Cochlear and vestibular gross and histologic anatomy (as seen from postauricular approach).

The otologic surgeon must have a clear understanding of the anatomy of the seventh and eighth cranial nerves from the labyrinth to the brain stem, as seen from the postauricular approach. The surgical anatomy of the seventh and eighth cranial nerves was studied in 64 transcochlear eighth-nerve sections and 33 retrolabyrinthine vestibular neurectomies. Analysis indicates the nerves rotate 90 degrees in their course from the ear to the brain. The key relationship is that the cochlear nerve is always the most inferior, rotating from anterior (medial) near the labyrinth to posterior (lateral) near the brain stem. The seventh (facial) nerve rotates from anterosuperior (medial superior) near the labyrinth to anteroinferior (medial inferior) near the brain stem. The seventh nerve is easily seen in the transcochlear approach and hidden from view in the retrolabyrinthine approach. Twenty-seven fixed nerve specimens were examined with an operating microscope before being prepared for sectioning. In 73% (19 of 26) a cleavage plane was seen on the lateral aspect of the eighth nerve (that portion of the nerve facing the surgeon in the retrolabyrinthine approach).