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Abstract
The purpose of our study was to investigate the topographical relationship between these nerves along their course from the brainstem through the internal acoustic canal IAC in the living human brain using MR imaging. We performed three-dimensional gradient echo balanced Fast Field Echo (3D bFFE) sequence oblique parasagittal MR imaging in 73 healthy subjects. The IACs were analyzed from the brainstem end of the IAC to the fundus in contiguous sections. At five levels, the topographical relationships between the facial and vestibulocochlear nerves (VCN) were recorded. In the lateral portions of the IACs where they separated from each other, the relative sizes of the individual nerves were examined. In general, the facial nerve (FN), which is a round structure, is located anteriorly and superiorly to the vestibulocochlear nerve throughout its course. The vestibulocochlear nerve is usually rectangular; however, it was found to be round and at times triangular in shape near the brainstem, before it became crescent-shaped at the porus in 89% of the cases. The superior vestibular nerve kept its posterosuperior position in the canal, and the inferior vestibular nerve (IVN) and the cochlear nerve (CN) travelled inferior to it. The superior and inferior vestibular nerves were divided by the falciform crest in 53% of the cases. The inferior vestibular nerve was the smallest nerve in 52% of the cases, and the cochlear nerve was the largest in 36% of the cases. To the best of our knowledge, this study is the largest in vivo MR study, and most of our findings differ from previous cadaver studies. Determination of these topographical relationships may facilitate our understanding of the complicated physiological relationships between the 7(th) and 8(th) nerve complexes during surgery in this region.
Highlights
Along their course from the brainstem to the internal acoustic canal, the relationship between the facial and vestibulocochlear nerves shows some variations [1, 2]
The vestibulocochlear nerve is usually rectangular; it was found to be round and at times triangular in shape near the brainstem, before it became crescent-shaped at the porus in 89% of the cases
The inferior vestibular nerve was the smallest nerve in 52% of the cases, and the cochlear nerve was the largest in 36% of the cases
Summary
Along their course from the brainstem to the internal acoustic canal, the relationship between the facial and vestibulocochlear nerves shows some variations [1, 2]. With the development of microsurgery and endoscopic surgery in the internal acoustic canal (IAC), the study of the topographical relationship between these two nerves in the human IAC becomes increasingly important for the diagnosis and surgical planning of acoustic schwannomas [2,3,4]. Cadaver studies always carry the risk that the anatomical relationships and shapes of the component of the nerves in the IAC and cerebellopontine cisterns may have been altered due to the absence of cerebellopontine fluid and blood supply as well as the formalin fixation, which results in the shrinkage of nerves [3,4,5,6,7]. Anatomical studies must be conducted in living human tissue to define the true anatomical relationships among these nerves within the IAC and cerebellopontine cistern [3]
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