THE ENDOLYMPHATIC SAC - MICROSURGICAL TOPOGRAPHIC ANATOMY

THE ENDOLYMPHATIC SAC is part of the membranous labyrinth; it plays an important role in the hearing mechanism. Injury to the endolymphatic sac may, over time, severely compromise hearing. The endolymphatic sac is located in a duplication of the dura of the posterior aspect of th e petrous pyramid and is, therefore, in the surgical field of many neu rosurgical operations performed on the posterolateral cranial base. Th e endolymphatic sac was exposed bilaterally in 10 anatomic specimens; the distance from the center of the sac to the posterior lip of the in ternal auditory meatus and to the XIth nerve in the jugular foramen wa s measured with a caliper. Also measured was the distance between the center of the sac and the closest point on the petrous ridge and the d istance between that point and the petro-sigmoid intersection. The pet ro-sigmoid intersection was defined as the point at which the medial a spect of the sigmoid sinus intersects the lateral aspect of the petrou s ridge. The dimensions of the sac were also recorded. On the average, the sac was found to be 15.7 mm posterosuperior (superolateral) to th e XIth nerve in the jugular foramen (range, 11.0-18.5 mm) and 13.3 mm posterior (lateral) to the internal auditory meatus (range, 10.0-18.0 mm). The center of the sac was 24.1 mm (mean value) (range, 20.0-28.0 mm) in front of the petro-sigmoid intersection at a point 11.5 mm (mea n value) (range, 8-17 mm) below the petrous ridge. The mean width and height of the sac were 3.83 (range, 2-6 mm) and 3.80 mm (range, 2.5-8 mm), respectively. The anatomical landmarks selected (XIth nerve, inte rnal auditory meatus, petrous ridge, petro-sigmoid intersection) are i dentifiable during neurosurgical procedures on the posterolateral cran ial base. Knowing the general relationships between these landmarks an d the endolymphatic sac may help the neurosurgeon to correctly localiz e the sac and to protect it during surgery to avoid inadvertent lesion s to the hearing mechanism.