Anatomical delineation of a safety zone for drilling the internal acoustic meatus during surgery for vestibular schwanomma by retrosigmoid suboccipital approach.

Drilling the internal acoustic meatus (IAM) is an important step during surgical removal of vestibular schwanommas. During this maneuver, the proximity of the jugular bulb and the saccus endolymphaticus to the IAM makes these structures prone to injury. There have been studies in the past on the relationship between a high jugular bulb (HJB) and the IAM, but there is no description of a safety area available for drilling the meatus. In the present study, measurements were made between the IAM and the nearest points on the top of the jugular bulb, the saccus endolymphaticus and the petrous ridge. The incidence of a HJB was also calculated. It was found that about 5 mm of bone could be safely drilled away from the posterior lip of the IAM without damaging either the jugular bulb or the saccus endolymphaticus in most of the cases. The incidence of HJB was found to be 38.6% with 18.1% having a very high bulb. Thus, a significant proportion of patients are likely to have a HJB. Since there are significant individual variations, a preoperative radiological study for delineation of important bony landmarks is necessary in all patients undergoing surgery for vestibular schwanommas.