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 the petrous pyramid and is, therefore, in the surgical field of many neurosurgical operations performed on the posterolateral cranial base. The endolymphatic sac was exposed bilaterally in 10 anatomic specimens; the distance from the center of the sac to the posterior lip of the internal auditory meatus and to the XIth nerve in the jugular foramen was 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 distance between that point and the petro-sigmoid intersection. The petro-sigmoid intersection was defined as the point at which the medial aspect of the sigmoid sinus intersects the lateral aspect of the petrous ridge. The dimensions of the sac were also recorded. On the average, the sac was found to be 15.7 mm posterosuperior (superolateral) to the 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 (mean 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, internal auditory meatus, petrous ridge, petro-sigmoid intersection) are identifiable during neurosurgical procedures on the posterolateral cranial base. Knowing the general relationships between these landmarks and the endolymphatic sac may help the neurosurgeon to correctly localize the sac and to protect it during surgery to avoid inadvertent lesions to the hearing mechanism. © by the Congress of Neurological Surgeons.