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Motor-evoked potentials (MEP) during brainstem surgery to preserve corticospinal function

  • Clinical Article
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Abstract

Background

Brainstem surgery bears a risk of damage to the corticospinal tract (CST). Motor-evoked potentials (MEPs) are used intraoperatively to monitor CST function in order to detect CST damage at a reversible stage and thus impede permanent neurological deficits. While the method of MEP is generally accepted, warning criteria in the context of brainstem surgery still have to be agreed on.

Method

We analyzed 104 consecutive patients who underwent microsurgical resection of lesions affecting the brainstem. Motor grade was documented prior to surgery, early postoperatively and at discharge. A baseline MEP stimulation intensity threshold was defined and intraoperative testing aimed to keep MEP response amplitude constant. MEPs were considered deteriorated and the surgical team was notified whenever the threshold was elevated by ≥20 mA or MEP response fell under 50%.

Findings

On the first postoperative day, 18 patients experienced new paresis that resolved by discharge in 11. MEPs deteriorated in 39 patients, and 16 of these showed new postoperative paresis, indicating a 41% risk of new paresis. In the remaining 2/18 patients, intraoperative MEPs were stable, although new paresis appeared postoperatively. In one of these patients, intraoperative hemorrhage caused postoperative swelling, and the new motor deficit persisted until discharge. Of all 104 patients, 7 deteriorated in motor grade at discharge, 92 remained unchanged, and 5 patients have improved.

Conclusions

Adjustment of surgical strategy contributed to good motor outcome in 33/39 patients. MEP monitoring may help significantly to prevent motor deficits during demanding neurosurgical procedures on the brainstem.

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Acknowledgements

We thank Dr. M. Bjeljac and E. Ciessynna for their help with MEP recordings in this case series.

Conflicts of interest

None.

Author information

Authors and Affiliations

  1. Klinik für Neurochirurgie, UniversitätsSpital Zürich, 8091, Zürich, Switzerland

    Johannes Sarnthein, Oliver Bozinov & Angelina Graziella Melone

  2. Center for Integrative Human Physiology, Universität Zürich, Zürich, Switzerland

    Johannes Sarnthein

  3. Dipartimento di Scienze Neurologiche, Università La Sapienza, Roma, Italy

    Angelina Graziella Melone

  4. International Neuroscience Institute, Hannover, Germany

    Helmut Bertalanffy

Authors
  1. Johannes Sarnthein
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  2. Oliver Bozinov
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  3. Angelina Graziella Melone
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  4. Helmut Bertalanffy
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Corresponding author

Correspondence to Johannes Sarnthein.

Additional information

Comment

In this article the authors report their experience with the use of IOM with MEPs in the resection of brainstem (BS) lesions. Surgery of brainstem lesions is certainly not free of potential ominous consequences, which makes the use of any intra-operative real time monitoring of nervous function located within the stem all the more important. The authors are therefore to be commended for their effort in trying to come up with clearer and more objective clinical rules to be pursued during these operations. The series is significant, although it includes a wide mixture of lesions differently affecting the BS.

The distinction between extra- and intra-axial lesions follows criteria that are not strictly related to the intrinsic nature of the lesion itself (i.e., ependymoma in the extra-axial group), but also according to the topography of the pathological growth to be excised.

As expected, lesions that grow outside the BS evoke fewer intra-operative MEP changes than those intimately arising from the cellular patrimony of the stem.

Reasons for neuroelectric MEP changes are several. The latency period between the initial aggression and the establishment of the predictable changes in MEPs is variable, as is the mechanism of the insult. Also, some changes may be severe but reversible, whereas others are not.

As mentioned in the text, mechanical causes may tend to evoke more immediate changes, whereas vascular (especially venous) insult or 'edema' may have a significant delay despite the fact that the consequences may end up being worse. The authors clearly demonstrate that they are well aware of the current limitations and caveats involved with the translation of the neurophysiological information into intraoperative surgical decisions.

The heart of the matter is that this technology is no longer dispensable given the possible adverse effects of the surgical manipulation of the BS and the need to minimize it. On the other hand we need clearer indications of what the intraoperative neurophsysiologic changes really mean in terms of determining transient or permanent morbidity so that we can act accordingly, either continuing to pursue the resection of the lesion or stopping short of doing so. The third alternative is to alter the surgical strategy, which is what the authors allude to in their text.

This issue is not without importance because there is little one can change in a pristine microsurgical technique itself; what can be altered is, essentially, the location of the dissection or the degree of the resection.

The results are very clearly documented. All patients who deteriorated postoperatively had intra-operative changes signaled to the surgeon. This means that despite the warning and the eventual change in strategy, not all consequences could be avoided.

What advice is there from the surgeon's standpoint taking into consideration the fact that in only 41% of cases did the deterioration of the warning signs predict a new motor deficit? This in fact is the bottom line: Should these warnings forbid us from pursuing a more radical resection or should one still carry on with it?

Of course, experience is foremost and the key to the question, albeit subjective, before we can come up with absolute boundaries for these IOM changes (MEP or others) more independent from the eye and soul of the surgeon, and the monitoring equipment as well.

Manuel Cunha e Sa

Almada, Portugal

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Sarnthein, J., Bozinov, O., Melone, A.G. et al. Motor-evoked potentials (MEP) during brainstem surgery to preserve corticospinal function. Acta Neurochir 153, 1753–1759 (2011). https://doi.org/10.1007/s00701-011-1065-7

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  • DOI: https://doi.org/10.1007/s00701-011-1065-7

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