Perspectives Commentary on: Motor Evoked Potential Monitoring During Surgery of Middle Cerebral Artery Aneurysms: A Cohort Study by Yue et al. World Neurosurg 82:1091-1099, 2014

Motor-Evoked Potentials in Cerebral Aneurysm Surgery Peter Nakaji

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he middle cerebral artery (MCA) is one of the aneurysm locations for which the preponderance of evidence continues to support open microsurgical clipping over endovascular therapy in most cases. Even craniotomy for aneurysm clipping, however, is not static from a technical standpoint. Indocyanine green angiography, minicraniotomy approaches, and other advances continue to be made. In the article by Yue et al. in a recent issue of WORLD NEUROSURGERY, Professor Ying Mao’s large and sophisticated cerebrovascular group at Huashan Hospital in Shanghai reviewed their experience with using motor-evoked potentials (MEPs) for pyramidal system monitoring during MCA aneurysm surgery. They looked prospectively at 2 nonrandomized cohorts of patients undergoing surgery for clipping of MCA aneurysms. They found that although decreases in the MEPs of 50% or 100% did not predict discharge outcome, they did predict motor outcome at their patients’ last mean follow-up of 39.1 months. The rationale for the use of MEPs in this setting is quite sound. Although direct damage to the relevant cortex and white matter tracts is not frequently an issue in aneurysm surgery, the vascular territory of the MCA includes them both. In the M1 segment, the lenticulostriate perforators perfuse the internal capsule and the upper portion of the pyramidal tracts. The distal MCA branches most often perfuse all but the most medial portion of the motor strip, but even the medial functions (i.e., trunk and leg control) may be compromised because of ischemia affecting their descending subcortical connections. Evidence that they can improve long-term outcome should be considered seriously.

Key words Aneurysm - Intraoperative monitoring - Middle cerebral artery - Motor evoked potential - Surgery -

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Abbreviations and Acronyms MCA: Middle cerebral artery MEP: Motor-evoked potential SSEP: Somatosensory-evoked potential

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Much of the controversy over the use of MEPs in cerebrovascular neurosurgery has less to do with their usefulness than with their finicky nature. A variety of confounding factors may affect their sensitivity and specificity, usually in the sense of obscuring the line between true- and false-positives. Such factors include anesthetic types and levels, blood pressure, temperature, and technical issues of the monitoring itself. For surgeries that often require the surgeon to maintain a laser-beam-like focus, the addition of one more unreliable annoyance can be too much. For aneurysm surgery, in particular, it may batter the nerves of the calmest surgeon to have a patient twitching every 5 minutes throughout surgery and every 2 minutes during the most crucial parts of microdissection and temporary clipping. Nonetheless, one gets used to such things, especially if the annoyance is worth it: the loss of MEPs at a relevant time may alert the operative team to intervene in a meaningful way to prevent a deficit. As the authors point out, the MEPs do not simply tell one that something bad is happening without the surgeon having recourse. On the contrary, there are many meaningful maneuvers that the team can use: blood pressure may be increased, temporary clips may be removed, or a permanent clip may be re evaluated for adjustment to restore better flow. Equally, if the MEPs are properly established in the beginning, there is significant reassurance in finding them maintained in surgery past the maneuvers that might have been expected to cause them to decrease. In general, MEPs are more sensitive than somatosensory-evoked potentials (SSEPs), partly as a consequence of being less robust. When they are still present, one is reassured that flow is adequate to the brain area of interest. In theory, by being more anterior, the motor cortex is also

Division of Neurological Surgery, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona, USA To whom correspondence should be addressed: Peter Nakaji, M.D. [E-mail: [email protected]] Citation: World Neurosurg. (2015) 83, 2:162-163. http://dx.doi.org/10.1016/j.wneu.2014.11.006

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PERSPECTIVES

more dependent on MCA blood flow than the sensory cortex. This effect is unreliable, however, because vascular territories are unpredictable. Furthermore, simple maintenance of MEPs during surgery does not guarantee that postoperative ischemia will not result, because the metabolic demand of the anesthetized brain is less than that of the awake brain. The present study still leaves room for refinement. It would be helpful to examine the combined predictive value of MEPs, SSEPs, and electroencephalography as well as with the use of multivariate analysis. Because they are routinely performed together, the combination of these techniques may be even more useful than any one alone. It would also provide evidence that MEPs supply more information than the more user-friendly SSEPs and electroencephalography modalities. Other desirable improvements would include randomization and separate enrollment of patients with ruptured and unruptured aneurysms. Some detractors will say that MEPs are yet one more complication and expense to add to an already complex surgery. In response to this criticism, I would point out that the cost of a deficit resulting from MCA surgery, which will likely result in hemiparesis or hemiplegia, aphasia, loss of a visual field, or some other tremendous detriment to the patient’s quality of life, is so catastrophically high that it justifies both enormous effort and expense on our part. Investment of even a large amount of

additional effort or money at the time of surgery can prevent a lifetime of heartbreak. The human cost of neurologic deficits, whether caused by disease or by its treatment, is incalculable. And although the financial cost is uncalculated, it is not beyond belief that in strict accounting terms, there is no country in the world in which the prevention of a deficit is not worthwhile, because the downstream costs are so great. In a world that is increasingly focused on providing value for money throughout a lifetime (as opposed to a single surgery, illness, or hospitalization), preserving and restoring neurologic function will take on the absolute priority that it should have deserved all along. Excellent cerebrovascular neurosurgery may be expensive, but not nearly as expensive as the excess morbidity and mortality it can treat. For the cerebrovascular neurosurgeon who has long battled for the well-being of the patient, it will be good to see the health care economy have this epiphany up-front costs be damned.

Citation: World Neurosurg. (2015) 83, 2:162-163. http://dx.doi.org/10.1016/j.wneu.2014.11.006 Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com 1878-8750/$ - see front matter ª 2015 Elsevier Inc. All rights reserved.

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WORLD NEUROSURGERY 83 [2]: 162-163, FEBRUARY 2015

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Motor-evoked potentials in cerebral aneurysm surgery.

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