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 interindividualand intraindividual reproducibilityof the magnetically evoked latencies were at least as good as those of electric stimulation when coil-skin distance was 5 2 cm. Key words: coil position electric stimulation magnetic stimulation motor evoked potentials peripheral nerves MUSCLE 81 NERVE 13681-686 1990
SIGNIFICANCE OF MAGNETIC COIL POSITION IN PERIPHERAL MOTOR NERVE STIMULATION MADS RAVNBORG, MD, MORTEN BLINKENBERG, MD, and KRlSTlAN DAHL. MSc
By magnetic stimulation of the cerebral cortex the total latencies of compound motor action potentials (CMAPs) can be measured.2s3The stimulation is painless and without significant side effects, which allows multiple stimulations. The promising aspect of the method is the possibility of central motor conduction analysis. For this analysis the peripheral component of the total latency must be determined for subtraction. However, stimulation with a magnetic coil, usually 10-14 cm in diameter, makes definite localization of the depolarization difficult. Furthermore, the stimulations are often submaximal and even minor displacements of the coil may have a pronounced effect on the evoked CMAPS.~ We made this study to define the optimal position of the magnetic coil in relation to a peripheral From the Departments of Neurology (Drs. Ravnborg and Blinkenberg) and Clinical Neurophysiology (Mr Dahl). Rigshospitalet, Copenhagen, Denmark Acknowledgement: The study was supported by grants from the Danish Multiple Sclerosis Society and "Fondsbprsvekselerer Henry Hansen og hustru, Carla Hansen, f. Westergaards legat". Address correpondence to Dr. Ravnborg. Dept. of Neurology, Rigshospitalet. Blegdarnsvej 9,2200 Copenhagen N.,Denmark. Accepted for publication September 4,1989. CCC 0148-639Xl30/080681-06$04.00 0 1990 John Wiley 8 Sons, Inc.
Magnetic Nerve Stimulation
nerve, searching for the most constant relationship to the electrically evoked CMAP with the least dependence on stimulus intensity and coil- nerve distance. Having found this coil position, we compared the reproducibility of magnetically evoked CMAPs with those obtained by electric stimulation. MATERIALS AND METHODS
10 healthy volunteers participated in the study (median age 30 years, range 22-38 years). The study was approved by the local Ethics Committee and informed consent was obtained from all subjects. A Dantec magnetic stimulator was used (Dantec, Copenhagen). This stimulator delivers a maximal output of 1,500 VDCand 10,000 A with a rise time of 150 ps. The stimulus intensity (SI) can be adjusted on a scale from O%, to 100%. The outer diameter of the stimulating coil is 14 cm. Fourteen copper windings are found within 2 through 6 cm from the center. Seven lines were drawn on the coil with 1-cm intervals from the center (L-0) to the outer rim (L-6) (Fig. 1). The recordings were made with a Counterpoint (Dantec, Copenhagen). A gain of 1 mVIcm was used for detection of onset latencies. Filter settings were: high-pass 1 Hz, low-pass 5 kHz. An AgIAgC1 surface electrode was fixed over the first dorsal interosseus muscle. The neurovascular bundle in the
MUSCLE & NERVE
August 1990
681
the wrist was used to record possible median nerve components of the first dorsal interosseus CMAP. The magnetic coil was placed on the skin with the center line (L-0) over and parallel to the line indicating the neurovascular bundle with the center of the coil at the stimulation point. In three volunteers the effect of reversing the coil current was studied. Amplitudes and latencies obtained in each volunteer for the two current directions were compared by the Wilcoxon test for paired data. On the basis of these results we chose to use only proximally directed coil-current for stimulation throughout the study. The coil was oriented tangentially to the surface contour. CMAPs were recorded from threshold to 100% SI at 10% intervals. This procedure was repeated as the coil was displaced parallelly in 1-cm steps in a lateral direction (Fig. 1). The complete stimulation procedure was repeated with PVC plates of 1-cm, 2-cm and 3-cm thickness interposed between coil and skin. Using the optimal coil position, the reproducibility of the CMAPs was analyzed in two persons at the four coil- skin distances. The stimulation procedures, including electrode attachment, were repeated 10 times.
