NI FEATURE: THE EDITORIAL DEBATE III-- PROS AND CONS
|Year : 2019 | Volume
| Issue : 1 | Page : 67--68
Preoperative facial nerve diffusion tensor imaging tractography for preservation of facial nerve function in surgery for large vestibular schwannomas
Division of Neurosurgery, University of Cape Town, Cape Town, South Africa
Dr. Allan Taylor
Division of Neurosurgery, University of Cape Town, Cape Town
|How to cite this article:|
Taylor A. Preoperative facial nerve diffusion tensor imaging tractography for preservation of facial nerve function in surgery for large vestibular schwannomas.Neurol India 2019;67:67-68
|How to cite this URL:|
Taylor A. Preoperative facial nerve diffusion tensor imaging tractography for preservation of facial nerve function in surgery for large vestibular schwannomas. Neurol India [serial online] 2019 [cited 2022 Jun 25 ];67:67-68
Available from: https://www.neurologyindia.com/text.asp?2019/67/1/67/253595
Diffusion tensor imaging (DTI) has been explored since 2006 as a technique to help with the pre-surgical localization of the VII nerve before operating on large vestibular schwannomas. Despite technical challenges, such as where to place seed sites, the presence of cerebrospinal fluid pulsation artifacts, and the presence of fluid close to the nerve in cystic tumors, it has been shown to have reliable concordance with surgical nerve identification in 85% of cases. Previous studies have focused on the technical possibilities of cranial nerve DTI imaging, and this is the first study examining the utility of DTI in the preservation of VII nerve function during total tumor removal. The authors are to be commended for conducting a randomized trial and for their outstanding results in VII nerve preservation. Anatomical and functional preservation was achieved in 90% of cases where the nerve position was localized on pre-operative DTI imaging. In the group where no DTI was done, the preservation rate was 63%. Unfortunately, the facial nerve function results are not graded in House Brackman grades. This would have been useful as the patient group had a high incidence of facial weakness before treatment, where only 19.7% of patients were reported to have a normal facial function. It is not clear if the patients classified as having functional preservation had improved, static, or worse function.
The ideal outcome with surgery for vestibular schwannoma is complete removal with preservation or restoration of function. Surgical results have improved steadily with the introduction of microsurgery, improved anatomical knowledge, intraoperative monitoring and imaging techniques. With a good technique, it is now possible to reliably preserve facial nerve function, facial sensation, and lower cranial nerve function. Although it is possible to preserve hearing in larger tumors, it requires the presence of a good function at the time of treatment and this is uncommon. When approaching the VII nerve, most surgeons locate the nerve at the internal auditory meatus and brainstem before attempting to dissect the nerve off the tumor capsule. The stretched nerve is usually located on the anterior surface of the tumor from the middle of the tumor to the superior surface (72.6%). It is rarely displaced inferiorly and even less often posteriorly. It is routine to use monitoring of VII nerve function during surgery and a posterior location of the nerve can be excluded before opening the capsule by using stimulation. If this step is followed, the surgeon can assume that in most cases, the stretched nerve is anterior to the tumor and at the level of the internal auditory meatus (IAM) or superior to this. If the nerve is not splayed then it can be followed from the IAM and the brainstem. Dissection should not put traction on the nerve away from the IAM or brainstem, as gentle traction at 90 degrees to the direction of the nerve is safer. For most patients, these assumptions and this technique will result in a good outcome.
Pre-operative identification of an inferiorly displaced facial nerve or a splayed nerve would be helpful in avoiding an intra-operative injury. It is often not possible to dissect a splayed nerve off the capsule without having a functional loss. Pre-surgery detection of the position of the VIIth nerve would allow an alternative surgical approach such as partial resection or planned grafting. If the nerve is identified as being inferior, then unnecessary retraction of this part of the capsule could be avoided. In this study, the technique did not identify the splayed nerves; it was only helpful in alerting the operating surgeon as to the location of the VIIth nerve. This information did improve results in this study but further work is required before concluding that this should be a routine investigation.
When trying to achieve good outcomes for patients, extra information should always be considered valuable. However, differences in technique, technical failures (10%), lack of concordance between the imaging and operative findings (2.5% in this study and 24.5% in others) means that DTI evaluation of the VIIth nerve should considered an evolving technique.
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