BJA Advance Access originally published online on January 19, 2007
British Journal of Anaesthesia 2007 98(3):362-365; doi:10.1093/bja/ael365
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Cortical somatosensory-evoked potentials during spine surgery in patients with neuromuscular and idiopathic scoliosis under propofol–remifentanil anaesthesia
1 Department of Anaesthesiology
2 Department of Orthopaedics, University of Duesseldorf, Duesseldorf, Germany
* Correspondence author: Department of Anaesthesiology, University of Duesseldorf, Moorenstr. 5, Postfach 101007, 40225 Duesseldorf, Germany. E-mail: Henning.Hermanns{at}uni-duesseldorf.de
Accepted for publication November 15, 2006.
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Abstract |
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BACKGROUND: Intraoperative monitoring of the spinal cord via cortical somatosensory-evoked potentials (SSEP) is a routine during spinal surgery. However, especially in neuromuscular scoliosis, the reliability of cortical SSEP has been questioned. Therefore, we compared the feasibility of cortical SSEP in idiopathic and neuromuscular scoliosis using anaesthetics known to have only minimal effect on SSEP recordings.
METHODS: Total intravenous anaesthesia with propofol and remifentanil as continuous infusion was standardized for all the patients. Median and tibial nerve cortical SSEP were monitored in 54 patients who underwent surgery for spinal deformity. Twenty-seven had idiopathic scoliosis and 27 had neuromuscular scoliosis. The portion of reproducible results and intraoperative changes were compared between the groups.
RESULTS: In both groups, cortical SSEP could be monitored with sufficient reliability. Only in two patients with idiopathic and four patients with neuromuscular scoliosis no reproducible traces could be obtained. The amplitudes in patients with neuromuscular scoliosis were lower than in those with idiopathic scoliosis, but not statistically significant. There were no postoperative neurological deficits. The number of false positive and true positive did not differ between the groups.
CONCLUSIONS: Assessment of cortical SSEP during spine surgery was equally effective and reliable in patients with neuromuscular scoliosis and in patients with idiopathic scoliosis, possibly as a result of propofol–remifentanil anaesthesia.
Keywords: monitoring, somatosensory evoked potentials; neuromuscular scoliosis; surgery, spinal; technique, total i.v. anaesthesia
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Introduction |
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The integrity of the spinal cord is potentially at risk during scoliosis surgery. Intraoperative monitoring of spinal cord function has been proved to reduce the risk of motor deficit or paraplegia1 and therefore is now considered mandatory during surgical procedures which bear a risk of irreversible damage to the spinal cord.2
The reliability of cortical somatosensory-evoked potentials (SSEP) in patients with neuromuscular scoliosis has been questioned repeatedly.3–5 In up to 30% of patients with neuromuscular scoliosis, reliable recordings could not be obtained.3 4 6 Furthermore, patients with unobtainable cortical SSEP were at an increased risk of postoperative neurological deficit.7 In those studies, balanced anaesthesia with the use of inhalation agents was performed. As propofol has only minimal effect on SSEP recordings in contrast to inhalation anaesthetics such as sevoflurane,8 isoflurane,9 and nitrous oxide,10 we chose total i.v. anaesthesia with propofol and remifentanil as the anaesthetic technique.
Our hypothesis was that under total i.v. anaesthesia, cortical SSEP could be monitored with equal reliability in both idiopathic and neuromuscular scoliosis.
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Patients and methods |
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In this prospective, non-randomized study, 54 patients who underwent surgical instrumental correction and fusion of scoliosis were included. Informed patient or parental consent was obtained. Data were collected over a period of 4 yr. Twenty-seven patients had idiopathic scoliosis and 27 presented with neuromuscular scoliosis. Among the 27 patients with neuromuscular scoliosis, 13 had cerebral palsy, 4 patients suffered from Rett syndrome, 3 had spinal muscular atrophy and 2 patients had Duchenne muscular dystrophy (DMD). In one patient, brain tumour was the cause for scoliosis, in another patient, a previous meningoencephalitis, one patient suffered from Charcot Marie Tooth disease, and in two patients, unknown syndromes that were associated with neuromuscular dysbalance and scoliosis.
