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NI FEATURE: THE EDITORIAL DEBATE III-- PROS AND CONS
Year : 2019  |  Volume : 67  |  Issue : 1  |  Page : 69-70

Sketching the facial nerve on vestibular schwannoma


Department of Neurosurgery, National Institute of Mental Health and Neurosciences, Bengaluru, Karnataka, India

Date of Web Publication7-Mar-2019

Correspondence Address:
Dr. Dhaval Shukla
Department of Neurosurgery, National Institute of Mental Health and Neurosciences, Bengaluru - 560 029, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0028-3886.253589

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How to cite this article:
Shukla D. Sketching the facial nerve on vestibular schwannoma. Neurol India 2019;67:69-70

How to cite this URL:
Shukla D. Sketching the facial nerve on vestibular schwannoma. Neurol India [serial online] 2019 [cited 2019 May 27];67:69-70. Available from: http://www.neurologyindia.com/text.asp?2019/67/1/69/253589




A significant number of patients undergoing surgery for vestibular schwannomas still experience a partial or total damage to facial nerve. The main reason is that the facial nerve can be displaced by the tumor, and therefore, its location is unknown to the surgeon before surgery. Awareness of the course of the facial nerve is crucial during surgery for vestibular schwannomas. The current strategy in vestibular schwannoma surgery is early intraoperative identification of the facial nerve by means of intraoperative monitoring. Intraoperative facial nerve monitoring (IOFNM) is used to localize the facial nerve during surgery. The IOFNM also accurately predicts favorable long-term function of the facial nerve.[1] The evidence-based guidelines by the Congress of Neurological Surgeons recommend that IOFNM should be routinely used during surgery for vestibular schwannomas to improve long-term facial function.[2] However, the knowledge of the position of the facial nerve prior to the surgery will be really useful to the surgeons. Various magnetic resonance imaging (MRI) studies have explored the optimal imaging sequence to enhance visualization of the facial nerve as it courses through the cerebrospinal fluid in the cisternal segment into the canalicular segment, where it might be deflected by the presence of the vestibular schwannoma. Specialized T2-weighted sequences, which highlight fluid–tissue interfaces, such as constructed interference in steady state, have a sensitivity of 63%–90% to delineate the facial nerve. The facial nerve is more likely to be visualized in smaller tumors, with a solid consistency.[3] The visualization of the course of the facial nerve diminishes with larger sized tumors because of nerve thinning, splaying of fibers, or obliteration of anatomical landmarks on standard MRI sequences when distorted by a tumor. Imaging studies designed to trace the course of the facial nerve preoperatively should theoretically enhance surgical safety. The use of diffusion tensor imaging (DTI) for nerve tracking has evolved as a reliable technique in this regard. DTI has further motivated investigation into three-dimensional fiber tractography to augment the visualization of nerves adjacent to a vestibular schwannoma. The latest systematic review showed that complete tracking of the nerve's course was obtained in 96.6% of cases and that surgical concordance with the preoperative tractography findings was obtained in 90.7% of cases.[4] The preoperative tractography for facial nerve identification is a useful adjunct in the surgical planning for large tumors.

In this issue, Samala et al., present their experience with tractography of the facial nerve for surgery of large vestibular schwannomas.[5] The authors reproduced their results of concordance on preoperative identification of the facial nerve with intraoperative findings. In addition, they also found that preoperative identification of the facial nerve helped to preserve the nerve more frequently. The results of this study are presented in an extremely simplified form for the readers to interpret. An inexperienced reader may feel that the tractography technique is extremely simple and can be easily replicated at his or her center.

The application of tractography to detailed morphological reconstruction of the cranial nerves is a recent technology; therefore, it has potential limitations. Using DTI tractography to predict facial nerve location nearby the tumor is, however, not an easy task, since several nerves are present in the tumor area, very close to each other. The major limitation is inability to distinguish the fibers of cochlear and facial nerves. The cranial nerve regions are at low signal-to-noise ratio and can present as complex patterns. For these reasons, the analysis of fiber tracking of cranial nerves requires a number of DTI subprotocols. The truthful reconstruction of the facial nerve is possible only by providing additional anatomical information to the tracking algorithms. The most important factors influencing visualization of the facial nerve are the selection of the region of interest and the set point of the fractional anisotropy threshold. The cooperation between an experienced surgeon and the neuroinformatics team is essential for the successful demonstration of the facial nerve. Besides technological limitations, the cause of discordance can be due to the cystic nature of schwannoma, through which the constructed tract can appear penetrated; and a very large size of the tumor, which can impede intraoperative identification of the facial nerve.

The loading of the reconstructed nerve course on the neuronavigation system can provide additional real-time and patient-specific information. The integration of the navigation system should decrease the duration of the surgery, helping the surgeons to preserve the facial nerve and to improve patients' outcome. In one study, intraoperative navigation confirmed the accuracy of facial nerve tractography in 94.4% of cases.[6] The implication of identification of the facial nerve and its impact on preservation of facial function have been less studied. Samala et al., have shown that identification of the facial nerve preoperatively had a significant impact on the facial nerve preservation. However, the operative technique and continuous intraoperative neuromonitoring also have influence on the functional outcome in these cases.



 
  References Top

1.
Mastronardi L, Campione A, Zomorodi A, Di Scipio E, Adornetti A, Fukushima T. Intraoperative identification and location of facial nerve: Type of facial nerve displacement—How to use monopolar stimulator. In: Advances in Vestibular Schwannoma Microneurosurgery. Cham: Springer International Publishing; 2019. p. 83-94.  Back to cited text no. 1
    
2.
Vivas EX, Carlson ML, Neff BA, Shepard NT, McCracken DJ, Sweeney AD, et al. Congress of neurological surgeons systematic review and evidence-based guidelines on intraoperative cranial nerve monitoring in vestibular schwannoma surgery. Neurosurgery 2018;82:E44-6.  Back to cited text no. 2
    
3.
Dunn IF, Bi WL, Mukundan S, Delman BN, Parish J, Atkins T, et al. Congress of neurological surgeons systematic review and evidence-based guidelines on the role of imaging in the diagnosis and management of patients with vestibular schwannomas. Neurosurgery 2018;82:E32-4.  Back to cited text no. 3
    
4.
Savardekar AR, Patra DP, Thakur JD, Narayan V, Mohammed N, Bollam P, et al. Preoperative diffusion tensor imaging–fiber tracking for facial nerve identification in vestibular schwannoma: A systematic review on its evolution and current status with a pooled data analysis of surgical concordance rates. Neurosurg Focus 2018;44:E5.  Back to cited text no. 4
    
5.
Samala R, Borkar S, Sharma R, Garg A, Suri A, Gupta D. Effectiveness of preoperative facial nerve diffusion tensor imaging tractography for preservation of facial nerve function in surgery for large vestibular schwannomas: Results of a prospective randomized study. Neurol India 2019;67:149-54.  Back to cited text no. 5
  [Full text]  
6.
Li H, Wang L, Hao S, Li D, Wu Z, Zhang L, et al. Identification of the facial nerve in relation to vestibular schwannoma using preoperative diffusion tensor tractography and intraoperative tractography-integrated neuronavigation system. World Neurosurg 2017;107:669-77.  Back to cited text no. 6
    




 

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