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Ann Thorac Surg 2002;74:S1873-S1876
© 2002 The Society of Thoracic Surgeons
a Department of Surgery, Vancouver General Hospital, Vancouver, BC, Canada
b Department of Neurosciences, King Faisal Specialist Hospital and Research Center, Riyadh, Saudi Arabia
* Address reprint requests to Dr Dong, EEG Lab, CP Ground Floor, Vancouver General Hospital, 855 West 12th Ave, Vancouver, BC V5Z 1M9 Canada
e-mail: cdong{at}interchg.ubc.ca
Presented at the Aortic Surgery Symposium VIII, May 2–3, 2002, New York, NY.
Abstract
BACKGROUND: Postoperative paraplegia is one of the most dreaded complications after descending thoracic and thoracoabdominal aneurysm surgery. In this study, intraoperative monitoring was applied during resection of descending thoracic and thoracoabdominal aneurysms to detect spinal cord ischemia and help prevent paraplegia.
METHODS: Fifty-six patients (descending thoracic, 25; thoracoabdominal, 31) were monitored intraoperatively with both motor- (MEP) and somatosensory- (SSEP) evoked potentials. MEPs were elicited with transcranial electrical stimulation and recorded from the spinal epidural space (D wave) or peripheral muscles (myogenic MEP). SSEPs were obtained with median and tibial nerve stimulation.
RESULTS: A total of 16 patients (28.6%) showed MEP evidence of spinal cord ischemia, only 4 of whom had delayed congruent SSEP changes. In 13 patients (23.2%), ischemic changes in MEPs were reversed by reimplanting segmental arteries or increasing blood flow or blood pressure. None of these 13 patients suffered acute paraplegia regardless of the status of SSEP at the end of the procedure, but 1 of them developed delayed postoperative paraplegia after multisystem failure. Three patients (5.4%) who had persistent loss of MEPs despite of recovery of SSEPs awoke paraplegic.
CONCLUSIONS: The results demonstrate that compared with SSEP, MEP, especially myogenic MEP, is more sensitive and specific in detection of spinal cord ischemia, and that intraoperative monitoring can indeed help prevent paraplegia.
Paraplegia is one of the most severe postoperative complications after descending thoracic and thoracoabdominal aneurysm surgery. This devastating neurologic deficit is caused by spinal cord ischemia resulting from temporary or permanent interruption of spinal cord blood supply. During the past two decades, several protective strategies have been developed to maintain spinal cord blood flow during and after aortic aneurysm surgery in order to reduce the incidence of postoperative paraplegia. These measures include retrograde aortic perfusion with atriofemoral bypass, cerebrospinal fluid (CSF) drainage, and revascularization of segmental arteries [1, 2]. However, efforts to further decrease the rate of paraplegia have been hampered by the inability to assess the adequacy of spinal cord blood supply during surgery [3]. If a specific protective surgical manipulation fails to reestablish spinal cord perfusion, irreversible spinal cord ischemic injury will not be found until the patient awakens.
An approach, aimed at monitoring the functional integrity of the spinal cord, may provide a solution to this problem. Using somatosensory- (SSEP) and motor- (MEP) evoked potentials, spinal cord function can be assessed intraoperatively and the status of spinal cord perfusion inferred. To improve postoperative neurologic outcome, spinal cord ischemia should be detected as early as possible so that protective strategies can be applied before ischemic cord injury becomes irreversible. SSEPs have been used in monitoring spinal cord function since the 1970s [4]. However, SSEPs only reflect conduction of sensory information in the posterior column, which has a different blood supply from that supplying the motoneural system located in the anterolateral part of the spinal cord. In aortic aneurysm surgery, results of SSEP spinal cord monitoring are disappointing. A large prospective study showed that SSEP monitoring had high false-negative (13%) and false-positive (67%) rates, and could not significantly improve neurologic outcome [5]. More recently, MEPs elicited with transcranial electrical stimulation (TCES), which exclusively assess the function of the spinal cord motoneural system, including the corticospinal tract and the anterior horn gray matter, have been employed in aortic aneurysm surgery [6–8].
