Electroencephalography and clinical Neurophysiology, 84 (1992) 315-320 © 1992 Elsevier Scientific Publishers Ireland, Ltd. 0168-5597/92/$05.00
315
EVOPOT 90202
Intraoperative spinal cord monitoring during surgery for aortic aneurysm: application of spinal cord evoked potential Yuzuru Okamoto, Masazumi Murakami, Takeo Nakagawa, Atsushi Murata and Hideshige Moriya Department of Orthopaedic Surgery, School of Medicine, Chiba Uniuersity, Chiba (Japan) (Accepted for publication: 24 February 1992)
Summary Spinal cord evoked potentials elicited by direct stimulation of the spinal cord were monitored in 21 patients during thoracic or thoraco-abdominal aortic aneurysm surgery. Flexible catheter-type electrodes were used for both stimulating and recording. The basic pattern of the spinal cord evoked potential consisted of an initial spike and a subsequent polyphasiccomponent. The earliest and most frequent alterations after cross-clampingof the aorta were changes in the configuration or amplitude of the polyphasic component. In 13 patients who exhibited no change except minor alterations of the polyphasic component during the initial test clamping for 15 or 20 min, subsequent graft replacements were safely performed without reimplantation of intercostal vessels. In 2 patients who had sudden cardiac arrests, the evoked potential completely disappeared. The polyphasic component disappeared first, followed by the initial spike. Another patient developed acute loss of the potential after the aneurysm was incised, presumably due to distal aortic hypoperfusion. In this case, prolonged distal hypotension resulted in flaccid paraplegia. Intraoperative monitoring of the spinal cord evoked potential is a useful method for the early detection of spinal cord ischemia during surgery requiring aortic occlusion. Key words: Spinal cord monitoring; Spinal cord evoked potential; Aortic aneurysm; Spinal cord ischemia; Paraplegia
Neurological complications due to spinal cord ischemia are among the major problems encountered in cardiovascular surgery requiring aortic occlusion. The most disastrous complication is permanent paraplegia. Various surgical techniques have been employed to prevent ischemic spinal cord injury (Wakabayashi and Connolly 1976). So far, however, these techniques have not obviated postoperative paraplegia. One reason is thought to be the extreme complexity and diversity of the blood supply to the spinal cord (Dommisse 1974). Spinal cord evoked potentials (SpEPs) elicited by direct stimulation of the spinal cord have been used clinically to monitor intraoperative spinal cord function in a wide variety of spine and spinal cord surgeries (Imai et al. 1984; Tamaki et al. 1984b, 1985b). We recently began using this technique for early detection of spinal cord ischemia during operations on the descending aorta. This report presents the authors' initial experiences with 21 consecutive patients undergoing surgical repair of thoracic or thoraco-abdominal aortic aneurysms.
Correspondence to: Dr. Y. Okamoto, Department of Orthopaedic Surgery, School of Medicine, Chiba University, 1o8-1 Inohana, Chiba-City, Chiba 280 (Japan).
Materials and methods
Patient population From October 1985 to March 1990, 21 patients underwent intraoperative monitoring of spinal cord evoked potentials (SpEPs) during surgical procedures for aortic lesions. There were 15 men and 6 women, with ages ranging between 40 and 74 years (mean 59.6 years). The clinical diagnoses were thoracic aortic aneurysms in 12 patients, thoraco-abdominal aortic aneurysms in 5 and dissecting aortic aneurysms (DeBakey III) in 4. In case 2, a Dacron patch was employed after excision of a localized saccular aneurysm. Graft replacement of the aneurysm with a Dacron tube was designed for the remaining 20 patients. To maintain distal aortic perfusion, an extra-anatomical temporary bypass was produced in 11 patients and femorofemoral bypass with a pump-oxygenator was used in the remaining 10. All patients were anesthetized with high doses of morphine. Partial pressures of oxygen and body temperature were well controlled except in two cases in whom there was some fluctuation (described later). A detailed profile of each patient is illustrated in Table I. Recording o f S p E P According to the method described by Tamaki et al.
(yrs), sex
58, 60, 59, 61,
54, 63, 65, 49,
no.
