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. Key Words: spinal nerve, magnetic coil, root stimulation, brachial plexus MUSCLE & NERVE 13:414-420 1990
A COMPARISON OF MAGNETIC AND ELECTRICAL STIMULATION OF SPINAL NERVES B.A. EVANS, MD, J.R. DAUBE, MD, and W.J. LITCHY, MD
T h e magnetic coil stimulator offers a theoretic advantage over conventional bipolar electrical stimulation in depdarizing deep nerves, since its time-varying magnetic field is not attenuated by intervening high-impedance structures such as skin and bone. This advantage might be exploited in the study of brachial plexus lesions through stimulation of the proximal cervical spinal nerves or the proximal brachial plexus. Percutaneous electrical stimulation of cervical spinal nerves is possible with special high-voltage, low-output impedance stimulators6 but is uncomfortable. Stimulation of cervical spinal nerves with a needle electrode placed onto a cervical lamina is a well described technique5 but is invasive and uncomfortable. The purpose of this study is to evaluate the use of the magnetic coil for stimulation of the cervical spinal nerves and to define its advantages, if any, with respect to conventional electrical stimulation with a needle cathode placed onto the C7 or C6 spinal lamina. Comparisons include the abil-
From the Division of Clinical Neurophysiology, Mayo Clinic, Rochester, Minnesota. Address reprint requests to Bruce A. Evans, MD. Mayo Clinic W-8A, 200 1st St. SW, Rochester, MN 55905 Presented in part at the AAEEiAEEGS Joint Symposium on Sornatosensory Evoked Potentials and Magnetlc Stimulation, San Diego. California, October 5, 1988. Accepied for publication June 1, 1989. CCC 0148-639X1901050414-07 $04.00 0 1990 John Wiley & Sons, Inc.
414
Cervical Spinal Nerve Stimulation
ity to supramaximally stimulate the appropriate nerves, the apparent site of the nerve trunk depolarization, and the discomfort involved in the different stimulation procedures.
METHODS
Six normal volunteers were studied using a Nicolet Viking two-channel EMG machine (Nicolet Biomedical Instruments, Madison, WI). Peripheral nerve conduction studies employed standard methods. Surface recording electrodes placed over the biceps muscle recorded compound muscle action potentials (CMAPs) with bipolar surface electrical stimulation of the musculocutaneous nerve at the axilla and Erbs point. Recording electrodes placed over the abductor digiti minimi (ADM) muscle recorded CMAPs with stimulation of the ulnar nerve at the wrist, elbow, upper arm, and at Erb's point. F-wave latency was obtained to the ADM with stimulation of the ulnar nerve at the wrist. Amplitude, area, and latency were recorded for the CMAP recorded at each site. Electrical spinal nerve stimulation was obtained using a 75-mm monopolar needle with a 3-mm bare tip placed onto the C7 spinal lamina with a surface anode plate placed lateral to it. Supramaximal CMAPs were obtained from the ADM. The needle was then repositioned over the C6 lamina and supramximal CMAPs recorded from the biceps muscle. The amplitude, area, and latency were recorded for the CMAP obtained from each stimulation site.
MUSCLE & NERVE
May 1990
Magnetic stimulation was carried out with a Novametrix Magstim Model 200 (Novametrix Medical Systems, Inc, Wallingford CT). The center of a 14-cm diameter magnetic coil was placed successively at points 2 cm apart defining a grid over the posterior neck and trunk. Although not the center of the the site of nerve depolari~ation,~ coil was used as a convenient point of reference for coil position. Positions which would be expected to result in significant stimulation of intracranial contents were, however, avoided. CMAPs were recorded simultaneously from the ADM and biceps electrodes in separate channels with each stimulation. Maximum stimulator output was used at each stimulation site. Amplitude, area, and latency were recorded for each CMAP. The study was repeated on one subject using a Cadwell MES10 magnetic stimulator (Cadwell Laboratories, Inc, Kennewick WA). Subjects were asked to comment on the discomfort of stimulation with the magnetic coil, with C7 lamina electrical stimulation, and with C6 lamina electrical stimulation, compared with the discomfort of the surface bipolar electrical stimulation of the ulnar nerve at Erb’s point.
RESULTS Optimum Magnetic Stimulator Head Placement.
