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COMMENTARY
Year : 2016  |  Volume : 64  |  Issue : 5  |  Page : 971-972

Localising cranial nerves in the cerebellopontine angle tumors using tractography prior to surgery for vestibular schwannomas


Department of Imaging Sciences and Interventional Radiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India

Date of Web Publication12-Sep-2016

Correspondence Address:
Chandrasekaharan Kesavadas
Department of Imaging Sciences and Interventional Radiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0028-3886.190281

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How to cite this article:
Kesavadas C. Localising cranial nerves in the cerebellopontine angle tumors using tractography prior to surgery for vestibular schwannomas. Neurol India 2016;64:971-2

How to cite this URL:
Kesavadas C. Localising cranial nerves in the cerebellopontine angle tumors using tractography prior to surgery for vestibular schwannomas. Neurol India [serial online] 2016 [cited 2019 Aug 23];64:971-2. Available from: http://www.neurologyindia.com/text.asp?2016/64/5/971/190281




Preservation of cranial nerve function is important during surgery for a vestibular schwannoma (VS). A good post-surgical quality of life in these patients requires the surgeon to preserve the function of the facial and cochlear nerves (especially if the patient had preserved hearing before surgery). In a case where the tumor is large, this could be a challenge because of the distorted anatomy and variation in the course of the nerves. Attempts have been made to understand the cranial nerve anatomy using various conventional and advanced magnetic resonance imaging (MRI) techniques. Earlier studies have focused on highly T2 weighted sequences such as three-dimensional (3-D) FIESTA (fast imaging employing steady state acquisition) to get magnetic resonance cisternography that can show the cranial nerve anatomy within the cisternal spaces.[1] This technique has been shown to be superior to 3D turbo spin echo T2 weighted images and hence has become a routine protocol in the evaluation of the anatomy in the cerebellopontine angle especially in cases of VSs. However in larger tumors, the demonstration of the cranial nerves by this technique is difficult.

Diffusion tensor tractography (DTT) is a three dimensional modeling technique utilized to visualize neural tracts using the data collected from diffusion tensor imaging. This visualization has clinical value for detecting changes in the anatomic course of the tract and its relation to a tumor. This information may be of importance in planning surgery. With the advent of DTT and its refinements over the last ten years, there have been several publications, including the one published by Borkar et al.,[2] in this issue, that have tried to evaluate the role of this technique in demonstrating the course of the facial nerve in relation to a vestibular schwannoma. Most of the studies looked at the concordance rate between the preoperative tractography findings and intraoperative position of the facial nerve during VS surgery. Though the number of patients is small, a good correlation of 89 to 100% is reported in most of the recent studies.

There are challenges in the tractography technique and limitations in the methods employed in many studies. A few articles have addressed these limitations.[3] The primary objective in most of the earlier studies was identification of only the facial nerve in relation to the tumor. Identification of fibers other than that of facial nerve and predicting whether a track that is visualized represents the facial nerve, vestibulocochlear nerve, or mere noise can be difficult.[4] Tracking strategies should individually identify vestibulocochlear nerve in patients who have retained functional hearing and this remains a challenge. While vestibulocochlear fibers enter the internal auditory meatus along the posterior or inferior wall, the facial nerve enters the meatus along the anterior or antero-superior wall. Tractography may be able to suggest the cochlear nerve course that can help in preserving the nerve during surgery. A recent study has combined DTT and contrast enhanced FIESTA to predict facial and cochlear nerve locations in relation to the VS.[5] DTT can also be difficult in the presence of cystic components within the tumor, and also in tumors penetrating fibers. In these difficult situations, it is better to rely on cranial nerve localization by neurophysiological monitoring.[3]

There is no uniformity in the tracking strategy used in previous studies resulting in discrepancies in the identification of cranial nerves. Several parameters during DTI data acquisition such as voxel size, number of diffusion encoding directions and post processing parameters such as fractional anisotropy (FA) threshold and turning angle can influence the results of tractography.[6] Our earlier experience with this technique has taught us to optimize the technique, both its sequence parameters and post processing methods, before starting data acquisition in subjects.[7] This optimization of the methods is necessary before using the results of DTT in neurosurgical patient management.

Recent articles have shown that an improvement in the technique can give better information about the anatomy of cranial nerves and its relation to the tumor.[3],[4],[5] Methods are being developed to increase the spatial resolution and decrease the artifacts in the DTI data. With the availability of even better DTI acquisition techniques, post-processing tractography protocols and co-registration techniques, there is a possibility that some of the challenges mentioned can be overcome.

 
  References Top

1.
Mikami T, Minamida Y, Yamaki T, Koyanagi I, Nonaka T, Houkin K: Cranial nerve assessment in posterior fossa tumors with fast imaging employing steady-state acquisition (FIESTA). Neurosurg Rev 2005;28:261-6.   Back to cited text no. 1
    
2.
Borkar SA, Garg A, Mankotia DS, Joseph SL, Suri A, Kumar R, et al. Prediction of facial nerve position in vestibular schwannoma using diffusion tensor imaging tractography and its intraoperative correlation Neurology India 2016;64:965-70.  Back to cited text no. 2
    
3.
Wei PH, Qi ZG, Chen G, Hu P, Li MC, Liang JT, et al. Identification of cranial nerves near large vestibular schwannomas using superselective diffusion tensor tractography: Experience with 23 cases. Acta Neurochir (Wien) 2015;157:1239-49.  Back to cited text no. 3
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4.
Yoshino M, Kin T, Ito A, Saito T, Nakagawa D, Ino K, et al. Feasibility of diffusion tensor tractography for preoperative prediction of the location of the facial and vestibulocochlear nerves in relation to vestibular schwannoma Acta Neurochir (Wien) 2015;157:939-46.  Back to cited text no. 4
    
5.
Yoshino M, Kin T, Ito A, Saito T, Nakagawa D, Ino K, et al. Combined use of diffusion tensor tractography and multifused contrast enhanced FIESTA for predicting facial and cochlear nerve positions in relation to vestibular schwannoma. J Neurosurg 2015;123:1480-8.   Back to cited text no. 5
[PUBMED]    
6.
Mukherjee P, Chung SW, Berman JI, Hess CP, Henry RG. Diffusion tensor MR imaging and fiber tractography: Technical considerations AJNR Am. J. Neuroradiol. 2008;29:843-852.  Back to cited text no. 6
    
7.
James JS, Kumari SR, Sreedharan RM, Thomas B, Radhkrishnan A, Kesavadas C. Analyzing functional, structural, and anatomical correlation of hemispheric language lateralization in healthy subjects using functional MRI, diffusion tensor imaging, and voxel-based morphometry Neurol India 2015;63:49-57.  Back to cited text no. 7
    




 

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