FIGURE 1. The experimental set-up. On the magnetic coil the lines used for a standardized displacement are seen separated by 1-cm intervals. At the arm the course of the ulnar nerve is indicated.
medial bicipital groove was localized by palpation, and its course was marked on the skin. On this nerve line, a point, 14 cm above the medial epicondyle, was indicated and defined as the stimulation point. CMAPs evoked by supramaximal electrical cathode stimulations were recorded. Stimulation of the median nerve just proximal to
mp1. (%I 100
-
90 --
60
--
40
--
30
-A
20
A
10 I I
1
I
L- 1
L-2
1-3
I
0 L-0
1
0 L-4
? ./
Y
L-5
Coil position FIGURE 2. Mean values of the relative CMAP amplitudes (Amag/Ae, x 100%) evoked by a 100% stimulus intensity at coil-skin distances of 0 cm (m), 1 cm (a),2 cm (+) and 3 cm (0).L-0 through L-6 indicates the coil line overlying the nerve during stimulation.
682
Magnetic Nerve Stimulation
MUSCLE & NERVE
August 1990
RESULTS
the SDs were minimal at L-3 (Fig. 3). ‘I‘he interindividual variations of the magnetically evoked amplitudes at 0 cni C-S distance and 90%- 100% SI were similar to those obtained by electric stimulation (Table 1). Under the same circumstances the intraindividual variations in magnetic stimulation were slightly higher than in electric stimulation (Table 2).
Neither the electric nor the magnetic stimuli were found painful by any of the volunteers. However, the magnetic stimulation was felt to be more violent because the direct excitation of the biceps induces sudden flexion of the elbow. A coil orientation with the coil current directed proximally in the half of the coil overlying the nerve yielded significantly higher amplitudes and shorter latencies ( P < 0.001) than the reverse. The ranges of the latencies for all positions and SI were smallest for the former.
The changes in latency were expressed by the difference between the electrically and the magnetically evoked CMAP latencies, Ld = I, - L,. I n this way differences due to unequal arm lengths were avoided. The Lds recorded at a C-S distance of 0 cm decreased slightly as the coil was displaced laterally. T h e Ld variation caused by changes in coil-skin distance decreased progressively as the coil wa5 displaced. The most marked change occurred from L-2 to L-3 (Fig. 4). At L-5 and L-6, stimulation at 3-cm C-S distance became insufficient. The interindividual variations of the latencies evoked by magnetic stimulation at C-S distances of 0 cm were smaller than those obtained by electric stimulation (Table 3). The intraindividual variations were smaller for magnetic than for electric stimulation at C-S distances from 0 to 2 cm (Table 4). Onset Latencies.
With coil- skin ( C - S) distances of 0 and 1 cm, CMAP amplitudes ( N l ) reached peak values at L-3 for all stimulus intensities. At a C-S distance of 2 cm, L-3 gave the largest amplitudes with stimulus intensities (SI) >60%. Relow this intensity and at 3-cm C-S distance, the line of maximal excitation moved towards the center. At L-3 and a C-S distance of 0 cm, an amplitude of supramaximal magnitude was elicited in 6 of the 10 subjects. However, the amplitude decreased rapidly with increasing C- S distance (Fig. 2). Supramaximal electric median nerve stimulation resulted in maximal amplitudes of 0.7 mV. For 0-cm C-S distance and SI >70%
Amplitudes.
sod.(%)
T
’*t 1
I
1
I
I
FIGURE 3. Standard deviations of the relative CMAP amplitudes at 0 cm coil-skin distance using stimulus intensities of 100% (m), 90% (o),80% (+) and 70% (0).