With coil electron current flow counterclockwise, the average optimum coil position for C5-6 spinal nerve stimulation, as defined by the center of the coil, was 2 cm below (range 2 cm above to 6 cm below) and 4 cm lateral (range 2 to 6 cm) to the C7 spinous process. The average optimum coil position for C8-T1 spinal nerve stimulation was 6 cm below (range 2 to 12 cm) and 4 cm lateral to (range 0 to 8 cm) the C7 spinous process. In these average positions (Fig. l), the appropriate roots and spinal nerves are tangentially oriented to the coil, and the induced tissue electron current flow is away from the spinal cord, ensuring a distal virtual cathode and avoiding anodal block. With coil electron current flow clockwise, the average optimal coil position for C8-Tl spinal nerve stimulation was 4 cm above the C7 spinous process (range 2 to 6 cm) and in the midline (range midline to 4 cm lateral). In this position (Fig. 2), the CS-T1 roots and spinal nerves are tangentially oriented to the lower edge of the coil, again with induced current flow away from the spinal cord and a distal virtual cathode. Similar
FIGURE 1. Average optimum stimulation position in six trials for C8-TI spinal nerve stimulation (left) and C5-6 spinal nerve stimulation (right) when the stimulator head current (electron) flow is counterclockwise.
Cervical Spinal Nerve Stimulation
MUSCLE & NERVE
May 1990
415
FIGURE 2. Average optimum stimulation position for C8-Tl spinal nerve stimulation when the stimulator head current (electron) tlow is clockwise.
placement with respect to the C5-6 spinal nerves was not attempted due to the intracranial stimulation which would accompany those positions. There was no consistent advantage in terms of amplitude of the response to orientations of the stimulator head resulting in clockwise or counterclockwise current flow, although the placement to obtain those maximum values differed considerably as shown in Figs. 1 and 2. Average maximal biceps CMAPs evoked by electrical and magnetic stimulation respectively were similar in amplitude (one-tailed paired-sample t-test, p < 0.57) and area (P < 0.85). Average maximal ADM CMAPs evoked by electrical and magnetic stimulation of the C8-T 1 spinal nerves showed a trend in favor of electrical stimulation which did not reach statistical significance for amplitude ( P < 0.12) or area (P < 0.13). In 3 subject trials, however, magnetic stimulationevoked CMAP amplitude and area were less than 80% of the electrically evoked CMAP. For CMAPs obtained with peripheral electrical percutaneous stimulation of the musculocutaneous and ulnar nerves, regression lines were plotted comparing amplitude and area (normalized to the value obtained with the most distal stimulation site) with the distance between the stimulation site and the G-1 recording electrode. This line was taken to represent the theoretical supramaximal Comparison of Evoked CMAPs.
416
Cervical Spinal Nerve Stimulation
CMAP amplitude or area for stimulation at a site a given distance from the G-1 muscle electrode, compensating to some degree for physiologic dispersion. Responses to spinal nerve stimulation with the needle electrode and with the magnetic coil were plotted on the same graphs (Figs. 3 and 4). For C5-6 spinal nerve stimulation, performance of the stimulation techniques was similar, although one magnetic stimulation trial resulted in amplitude and area far below the regression line. For C8-TI spinal nerve stimulation, one needle stimulation evoked response and three magnetic stimulation evoked responses fell considerably below the regression line. Almost identical amplitudes and latencies were obtained in a repeat study of one subject using the Cadwell MES- 10 magnetic stimulator. A theoretic latency for conduction of the fastest motor fibers from the spinal cord to the ADM G-1 electrode can be derived from the F-wave latency and the distal latency obtained with ulnar nerve stimulation [(F-wave latency + Distal latency)/2]. The average C8-T 1 electrical needle stimulation obtained latency was 0.95 msec shorter than this theoretic cord-(;- 1 latency. Allowing for central turnaround time in the F response, the site of needle electrode C8-T1 root depolarization appears to be near the spinal cord (Table 1).
Apparent Site of Spinal Nerve Stimulation.
MUSCLE & NERVE
May 1990
Biceps CMAP - Amplitude
1.4
1.2
normalized amplitude
1 A n n
I .(I J
:I
..* ..*
0
00
-
-
0
;
b 0
.
,
,
,
0
.
,
0.2 11x1
0
?(XI
, mi
3(XI
distance
Biceps CMAP - Area
0.2
1 mi
0
,
I
I
mi
XXI
4(X1
distance FIGURE 3. Biceps CMAP amplitude (top) and area (bottom) as a function of the distance between the stimulation site and the recording electrodes. Open circles represent conventional percutaneous electrical stimulation at the axilla and at Erbs point.