In this study, we report our results of spinal cord monitoring during repair of descending thoracic (DTA) and thoracoabdominal aneurysms (TAA) using both MEPs and SSEPs. The relative contributions of SSEP and MEP to detection and prevention of postoperative neurologic deficits was evaluated.
Material and methods
Patients
Fifty-six consecutive patients (female, 26; male, 30; age, 29 to 78 years, mean 67 years) undergoing DTA (n = 25) or TAA (n = 31) repair between August 1996 and October 2001 were included in this study. Each patient had preoperative SSEP baseline studies and gave informed consent for TCES.
MEP recording
MEPs were elicited with TCES applied through spiral needle electrodes placed at C1 and C2 scalp sites (International 10 to 20 System) and recorded from the spinal epidural space (D wave, 16 patients) or peripheral limb muscles (myogenic MEP, 37 patients) or from both loci (3 patients). For D wave recording, a bipolar electrode was placed at a low thoracic location (T7–T11) through a 17-gauge Tuohy needle before induction, and its location was documented by chest radiographs. Single-pulse TCES (duration, 50 µsec; intensity, 250 to 1000 V; rate, 0.4 Hz) was used. Each D wave recording was obtained by averaging five to 20 responses, requiring 12 to 50 seconds. A decrease in the amplitude by 50% of baseline obtained before aortic cross-clamping was considered to represent cord ischemia.
Myogenic MEPs (mMEPs) required multipulse TCES and were recorded from needle electrodes inserted into limb muscles, including the first dorsal interosseous, tibialis anterior, and abductor hallucis muscles. Responses from the first dorsal interosseous were used as a control. Multipulse TCES consisted of three to five pulses of 250 to 700 V, with an interstimulus interval of 1 to 4 ms. mMEPs could be obtained instantaneously after TCES and did not need averaging. Because of their large variability, mMEPs were classified as present or absent, and no effort was made to measure their amplitude.
Cannulation of the left femoral artery for retrograde aortic perfusion frequently caused left leg ischemia and prevented mMEP monitoring from this leg. To overcome this problem, an end-to-side 8-mm Dacron graft was introduced for the left femoral artery in later cases, allowing the left leg to remain perfused and available for monitoring.
SSEP recording
Peripheral and cortical responses of bilateral median and tibial SSEPs were obtained in all the patients after stimulation of the median nerve at the wrist and the posterior tibial nerve at the ankle, respectively. Median SSEPs were used as a systemic control, whereas peripheral responses were collected to differentiate cord ischemia from limb ischemia. Although subcortical and lumbosacral potentials were also recorded in some initial cases, they were eventually abandoned because of their poor signal-to-noise ratios, which necessitated prolonged averaging time and prevented prompt surgical feedback. To obtain a reproducible cortical SSEP, responses to at least 200 electrical stimuli were needed for averaging. The stimulus (duration, 0.2 ms; intensity, 25 or 50 mA for median or tibial nerve stimulation, respectively) was delivered at a rate of 5.1 or 7.1 Hz. Responses were band-pass filtered between 30 and 300 Hz and between 150 and 1,000 Hz for cortical and peripheral SSEPs, respectively. A decrease in the amplitude of cortical responses by 50% from baseline was considered significant.
Anesthetic considerations
Anesthesia consisted of Propofol and narcotic infusion without inhalational agents. Neuromuscular blockade was omitted after induction, when mMEPs were recorded.
Results
Neurophysiological spinal cord monitoring, combined with other protective adjuncts (eg, retrograde aortic perfusion, CSF drainage, and moderate or deep hypothermia), was performed for 56 patients with DTA (n = 25) or TAA (n = 31) during repair of their aortic aneurysms. In this series of patients, in-hospital mortality was 5.4% (3 of 56 patients). The causes of death include cardiac (n = 1), respiratory (n = 1), and renal failure (n = 1).