Magnetic Nerve Stimulation
MUSCLE & NERVE
August 1990
683
Table 1. The interindividual variations of CMAP amplitudes. Magnetic stimulus intensities
C-S dist. 0 cm 1 cm 2 cm 3 cm
100%
90%
80%
70%
Electric stimulation
N
12.2 (2.48) 8.7 (2.63) 4.1 (3.13) 1.7 (1.92)
12.1 (2.69) 7.3 (2.93) 3.1 (2.93) 0.7 (0.61)
11. I (3.47) 5.8 (3.36) 1.8 (1.77) 0.7 (0.29)
10.3 (4.23) 4.0 (3.1) 0.9 (0.59) 0.7 (0.10)
12.5 (2.47) 13.0 (3.32) 13.3 (3.01) 13.6 (2.50)
10 10 9 6
Means and standard deviations () of the absolute amplitudes (mV) evoked by magnetic stimulation in the optimal position (L-3) and with coil-skin distances of 0 cm through 3 cm The values obtained by electric stimulation during each of the 4 sessions are shown to the right N = number of sublects
Table 2. The intraindividual variation of CMAP amplitudes. Coil-skin distance during magnetic stimulation
0 cm
1 cm
2 cm
3 cm
Electric stimulation
14.4 (3.27) 15.4 (2.65) A : l O , B:10
12.8 (3.27) 15.2 (2.29) A:10, B:10
5.7 (3.72) 12.6 (2.23) A:IO, B:10
0.4 (0.24) 2.7 (1.62) A:9, B:10
14.3 (3.62) 16.2 (2.07) A:10, B:10
Subject A
0
N
Means and standard deviations (j of the amplitudes (mVj in 2 sublects (A and 6’) obtained at 10 separate occasions. The results of stimulation with 100% stimulus intensity and coil-skin distances of 0 through 3 cm are shown. N = number of observations. A: Coil current directed proximally. 5: Coil current directed distally
Table 3. The interindividual variation of CMAP onset latencies Magnetic stimulus intensities C-S dist. 0 cm 1 cm 2 cm 3 cm
100%
90%
80%
70%
Electric stimulation
N
10.14 (0.81) 10.74 (1.05) 11.03 (1.26) 11.58 (2.09)
10.27 (0.83) 10.94 (1.08) 10.99 (1.21) 11.50 (1.46)
10.30 (0.83) 10.96 (1.03) 11.33 (1.21) 11.23 (1.45)
10.49 (0.89) 11 29 (1.19) 11.53 (1.26) 11.30 (1.39)
11.42 (1.02) 11.69 (1.07) 11.58 (1.12) 11.54 (1.08)
10 10 9 6
Means and standard deviations (j of the latencies (msec) evoked by magnetic stimulation in the optimal position (L-3) and with coil-skin distances of 0 cm through 3 cm. The values obtained by electric stimulation during each of the 4 sessions are shown to the right. N = number of subjects.
DISCUSSION
Electric stimulation of superficial nerves in the limbs is well tolerated by the patients, and there is no need for a magnetic technique for these purposes. However, electric stimulation of profound nerves is painful. Therefore, magnetic stimulation may be a valuable technique for painfree stimulation of deep-lying nerves. The value of magnetically evoked motor potentials (MEMPs) in peripheral nerve studies has been analyzed in several ~ t u d i e s . ” ~ One6 * ~ states that MEMPs do not differ significantly from CMAPs obtained by electric stimulation, but the confidence limits are not specified. Evans et all state that MEMPs are less reliable than electrically evoked motor po-
684
Magnetic Nerve Stimulation
ing to the poorly defined site of depolarization larization and because stimulation is often submaximal. However, this statement is not documented. Physical mapping of the magnetic fields induced by magnetic coils may be used for prediction of the optimal coil-nerve relationship. J. Nilsson et a14 used a I-cm measuring loop for such a mapping. The loop was oriented parallel to the coil regardless of the distance between the loop and the coil. However, a true magnetic vector can be obtained only by repeated measurings with the loop oriented parallel and perpendicular to the coil respectively. Another problem is that all nerves have longitudinal extensions while magnetic coils are circular. Therefore, the part of a
MUSCLE & NERVE
August 1990
1
0.5
0
-0.5
L- 1
L-0
L-2
L-3
L-5
L-4
L-6
Coil position FIGURE 4. Mean values of Ld (the difference between electrically and magnetically evoked latencies) using 80% stimulus intensity at coil-skin distances of 0 cm (m), 1 cm (a),2 cm (+) and 3 cm (0).