Table 1. (A) Relative site of stimulation: ADM
A
B C D E
F
Table 1. (B) Biceps
~
~~
Subject
Needle Magnetic G1-cord Cord-needle Needlelatency latency latency diff magnetic (rnsec) (msec) (msec)' (msec) diff (cm)+ 14.9 15.1
13.2 14.2 14.9 13.4 14.0 16.7
15.0 14.1 15.4 17.3
15.4 16.4 16.1 15.6 15.8 18.2
Average
+
0.5 1.3
1.1 1.5 0.4 0.9 0.95 rnsec
10.2 5.4 0.6 4.2 8.2 3.5 5.4 cm
*(F-wave latency Distal latency)/Z +(Needle root latency magnetic foot latency) x proximal conduction velocity x 0 1
-
Cervical Spinal Nerve Stimulation
Subject
C
F E D A B Average
Needle latency (rnsec) 5.8 5.9 5.8 5.9 6.0 5.8
Magnetic latency (msec)
Needlemagnetic diff (crn)*
5.6
1.5
5.I
5.3
5.4 5.2 5.4 5.3
3.4 4.4 4.1 3.0 3.6
'(Needle root latency - magnetic root latency) x proximal conduction velocity x 0 1
MUSCLE & NERVE
May 1990
417
ADM CMAP - Amplitude o
pcnphrral
root ncdlc
rn
rruit magnctic
normalized amplitude
**
ADM CMAP - Area o
area
peripheral
rmt n d l e
rmt magnctic
", I(XX1
(1.4
02 0
2tX)
4(*
H)o
8(W)
distance FIGURE 4. ADM CMAP amplitude (top) and area (bottom) as a function of the distance between the stimulation site and the recording electrodes. Open circles represent conventional percutaneous electrical stirnulation at the wrist, elbow, upper arm, and at Erb's point.
The average site of magnetic coil C8-T1 spinal nerve depolarization with the coil orientation and position where the maximal CMAP amplitude/ area was obtained was 5.4 cm more distal than with C8-Tl needle stimulation based on the difference in latencies and the proximal conduction velocity (Table 1). On the average, this depolarization point would be in the region of the proximal medial cord of the brachial plexus. Shifting the stimulator coil medially from the optimum amplitude position up to 4 cm resulted, on the average, in stimulation 1.8 cm closer to the cord, but with a marked loss of amplitude (Table 2). The average site of magnetic coil C5-6 spinal nerve stimulation with the coil orientation and position where the maximal CMAP amplitudelarea was obtained was 3.6 cm more distal than with C5- 6 needle stimulation, based on the difference
418
Cervical Spinal Nerve Stimulation
in latencies and the proximal conduction velocity. On the average, this depolarization point would be in the region of the upper trunk of the brachial plexus proximal to the divisions. Shifting the stimulator coil medially from the optimum amplitude position up to 4 cm resulted, on the average, in stimulation 3.4 cm closer to the cord and approximating the estimated site of depolarization with needle stimulation based on latency, but with a substantial loss of amplitude (Table 2). C8-T 1 needle spinal nerve stimulation was felt to be slightly more uncomfortable than magnetic stimulation. C5 - 6 needle spinal nerve stimulation was felt to be more uncomfortable than magnetic stimulation, but less so than percutaneous stimulation at Erb's point. Comparison of Discomfort.