1 2 3 4
5 6 7 8
69, M 65, M
49, 58, 67, 40, 70, 69, 74, 59,
60, M
11 12
13 14 15 16 17 18 19 20
21
DB III b
TAAA DB III b TAA TAA TAA TAAA TAA TAA
TAA TAA
TAA TAAA
TAA DB III a DB III b TAA
TAA TAA TAAA TAAA
Diagnosis
D5
distal to LSA D3 D5 D5 . . D6 D3 D6
distal to LSA ascending aorta
D5 distal to LSA
distal to LSA distal to LSA distal to LSA D5
Ds localized in distal to LSA D8 D10
Proximal
135 unidentified
145 260
unidentified 120 160 120
60 75 110 305
Duration (rain)
D12
142
proximal to SMA 143 D9 105 D10 80 Dn 132 . . proximal to CA 85 Dl0 67 Dn 74
D5 D6
D8 proximal to CIA
D12 D10 proximal to CA Dio
proximal to CA proximal to CIA
D10
Distal
Aortic cross-clamping placement
60-100
4 0 - 80 6 0 - 90 8 0 - 90
60-100 5 0 - 70 4 0 - 80 70-110
5 0 - 90 40-110
5 0 - 90 3 0 - 50
70-120 unknown 7 0 - 90 80-120
unknown 8 0 - 90 70-110 70- 80
pressure during occlusion (mm Hg)
Distal aortic
none none none none none 26% decrease none latency shortening (0.3 msec) 47% decrease
none 85% decrease
none total disappearance
none none 56% decrease latency prolongation (0.9 msec) total disappearance none total disappearance none
Spike
SpEP changes
39% decrease
33% increase none none none none 25% decrease none 30% increase
33% decrease 77% decrease
pattern alteration none
none
32% decrease none 45% decrease marked increase (scaled out)
Polyphasic
yes
yes yes no
yes yes
no
yes yes no
yes yes no
Recovery
none
none none none none none none none none
none (died)
none paraplegia
(died) none none none
none none none none
complication
Neurological
massive bleeding from intercostal arteries c
distal hypoperfusion ~
total o c c l u s i o n / distal hypoperfusion b,c
cardiac arrest c
cardiac arrest c
total occlusion b.c b
Comments
a T A A = thoracic aortic aneurysm; T A A A = thoraco-abdominal aortic aneurysm; DB l i i a and DB III b = dissecting aortic aneurysm; DeBakey's type III a and IIl b, respectively; LSA = left subclavian artery; CA = celiac artery; C IA = common lilac artery. b Patients undergoing reimplantation of intercostal vessels. c Presumed causes of SpEP changes.
M F F M M F F M
60, M 42, M
9 10
M F M M
F M M M
Age
Case
Profile of 21 patients with spinal cord monitoring during aortic occlusion a
TABLE I
r-
0 rn
0
©
SPINAL CORD MONITORING FOR AORTIC ANEURYSM
(1984b), specially designed flexible catheter-type electrodes (Unique Medical Co. Ltd., Tokyo) were inserted by means of a Tuohy's needle into the subarachnoid space at the level of the conus medullaris and into the epidural space at the cervical level. One (mainly the rostral electrode) was used for stimulating, and the other for recording. Square-wave pulses of 0.2 msec duration and supramaximal intensity (10-20 mA) were applied at a rate of 10-30/sec. Records were obtained using one of the following: DISA N-2000 or Medelec MS-6 or MS-92B. Low and high frequency filters were at 20 and 2000 Hz, respectively, and the evoked potentials were averaged 20-100 times to eliminate electrical noise. The electrodes were removed more than 24 h after the operations in view of the hemorrhagic tendency due to the intraoperative use of heparin.
Results
Operative results The only patient receiving a Dacron patch (case 2) had an uneventful perioperative course. Of the remaining 20 patients who were scheduled to undergo a Dracon graft replacement, one (case 17) developed systemic hypotension (mean aortic pressure 30 mm Hg) due to acute heart failure prior to aortic occlusion, and the subsequent graft replacement was aborted. Two patients (cases 5 and 12) developed acute heart failure during aortic occlusion and died intraoperatively. One patient (case 10) with an extensive thoraco-abdominal aneurysm exhibited permanent paraplegia postoperatively. The remaining 16 patients successfully underwent graft replacements and were awakened without neurological complications.