nerve underlying a stimulation coil will be exposed to a variety of magnetic field strengths and directions in one stimulation. The effect of a possible spatial summation of the induced voltages on the nerve cannot readily be predicted by physical mapping of the magnetic fields, when using small loops placed in various positions over the coil. A direct biological mapping of the depolarizing effects is adequate for an evaluation of the optimal position of the coil. In the present study we focused on the effects of changing coil-nerve geometry on onset latency and N1-amplitude. In contrast to Evans et al,' we
found a proximally directed coil current significantly more effective than the reverse. In this way the current induced in the tissue is directed distally as in standard cathodal stimulation. For a standard Dantec magnetic coil with a diameter of 14 cm, we found optimal excitation when the center of the coil was displaced 3 cm laterally to the nerve trunk. In this position, the latencies of magnetic CMAPs were at least as reproducible as the electric CMAPs at C-S distances up to 2 cm, both intraindividually and interindividually. In our experimental setup it was not possible to simulate deep-nerve electric stimulation. Thus, we were not
Table 4. The intraindividual variation of CMAP latencies. ~
Coil-skin distance during magnetic stimulation 0 cm
1 cm
2 cm
3 ern
Electric stirnulation
10.5 (0.31) 10.3 (0.28) A:10. B:10
10.9 (0.39) 10.4 (0.26) A:10, B:lO
11.4 (0.62) 10.6 (0.31) A:10, B:10
12.7 (1.36) 10.9 (0.34) A:9, B:lO
11.5 (0.60) 11.6 (0.34) A:10, B:lO
Subject A B N
Means and standard denations ( j of the latencies (msecj in 2 subjects ( A and 6) obtained at 10 separate occasions. The data represent optimal coil position (L-3), and with coil-skin distances of 0 through 3 cm and a stimulus intensity of 700%. N = number of observations. A: Coil current dfrected proximally. 6: Coil current directed distally.
Magnetic Nerve stimulation
MUSCLE & NERVE
August 1990
685
able to compare magnetic and electric stimulation of deeper lying nerves. Magnetic stimulation of nervous tissue involves more variables than the traditional electric stimulation, and our results emphasize the importance of standardization of these variables. The results of our standardization, however, cannot as a mat-
ter of course be applied to other coil sizes and manufactures. CMAP latencies evoked by magnetic stimulations of a straight peripheral nerve seem as reproducible as those obtained by electric stimulation if the magnetic coil is in the optimal position and the coil-nerve distance is 1 2 cm.
REFERENCES
1. Evans BA, Litchy WJ, Daube JR: T h e utility of magnetic stimulation for routine peripheral nerve conduction studies. Muscle Nerve 1988; 11:1074-1078. 2. Hess CW, Mills KR, Murray NMF: Responses in small hand muscles from magnetic stimulation of the human brain. J Phy~Zol1987; 3881397-419. 3. Ingram DA, Thompson AJ, Swash M: Central motor conduction in multiple sclerosis: Evaluation of abnormalities revealed by transcutaneous magnetic stimulation of the brain. J Nezirol Neurosurg Psychiatry 1988; 51 :487-494.
686
Magnetic Nerve Stimulation
4. Nilsson J , Panizza M, Cohen LG, Hallett M : Distinct mapping of induced voltages produced by different magnetic coils. AAEEIAEEGS Joint Sjmpo~ium, October 5th, 1988, poster 38. 5. Smith S.JM, Murray NMF: Electrical and magnetic stimulation of lower limb nerves and roots. M u s c l r A'rrur 1986;
9:652-653. 6. Spire J P , Maselli RA, Soliven B, McCaffrey M, Welsh D, Grate JR, O'Hira T: Magnetic stimulation of the human peripheral nervous system. Muscle Nerve 1987; 10:543.