MUSCLE & NERVE
May 1990
Table 2. Latency shifts with lateral displacement from the optimum amplitude site (0 cm) (average n Biceps Lateral shift (relative to 0) Amplitude (relative to 0) Depolarization point shift (cm) (Avg CV 69 mlsec) ADM Lateral shift (relative to 0) Amplitude (relative to 0) Depolarization point shift (cm) (Avg CV 69 m/sec)
6)
-4 cm 63 34
-2 cm a3 21
0 cm 10 0
2 cm 90 -1 4
4 cm 49
-4 cm
-2 cm
66 06
0 cm 10 0
2 cm
23 18
4 cm a2 -0 6
DISCUSSION
In a previous study,4 we found that current magnetic stimulation devices were inferior to percutaneous electrical stimulation in the performance of routine peripheral nerve conduction studies. This is due mainly to an inability to reliably obtain supramaximal stimulation and to uncertainty with regard to the precise site of depolarization. The possible advantages of magnetic stimulation in the study of deeper structures such as the spinal nerves or proximal brachial or lumbar plexuses have prompted several descriptions of such use. The first' did not directly compare the technique to electrical stimulation with a needle electrode, and the ADM CMAP amplitudes recorded are far smaller than would be expected with supramaxima1 stimulation. Three other also demonstrated the practicality of obtaining a recordable response with spinal nerve or proximal plexus stimulation, without direct comparison of the electrical stimulation in the same patients. Conway, et a13 reported a series of 12 patients with cervical magnetic stimulation, 4 of whom had electrical spinal nerve stimulation with a needle electrode. No comparison of CMAP amplitudes is mentioned. We conclude from the present study that magnetic stimulation of the C5-6 spinal nerves results in CMAP amplitudes and areas comparable to those obtained with needle stimulation in most subjects. Magnetic stimulation of the deeper C8T1 spinal nerves results in amplitudes and areas that are often less than those obtained with needle electrical stimulation. This is significant since recognition of the presence or absence of conduction block is an important goal when studying proximal nerves. Conway3 compared the results of 2 magnetic coil placements involving the use of cervical surface landmarks. The marked variability we find in the coil position giving the maximal CMAP ampli-
Cervical Spinal Nerve Stimulation
=
-4 1
95 0
tude/area suggests that no single landmark scheme can be successful in located the optimal placement in all patients, and that systematic searching movement of the coil head with numerous repetitions is necessary. Initial placement may be guided by visualizing the course of the desired roots or spinal nerves describing a line parallel to a tangent but overlapping the edge of the coil by 2 or 3 cm. Maximum amplitudes usually result when the direction of the electron current flow in the stimulator head is towards the midline along the edge of the coil overlapping the course of the nerve. The electrical needle stimulation takes less time since the responses may be obtained with stimulation at the initial electrode position, as in our experience trial and error movement is seldom necessary. We find that stimulation with a needle electrode results in depolarization of the spinal nerves close to the spinal cord, similar to the results of using special high-voltage stimulators and percutaneous electrical stimulation in the midline.6 Conway3 found electrically and magnetically induced latencies similar, but we found that magnetic stimulation at sites yielding maximal amplitude/area CMAPs was taking place several cm distal to the site of electrical needle stimulation. Medial shifts of the magnetic coil from this position appeared to result in somewhat more proximal stimulation, but at the cost of lost amplitude of the response. This suggests that electrical stimulation is preferable in the study of possible proximal lesions. Magnetic stimulation was judged less painful, especially at C5-6, but was not without discomfort as is sometimes claimed. Only one of the six subjects felt that the needle electrode stimulation was unacceptably painful and more painful than SUpramaximal percutaneous stimulation at Erb's point.
MUSCLE 8, NERVE
May 1990
419
We conclude that the current generation of magnetic stimulators offers little advantage other than comfort and several disadvantages compared
to electrical stimulation with a needle electrode when studying cervical nerve spinal nerves and the proximal brachial plexus.
REFERENCES 1. Chokroverty S, DiLullo J: Magnetic coil stimulation of the lumbosacral roots and proximal nerves. Muscle Nerve 1988; 11:996-997. 2 . Chokroverty S, Sachdeo R, Duvoisin RC: Magnetic stimulation of the lumbosacral roots: A new diagnostic technique. Neurology 1988; 38(suppl 1):387. 3. Conway RR, Hof J, Buckelew S: Lower cervical magnetic stimulation: Comparison with C8 needle root stimulation and supraclavicular stimulation. Muscle Nerve 1988; 11:977. 4. Evans BA, Litchy WJ, Daube JRD: The utility of magnetic stimulation for routine peripheral nerve conduction studies. Muscle Nerve 1988; 11:1074- 1078. 5. MacLean IC: Nerve root stimulation to evaluate conduction
420
Cervical Spinal Nerve Stimulation
across the brachial and lumbosacral plexuses. Third Annual Continuing Education Course, American Association of Electromyography and Electrodiagnosis, September 25, 1980, Philadelphia, PA. 6. Mills KR, Murray NMF: Electrical stimulation over the human vertebral column: Which neural elements are excited? Electroenceph Clin Neurophysiol 1986; 63:582-589. 7. Santamaria J, King PJL, Cros D, Chiappa KH: Cervical magnetic stimulation: Roots or spinal nerves. Neurologsl 1988; 38(suppl 1):199. 8. Spire JP, Maselli RA, Soliven B, McCaffrey M, Welsh D, Crate JR, OHira T : Magnetic stimulation of the human peripheral nervous system. Muscle Nerue 1987; 10:643.
