Microsurgical anatomy of the cochlear nuclei

OBJECTIVE: The dorsal and ventral cochlear nuclei are exposed when treating lesions involving the fourth ventricle and the cerebellopontine angle. Precisely locating the nuclei is important in the placement of the auditory brainstem implantation and in intraoperative monitoring of auditory evoked potentials. This study examined the microsurgical anatomy of the cochlear nuclei. METHODS: The relationships and the surgical approaches to the nuclei were examined in 20 formalin-fixed specimens with the aid of an operating microscope. RESULTS: The dorsal cochlear nucleus is located in the floor of the lateral recess of the fourth ventricle on the posterior surface of the inferior cerebellar peduncle where it produces a prominence, the auditory tubercle. The ventral cochlear nucleus is positioned on the lateral surface of the inferior cerebellar peduncle where it often straddles the anterior edge of the foramen of Luschka at the junction of the lateral recess and cerebellopontine angle and produces less of a prominence on the surface of the brainstem than on the dorsal nucleus. CONCLUSION: Both the dorsal and ventral cochlear nuclei can be exposed after opening the foramen of Luschka and the lateral recess of the fourth ventricle by the retrosigmoid or translabyrinthine approach. The dorsal cochlear nucleus is positioned so that it would be damaged by a tumor within or approaches to the floor of the lateral recess. The ventral cochlear nucleus is positioned so that it would be vulnerable to damage by tumors within or operative approaches to the cerebellopontine angle. The dorsal cochlear nucleus and the adjacent intraventricular part of the ventral cochlear nucleus have anatomic characteristics that make them the preferable site of placement for the auditory brain stem implant.