Basic pattern of SpEP If both the electrodes are placed properly, SpEP can be recorded with either the rostral or caudal electrode (Fig. 1). In this series, SpEP was principally elicited by rostral stimulation and recorded caudally (descending SpEP). In some cases in which the descending SpEP could not be appropriately recorded, presumably due to incorrect placement of the caudal electrode, the ascending SpEP, using the caudal electrode to stimulate and the rostral to record, was used for monitoring. The basic SpEP pattern consisted of an initial spike and a subsequent polyphasic component (Fig. 1). As the wave configurations varied from patient to patient, the evoked potential initially recorded prior to the surgical procedure on the aorta was considered the control pattern for that patient. The peak-to-peak amplitudes of both the initial spike and the largest component of the polyphasic response were used as the main
317
initialspike
polyphasiccomponent
descendingSpEP
ascendingSpEP
+25JUV 2ms Fig. 1. Basic pattern of SpEP. Upper trace: descending SpEP elicited by rostral stimulation. Lower trace: ascending SpEP elicited by caudal stimulation. Typical wave form of SpEP consists of an initial spike and a polyphasic component. Upward deflections are negative in this and all other figures.
indicators, and pared with the cording to the peak latency of
an alteration of more than 20% comcontrol was considered significant accriteria of Tamaki et al. (1984b). The the spike was also used as an indicator.
SpEP changes during aortic occlusion An initial test clamping was performed in most of the cases before the aneurysm was incised. Both proximal and distal clamping was done, and the evoked potential was observed for 15 or 20 min to estimate collateral circulation to the spinal cord. The earliest and most frequent SpEP alterations after cross-clamping of the aorta were changes in the configuration or amplitude of the polyphasic component. Six patients (cases 1, 4, 8, 11, 13 and 20) demonstrated alterations of the polyphasic component without any change in the amplitude of the spike. In 13 patients who showed no change in the spike amplitude during the test clamping, subsequent graft replacements were successfully performed without reimplantation of intercostal vessels. Significant changes in the amplitude of the initial spike were observed in 7 patients (cases 3, 5, 7, 10, 12, 18 and 21), including 3 patients with total disappearance of SpEP. Among them, 2 patients (cases 5 and 7) developed sudden cardiac arrest during aortic occlusion and the evoked potential gradually faded out, first the polyphasic component and then the initial spike (Fig. 2). In case 10, SpEP amplitude decreased immediately after aortic cross-clamping and disappeared after the aneurysm was incised. The evoked potential did not return during the remainder of the surgery and the patient was found to have flaccid paraplegia postoperatively (Fig. 3). Changes in peak latency of the initial spike were observed in 2 patients. One was prolongation by 0.9 msec (case 4) and the other was shortening by 0.3 msec
318
Y. O K A M O T O E T AL.
(case 20). Since these alterations in latency were not accompanied by any change in the spike amplitude, body temperature fluctuation (a fall of 2.0°C and a rise of 1.1°C, respectively) was thought to be the main cause.
conlro] -.-,-- 14:44
aortic clamp
-4-- Itt: tt 7
aneurysm incised
l,uV
i; 2ms
Discussion
Paraplegia is one of the most devastating complications of cardiovascular surgery requiring temporary occlusion of the aorta. The frequency of paraplegia following surgical repair of a thoracic or thoraco-abdominal aortic aneurysm is considerable and estimated to be approximately 5% (Wakabayashi et al. 1976; Crawford et al. 1981). The exact causative mechanism of the ischemic spinal cord injury remains unclear because the circulation of the spinal cord is extremely complex and the patterns vary with the individual
2ms
~,
~",~,,~".b~ J.