Sixteen patients (28.6%) had significant MEP changes (Table 1). One of these MEP events was revealed by loss of the D wave recorded at T10 level (patient 1), and others by disappearance of mMEPs obtained from the lower extremities (patients 2 to 16). In only 4 of these patients (patients 2 to 5) was loss of MEPs accompanied by a significant deterioration in cortical tibial SSEPs. These congruent SSEP alterations occurred with a delay of 3 to 12 minutes compared with the onset of mMEP changes.
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Three patients (patients 3, 4, and 13) with persistent loss of MEPs from the lower extremities developed postoperative paraplegia, despite recovery of tibial SSEPs at the end of the procedure. In patient 3, both D wave and mMEPs were monitored. mMEPs from the lower extremities disappeared after cross-clamping and remained absent during the rest of the procedure, despite segment artery reimplantation. The D wave recorded from the epidural electrodes inadvertently placed at T5 level did not show any significant change, suggesting that the cord infarction occurred below T5. Tibial SSEP evidence of cord ischemia was not observed until 12 minutes after loss of leg mMEPs and returned to above 50% of baseline during closure. For patients 4 and 13, D wave recording was not attempted. In patient 4, mMEPs and SSEPs showed similar changes to those in patient 3, predicting postoperative paraplegia but preserved posterior column function. In patient 13, leg mMEPs were lost during cross-clamping and did not return at the end of the procedure. Loss of leg mMEPs was not accompanied by significant changes in tibial SSEPs. This patient awoke paraplegic and unfortunately died of respiratory failure.
Comment
In this study, spinal cord function in 56 patients with descending aortic aneurysms was monitored intraoperatively with both SSEPs and MEPs. Sixteen patients had significant MEP changes, among which only 4 patients had delayed accompanying SSEP alteration. Loss of MEPs from the lower extremities suggested anterior cord ischemia and prompted intervention to restore blood supply to the spinal cord. Although 1 developed delayed postoperative paraplegia, all 13 patients with restoration of MEPs were not paraplegic immediately after surgery, independent of the status of SSEPs. All 3 patients with persistent loss of MEPs suffered postoperative paraplegia (5.4%) despite SSEP recovery. The results convincingly demonstrate that MEPs are extremely accurate in predicting neurologic outcome, and are more sensitive and specific than SSEPs in detection of spinal cord ischemia. Intraoperative neurophysiological monitoring can help prevent paraplegia for patients undergoing aortic aneurysm surgery.
It is worthwhile to mention that for the patients included in this study, there was no monitoring in the interval between the end of operation and awakening. Although it did not occur in our series, spinal cord ischemia can develop during this sensitive postoperative period. This is especially true if MEPs and SSEPs are not stable at the end of the operation, which suggests that spinal cord blood supply is at a marginal level and vulnerable. Because SSEP monitoring is feasible during this specific interval and can provide some information regarding spinal cord function, we recommend that SSEPs be monitored postoperatively if necessary.
Acknowledgments
We thank Karin Liddle, Nimira Bapoo, and Peter Van Rienen, IOM technologists at Vancouver General Hospital, for their dedicated technical support.
References
This article has been cited by other articles:
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A. Winnerkvist, R. E. Anderson, L.-O. Hansson, L. Rosengren, A. E. Estrera, T. T.T. Huynh, E. E. Porat, and H. J. Safi Multilevel somatosensory evoked potentials and cerebrospinal proteins: indicators of spinal cord injury in thoracoabdominal aortic aneurysm surgery Eur. J. Cardiothorac. Surg., April 1, 2007; 31(4): 637 - 642. [Abstract] [Full Text] [PDF]
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H. Kumagai, M. Isaka, Y. Sugawara, K. Okada, K. Imai, K. Orihashi, and T. Sueda Intra-aortic injection of propofol prevents spinal cord injury during aortic surgery. Eur. J. Cardiothorac. Surg., May 1, 2006; 29(5): 714 - 719. [Abstract] [Full Text] [PDF]
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