were used to monitor cerebral function during 26 carotid endarterectomies. The ... Key-Words:Carotid endarterectomy -- monitoring -- somatosensory evoked ...
ltal. J. Neurol. Sci. 10:315-320, 1989
Considerations after intraoperative monitoring of somatosensory evoked potentials during carotid endarterectomy Carenini L. Bottacchi E. CamerlingoM., Mamoli A.
Reparto di Neurologia e Neurofisiopatologia, Ospedale Regionale, Aosta
Somatosensory evoked potentials (SEPs) following median nerve stimulation were used to monitor cerebral function during 26 carotid endarterectomies. The patients with minor SEP variations had no neurological deficits on regaining consciousness while the one with more serious SEP variations had a transient deficit. The method thus seems useful in the early detection of ischemic brain impairment.
Key-Words:Carotid endarterectomy - - monitoring -- somatosensory evoked potentials.
Introduction To prevent ischemic brain damage in the course of carotid endarterectomy (CEA) it is necessary to assess the compensatory capacity of the collateral intracranial circulation. The methods used to date include measurement of stump pressure [13,7], intraoperative EEG monitoring [2,15,16] and measurement of regional cerebral blood flow [16]. Recent experimental [9,12] and clinical [8,10,11] studies suggest that intraoperative monitoring of the somatosensory evoked potentials (SEPs) is a sensitive and reliable method of detecting central nervous system (CNS) impairment due to hypopeffusion and so may help to select candidates for carotid shunting during CEA. To test the method we monitored SEPs following median nerve stimulation in the course of 26 CEAs.
Patients and method The intraoperative SEPs were recorded in 26
CEAs performed on 23 patients, 3 of whom underwent a second operation on the other side. The patients, 13 men and 10 women, had an age range of 46 to 82 years with a mean of 64.9 +9.5 years. 3 of them had had a nondisabling major stroke, 7 a minor stroke, 10 one or more transient ischemic attacks and 3 had no symptoms. All had undergone dopplersonography, cerebral CT scanning and angiography. In 10 patients CT had revealed a small hypodense area consistent with the clinical symptoms, in 3 modest cerebral atrophy and in 10 a normal pattern. Angiography had disclosed unilateral lesions of the carotid artery in one patient only and bilateral lesions in the rest. Of the 26 carotid arteries operated on 11 presented angiographic stenosis in excess of 90%, 14 between 50% and 90% and one an ulcerated plaque. 13 operations were performed on the right and 13 on the left internal carotid artery. All the operations were performed under general anesthesia, induced by thiopenial 5 mg/Kg, and maintained by neuroleptoanalgesics with barbiturate, fentanyl and droperidole in divided doses. Controlled 315
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analysis time of 50 msec and 512 responses were averaged per pass. One recording was taken before and one during the operation, one 24-h later and another a week later. During surgery, the SEPs were continuously monitored and their morphology, latencies, and amplitude were compared serially. The responses, directly reproduced on paper after each acquisition, were ~ s e s s e d for N20 and P25 latency, N20/P25 amplitude and for morphology. The neurophysiological examination was done with an Interspec Medical Neurotrac two-channel equipment. The patients were assessed clinically before the operation, 12 h after and 7 days later.
ventilation was set up in all patients with a 50o nitrous oxide mixture, the volume and rate being such as to keep the PaCO2 between 30 and 35 mmHg. Isoflurane (0,5%) was added in 4 operations. All the patients were monitored by electrocardiogram, systemic blood pressure (BP) measurement add blood gas analysis. The carotid stump pressure was measured at the time of clamping. The median nerve was stimulated at the wrist and the SEPs were recorded from the contralateral scalp. Both median nerves were stimulated successively. The recording electrode was positioned 7 era, laterally and 2 cm. posteriorly to the Cz, according to the 10-20 International System, the reference electrode at Fpz and the ground on the arm. Both stimulating and recording electrodes were AgC 1 cup electrodes fixed with collodion. The interelectrode resistance was kept under 5 Khom. The nerve was stimulated at a frequency o f 5 Hz with monophasic square wave impulses of intensity barely above the motor threshold and lasting 0.2 msec. The recording was done with an
Results
Table I summmarized the SEP variations on the homolateral hemisphere (operative side) during the period of carotid clamping. In One patient the recording could not be evaluated due to technical shortcomings. Only 5 patients (cases
TABLE I. Changes of the median SEP during carotid clamping C a s e n.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
N20/P25 amplitude % --21.1 - 33.3 - 69.0 0 0 - 37.8 - 56.7 -42.7 * 0 0 - 23.6 . . - 24.6 0 - 24.4 --38.8 0 0 - 19.1 0 -- 70.0 0 - 68.2 0 -24.0
N20 latency msec.
