Background and purpose: Evoked potentials from the spinal cord are used during spinal surgery to alert the surgeon of potential damage and so enable timely intervention to protect the cord. Stimuli are applied to a nerve in the leg and the evoked potentials recorded from the epidural space in the cervical or upper thoracic region. Amplitudes are monitored for changes that might indicate trauma to the spinal cord.
Evoked potentials were originally recorded from a single electrode in the epidural space referred to a remote electrode on the patient's back (monopolar response). Subsequently many centres have used bipolar electrodes (two electrodes about 15 mm apart in the epidural space). This latter arrangement is less prone to interference (noise), since the electrodes are closer together, but we know of no critical comparison of the two montages.
Aims: In this study we compare the noise superimposed on the monopolar and bipolar responses. We also investigate whether or not similar amplitude changes are seen by both techniques when the response changes.
Methods: Bipolar and monopolar responses were recorded using a bipolar epidural electrode with an additional remote reference electrode on the patient's back. The monopolar response was obtained between one of the two bipolar contacts and the remote reference. A second monopolar response is obtained from the other bipolar contact. Monopolar and bipolar responses were averaged simultaneously. Separate responses were obtained for stimuli applied to left and right legs.
Data were collected from 98 surgical operations. Where the responses were constant (or had a constant linear trend), variances about the constant (or trend line) were compared for monopolar and bipolar responses. Where there was a noticeable amplitude drop (25% or greater) in either the monopolar or bipolar response, we noted whether there was a corresponding change in the other.
Results and discussion: Of the 98 patients, the response amplitude was constant or had a constant linear trend in 26 cases. Amplitudes ranged from 0.2 uV to 3.4 uV for monopolar and from 0.1 uV to 3.4 uV for bipolar recordings. Paired t-tests showed no significant differences between monopolar and bipolar amplitudes. We assume standard deviation is independent of amplitude. The mean standard deviations were 0.18 uV (monopolar) and 0.12 uV (bipolar) showing a significantly smaller variance of the bipolar recordings (F = 2.16, p < 0.05, df = 25, 25).
The table shows the number of occasions in which there was a noticeable amplitude change in the corresponding monopolar or bipolar response. Of interest are the 34 occasions (17% of this sample) when there was a change of the bipolar response but no corresponding change in one or other of the monopolar responses.
| . | . | > 25% drop of bipolar response? | |
|---|---|---|---|
| . | . | NO | YES |
| > 25% drop of monopolar response? | NO | 148 | 34 |
| YES | 7 | 7 | |
The bipolar response is the difference of the two monopolar responses. The two monopolar recordings are only 15 mm apart in the epidural space. If either one shows that the evoked potential is unchanged, then we can assume that there has been no detectable trauma to the cord. A loss of one, but not both, of the monopolar responses must have a technical explanation, but this situation would not be distinguished by the bipolar recording.
Conclusions: It is easy to be seduced by the lower noise associated with bipolar recordings. Indeed in a few, very noisy, cases only a bipolar recording can be used at all. However, we have found a potentially higher false-positive rate for bipolar as compared to dual monopolar recordings.
Bowers EJ, McArdle FJ, Griffiths CJ. Monopolar versus bipolar recordings of spinal cord evoked responses for intraoperative monitoring. In: Morgan HM, Starritt HC, Duck FA, eds. Proceedings of the 9th Annual Scientific Meeting, IPEM 2003; 107-8.
Ms Emma Bowers, Freeman Unit, Newcastle upon Tyne. Tel: +44 (0)191-223-6161.
Regional Medical Physics Department, Freeman Hospital, Newcastle upon Tyne, NE7 7DN, UK.
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