17.51
t
cardiac arrest 18.17
,82,
1:
18.33
18.45 18.52 --
F
^
.,~ x ~ . _ . ~ . _ ~
CPB on 19.07
~
19.37
lOpV
L
zo.3o
14;53
,L
Fig. 3. Intraoperative SpEP in case 10. The initial spike (arrow heads) decreased in amplitude after aortic cross-clamping, but the aneurysm was unfortunately incised before an alarm was given. The distal aortic pressure fell to approximately 40 m m Hg and the latency of the spike was markedly prolonged. SpEP then rapidly disappeared. No recovery of the potentials was observed and the patient was found to have flaccid paraplegia postoperatively.
control
315
EVOPOT 90202
Intraoperative spinal cord monitoring during surgery for aortic aneurysm: application of spinal cord evoked potential Yuzuru Okamoto, Masazumi Murakami, Takeo Nakagawa, Atsushi Murata and Hideshige Moriya Department of Orthopaedic Surgery, School of Medicine, Chiba Uniuersity, Chiba (Japan) (Accepted for publication: 24 February 1992)
Summary Spinal cord evoked potentials elicited by direct stimulation of the spinal cord were monitored in 21 patients during thoracic or thoraco-abdominal aortic aneurysm surgery. Flexible catheter-type electrodes were used for both stimulating and recording. The basic pattern of the spinal cord evoked potential consisted of an initial spike and a subsequent polyphasiccomponent. The earliest and most frequent alterations after cross-clampingof the aorta were changes in the configuration or amplitude of the polyphasic component. In 13 patients who exhibited no change except minor alterations of the polyphasic component during the initial test clamping for 15 or 20 min, subsequent graft replacements were safely performed without reimplantation of intercostal vessels. In 2 patients who had sudden cardiac arrests, the evoked potential completely disappeared. The polyphasic component disappeared first, followed by the initial spike. Another patient developed acute loss of the potential after the aneurysm was incised, presumably due to distal aortic hypoperfusion. In this case, prolonged distal hypotension resulted in flaccid paraplegia. Intraoperative monitoring of the spinal cord evoked potential is a useful method for the early detection of spinal cord ischemia during surgery requiring aortic occlusion. Key words: Spinal cord monitoring; Spinal cord evoked potential; Aortic aneurysm; Spinal cord ischemia; Paraplegia
Neurological complications due to spinal cord ischemia are among the major problems encountered in cardiovascular surgery requiring aortic occlusion. The most disastrous complication is permanent paraplegia. Various surgical techniques have been employed to prevent ischemic spinal cord injury (Wakabayashi and Connolly 1976). So far, however, these techniques have not obviated postoperative paraplegia. One reason is thought to be the extreme complexity and diversity of the blood supply to the spinal cord (Dommisse 1974). Spinal cord evoked potentials (SpEPs) elicited by direct stimulation of the spinal cord have been used clinically to monitor intraoperative spinal cord function in a wide variety of spine and spinal cord surgeries (Imai et al. 1984; Tamaki et al. 1984b, 1985b). We recently began using this technique for early detection of spinal cord ischemia during operations on the descending aorta. This report presents the authors' initial experiences with 21 consecutive patients undergoing surgical repair of thoracic or thoraco-abdominal aortic aneurysms.
Correspondence to: Dr. Y. Okamoto, Department of Orthopaedic Surgery, School of Medicine, Chiba University, 1o8-1 Inohana, Chiba-City, Chiba 280 (Japan).
Materials and methods
Patient population From October 1985 to March 1990, 21 patients underwent intraoperative monitoring of spinal cord evoked potentials (SpEPs) during surgical procedures for aortic lesions. There were 15 men and 6 women, with ages ranging between 40 and 74 years (mean 59.6 years). The clinical diagnoses were thoracic aortic aneurysms in 12 patients, thoraco-abdominal aortic aneurysms in 5 and dissecting aortic aneurysms (DeBakey III) in 4. In case 2, a Dacron patch was employed after excision of a localized saccular aneurysm. Graft replacement of the aneurysm with a Dacron tube was designed for the remaining 20 patients. To maintain distal aortic perfusion, an extra-anatomical temporary bypass was produced in 11 patients and femorofemoral bypass with a pump-oxygenator was used in the remaining 10. All patients were anesthetized with high doses of morphine. Partial pressures of oxygen and body temperature were well controlled except in two cases in whom there was some fluctuation (described later). A detailed profile of each patient is illustrated in Table I. Recording o f S p E P According to the method described by Tamaki et al.
(yrs), sex
58, 60, 59, 61,
54, 63, 65, 49,
no.