MUSCLE & NERVE
August 1990
SIGNIFICANCE OF MAGNETIC COIL POSITION IN PERIPHERAL MOTOR NERVE STIMULATION MADS RAVNBORG, MD, MORTEN BLINKENBERG, MD, and KRlSTlAN DAHL. MSc
By magnetic stimulation of the cerebral cortex the total latencies of compound motor action potentials (CMAPs) can be measured.2s3The stimulation is painless and without significant side effects, which allows multiple stimulations. The promising aspect of the method is the possibility of central motor conduction analysis. For this analysis the peripheral component of the total latency must be determined for subtraction. However, stimulation with a magnetic coil, usually 10-14 cm in diameter, makes definite localization of the depolarization difficult. Furthermore, the stimulations are often submaximal and even minor displacements of the coil may have a pronounced effect on the evoked CMAPS.~ We made this study to define the optimal position of the magnetic coil in relation to a peripheral From the Departments of Neurology (Drs. Ravnborg and Blinkenberg) and Clinical Neurophysiology (Mr Dahl). Rigshospitalet, Copenhagen, Denmark Acknowledgement: The study was supported by grants from the Danish Multiple Sclerosis Society and "Fondsbprsvekselerer Henry Hansen og hustru, Carla Hansen, f. Westergaards legat". Address correpondence to Dr. Ravnborg. Dept. of Neurology, Rigshospitalet. Blegdarnsvej 9,2200 Copenhagen N.,Denmark. Accepted for publication September 4,1989. CCC 0148-639Xl30/080681-06$04.00 0 1990 John Wiley 8 Sons, Inc.
Magnetic Nerve Stimulation
nerve, searching for the most constant relationship to the electrically evoked CMAP with the least dependence on stimulus intensity and coil- nerve distance. Having found this coil position, we compared the reproducibility of magnetically evoked CMAPs with those obtained by electric stimulation. MATERIALS AND METHODS
10 healthy volunteers participated in the study (median age 30 years, range 22-38 years). The study was approved by the local Ethics Committee and informed consent was obtained from all subjects. A Dantec magnetic stimulator was used (Dantec, Copenhagen). This stimulator delivers a maximal output of 1,500 VDCand 10,000 A with a rise time of 150 ps. The stimulus intensity (SI) can be adjusted on a scale from O%, to 100%. The outer diameter of the stimulating coil is 14 cm. Fourteen copper windings are found within 2 through 6 cm from the center. Seven lines were drawn on the coil with 1-cm intervals from the center (L-0) to the outer rim (L-6) (Fig. 1). The recordings were made with a Counterpoint (Dantec, Copenhagen). A gain of 1 mVIcm was used for detection of onset latencies. Filter settings were: high-pass 1 Hz, low-pass 5 kHz. An AgIAgC1 surface electrode was fixed over the first dorsal interosseus muscle. The neurovascular bundle in the
MUSCLE & NERVE
August 1990
681
the wrist was used to record possible median nerve components of the first dorsal interosseus CMAP. The magnetic coil was placed on the skin with the center line (L-0) over and parallel to the line indicating the neurovascular bundle with the center of the coil at the stimulation point. In three volunteers the effect of reversing the coil current was studied. Amplitudes and latencies obtained in each volunteer for the two current directions were compared by the Wilcoxon test for paired data. On the basis of these results we chose to use only proximally directed coil-current for stimulation throughout the study. The coil was oriented tangentially to the surface contour. CMAPs were recorded from threshold to 100% SI at 10% intervals. This procedure was repeated as the coil was displaced parallelly in 1-cm steps in a lateral direction (Fig. 1). The complete stimulation procedure was repeated with PVC plates of 1-cm, 2-cm and 3-cm thickness interposed between coil and skin. Using the optimal coil position, the reproducibility of the CMAPs was analyzed in two persons at the four coil- skin distances. The stimulation procedures, including electrode attachment, were repeated 10 times.
