Enhancing hearing preservation in endoscopic-assisted excision of acoustic neuroma via the retrosigmoid approach

Surgeons using the operating microscope are able to make use of numerous landmarks described for the lateral limits of dissection to preserve hearing in acoustic neuroma surgery via the retrosigmoid approach. Similar landmarks for hearing preservation described specifically for the endoscopic-assisted technique, are lacking. By analysing computed tomography (CT) scans of temporal bones, it was observed that to reach within 3 mm of the lateral end of the internal auditory meatus (IAM) via a 3 cm retrosigmoid craniotomy, drilling should be up to about 3 mm medial to the opening of the vestibular aqueduct. It was hypothesized that in surgery, by keeping 3 mm medial to the opening of the vestibular aqueduct, the integrity of inner ear structures would be preserved. This hypothesis was tested in 30 temporal bones and was found to be true. In addition, the lateral end of the IAM up to the transverse crest could be viewed by the 30-degree rigid angled endoscope. This landmark could, therefore, be utilized in the endoscopic-assisted technique to predict the optimal amount of bone to be removed at a stage before the internal auditory meatal dura is opened when the intact dura affords added protection to the meatal contents during drilling. Well designed dural flaps on the posterior petrous bone could be created by making a longitudinal incision not more than 7 mm from the superior border of petrous bone and a transverse incision at least 17 mm from sigmoid. These flaps minimize injury to the endolymphatic sac and protect the cochlear nerve and vasculature that when damaged, may result in hearing loss.