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Ann Thorac Surg 1999;67:1943-1946
© 1999 The Society of Thoracic Surgeons

Sensitivity, specificity, and surgical impact of somatosensory evoked potentials in descending aorta surgery

^a Cliniques Universitaires Saint-Luc, U.C.L. Brussels, Brussels, Belgium

Address reprint requests to Dr Guerit, Service Potentials Evoques Cliniques Universitaires Saint-Luc, Avenue Hippocrate, 10 B-1200 Brussels, Belgium
e-mail: guerit{at}nchm.ucl.ac.be

Presented at the Aortic Surgery Symposium VI, April 30–May 1, 1998, New York, NY.

	Abstract

Top Abstract Introduction Patients and methods Results Discussion Conclusions References

 
Background. We evaluate the efficiency of multilevel somatosensory evoked potential (SEP) monitoring for intraoperative re-establishment of blood flow to the spinal cord in 63 patients undergoing descending aorta repair.

Methods. The presence of critical vessels in a cross-clamped aortic segment was ascertained by a 15 minute SEP observation period while the segment between the clamps was vented to drain out the collateral flow.

Results. SEPs influenced the surgical strategy in 17 cases (27%): use of the Biomedicus in 1 traumatic rupture; critical vessel reimplantation or distal clamp replacement in 13 cases of segmental spinal ischemia; and hastening the procedure or proximal clamp replacement in 3 cases of left carotid ischemia. There were no cases of unexplained multilevel SEP abnormalities. Immediate paraplegia was observed in 2 cases (1 pre-existing; 1 forecast by a 199-minute period of SEP absence due to segmental ischemia); 2 patients presented delayed paraplegias despite unchanged intraoperative SEPs, and 1 case presented a transient paraplegia due to lower motoneuronal involvement.

Conclusions. SEPs efficiently identified critical vessels to be reimplanted in order to avoid immediate paraplegia. However, systematic additional vessel reimplantation, if technically feasible, and prolongation of SEP monitoring during the postoperative period with careful blood pressure control are needed to prevent delayed paraplegia.

	Introduction

Top Abstract Introduction Patients and methods Results Discussion Conclusions References

 
One way to prevent spinal cord ischemia in descending aorta replacement is the intraoperative neurophysiological identification of the segmental arteries supplying the spinal cord [1]. Somatosensory evoked potentials (SEPs) have been used for 15 years [2, 3] but their use was challenged in an influential paper by Crawford and associates [4] who reported, first, a high number of false positives, and, second, a low sensitivity, since some patients developed paraplegia despite unchanged SEPs or SEPs that returned to normal after a period of disappearance. SEPs were considered to test the posterior columns of the spinal cord, which are not as sensitive to the ischemic processes as the anterior pathways. Some suggested replacing SEPs with motor EPs (MEPs). However, there is only one study [5] that reports on the clinical use of MEPs in a relatively large series. Thus, the issue of which technique should be used remains a matter of debate. This paper will address this issue, both practically and theoretically. Practically, our results with SEP monitoring will be compared to those obtained by de Haan and associates [5] in a comparable group using MEP monitoring. Theoretically, the advantages and drawbacks of SEPs and MEPs will be discussed on the basis of test sensitivity, specificity, feasibility, and impact on surgical strategy.

	Patients and methods

Top Abstract Introduction Patients and methods Results Discussion Conclusions References

 
Sixty-three consecutive operations on the descending aorta performed between May 1987 and February 1998 are considered (Table 1 ). A dissection of the aorta was found in 14 cases (22.2%) and 25 cases (39.7%) were operated on as emergencies. The pathology of the aorta was confined to the isthmus in 21 cases (group I, 33.3%), involved the whole descending thoracic aorta in 26 cases (group T, 41.3%) and also involved the abdominal aorta in 16 cases (group TA, 25.4%). The preoperative neurological history was not relevant in 62 cases. One patient with an acute thoracic dissection was paraplegic with a T-10 sensory level preoperatively.