FIGURE 1. The experimental set-up. On the magnetic coil the lines used for a standardized displacement are seen separated by 1-cm intervals. At the arm the course of the ulnar nerve is indicated.
medial bicipital groove was localized by palpation, and its course was marked on the skin. On this nerve line, a point, 14 cm above the medial epicondyle, was indicated and defined as the stimulation point. CMAPs evoked by supramaximal electrical cathode stimulations were recorded. Stimulation of the median nerve just proximal to
mp1. (%I 100
-
90 --
60
--
40
--
30
-A
20
A
10 I I
1
I
L- 1
L-2
1-3
I
0 L-0
1
0 L-4
? ./
Y
L-5
Coil position FIGURE 2. Mean values of the relative CMAP amplitudes (Amag/Ae, x 100%) evoked by a 100% stimulus intensity at coil-skin distances of 0 cm (m), 1 cm (a),2 cm (+) and 3 cm (0).L-0 through L-6 indicates the coil line overlying the nerve during stimulation.
682
Magnetic Nerve Stimulation
MUSCLE & NERVE
August 1990
RESULTS
the SDs were minimal at L-3 (Fig. 3). ‘I‘he interindividual variations of the magnetically evoked amplitudes at 0 cni C-S distance and 90%- 100% SI were similar to those obtained by electric stimulation (Table 1). Under the same circumstances the intraindividual variations in magnetic stimulation were slightly higher than in electric stimulation (Table 2).
Neither the electric nor the magnetic stimuli were found painful by any of the volunteers. However, the magnetic stimulation was felt to be more violent because the direct excitation of the biceps induces sudden flexion of the elbow. A coil orientation with the coil current directed proximally in the half of the coil overlying the nerve yielded significantly higher amplitudes and shorter latencies ( P < 0.001) than the reverse. The ranges of the latencies for all positions and SI were smallest for the former.
The changes in latency were expressed by the difference between the electrically and the magnetically evoked CMAP latencies, Ld = I, - L,. I n this way differences due to unequal arm lengths were avoided. The Lds recorded at a C-S distance of 0 cm decreased slightly as the coil was displaced laterally. T h e Ld variation caused by changes in coil-skin distance decreased progressively as the coil wa5 displaced. The most marked change occurred from L-2 to L-3 (Fig. 4). At L-5 and L-6, stimulation at 3-cm C-S distance became insufficient. The interindividual variations of the latencies evoked by magnetic stimulation at C-S distances of 0 cm were smaller than those obtained by electric stimulation (Table 3). The intraindividual variations were smaller for magnetic than for electric stimulation at C-S distances from 0 to 2 cm (Table 4). Onset Latencies.
With coil- skin ( C - S) distances of 0 and 1 cm, CMAP amplitudes ( N l ) reached peak values at L-3 for all stimulus intensities. At a C-S distance of 2 cm, L-3 gave the largest amplitudes with stimulus intensities (SI) >60%. Relow this intensity and at 3-cm C-S distance, the line of maximal excitation moved towards the center. At L-3 and a C-S distance of 0 cm, an amplitude of supramaximal magnitude was elicited in 6 of the 10 subjects. However, the amplitude decreased rapidly with increasing C- S distance (Fig. 2). Supramaximal electric median nerve stimulation resulted in maximal amplitudes of 0.7 mV. For 0-cm C-S distance and SI >70%
Amplitudes.
sod.(%)
T
’*t 1
I
1
I
I
FIGURE 3. Standard deviations of the relative CMAP amplitudes at 0 cm coil-skin distance using stimulus intensities of 100% (m), 90% (o),80% (+) and 70% (0).