.
+0.4 + 0.8 + 0.4 + 0.4 + 0.4 + 0.4 + 2.0 +0.8 * 0 0 + 1.2 . . 0 0 0 0 0 0 0 + 0.4 + 0.5 0 0 0 +0.4
P25 latency msec. + + + + + + + +
1.2 1.6 1.2 1.6 0.4 1.2 2.0 1.6 * + 1.2 + 0.4 + 2.0 . + 0.8 0 + 0.5 +0.8 0 0 + 1.6 + 1.5 + 0.9 0 + 1.2 0 +0.5
Clamping duration min.
Stump pressure mmHg
63 37 42 33 44 28 37 40 38 37 39 72 25 36 64 34 48 34 23 22 35 64 32 64 35 46
40 60 75 45 70 55 60 80 75 75 60 100 26 70 100 55 100 90 45 90 100 100 95 42 100 100
= decrease; -6 ~ increase; 0 = no change % z the variation of amplitude is expressed as percentage of amplitude before clamping * = the recording could not be evaluated due to technical shortcomings ** 16 minutes after clamping there was a flattening of SEP with suppression of the N20 and P25 components.
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Fig. 1. Case no. 7. Median SEP during left carotid endarterectomy. Marked changes were noted after carotid clamping over both hemispheres. Minutes of clamping are shown to the right of the traces. Positivity is upward.
15,18,19,23,25) presented no S EP changes during clamping. In 19 cases there was a symmetrical SEP variation on both hemispheres (Fig. l). In these cases there was always a lengthening of the P25 latency, by less than l msec. in 7 cases and between i and 2 msec. in the others. In 12 cases there was also an increase of the N20 latency, by less than l msec. in l0 and between l and 2 msec. in 2 cases.
Of these 19 cases 14 presented also a reduction of amplitude (N20/P25), almost symmetrical on the two hemispheres, of under 50% in 10 cases, of between 50% and 70% in 4 cases. In one patient (case 13) (Fig. 2) only a unilateral flattening of the SEP occurred about 16 minutes after clamping but the various components quickly reappeared after clamp removal, with a gradual return to the baseline amplitude and latency. Total clamp time was 25 minutes and the SEP was flat for 9 minutes. This patient was the only one to have had a very low stump pressure (26 mmHg). In 4 other cases the stump pressure was between 40 and 45 mmHg and in other 55 mmHg or more. All SEP variations that occurred in our patients receded after clamp removal and the patterns observed 24 h and 7 days after the operation were substantially the same as beforehand. Only one patient, the one with unilateral SEP flattening, had a slight right motor hemisyndrome on regaining consciousness but it cleared in 24-48 h. None
of the other patients presented new neurological deficits. Regarding SEP variations during the induction of anesthesia, we found only small variations in the early components, ie a slight reduction in N20/P25 amplitude, under general anesthesia with barbiturate and neuroleptics. But with isoflurane, used in 4 operations, we noted definite changes in the morphology, amplitude and latency of SEPs. Systemic BP was monitored throughout CEA and sudden variations in BP were accompanied by a rapid variation in SEPs on both hemispheres with a tendency to reduced amplitude and increased latency of the peaks. These changes were, however, transient and on restoration of a strong BP the SEP record swiftly normalized. Discussion Surgical treatment for extracranial carotid lesions has become increasingly frequent in the past few years and CEA seems to be indicated for patients with tight stenosis or potentially emboligenous plaques. Neurological complications are, however, possible and, in the attempt to lower the risk of intraoperative ischemia, gome surgeons practise routine shunting of the carotid artery while others recommend this measure only in selected 317
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Fig. 2. Case no 13. Left carotid endarterectomy. Changes of the median SEP over the clamped hemisphere during carotid clamping (on the left side) and after clamp removal (on the right side). Minutes of clamping or after clamp removal are shown on the right of traces. Positivity is upward.