Anaesthesia was standardized in all patients. After placement of an i.v. cannula, anaesthesia was induced with propofol (2 mg kg–1) and remifentanil (1 µg kg–1), and maintained with continuous infusion of propofol (6 mg kg–1 h–1) and remifentanil (0.2–0.4 µg kg–1 min–1). If necessary, anaesthetic depth was increased by temporarily elevating the remifentanil infusion rate. Neuromuscular blocking drugs were avoided to minimize difficulties during extubation in patients with impaired neuromuscular status; no patient received neuromuscular block at any time. After induction of anaesthesia, an arterial line, a central venous catheter and a urinary catheter was placed. During the operation, patients were kept normothermic with air and infusion warmers, and blood gases and haemoglobin concentration were checked intermittently. Blood pressure was measured invasively and mean arterial blood pressure (MAP) was kept above 50 mm Hg, using noradrenaline infusion if necessary. Ventilation with 50% oxygen was adjusted so that arterial carbon dioxide partial pressure was maintained between 4.6 and 5.3 kPa.
Cortical SSEP were recorded by alternate right and left posterior tibial and both median nerve stimulation with needle electrodes (duration 0.5 ms, frequency 4.3 Hz, current 20 mA). Evoked potentials were recorded using corkscrew electrodes (Nicolet Biomedical, Madison WI, USA; impedance <3 k
) placed at Cz', C3 and C4 with the reference electrode at Fz according to the 10–20 international system of EEG electrode placement.11 A bandpass filter of 20 Hz to 20 kHz was used. An analysis time of 100 ms was used and 250 sweeps were averaged. Evoked potentials of the median nerves were monitored to control for the effect of anaesthesia and haemodynamic changes on stimulation. The baseline recordings were obtained after induction of anaesthesia when a steady anaesthetic state was reached.
The warning criteria for significant change consisted of an amplitude decrease of more than 50% or a latency increase of more than 10% or both.1 When the SSEP data met these criteria and technical difficulties were ruled out, blood pressure and haemoglobin concentration were increased if possible and the surgeon was informed. When data continued to meet warning criteria, a Stagnara wake-up test was performed, if applicable.12
The recordings were classified according to Szalay and colleagues:13 true positive was defined as a change in waveform possibly related to surgery, false positive was a change unrelated to surgical events, true negative was unchanged waveforms and absence of neurological sequelae, and false negative was a postoperative neurological deficit without changes in SSEP traces.
Power analysis revealed a sample size of 25 per group with an expected difference of 30%, 
= 0.05 and a power of 80%. The portion of successful recordings between the two groups was compared with two-tailed Fisher's exact test. The amplitudes of the recordings for the two groups were analysed using the paired Student's t-test with P < 0.05 considered significant. Data are presented as means (SD) or median [range].
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Results |
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Cortical SSEP were recordable and reproducible in 48 of 54 patients (88.9%). Patient characteristics for both groups are described in Table 1.
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Among the patients with idiopathic scoliosis, reproducible potentials could be obtained in 25 of 27 patients (92.6%), while traces could be satisfactorily reproduced in 23 of 27 patients with neuromuscular scoliosis (85.2%) (P = 0.67). In patients with cerebral palsy, an adequate recording could be achieved in 12 of 13 patients (92.3%). Whether or not SSEP could be obtained was unrelated to age. In no patients, postoperative neurological changes were observed.
The amplitudes of SSEP tended to be slightly lower in patients with neuromuscular scoliosis before and after operation, but this was not statistically significant. Furthermore, in both groups, the average maximum amplitude reduction was similar between 20 and 32% (Table 2). The results from our recordings were classified according to Szalay and colleagues.13 Again there was no statistical difference between both groups (Table 3).
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In two patients with neuromuscular scoliosis, a reduction in amplitude of more than 50% from baseline was noted that could be related to surgical events. In one patient, a 16-yr-old boy with DMD, significant amplitude reduction was noted during correction manoeuvre, and in another patient, a 15-yr-old boy with cerebral palsy, amplitude reduction occurred during sublaminar wire passage. Likewise, in a 16-yr-old boy with idiopathic scoliosis, amplitude reduction was observed after correction. All potentials recovered after adjustment of surgical technique. In two of the three patients classified as true positive, wake-up tests were performed and uneventful, while it was omitted in the boy with cerebral palsy because of severe mental retardation. The potentials remained stable until the end of the operation, and no changes in neurological status were noted after the operation.
