertex; facial area stimulated 4 cm lateral to vertex on line connecting vertex and external auditory meatus; lower cervical stimulation over the spinous process of C7; voluntary contraction between 15% and 75% of maximum had similar effect, 15% to 25% used; in some normal subjects, no response could be elicited from tibialis anterior. Room temperature 20°C to 22°C.

Cros et al. (1990)131

s = 21 (21−47 y.o.)                       Evoked Motor Responses of the Brain, Spinal Cord, and Roots

Two stimulators used (with equivalent results) were Novametrix Magstim 200 (1.5 tesla) and Cadwell MES-10 (2.2 tesla); maximum responses occurred with coil centered on spinous process: over C3 or C4 for biceps response, over C4 for triceps response, and C5 for abductor digiti V response.