The Big Question:
The authors (Bander, et al.) point out that monopolar stimulation for direct cortical brain mapping is quickly become the standard when compared to the previous method of mapping: â€œlow-frequency bipolar stimulation mappingâ€ (also known as the Penfield method). However, the authors wanted to compare using what they refer to as â€œtrain-of-fiveâ€ (also referred to as multipulse or pulse-train) stimulation to the low-frequency bipolar simulation mapping.
The idea behind this experiment is twofold. First is that the potential risk for tissue damage is possibly higher based off data recorded in animal studies (p. 1). And second, â€œmonopolar [train-of-five] stimulation [causes] diffuse, radial spread of electrical stimulation that leads to spatially inaccurate motor cortex identificationâ€ (p.2).
Thirteen patients were used in this study. The two things this study wanted to compare was the reliability of locating the motor cortex through direct cortical stimulation when comparing low-frequency bipolar stimulation and train-of-five bipolar stimulation and the occurrence of intraoperative seizures (a known risk of any direct cortical stimulation, especially low-frequency bipolar stimulation).
The authorsâ€™ used four steps for motor mapping and monitoring during these cases. First they would use SSEP phase reversal testing to identify the central sulcus. Next they would identify the regions of motor cortex at risk using the train-of-five bipolar stimulation technique while using the strip used for phase reversal to watch for after discharges through EEG monitoring. Then they would use low-frequency bipolar stimulation to see if they could re-identify those same areas of the motor cortex they had previously mapped using train-of-five stimulation. Finally they would run direct cortical MEPs using the strip for the duration of the resection at a rate of every â€œ2â€“15 secondsâ€ (p. 3).
When comparing methods the authors identified the motor cortex in all 13 patients using the train-of-five technique (max stim intensity = 53 V Â± 17.7 V) compared to only 4 times with the low-frequency stimulation technique (max stim intensity = 8 mA Â± 2.2 mA).
No seizures occurred when using the train-of-five technique while two seizures occurred during the low-frequency stimulation technique along with two instances of after discharges that did not progress to seizures. These number line up very closely with other studies testing the seizure frequency when using the low-frequency (or Penfield) technique.
The authors acknowledge a â€œlack of comparison with a monopolar [train-of-five] stimulationâ€ (along with â€œsmall sample sizeâ€) as limitations to this study however I would say neither of these should have a big impact on the whether this information is useful and the technique should be further tested and verified.
Comparison to monopolar direct cortical stimulation would likely be of little use considering this technique (monopolar multipulse stimulation) is already becoming the mainstream method for cortical mapping. However, if it could be demonstrated that the direct risk of tissue damage is a serious factor linked to monopolar stimulation, the bipolar pulse train technique presented by the authors could be relevant very quickly. Also, though the sample size is small successful recording in 100% of patients indicates a high potential for reliability (in my opinion).
The technique of bipolar/monopolar, Penfield/Multipulse techniques have been compared in subcortical mapping by Szelenyi, et al. in 2011. They found that multipulse stimulation, whether with a monopolar or bipolar probe, was superior for stimulation for subcortical mapping versus the low-frequency (50 Hz) stimulation technique.
This article appears to offer a promising, potentially reliable stimulation alternative in an area of IONM that has received a lot of attention in recent years.
- Neurosurg Focus. 2020 Feb 1; 48(2):E4. doi: 10.3171/2019.11.FOCUS19776. Use of the train-of-five bipolar technique to provide reliable, spatially accurate motor cortex identification in asleep patients. Bander ED, Shelkov E, Modik O, Kandula P, Karceski SC, Ramakrishna R1
- Clin Neurophysiol. 2011 Jul; 122(7):1470-5. doi: 10.1016/j.clinph.2010.12.055. Intra-operative subcortical electrical stimulation: a comparison of two methods. SzelÃ©nyi A1, Senft C, Jardan M, Forster MT, Franz K, Seifert V, Vatter H.
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