Magnetic Nerve Stimulation
MUSCLE & NERVE
August 1990
683
Table 1. The interindividual variations of CMAP amplitudes. Magnetic stimulus intensities
C-S dist. 0 cm 1 cm 2 cm 3 cm
100%
90%
80%
70%
Electric stimulation
N
12.2 (2.48) 8.7 (2.63) 4.1 (3.13) 1.7 (1.92)
12.1 (2.69) 7.3 (2.93) 3.1 (2.93) 0.7 (0.61)
11. I (3.47) 5.8 (3.36) 1.8 (1.77) 0.7 (0.29)
10.3 (4.23) 4.0 (3.1) 0.9 (0.59) 0.7 (0.10)
12.5 (2.47) 13.0 (3.32) 13.3 (3.01) 13.6 (2.50)
10 10 9 6
Means and standard deviations () of the absolute amplitudes (mV) evoked by magnetic stimulation in the optimal position (L-3) and with coil-skin distances of 0 cm through 3 cm The values obtained by electric stimulation during each of the 4 sessions are shown to the right N = number of sublects
Table 2. The intraindividual variation of CMAP amplitudes. Coil-skin distance during magnetic stimulation
0 cm
1 cm
2 cm
3 cm
Electric stimulation
14.4 (3.27) 15.4 (2.65) A : l O , B:10
12.8 (3.27) 15.2 (2.29) A:10, B:10
5.7 (3.72) 12.6 (2.23) A:IO, B:10
0.4 (0.24) 2.7 (1.62) A:9, B:10
14.3 (3.62) 16.2 (2.07) A:10, B:10
Subject A
0
N
Means and standard deviations (j of the amplitudes (mVj in 2 sublects (A and 6’) obtained at 10 separate occasions. The results of stimulation with 100% stimulus intensity and coil-skin distances of 0 through 3 cm are shown. N = number of observations. A: Coil current directed proximally. 5: Coil current directed distally
Table 3. The interindividual variation of CMAP onset latencies Magnetic stimulus intensities C-S dist. 0 cm 1 cm 2 cm 3 cm
100%
90%
80%
70%
Electric stimulation
N
10.14 (0.81) 10.74 (1.05) 11.03 (1.26) 11.58 (2.09)
10.27 (0.83) 10.94 (1.08) 10.99 (1.21) 11.50 (1.46)
10.30 (0.83) 10.96 (1.03) 11.33 (1.21) 11.23 (1.45)
10.49 (0.89) 11 29 (1.19) 11.53 (1.26) 11.30 (1.39)
11.42 (1.02) 11.69 (1.07) 11.58 (1.12) 11.54 (1.08)
10 10 9 6
Means and standard deviations (j of the latencies (msec) evoked by magnetic stimulation in the optimal position (L-3) and with coil-skin distances of 0 cm through 3 cm. The values obtained by electric stimulation during each of the 4 sessions are shown to the right. N = number of subjects.
DISCUSSION
Electric stimulation of superficial nerves in the limbs is well tolerated by the patients, and there is no need for a magnetic technique for these purposes. However, electric stimulation of profound nerves is painful. Therefore, magnetic stimulation may be a valuable technique for painfree stimulation of deep-lying nerves. The value of magnetically evoked motor potentials (MEMPs) in peripheral nerve studies has been analyzed in several ~ t u d i e s . ” ~ One6 * ~ states that MEMPs do not differ significantly from CMAPs obtained by electric stimulation, but the confidence limits are not specified. Evans et all state that MEMPs are less reliable than electrically evoked motor po-
684
Magnetic Nerve Stimulation
ing to the poorly defined site of depolarization larization and because stimulation is often submaximal. However, this statement is not documented. Physical mapping of the magnetic fields induced by magnetic coils may be used for prediction of the optimal coil-nerve relationship. J. Nilsson et a14 used a I-cm measuring loop for such a mapping. The loop was oriented parallel to the coil regardless of the distance between the loop and the coil. However, a true magnetic vector can be obtained only by repeated measurings with the loop oriented parallel and perpendicular to the coil respectively. Another problem is that all nerves have longitudinal extensions while magnetic coils are circular. Therefore, the part of a
MUSCLE & NERVE
August 1990
1
0.5
0
-0.5
L- 1
L-0
L-2
L-3
L-5
L-4
L-6
Coil position FIGURE 4. Mean values of Ld (the difference between electrically and magnetically evoked latencies) using 80% stimulus intensity at coil-skin distances of 0 cm (m), 1 cm (a),2 cm (+) and 3 cm (0).