1 2 3 4
5 6 7 8
69, M 65, M
49, 58, 67, 40, 70, 69, 74, 59,
60, M
11 12
13 14 15 16 17 18 19 20
21
DB III b
TAAA DB III b TAA TAA TAA TAAA TAA TAA
TAA TAA
TAA TAAA
TAA DB III a DB III b TAA
TAA TAA TAAA TAAA
Diagnosis
D5
distal to LSA D3 D5 D5 . . D6 D3 D6
distal to LSA ascending aorta
D5 distal to LSA
distal to LSA distal to LSA distal to LSA D5
Ds localized in distal to LSA D8 D10
Proximal
135 unidentified
145 260
unidentified 120 160 120
60 75 110 305
Duration (rain)
D12
142
proximal to SMA 143 D9 105 D10 80 Dn 132 . . proximal to CA 85 Dl0 67 Dn 74
D5 D6
D8 proximal to CIA
D12 D10 proximal to CA Dio
proximal to CA proximal to CIA
D10
Distal
Aortic cross-clamping placement
60-100
4 0 - 80 6 0 - 90 8 0 - 90
60-100 5 0 - 70 4 0 - 80 70-110
5 0 - 90 40-110
5 0 - 90 3 0 - 50
70-120 unknown 7 0 - 90 80-120
unknown 8 0 - 90 70-110 70- 80
pressure during occlusion (mm Hg)
Distal aortic
none none none none none 26% decrease none latency shortening (0.3 msec) 47% decrease
none 85% decrease
none total disappearance
none none 56% decrease latency prolongation (0.9 msec) total disappearance none total disappearance none
Spike
SpEP changes
39% decrease
33% increase none none none none 25% decrease none 30% increase
33% decrease 77% decrease
pattern alteration none
none
32% decrease none 45% decrease marked increase (scaled out)
Polyphasic
yes
yes yes no
yes yes
no
yes yes no
yes yes no
Recovery
none
none none none none none none none none
none (died)
none paraplegia
(died) none none none
none none none none
complication
Neurological
massive bleeding from intercostal arteries c
distal hypoperfusion ~
total o c c l u s i o n / distal hypoperfusion b,c
cardiac arrest c
cardiac arrest c
total occlusion b.c b
Comments
a T A A = thoracic aortic aneurysm; T A A A = thoraco-abdominal aortic aneurysm; DB l i i a and DB III b = dissecting aortic aneurysm; DeBakey's type III a and IIl b, respectively; LSA = left subclavian artery; CA = celiac artery; C IA = common lilac artery. b Patients undergoing reimplantation of intercostal vessels. c Presumed causes of SpEP changes.
M F F M M F F M
60, M 42, M
9 10
M F M M
F M M M
Age
Case
Profile of 21 patients with spinal cord monitoring during aortic occlusion a
TABLE I
r-
0 rn
0
©
SPINAL CORD MONITORING FOR AORTIC ANEURYSM
(1984b), specially designed flexible catheter-type electrodes (Unique Medical Co. Ltd., Tokyo) were inserted by means of a Tuohy's needle into the subarachnoid space at the level of the conus medullaris and into the epidural space at the cervical level. One (mainly the rostral electrode) was used for stimulating, and the other for recording. Square-wave pulses of 0.2 msec duration and supramaximal intensity (10-20 mA) were applied at a rate of 10-30/sec. Records were obtained using one of the following: DISA N-2000 or Medelec MS-6 or MS-92B. Low and high frequency filters were at 20 and 2000 Hz, respectively, and the evoked potentials were averaged 20-100 times to eliminate electrical noise. The electrodes were removed more than 24 h after the operations in view of the hemorrhagic tendency due to the intraoperative use of heparin.
Results
Operative results The only patient receiving a Dacron patch (case 2) had an uneventful perioperative course. Of the remaining 20 patients who were scheduled to undergo a Dracon graft replacement, one (case 17) developed systemic hypotension (mean aortic pressure 30 mm Hg) due to acute heart failure prior to aortic occlusion, and the subsequent graft replacement was aborted. Two patients (cases 5 and 12) developed acute heart failure during aortic occlusion and died intraoperatively. One patient (case 10) with an extensive thoraco-abdominal aneurysm exhibited permanent paraplegia postoperatively. The remaining 16 patients successfully underwent graft replacements and were awakened without neurological complications.