MUSCLE & NERVE
May 1990
A COMPARISON OF MAGNETIC AND ELECTRICAL STIMULATION OF SPINAL NERVES B.A. EVANS, MD, J.R. DAUBE, MD, and W.J. LITCHY, MD
T h e magnetic coil stimulator offers a theoretic advantage over conventional bipolar electrical stimulation in depdarizing deep nerves, since its time-varying magnetic field is not attenuated by intervening high-impedance structures such as skin and bone. This advantage might be exploited in the study of brachial plexus lesions through stimulation of the proximal cervical spinal nerves or the proximal brachial plexus. Percutaneous electrical stimulation of cervical spinal nerves is possible with special high-voltage, low-output impedance stimulators6 but is uncomfortable. Stimulation of cervical spinal nerves with a needle electrode placed onto a cervical lamina is a well described technique5 but is invasive and uncomfortable. The purpose of this study is to evaluate the use of the magnetic coil for stimulation of the cervical spinal nerves and to define its advantages, if any, with respect to conventional electrical stimulation with a needle cathode placed onto the C7 or C6 spinal lamina. Comparisons include the abil-
From the Division of Clinical Neurophysiology, Mayo Clinic, Rochester, Minnesota. Address reprint requests to Bruce A. Evans, MD. Mayo Clinic W-8A, 200 1st St. SW, Rochester, MN 55905 Presented in part at the AAEEiAEEGS Joint Symposium on Sornatosensory Evoked Potentials and Magnetlc Stimulation, San Diego. California, October 5, 1988. Accepied for publication June 1, 1989. CCC 0148-639X1901050414-07 $04.00 0 1990 John Wiley & Sons, Inc.
414
Cervical Spinal Nerve Stimulation
ity to supramaximally stimulate the appropriate nerves, the apparent site of the nerve trunk depolarization, and the discomfort involved in the different stimulation procedures.
METHODS
Six normal volunteers were studied using a Nicolet Viking two-channel EMG machine (Nicolet Biomedical Instruments, Madison, WI). Peripheral nerve conduction studies employed standard methods. Surface recording electrodes placed over the biceps muscle recorded compound muscle action potentials (CMAPs) with bipolar surface electrical stimulation of the musculocutaneous nerve at the axilla and Erbs point. Recording electrodes placed over the abductor digiti minimi (ADM) muscle recorded CMAPs with stimulation of the ulnar nerve at the wrist, elbow, upper arm, and at Erb's point. F-wave latency was obtained to the ADM with stimulation of the ulnar nerve at the wrist. Amplitude, area, and latency were recorded for the CMAP recorded at each site. Electrical spinal nerve stimulation was obtained using a 75-mm monopolar needle with a 3-mm bare tip placed onto the C7 spinal lamina with a surface anode plate placed lateral to it. Supramaximal CMAPs were obtained from the ADM. The needle was then repositioned over the C6 lamina and supramximal CMAPs recorded from the biceps muscle. The amplitude, area, and latency were recorded for the CMAP obtained from each stimulation site.
MUSCLE & NERVE
May 1990
Magnetic stimulation was carried out with a Novametrix Magstim Model 200 (Novametrix Medical Systems, Inc, Wallingford CT). The center of a 14-cm diameter magnetic coil was placed successively at points 2 cm apart defining a grid over the posterior neck and trunk. Although not the center of the the site of nerve depolari~ation,~ coil was used as a convenient point of reference for coil position. Positions which would be expected to result in significant stimulation of intracranial contents were, however, avoided. CMAPs were recorded simultaneously from the ADM and biceps electrodes in separate channels with each stimulation. Maximum stimulator output was used at each stimulation site. Amplitude, area, and latency were recorded for each CMAP. The study was repeated on one subject using a Cadwell MES10 magnetic stimulator (Cadwell Laboratories, Inc, Kennewick WA). Subjects were asked to comment on the discomfort of stimulation with the magnetic coil, with C7 lamina electrical stimulation, and with C6 lamina electrical stimulation, compared with the discomfort of the surface bipolar electrical stimulation of the ulnar nerve at Erb’s point.
RESULTS Optimum Magnetic Stimulator Head Placement.
