Anatomical study of the central myelin portion and transitional zone of the vestibulocochlear nerve

The aim of this study was to evaluate gross and microscopic anatomical features of the vestibulocochlear nerve or eighth cranial nerve (CNVIII) from fresh cadavers, especially the nerve's central myelin portion (CMP) and transitional zone (TZ), and to consider any pathological implications.

Six fresh cadavers were used to examine the CNVIII. Its cisternal length from brainstem to internal auditory meatus was measured. Longitudinal sections were stained to make following measurements: the diameter where the nerve enters the brainstem, the diameter where the TZ begins, the distance to the most distal part of TZ from the brainstem, and the depth of the TZ. The volume of the CMP was calculated as well.

The cisternal length of ten CNVIIIs measured between 14.2 and 19.2 mm (16.48 ± 1.78 mm). The thickness where the CNVIII enters the brainstem was between 1.21 and 3.16 mm (2.31 ± 0.68 mm); the thickness where the TZ begins was between 1.07 and 2.21 mm (1.44 ± 0.38 mm); the distance of the most distal part of the TZ from the brainstem was between 9.28 and 13.84 mm (11.50 ± 1.56 mm); the depth of the TZ was between 0.56 and 1.28 mm (0.81 ± 0.27 mm). The volume of the CMP was between 17.34 and 53.87 mm(3) (33.98 ± 13.74 mm(3)). The measurements were compared to trigeminal, facial, glossopharyngeal and vagus nerves. CNVIII was the nerve with the longest CMP.

The measurements showed that the CMP of CNVIII was very long. The implication of this length in the dysfunctional syndromes of this nerve, its propensity to harbor schwannomas, which most frequently arise at the porus of the auditory meatus, and the vulnerability to damages are discussed.