In 54 cases, the perfusion of the inferior aorta was maintained by means of either an aorta-aorta (or femoral) or a left atrium-aorta (or femoral) bypass using a centrifugal pump (Biomedicus Medtronics, Prairie, MN) with heparin-coated tubing. After initiation of bypass, 2 aortic clamps were applied above and below the aortic lesions, and SEPs were recorded for 15 minutes while the aortic segment comprised between the clamps was vented to drain out the intercostal or lumbar collateral flow. If there were no SEP modifications in the interval, the surgeon was allowed to carry on with the procedure. If spinal ischemia was detected, the inferior clamp was removed until SEP normalization, and re-applied closer to the superior clamp for another 15 minute interval with simultaneous SEP recording. By doing this, the surgeon was able to identify the intercostal or lumbar arteries to be reimplanted in the aortic prosthesis. In the case of a thoraco-abdominal aneurysm, the inferior aortic clamp was first applied as close as possible to the superior one, allowing for a safer proximal anastomosis.

In the 9 other cases, no bypass was used because of catastrophic aortic hemorrhage in 2 cases, the presence of severe concomitant injuries in 2, technical impossibility in 1, and of surgeon’s preference in 4.

Neurophysiological techniques
Our recording techniques have been described previously [3, 6]. SEPs were recorded on 4 channels (peripheral, lumbar, brainstem, and parietal) by alternate left and right posterior tibial nerve stimulation at the ankle. Five neurophysiological events were identified in a sub-group of these patients [6]: type 1: distal spinal ischemia related to proximal aortic cross-clamping in the absence of bypass or natural collaterals; type 2: nerve ischemia related to common femoral artery cross-clamping; type 3: segmental spinal ischemia due to the exclusion of critical arteries; type 4: ischemia of the left carotid artery; type 5: brain hypoperfusion due to systemic hypotension.

	Results

Top Abstract Introduction Patients and methods Results Discussion Conclusions References

 
Patient outcome
The mortality rate was 7/63 patients (11.1%)(Table 2 ). Three patients died intraoperatively, and 4 patients died postoperatively from non-neurological causes. A new paraplegia or paraparesis was observed in 3 patients. In one thoracoabdominal case, the problem was forecast on the basis of a 119-minute period of SEP absence due to Type 3 alterations. One thoracic case presented with delayed paraplegia after a prolonged (4h) episode of relative hypotension. These 2 patients died without recovery. One thoracoabdominal patient was neurologically normal after the operation, but developed transient, blood pressure-dependent paraparesis attacks; he was neurologically normal between the attacks. This gives a 4.8% overall rate of new paraplegia. Interestingly, 3 other patients presented transient lower motoneuron problems without MRI evidence of spinal-cord ischemia.

Impact of SEP alterations on surgical strategy
The observation of type 3 abnormalities gave rise to a specific surgical riposte in all 13 cases (reimplantation of intercostal, lumbar, or sacral arteries in 11 cases; suturing of 1 profusely backbleeding intercostal in 1 case, repositioning the distal clamp to a more proximal level in 1 case). Type 4 abnormalities also gave rise to a specific riposte in all 3 cases (repositioning of the proximal cross-clamp in 2 cases, hastening the proximal suture in 1 case). By contrast, the observation of type 1 abnormalities prompted a surgical decision in only 1/12 cases (use of the Biomedicus without heparin in one traumatic rupture with multiple injuries). Overall, SEP monitoring prompted a modification of the surgical attitude in 17 cases (27%) in our whole group: in 9 cases (56.3%), 5 cases (19.2%), and 3 cases (14.3%) in the TA, T, and I groups respectively. The surgical riposte always caused SEP recovery.

	Discussion

Top Abstract Introduction Patients and methods Results Discussion Conclusions References

 
Three criteria should determine the choice of MEPs or SEPs to monitor the spinal cord in descending aorta surgery: the test feasibility, sensitivity (number of unforecast paraplegias), and specificity (percentage of other interfering factors than spinal cord ischemia).

Test feasibility
Test feasibility mainly depends on anesthetic constraints. MEPs are far more sensitive than SEPs to most anesthetic drugs (reviewed in [7]) and cannot be recorded in the presence of a complete neuromuscular block. Reliable MEP recording actually requires a combination of special anesthetic regimens (ketamine or etomidate/fentanyl), special stimulation techniques (paired electrical stimuli of high intensity), and partial neuromuscular blockade [5]. Moreover, electrical stimulations are painful and may in no way be used during the postoperative period in awake, sedated patients. It is theoretically possible to bypass these influences by directly stimulating the spinal cord and recording spinal cord [8] or neurogenic [9] potentials. However, some doubt remains about the exact structures (orthodromic motor or antidromic sensory pathways) that are then stimulated [10].