With coil electron current flow counterclockwise, the average optimum coil position for C5-6 spinal nerve stimulation, as defined by the center of the coil, was 2 cm below (range 2 cm above to 6 cm below) and 4 cm lateral (range 2 to 6 cm) to the C7 spinous process. The average optimum coil position for C8-T1 spinal nerve stimulation was 6 cm below (range 2 to 12 cm) and 4 cm lateral to (range 0 to 8 cm) the C7 spinous process. In these average positions (Fig. l), the appropriate roots and spinal nerves are tangentially oriented to the coil, and the induced tissue electron current flow is away from the spinal cord, ensuring a distal virtual cathode and avoiding anodal block. With coil electron current flow clockwise, the average optimal coil position for C8-Tl spinal nerve stimulation was 4 cm above the C7 spinous process (range 2 to 6 cm) and in the midline (range midline to 4 cm lateral). In this position (Fig. 2), the CS-T1 roots and spinal nerves are tangentially oriented to the lower edge of the coil, again with induced current flow away from the spinal cord and a distal virtual cathode. Similar
FIGURE 1. Average optimum stimulation position in six trials for C8-TI spinal nerve stimulation (left) and C5-6 spinal nerve stimulation (right) when the stimulator head current (electron) flow is counterclockwise.
Cervical Spinal Nerve Stimulation
MUSCLE & NERVE
May 1990
415
FIGURE 2. Average optimum stimulation position for C8-Tl spinal nerve stimulation when the stimulator head current (electron) tlow is clockwise.
placement with respect to the C5-6 spinal nerves was not attempted due to the intracranial stimulation which would accompany those positions. There was no consistent advantage in terms of amplitude of the response to orientations of the stimulator head resulting in clockwise or counterclockwise current flow, although the placement to obtain those maximum values differed considerably as shown in Figs. 1 and 2. Average maximal biceps CMAPs evoked by electrical and magnetic stimulation respectively were similar in amplitude (one-tailed paired-sample t-test, p < 0.57) and area (P < 0.85). Average maximal ADM CMAPs evoked by electrical and magnetic stimulation of the C8-T 1 spinal nerves showed a trend in favor of electrical stimulation which did not reach statistical significance for amplitude ( P < 0.12) or area (P < 0.13). In 3 subject trials, however, magnetic stimulationevoked CMAP amplitude and area were less than 80% of the electrically evoked CMAP. For CMAPs obtained with peripheral electrical percutaneous stimulation of the musculocutaneous and ulnar nerves, regression lines were plotted comparing amplitude and area (normalized to the value obtained with the most distal stimulation site) with the distance between the stimulation site and the G-1 recording electrode. This line was taken to represent the theoretical supramaximal Comparison of Evoked CMAPs.
416
Cervical Spinal Nerve Stimulation
CMAP amplitude or area for stimulation at a site a given distance from the G-1 muscle electrode, compensating to some degree for physiologic dispersion. Responses to spinal nerve stimulation with the needle electrode and with the magnetic coil were plotted on the same graphs (Figs. 3 and 4). For C5-6 spinal nerve stimulation, performance of the stimulation techniques was similar, although one magnetic stimulation trial resulted in amplitude and area far below the regression line. For C8-TI spinal nerve stimulation, one needle stimulation evoked response and three magnetic stimulation evoked responses fell considerably below the regression line. Almost identical amplitudes and latencies were obtained in a repeat study of one subject using the Cadwell MES- 10 magnetic stimulator. A theoretic latency for conduction of the fastest motor fibers from the spinal cord to the ADM G-1 electrode can be derived from the F-wave latency and the distal latency obtained with ulnar nerve stimulation [(F-wave latency + Distal latency)/2]. The average C8-T 1 electrical needle stimulation obtained latency was 0.95 msec shorter than this theoretic cord-(;- 1 latency. Allowing for central turnaround time in the F response, the site of needle electrode C8-T1 root depolarization appears to be near the spinal cord (Table 1).
Apparent Site of Spinal Nerve Stimulation.
MUSCLE & NERVE
May 1990
Biceps CMAP - Amplitude
1.4
1.2
normalized amplitude
1 A n n
I .(I J
:I
..* ..*
0
00
-
-
0
;
b 0
.
,
,
,
0
.
,
0.2 11x1
0
?(XI
, mi
3(XI
distance
Biceps CMAP - Area
0.2
1 mi
0
,
I
I
mi
XXI
4(X1
distance FIGURE 3. Biceps CMAP amplitude (top) and area (bottom) as a function of the distance between the stimulation site and the recording electrodes. Open circles represent conventional percutaneous electrical stimulation at the axilla and at Erbs point.