Among the four patients in the neuromuscular scoliosis group who were classified as false positive, temporary hypotension was the cause (rapid temporary MAP decrease to 30–50 mm Hg), and the potentials recovered after increasing blood pressure with volume or catecholamine infusion or both. In one patient, massive haemorrhage occurred, resulting in significant amplitude reduction. After an adequate blood transfusion, all potentials returned to baseline values. In one patient, intraoperative circulatory collapse occurred leading to a significant amplitude reduction. After successful resuscitation, potentials reverted to preoperative values.14 In both patients in the idiopathic scoliosis group, the false positive recordings occurred during transitional hypotension (decrease up to MAP of 40 mm Hg) and resolved with normalization of blood pressure.
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Discussion |
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TopAbstractIntroductionPatients and methodsResultsDiscussionReferences |
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Assessment of cortical SSEP during spine surgery was similarly recordable, effective, and reliable in patients with neuromuscular scoliosis and in patients with idiopathic scoliosis, possibly as a result of propofol–remifentanil anaesthesia.
Spinal cord monitoring is performed to reduce the risk of neurological deficit with surgical correction of spinal deformity.2 As the incidence of neurological deterioration during neuromuscular scoliosis surgery is increased, suggesting a higher sensitivity of the spinal cord to injury in the neuromuscular patient,15 effective monitoring in these patients is crucial, especially when wake-up tests are not feasible because of mental retardation. Yet, the efficacy of spinal cord monitoring, especially of cortical SSEP in patients with neuromuscular scoliosis has repeatedly been questioned.3 4 16 17
During balanced anaesthesia, cortical SSEP recorded from patients with neuromuscular scoliosis were of poor morphology or absent in approximately 30% of the patients.3 4 6 In contrast to these results, we could obtain reliable cortical SSEP in approximately 90% of our patients, and there was no difference between patients with neuromuscular and idiopathic scoliosis. The most likely explanation of this increased quality of neuromonitoring is the standardized propofol-based anaesthesia, which has been shown to affect SSEP monitoring to a lesser degree.8–10 One may argue that a shorter acting volatile anaesthetic such as desflurane might have less effect on SSEP during scoliosis surgery, but it has been shown that desflurane depressed SSEP amplitude to a comparable extent as isoflurane.18
As the risk of postoperative neurological deficit is increased for spinal surgery patients with unobtainable evoked potentials,7 reliable spinal cord monitoring in this patient group is highly desirable.
Subcortical SSEP have been successfully and reliably recorded in patients with neuromuscular scoliosis via electrodes introduced into the epidural space.19 20 These potentials are very useful and, in some respect, superior to cortical SSEP, particularly because they are less susceptible to the depressing effects of volatile anaesthetics.21 However, the advantage of cortical recordings is the relatively easy application, lack of interference with surgery, low risk, and the larger amplitudes and signal-to-noise ratio.16 As the effect of anaesthetic agents is more pronounced on synaptic transmission than on axonal conduction, responses recorded from polysynaptic pathways are more affected by anaesthesia. This is the reason why cortical SSEP are more affected by anaesthesia than spinal cord and subcortical recordings.22 Therefore, it seems even more important to use anaesthetics with minimal effect on SSEP.
Several studies revealed that propofol affects cortical SSEP to a much lesser extent than other anaesthetics such as nitrous oxide,10 sevoflurane,8 and isoflurane,9 making propofol a recommended anaesthetic of choice during spinal cord monitoring. The only study in which a comparative effect of sevoflurane/nitrous oxide and propofol on SSEP was observed, anaesthetic depth was controlled only clinically,23 whereas other studies which controlled anaesthetic depth by bispectral index monitoring showed that volatile anaesthetics altered SSEP, whereas propofol did not.8 9
Opioids cause only modest SSEP amplitude depression. Continuous infusion seems to affect SSEP less than the injection of boluses.24 Compared with the combination of fentanyl and nitrous oxide, remifentanil reduces cortical amplitude less, with lower amplitude variability,25 making continuous remifentanil infusion an attractive anaesthetic for the patients who require intraoperative spinal cord monitoring.
In our prospective study, satisfactory recordings could be obtained in patients with idiopathic scoliosis and in those with neuromuscular scoliosis under propofol–remifentanil anaesthesia. Therefore, at least under propofol–remifentanil anaesthesia cortical SSEP monitoring is equally sensitive and specific for patients with neuromuscular and idiopathic scoliosis.
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References |
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