The cochlea in skull base surgery: an anatomy study

OBJECTIVEThe object of this study was to examine the relationships of the cochlea as a guide for avoiding both cochlear damage with loss of hearing in middle fossa approaches and injury to adjacent structures in approaches directed through the cochlea.METHODSTwenty adult cadaveric middle fossae were examined using magnifications of ×3 to ×40.RESULTSThe 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. Approximately 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.CONCLUSIONSAn 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.

Dissection and Exposure of the Whole Course of Deep Nerves in Human Head Specimens after Decalcification

The whole course of the chorda tympani nerve, nerve of pterygoid canal, and facial nerves and their relationships with surrounding structures are complex. After reviewing the literature, it was found that details of the whole course of these deep nerves are rarely reported and specimens displaying these nerves are rarely seen in the dissecting room, anatomical museum, or atlases. Dissections were performed on 16 decalcified human head specimens, exposing the chorda tympani and the nerve connection between the geniculate and pterygopalatine ganglia. Measurements of nerve lengths, branching distances, and ganglia size were taken. The chorda tympani is a very fine nerve (0.44 mm in diameter within the tympanic cavity) and approximately 54 mm in length. The mean length of the facial nerve from opening of internal acoustic meatus to stylomastoid foramen was 52.5 mm. The mean length of the greater petrosal nerve was 26.1 mm and nerve of the pterygoid canal was 15.1 mm.

Blood Supply of the Facial Nerve in the Middle Fossa: The Petrosal Artery

Objective: To define the arterial supply to the facial nerve that crosses the floor of the middle cranial fossa. Methods: Twenty-five middle fossae from adult cadaveric-injected specimens were examined under 3 to 40× magnification. Results: The petrosal branch of the middle meningeal artery is the sole source of supply that crossed the floor of the middle fossa to irrigate the facial nerve. The petrosal artery usually arises from the first 10-mm segment of the middle meningeal artery after it passes through the foramen spinosum, but it can arise within or just below the foramen spinosum. The petrosal artery is commonly partially or completely hidden in the bone below the middle fossa floor. It most commonly reaches the facial nerve by passing through the bone enclosing the geniculate ganglion and tympanic segment of the nerve and less commonly by passing through the hiatus of the greater petrosal nerve. The petrosal artery frequently gives rise to a branch to the trigeminal nerve. The middle meningeal artery was absent in one of the 25 middle fossae, and a petrosal artery could not be identified in four middle fossae. The petrosal arteries were divided into three types based on their pattern of supply to the facial nerve. Conclusion: The petrosal artery is at risk of being damaged during procedures in which the dura is elevated from the floor of the middle fossa, the middle fossa floor is drilled, or the middle meningeal artery is embolized or sacrificed. Several recommendations are offered to avoid damaging the facial nerve supply while performing such interventions.

The Course of the Lesser Petrosal Nerve on the Middle Cranial Fossa

Objective: The course of the lesser petrosal nerve is not well understood and may be confused with the course of the greater petrosal nerve during middle fossa surgery. The objective was to examine the course of the lesser petrosal nerve along the floor of the middle cranial fossa from the region of the geniculate ganglion to its outlet from the skull base. There are no studies focused on the course of this nerve in relationship to the floor of the middle cranial fossa. Methods: Twenty middle fossae from adult cadaveric specimens were examined using 3 to 40× magnification. Results: The lesser petrosal nerve was partially exposed on the floor of the middle fossa without drilling in 75% of the middle fossae and totally covered by thin bone in 25%. It crossed the floor anterior to the greater petrosal nerve and exited the middle fossa through the canaliculus innominatus in 14 cases, foramen spinosum in 3 cases, and the sphenopetrosal suture in 3 cases. The course of the lesser petrosal nerve has been shown in textbooks to be parallel to the greater petrosal nerve. However, the lesser and greater petrosal nerves diverged in the area medial to the geniculate ganglion in 90% of middle fossae with the angle of divergence averaging 11.6 degrees. The course of the lesser petrosal nerve was divided into three patterns based on the site of confluence of the three bundles of fibers forming the nerve. Conclusion: The relationships of the lesser petrosal nerve in the middle cranial fossa have been described. An understanding of these relationships will reduce the likelihood of it being confused with the greater petrosal nerve during surgical approaches to the middle fossa.