Table 1. (A) Relative site of stimulation: ADM
A
B C D E
F
Table 1. (B) Biceps
~
~~
Subject
Needle Magnetic G1-cord Cord-needle Needlelatency latency latency diff magnetic (rnsec) (msec) (msec)' (msec) diff (cm)+ 14.9 15.1
13.2 14.2 14.9 13.4 14.0 16.7
15.0 14.1 15.4 17.3
15.4 16.4 16.1 15.6 15.8 18.2
Average
+
0.5 1.3
1.1 1.5 0.4 0.9 0.95 rnsec
10.2 5.4 0.6 4.2 8.2 3.5 5.4 cm
*(F-wave latency Distal latency)/Z +(Needle root latency magnetic foot latency) x proximal conduction velocity x 0 1
-
Cervical Spinal Nerve Stimulation
Subject
C
F E D A B Average
Needle latency (rnsec) 5.8 5.9 5.8 5.9 6.0 5.8
Magnetic latency (msec)
Needlemagnetic diff (crn)*
5.6
1.5
5.I
5.3
5.4 5.2 5.4 5.3
3.4 4.4 4.1 3.0 3.6
'(Needle root latency - magnetic root latency) x proximal conduction velocity x 0 1
MUSCLE & NERVE
May 1990
417
ADM CMAP - Amplitude o
pcnphrral
root ncdlc
rn
rruit magnctic
normalized amplitude
**
ADM CMAP - Area o
area
peripheral
rmt n d l e
rmt magnctic
", I(XX1
(1.4
02 0
2tX)
4(*
H)o
8(W)
distance FIGURE 4. ADM CMAP amplitude (top) and area (bottom) as a function of the distance between the stimulation site and the recording electrodes. Open circles represent conventional percutaneous electrical stirnulation at the wrist, elbow, upper arm, and at Erb's point.
The average site of magnetic coil C8-T1 spinal nerve depolarization with the coil orientation and position where the maximal CMAP amplitude/ area was obtained was 5.4 cm more distal than with C8-Tl needle stimulation based on the difference in latencies and the proximal conduction velocity (Table 1). On the average, this depolarization point would be in the region of the proximal medial cord of the brachial plexus. Shifting the stimulator coil medially from the optimum amplitude position up to 4 cm resulted, on the average, in stimulation 1.8 cm closer to the cord, but with a marked loss of amplitude (Table 2). The average site of magnetic coil C5-6 spinal nerve stimulation with the coil orientation and position where the maximal CMAP amplitudelarea was obtained was 3.6 cm more distal than with C5- 6 needle stimulation, based on the difference
418
Cervical Spinal Nerve Stimulation
in latencies and the proximal conduction velocity. On the average, this depolarization point would be in the region of the upper trunk of the brachial plexus proximal to the divisions. Shifting the stimulator coil medially from the optimum amplitude position up to 4 cm resulted, on the average, in stimulation 3.4 cm closer to the cord and approximating the estimated site of depolarization with needle stimulation based on latency, but with a substantial loss of amplitude (Table 2). C8-T 1 needle spinal nerve stimulation was felt to be slightly more uncomfortable than magnetic stimulation. C5 - 6 needle spinal nerve stimulation was felt to be more uncomfortable than magnetic stimulation, but less so than percutaneous stimulation at Erb's point. Comparison of Discomfort.
