Preoperative visualization of the facial nerve using diffusion tensor imaging fibre tracking in patients with large vestibular schwannomas

Abstract

Surgical treatment of vestibular schwannoma (VS) aims to complete tumour removal and preservation of neurological functions and quality of life [ 1, 2 ]. Early visualization of the facial nerve (FN), based on its relation to anatomical landmarks and on the application of electrical stimulation and EMG monitoring, is among the main prerequisites for its preservation. In large VS, however, the nerve in its cerebellopontine angle (CPA) segment is displaced by the expanding tumour in an unpredictable manner and is often changed morphologically. Therefore, an imagingbased technique that demonstrates the location of the nerve in relation to the VS preoperatively could be of help to the surgeon. The advances in MR imaging have improved the visualization of cranial nerves [ 3 - 7 ]. In the cerebellopontine angle T2-weighted sequences such as constructive interference in steady state (CISS) allow a detailed anatomical visualization of nerve anatomy in normal subjects. FN can be clearly depicted both in its cisternal segment [ 4, 6 ] and in its course in the internal auditory [ 7, 8 ]. This anatomy changes radically in the presence of a space-occupying lesion such as VS, and the ability of FN depiction with the standard anatomical MR imaging sequences decreases. In a previous study [ 9 ] using T2-weighted fast spin-echo MR images, the spatial relationship between FN and the tumour was identifi ed in only 2 (9 %) of the patients harbouring VS. Furthermore, correlating these results with the dimension of the lesions, it has been observed that these 2 cases had lesions not exceeding 11 mm in maximal diameter (VS class T1-T2).