LABORATORY INVESTIGATION J Neurosurg 125:1094–1104, 2016 The cochlea in skull base surgery: an anatomy study Jian Wang, MD, PhD,1 Fumitaka Yoshioka, MD,4 Wonil Joo, MD,2 Noritaka Komune, MD, PhD,3,4 Vicent Quilis-Quesada, MD,4 and Albert L. Rhoton Jr., MD4 1Department of Neurosurgery/Neuro-Oncology, Sun Yat-sen University Cancer Center, Guangzhou, Guangdong, People’s Republic of China; 2Department of Neurosurgery, Catholic University of Korea, Uijeongbu St. Mary’s Hopsital, Gyeonggi-do, Korea; 3Department of Otorhinolaryngology, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan; and 4Department of Neurosurgery, University of Florida, Gainesville, Florida OBJECTIVE The object of this study was to examine the relationships of the cochlea as a guide for avoiding both co- chlear damage with loss of hearing in middle fossa approaches and injury to adjacent structures in approaches directed through the cochlea. METHODS Twenty adult cadaveric middle fossae were examined using magnifications of ×3 to ×40. RESULTS The cochlea sits below the floor of the middle fossa in the area between and below the labyrinthine segment of the facial nerve and greater petrosal nerve (GPN) and adjacent to the lateral genu of the petrous carotid. Approxi- mately one-third of the cochlea extends below the medial edge of the labyrinthine segment of the facial nerve, geniculate ganglion, and proximal part of the GPN. The medial part of the basal and middle turns are the parts at greatest risk in drilling the floor of the middle fossa to expose the nerves in middle fossa approaches to the internal acoustic meatus and in anterior petrosectomy approaches. Resection of the cochlea is used selectively in extending approaches through the mastoid toward the lateral edge of the clivus and front of the brainstem. CONCLUSIONS An understanding of the location and relationships of the cochlea will reduce the likelihood of cochlear damage with hearing loss in approaches directed through the middle fossa and reduce the incidence of injury to adjacent structures in approaches directed through the cochlea. http://thejns.org/doi/abs/10.3171/2015.8.JNS151325 KEY WORDS cochlea; facial nerve; internal acoustic meatus; internal carotid artery; microsurgical anatomy; skull base surgery HE cochlea is the most anterior and medial part of er with hearing. The basal cochlear turn bulges laterally the vestibulocochlear labyrinth. It lies anteromedial toward the tympanic cavity where it underlies the promon- to the vestibule and semicircular canals, where it is tory, and its medial part lies below the floor of the middle Tembedded in the petrous part of the temporal bone adjacent fossa just posterior and medial to the geniculate ganglion to the petrous segment of the internal carotid artery, facial in the angle between the labyrinthine segment of the facial nerve, geniculate ganglion, internal acoustic meatus, and nerve and the greater petrosal nerve (GPN).1,3,23 Few stud- semicircular canals. The middle fossa surgical approaches ies have examined the exact location of the cochlea and directed through the temporal bone to the internal acous- its relationships with surround structures.1,7 This study ex- tic meatus, cerebellopontine angle, and petroclival region amined the relationships important in avoiding damage to pass near but usually spare the cochlea. One goal of the the cochlea during approaches directed through the middle middle cranial approaches to the internal acoustic meatus fossa and injury to adjacent structures in approaches di- and anterior petrous apex is to preserve the cochlea togeth- rected through the cochlea (Fig. 1). ABBREVIATIONS GPN = greater petrosal nerve. SUBMITTED June 8, 2015. ACCEPTED August 7, 2015. INCLUDE WHEN CITING Published online January 29, 2016; DOI: 10.3171/2015.8.JNS151325. 1094 J Neurosurg Volume 125 • November 2016 ©AANS, 2016 Unauthenticated | Downloaded 09/23/21 06:19 PM UTC Cochlea in skull base surgery FIG. 1. A: Superior view of the right middle fossa exposing the trigeminal nerve, GPN, and middle meningeal artery. B: The floor of the middle fossa has been drilled to expose the internal acoustic meatus. The basal turn of the cochlea has been exposed in the angle between the facial nerve and the GPN. Superior view, stepwise dissection of another right middle fossa (C–F). C: Bone has been removed to expose the meatal and labyrinthine segments of the facial nerve and the geniculate ganglion. D: Additional bone has been removed to expose the cochlea in the bone in the angle between the facial nerve and the GPN, the nerves in the internal acoustic meatus, and the tympanic cavity and eustachian tube. E: Additional drilling exposes the cochlea in the angle between the facial nerve and the GPN. The second turn of the cochlea is exposed below the GPN, and the apical turn is exposed pos- terolateral to the trochleariform process. The lateral edge of the basal and middle