MUSCLE & NERVE
May 1990
Table 2. Latency shifts with lateral displacement from the optimum amplitude site (0 cm) (average n Biceps Lateral shift (relative to 0) Amplitude (relative to 0) Depolarization point shift (cm) (Avg CV 69 mlsec) ADM Lateral shift (relative to 0) Amplitude (relative to 0) Depolarization point shift (cm) (Avg CV 69 m/sec)
6)
-4 cm 63 34
-2 cm a3 21
0 cm 10 0
2 cm 90 -1 4
4 cm 49
-4 cm
-2 cm
66 06
0 cm 10 0
2 cm
23 18
4 cm a2 -0 6
DISCUSSION
In a previous study,4 we found that current magnetic stimulation devices were inferior to percutaneous electrical stimulation in the performance of routine peripheral nerve conduction studies. This is due mainly to an inability to reliably obtain supramaximal stimulation and to uncertainty with regard to the precise site of depolarization. The possible advantages of magnetic stimulation in the study of deeper structures such as the spinal nerves or proximal brachial or lumbar plexuses have prompted several descriptions of such use. The first' did not directly compare the technique to electrical stimulation with a needle electrode, and the ADM CMAP amplitudes recorded are far smaller than would be expected with supramaxima1 stimulation. Three other also demonstrated the practicality of obtaining a recordable response with spinal nerve or proximal plexus stimulation, without direct comparison of the electrical stimulation in the same patients. Conway, et a13 reported a series of 12 patients with cervical magnetic stimulation, 4 of whom had electrical spinal nerve stimulation with a needle electrode. No comparison of CMAP amplitudes is mentioned. We conclude from the present study that magnetic stimulation of the C5-6 spinal nerves results in CMAP amplitudes and areas comparable to those obtained with needle stimulation in most subjects. Magnetic stimulation of the deeper C8T1 spinal nerves results in amplitudes and areas that are often less than those obtained with needle electrical stimulation. This is significant since recognition of the presence or absence of conduction block is an important goal when studying proximal nerves. Conway3 compared the results of 2 magnetic coil placements involving the use of cervical surface landmarks. The marked variability we find in the coil position giving the maximal CMAP ampli-
Cervical Spinal Nerve Stimulation
=
-4 1
95 0
tude/area suggests that no single landmark scheme can be successful in located the optimal placement in all patients, and that systematic searching movement of the coil head with numerous repetitions is necessary. Initial placement may be guided by visualizing the course of the desired roots or spinal nerves describing a line parallel to a tangent but overlapping the edge of the coil by 2 or 3 cm. Maximum amplitudes usually result when the direction of the electron current flow in the stimulator head is towards the midline along the edge of the coil overlapping the course of the nerve. The electrical needle stimulation takes less time since the responses may be obtained with stimulation at the initial electrode position, as in our experience trial and error movement is seldom necessary. We find that stimulation with a needle electrode results in depolarization of the spinal nerves close to the spinal cord, similar to the results of using special high-voltage stimulators and percutaneous electrical stimulation in the midline.6 Conway3 found electrically and magnetically induced latencies similar, but we found that magnetic stimulation at sites yielding maximal amplitude/area CMAPs was taking place several cm distal to the site of electrical needle stimulation. Medial shifts of the magnetic coil from this position appeared to result in somewhat more proximal stimulation, but at the cost of lost amplitude of the response. This suggests that electrical stimulation is preferable in the study of possible proximal lesions. Magnetic stimulation was judged less painful, especially at C5-6, but was not without discomfort as is sometimes claimed. Only one of the six subjects felt that the needle electrode stimulation was unacceptably painful and more painful than SUpramaximal percutaneous stimulation at Erb's point.
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We conclude that the current generation of magnetic stimulators offers little advantage other than comfort and several disadvantages compared
to electrical stimulation with a needle electrode when studying cervical nerve spinal nerves and the proximal brachial plexus.
REFERENCES 1. Chokroverty S, DiLullo J: Magnetic coil stimulation of the lumbosacral roots and proximal nerves. Muscle Nerve 1988; 11:996-997. 2 . Chokroverty S, Sachdeo R, Duvoisin RC: Magnetic stimulation of the lumbosacral roots: A new diagnostic technique. Neurology 1988; 38(suppl 1):387. 3. Conway RR, Hof J, Buckelew S: Lower cervical magnetic stimulation: Comparison with C8 needle root stimulation and supraclavicular stimulation. Muscle Nerve 1988; 11:977. 4. Evans BA, Litchy WJ, Daube JRD: The utility of magnetic stimulation for routine peripheral nerve conduction studies. Muscle Nerve 1988; 11:1074- 1078. 5. MacLean IC: Nerve root stimulation to evaluate conduction
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across the brachial and lumbosacral plexuses. Third Annual Continuing Education Course, American Association of Electromyography and Electrodiagnosis, September 25, 1980, Philadelphia, PA. 6. Mills KR, Murray NMF: Electrical stimulation over the human vertebral column: Which neural elements are excited? Electroenceph Clin Neurophysiol 1986; 63:582-589. 7. Santamaria J, King PJL, Cros D, Chiappa KH: Cervical magnetic stimulation: Roots or spinal nerves. Neurologsl 1988; 38(suppl 1):199. 8. Spire JP, Maselli RA, Soliven B, McCaffrey M, Welsh D, Crate JR, OHira T : Magnetic stimulation of the human peripheral nervous system. Muscle Nerue 1987; 10:643.
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