Test sensitivity
Our results show that SEPs are sensitive to several intraoperative events: distal (type 1) or segmental (type 3) spinal cord ischemia, nerve ischemia (type 2), and brain ischemia (type 4). The same types of MEP alterations were obtained by de Haan and associates [5]: 3/20 patients (15%) presented MEP alterations equivalent to our type 1 SEP alterations, and 3 patients (15%) presented abnormalities clearly due to the exclusion of culprit vessels. Six cases (30%) presented abnormalities that recovered after distal aortic flow restoration; it is hard to decide whether these abnormalities correspond to distal or segmental spinal cord ischemia. Eight patients presented unilateral MEP abnormalities occurring within a mean delay of 37 minutes after occlusion of the left femoral artery, equivalent to our type 2 alterations.

The clinical results of de Haan and associates are also close to ours. Actually, 3 of their 20 patients (15%) became paraplegic (2 forecast and 1 delayed) and 1 patient transiently presented a right-leg lower motor neuron involvement. In our whole series, 4/63 patients (6.3%) were paraplegic on the day after the operation and 3 patients manifested transient lower-limb lower motor neuron involvement. A better comparison can be achieved by limiting our series to the 16 thoracoabdominal cases, in which case 2 new paraplegias (12.5%) were observed (1 forecast, and 1 delayed). Thus, our series and that of de Haan and associates [5] look similar with respect to clinical results.

That both multilevel SEPs and MEPs look equivalent with regard to types of abnormalities and clinical results is likely to be explained by the fact that both actually test spinal cord structures which are equally sensitive to spinal cord ischemia due to aortic cross-clamping. Indeed, both techniques evaluate the grey matter within the lumbar enlargement (the anterior horn cells for MEPs, the posterior horns for SEPs), which are the most sensitive to ischemia [11]. We feel that the argument that SEPs only test the posterior columns while MEPs should be more sensitive because they test the pyramidal tract is overly simplistic.

Test specificity
Although cortical SEP alterations are not specific for spinal cord ischemia, multilevel SEP recording always permits one to distinguish between alterations due to spinal cord ischemia and those resulting from peripheral nerve or brain ischemia. Moreover, it has always been possible to differentiate distal from segmental spinal cord ischemia. Although MEPs should be able to differentiate spinal cord ischemia (bilateral MEP alterations) from unilateral nerve or brain ischemia (unilateral MEP alterations), they do not allow one to differentiate distal from segmental spinal cord ischemia.

Problem of delayed paraplegia
More than 50% of postoperative paraplegia is delayed [4, 12]. We agree with Griepp and associates [12] that one way to prevent delayed paraplegia is to continue monitoring during the postoperative period. This can only be done with SEPs, since double train electrical MEPs are painful, and cannot be used in awake patients, while magnetic MEPs, although less painful, cannot reliably be obtained in sedated patients.

	Conclusions

Top Abstract Introduction Patients and methods Results Discussion Conclusions References

 
We did not demonstrate any difference in the sensitivity of multilevel SEPs and MEPs. This can probably be explained by the similarity of the nervous structures tested. Multilevel SEPs appear to be as specific, but are more readily carried out both intraoperatively and postoperatively than MEPs. In no fewer than 17 cases (27%), SEP monitoring detected changes that provoked a surgical response which invariably led to SEP recovery. It seems likely, therefore, that SEP monitoring helped prevent some neurological deficits in our patients.

	References

Top Abstract Introduction Patients and methods Results Discussion Conclusions References

 

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6. Guerit J.M., Etienne P.Y., Dion R. An explanation of the high <<false-positive>> rate of lower-limb SEPs in descending aorta surgery. In: Jones S.J., Boyd S., Hetreed M., Smith N.J., eds. Handbook of Spinal Cord Monitoring. Dordrecht: Kluwer Academic Publishers, 1994:135-145.
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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Robert Verhelst Robert A. Dion Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Guerit, J.-M. Articles by Dion, R. A. Search for Related Content PubMed PubMed Citation Articles by Guerit, J.-M. Articles by Dion, R. A.