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cases, according to the intraoperative data on cerebral function [5,14]. Although our series is small, our findings seem to confirm that short latency SEPs following median nerve stimulation may be a useful signal in the early detection of CNS impairment due to ischemia. Indeed, only one patient, who had a stump pressure of 26 mmHg, presented unilateral flattening of the record during carotid clamping with suppression of the N20 and P25 components; despite gradual recovery of the SEPs immediately after clamp removal, the patient had a neurological deficit on regaining consciousness, but he was the only one to be thus affected. All the other patients, who showed either symmetrical variations on both hemispheres or no SEP variations at all, were free from neurological deficits. The clinical applicability of a monitoring technique in the operating room ultimately depends on its sensitivity and reliability. SEPs seem to be more sensitive and reliable as a monitoring tool than EEG. Fergurson et al [5] monitored the EEG during 211 CEAs, performed without a shunt irrespective of the EEG findings. They found significant changes in 63 patients (30%), though only 4 had an intraoperative stroke. EEG monitoring 318
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thus seems to carry a high rate of false positives. McPherson et al [11],reporting on 25 patients who underwent CEA, stressed the greater reliability of SEP over EEG monitoring. Our findings, in line with published data [4,6], stress the fact that the reliability of intraoperative SEP monitoring depends partly on strict control of general factors, such as systemic BP and type of anesthesia, which can cause significant variations in potential. The early components of SEPs seem to be fairy resistant to barbiturates but are profoundly altered by isoflurane. The correlation between SEP cha.nges and cerebral blood flow (CBF) changes has been clearly established in experimental works. Branston et al [~ab ] described a lessening of SEP amplitude in the oon when the CBF fell below a given critical level. Lesnick et al [9], working with cats, studied the successive changes in SEPs following progressive ischemia and correlated them with the regional CBF and metabolic changes. According to these workers, following ischemia the evoked potential presents first a lengthening of latency of cortical wave V and then a lessening of amplitude to the point of total abolition of the response.
Carenini L.: lntraoperative monitoring of SEP potentials during carotid endarterectomy
In our patients the variations in SEPs observed during clamping of the carotid artery seem to tally with the experimental fndings. The most frequent changes were a progressive lengthening of latency of P25, considered to be of cortical origin [3], and a subsequent reduction of N20/P25 amplitude. A delay of the conical components seems to correlate with mild ischemia of the white matter and the subsequent reduction of amplitude to a reduction of cortical blood flow, while the abolition of SEPs would occur only when the blood flow to the white matter and the metabolism fall to critical levels [9]. In those of our patients who had no postoperative neurological deficits the lengthen-
ing of P25 latency never exceeded 2 msec and the reduction of N20/P25 amplitude was less than 50% in the most of the cases; these patients had a strong stump pressure. In sum, changes in SEPs seem to be a useful early warning of cerebral ischemic impairment during carotid clamping although further data on larger series with standardized clinical, neurophysiological and anestetic protocols are necessary to prove the soundness of the method. Other points that remain to be established are the exact limits of tolerance to intraoperative SEP changes: the degree and type of changes and how long these changes can last before irreversible C N S damage sets in.
Sommario I Potenziali Evocati Somatosensoriali (PES) da stimolazione del nervo mediano sono stati usati come metodo per monitorare lafunzione cerebrale durante endoarteriectomia carotidea. Sono stati eseguiti 26 interventi di endoarteriectomia carotidea in 23 pazienti. Durante il clampaggio carotideo, pif~frequentemente si d osservato un aumento della latenza della P25, che in nessun caso d risultato superiore ai 2 msec. In un solo paziente si ~ registrato un appiattimento monolaterale della traccia con scomparsa delle componenti N20 e P25; tale paziente era l'unico a presentare un deficit neurologico al risveglio. I nostri dati sembrano suggerire l'utilitiz del monitoraggio intraoperatorio del PES del mediano, alfine di svelare una sofferenza ischemica cerebrale.
Address reprint requests to: Dr. Lorenzo Carenini U.O. di Neurologia e Neurofisiopatologia Ospedale Regionale Viale Ginevra 3 - 11100 Aosta
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