nerve underlying a stimulation coil will be exposed to a variety of magnetic field strengths and directions in one stimulation. The effect of a possible spatial summation of the induced voltages on the nerve cannot readily be predicted by physical mapping of the magnetic fields, when using small loops placed in various positions over the coil. A direct biological mapping of the depolarizing effects is adequate for an evaluation of the optimal position of the coil. In the present study we focused on the effects of changing coil-nerve geometry on onset latency and N1-amplitude. In contrast to Evans et al,' we
found a proximally directed coil current significantly more effective than the reverse. In this way the current induced in the tissue is directed distally as in standard cathodal stimulation. For a standard Dantec magnetic coil with a diameter of 14 cm, we found optimal excitation when the center of the coil was displaced 3 cm laterally to the nerve trunk. In this position, the latencies of magnetic CMAPs were at least as reproducible as the electric CMAPs at C-S distances up to 2 cm, both intraindividually and interindividually. In our experimental setup it was not possible to simulate deep-nerve electric stimulation. Thus, we were not
Table 4. The intraindividual variation of CMAP latencies. ~
Coil-skin distance during magnetic stimulation 0 cm
1 cm
2 cm
3 ern
Electric stirnulation
10.5 (0.31) 10.3 (0.28) A:10. B:10
10.9 (0.39) 10.4 (0.26) A:10, B:lO
11.4 (0.62) 10.6 (0.31) A:10, B:10
12.7 (1.36) 10.9 (0.34) A:9, B:lO
11.5 (0.60) 11.6 (0.34) A:10, B:lO
Subject A B N
Means and standard denations ( j of the latencies (msecj in 2 subjects ( A and 6) obtained at 10 separate occasions. The data represent optimal coil position (L-3), and with coil-skin distances of 0 through 3 cm and a stimulus intensity of 700%. N = number of observations. A: Coil current dfrected proximally. 6: Coil current directed distally.
Magnetic Nerve stimulation
MUSCLE & NERVE
August 1990
685
able to compare magnetic and electric stimulation of deeper lying nerves. Magnetic stimulation of nervous tissue involves more variables than the traditional electric stimulation, and our results emphasize the importance of standardization of these variables. The results of our standardization, however, cannot as a mat-
ter of course be applied to other coil sizes and manufactures. CMAP latencies evoked by magnetic stimulations of a straight peripheral nerve seem as reproducible as those obtained by electric stimulation if the magnetic coil is in the optimal position and the coil-nerve distance is 1 2 cm.
REFERENCES
1. Evans BA, Litchy WJ, Daube JR: T h e utility of magnetic stimulation for routine peripheral nerve conduction studies. Muscle Nerve 1988; 11:1074-1078. 2. Hess CW, Mills KR, Murray NMF: Responses in small hand muscles from magnetic stimulation of the human brain. J Phy~Zol1987; 3881397-419. 3. Ingram DA, Thompson AJ, Swash M: Central motor conduction in multiple sclerosis: Evaluation of abnormalities revealed by transcutaneous magnetic stimulation of the brain. J Nezirol Neurosurg Psychiatry 1988; 51 :487-494.
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4. Nilsson J , Panizza M, Cohen LG, Hallett M : Distinct mapping of induced voltages produced by different magnetic coils. AAEEIAEEGS Joint Sjmpo~ium, October 5th, 1988, poster 38. 5. Smith S.JM, Murray NMF: Electrical and magnetic stimulation of lower limb nerves and roots. M u s c l r A'rrur 1986;
9:652-653. 6. Spire J P , Maselli RA, Soliven B, McCaffrey M, Welsh D, Grate JR, O'Hira T: Magnetic stimulation of the human peripheral nervous system. Muscle Nerve 1987; 10:543.
MUSCLE & NERVE
August 1990