turns of the cochlea extends below the medial edge of the labyrinthine segment of the facial nerve and geniculate ganglion. F: The meatal segment of the facial nerve has been retracted to expose the cochlear nerve proximal to the cochlea. A. = artery; Ac. = acoustic; Arc. = arcuate; Car. = carotid; CN = cranial nerve; Coch. = cochlear; Depress. = depression; Emin. = eminence; Eust. = eustachian; Ext. = external; Gang. = ganglion; Gen. = geniculate; Gr. = greater; Int. = internal; Intermed. = intermedius; Laby. = labyrinthine; Lat. = lateral; Less. = lesser; M. = muscle; Memb. = membrane; Men. = meningeal; Mid. = middle; N. = nerve; Nerv. = nervus; P.C.A. = posterior cerebellar artery; Pet. = petrosal, petrous; Post. = posterior; Proc. = process; Seg. = segment; Sup. = superior; Tens. = tensor; Troch. = trochleari- form; Tymp. = tympani, tympanic. Methods which the vessels were injected with colored silicon were examined using magnifications of ×3 to ×20. The cochlea Twenty temporal bones from cadaveric specimens in was exposed from laterally through the mastoid and tym- J Neurosurg Volume 125 • November 2016 1095 Unauthenticated | Downloaded 09/23/21 06:19 PM UTC J. Wang et al. TABLE 1. Relationships of the cochlea as illustrated in Fig. 3 Measurements Average Range Angles in Fig. 3A & B (°) a Modiolar axis & sagittal plane 26.5 22.2–32.4 b Modiolar axis & coronal plane 58.4 53.6–67.3 c Axis along outer diameter of basal turn & long axis of superior semicircular canal 104 95–112 d Axis along outer diameter of basal turn & long axis of internal acoustic meatus 58.9 51.2–63.3 Lines in Fig. 3C & D (mm) a Anterior edge of basal turn to GPN 3.9 3.4–4.4 b Medial edge of basal turn to trigeminal nerve 8.4 7.2–11.2 c GPN to petrous ridge 14.9 12.7–17.4 d Posterior edge of basal turn to petrous ridge 9.7 7.8–11.3 e Medial edge of basal turn to medial side of labyrinthine segment of facial nerve 4.3 3.7–5.1 f Distance btwn horizontal (axial) planes passing through lower edge of basal turn & upper edge of jugular bulb 5.3 3.9 –7.4 g Shortest distance btwn basal turn & posterior genu of petrous carotid 1.9 1.1–3.2 Not shown in Fig. 3 — Labyrinthine segment of facial nerve to superior edge of basal turn (mm) 0.4 0.2–0.5 — Trochleariform process to modiolar apex (mm) 4.7 4.5–5.1 — Superficial edge of basal turn to middle fossa floor (mm) 3.9 3.4–4.6 — Modiolar axis & degrees below horizontal (axial) plane (°) 8.6 5.5–10.3 — = not applicable. panic cavity, from anteriorly through the pterygopalatine Lateral and Anterior View and infratemporal fossae, from above through the floor of As viewed from laterally in the transmastoid approach- the middle fossa, and from posteriorly through the posteri- es through the facial recess, the round window, located or surface of the temporal bone. Blue lined or skeletonized between the mastoid segment of the facial nerve posteri- structures in which the bone was carefully removed with a orly and the chorda tympani anteriorly, is the most lateral high-speed drill to a thinness at which the structures could part of the cochlea (Fig. 4). The round window is located be seen through the thinned bone were used in examining inferior to the oval window and opens in a posterolateral the relationship of the cochlea to the facial nerve, vestibu- direction under the overhanging edge of the promontory. locochlear nerve, GPN, chorda tympani, geniculate gan- The oval window, in which the footplate of the stapes sits, glion, internal and external acoustic meati, tensor tympani is located posterosuperior to the promontory and, in the muscle, eustachian tube, and petrous carotid. Finally, the dry bone with the stapes removed, connects the tympanic cochlear turns were identified with meticulous drilling. cavity to the vestibule. The cochlea is hidden anterome- Selective distances and angles were measured (Table 1). dial to the semicircular canals, vestibule, facial nerve, and auditory ossicles in the lateral view through the mastoid Results (Figs. 1 and 4). The central cochlear axis is directed along the modio- The semicircular canals are positioned above the jugu- lus, the osseous cone-shaped central pillar of the cochlea lar bulb (Fig. 4A and D). The tympanic segment of the fa- from which the osseous spiral lamina projects into the cial nerve passes below the lateral semicircular canal and bony canal of the cochlea to give the modiolus a screw- turns downward to form the mastoid (descending or verti- like appearance. The osseous cochlear canal spirals for cal) segment of the facial nerve, which exits the stylomas- about 2.5–2.75 turns around its central bony core, the mo- toid foramen. The labyrinthine and tympanic segments of diolus, and is approximately 35 mm in length (Figs. 1 and the facial nerve and the geniculate ganglion have an inti- 2).4 The cochlea, from base to apex, measures 5 mm along mate relationship with the basal turn of the cochlea.