Basic pattern of SpEP If both the electrodes are placed properly, SpEP can be recorded with either the rostral or caudal electrode (Fig. 1). In this series, SpEP was principally elicited by rostral stimulation and recorded caudally (descending SpEP). In some cases in which the descending SpEP could not be appropriately recorded, presumably due to incorrect placement of the caudal electrode, the ascending SpEP, using the caudal electrode to stimulate and the rostral to record, was used for monitoring. The basic SpEP pattern consisted of an initial spike and a subsequent polyphasic component (Fig. 1). As the wave configurations varied from patient to patient, the evoked potential initially recorded prior to the surgical procedure on the aorta was considered the control pattern for that patient. The peak-to-peak amplitudes of both the initial spike and the largest component of the polyphasic response were used as the main
317
initialspike
polyphasiccomponent
descendingSpEP
ascendingSpEP
+25JUV 2ms Fig. 1. Basic pattern of SpEP. Upper trace: descending SpEP elicited by rostral stimulation. Lower trace: ascending SpEP elicited by caudal stimulation. Typical wave form of SpEP consists of an initial spike and a polyphasic component. Upward deflections are negative in this and all other figures.
indicators, and pared with the cording to the peak latency of
an alteration of more than 20% comcontrol was considered significant accriteria of Tamaki et al. (1984b). The the spike was also used as an indicator.
SpEP changes during aortic occlusion An initial test clamping was performed in most of the cases before the aneurysm was incised. Both proximal and distal clamping was done, and the evoked potential was observed for 15 or 20 min to estimate collateral circulation to the spinal cord. The earliest and most frequent SpEP alterations after cross-clamping of the aorta were changes in the configuration or amplitude of the polyphasic component. Six patients (cases 1, 4, 8, 11, 13 and 20) demonstrated alterations of the polyphasic component without any change in the amplitude of the spike. In 13 patients who showed no change in the spike amplitude during the test clamping, subsequent graft replacements were successfully performed without reimplantation of intercostal vessels. Significant changes in the amplitude of the initial spike were observed in 7 patients (cases 3, 5, 7, 10, 12, 18 and 21), including 3 patients with total disappearance of SpEP. Among them, 2 patients (cases 5 and 7) developed sudden cardiac arrest during aortic occlusion and the evoked potential gradually faded out, first the polyphasic component and then the initial spike (Fig. 2). In case 10, SpEP amplitude decreased immediately after aortic cross-clamping and disappeared after the aneurysm was incised. The evoked potential did not return during the remainder of the surgery and the patient was found to have flaccid paraplegia postoperatively (Fig. 3). Changes in peak latency of the initial spike were observed in 2 patients. One was prolongation by 0.9 msec (case 4) and the other was shortening by 0.3 msec
318
Y. O K A M O T O E T AL.
(case 20). Since these alterations in latency were not accompanied by any change in the spike amplitude, body temperature fluctuation (a fall of 2.0°C and a rise of 1.1°C, respectively) was thought to be the main cause.
conlro] -.-,-- 14:44
aortic clamp
-4-- Itt: tt 7
aneurysm incised
l,uV
i; 2ms
Discussion
Paraplegia is one of the most devastating complications of cardiovascular surgery requiring temporary occlusion of the aorta. The frequency of paraplegia following surgical repair of a thoracic or thoraco-abdominal aortic aneurysm is considerable and estimated to be approximately 5% (Wakabayashi et al. 1976; Crawford et al. 1981). The exact causative mechanism of the ischemic spinal cord injury remains unclear because the circulation of the spinal cord is extremely complex and the patterns vary with the individual
2ms
~,
~",~,,~".b~ J.
17.51
t
cardiac arrest 18.17
,82,
1:
18.33
18.45 18.52 --
F
^
.,~ x ~ . _ . ~ . _ ~
CPB on 19.07
~
19.37
lOpV
L
zo.3o
14;53
,L
Fig. 3. Intraoperative SpEP in case 10. The initial spike (arrow heads) decreased in amplitude after aortic cross-clamping, but the aneurysm was unfortunately incised before an alarm was given. The distal aortic pressure fell to approximately 40 m m Hg and the latency of the spike was markedly prolonged. SpEP then rapidly disappeared. No recovery of the potentials was observed and the patient was found to have flaccid paraplegia postoperatively.
control
