The unrecognized rotation of the vestibular and cochlear nerves from the labyrinth to the brain stem: its implications to surgery of the eighth cranial nerve

The cochlear and vestibular nerves rotate 90 degrees from the inner ear to the brain stem. Most of the rotation occurs within the internal auditory canal (IAC); only minimal rotation occurs in the cerebellopontine (CP) angle. At the labyrinthine end of the IAC, the cochlear nerve--which at first lies anterior to the inferior vestibular nerve (saccular nerve)--rapidly fuses with the inferior vestibular nerve. It then rotates to become inferior as the nerves leave the porus acousticus. The cochleovestibular (C-V) cleavage plane lies in a superior-inferior direction in the lateral IAC and rotates to become anterior-posterior in the CP angle. In 25% of patients in whom no C-V cleavage plane can be seen, it is not possible to completely transect all vestibular fibers. The surgical implications are that the most complete vestibular neurectomy can be done only in the lateral IAC, the cochlear and inferior vestibular nerves, because of their intimate association, should not be separated in the mid-IAC, in order to prevent damage to the cochlear nerve, and to create a complete denervation of the vestibular labyrinth, only the posterior ampullary nerve along with the superior vestibular nerve should be transected.     

The vestibulocochlear nerve (VIII)

The vestibulocochlear nerve (8th cranial nerve) is a sensory nerve. It is made up of two nerves, the cochlear, which transmits sound and the vestibular which controls balance. It is an intracranial nerve which runs from the sensory receptors in the internal ear to the brain stem nuclei and finally to the auditory areas: the post-central gyrus and superior temporal auditory cortex. The most common lesions responsible for damage to VIII are vestibular Schwannomas. This report reviews the anatomy and various investigations of the nerve.     

Occurrence of vestibular and facial nerve injury following cerebellopontine angle operations

Summary To elucidate how surgery in the cerebellopontine (CP) angle may cause vestibular and facial nerve injury, the 7th and 8th cranial nerves of dogs were manipulated as in human surgery along with monitoring of auditory evoked brain stem responses. Postoperatively, histological examinations were performed to investigate the effect of the surgical manipulations.We found that the occurrence of vestibular, facial and cochlear nerve injury was dependent on the direction of theexcessive movement of the nerves in the cerebellopontine (CP) angle. Caudal-to-rostral shift of the nerve trunk most effectively avulsed the vestibular nerve. Haemorrhages were revealed between the vestibular ganglion and the fundus of the internal auditory canal. This caudal-to-rostral retraction could also damage the facial nerve in its intrapetrous labyrinthine portion. This was likely to be one of the pathophysiological mechanisms responsible for postoperative facial nerve palsy occasionally observed in human cases.Rostral-to-caudal retraction of the cerebellum damaged the cochlear nerve selectively. Although caudal-to-rostral retraction, instead of lateral-to-medial one, has been recommended to protect the cochlear nerve, this retraction was shown to be dangerous to the vestibular nerve if excessive.The clinical significance of the fragility of the vestibular nerve was discussed and the importance of preserving the vestibular nerve function is stressed.     

Cochlear nucleus anatomy related to central electroauditory prosthesis implantation

The central electroauditory prosthesis is now used to stimulate the cochlear nuclei to obtain auditory perception in patients with bilateral cochlear nerve transection who are undergoing bilateral acoustic tumor removal. In this study, we used fixed cadaver specimens to identify visible landmarks for accurate placement of the central electroauditory prosthesis through a combined suboccipital-translabyrinthine opening. Histologic features of the regions of probable implantation of the central electroauditory prosthesis were also investigated. We found that the following landmarks might have surgical significance: (1) the tenia of the inferior velum of the fourth ventricle, which crosses the surface of the ventral cochlear nucleus and the vestibulocochlear nerve; (2) the angle between the vestibulocochlear and glossopharyngeal nerves; and (3) the foramen of Luschka. It is suggested that an incision be made in the tenia for insertion of the prosthesis into the lateral recess and eventual placement on the ventral cochlear nucleus surface. To study regions of potential stimulation, we injected ink into different sites on the exposed surface of the cochlear nuclei. We then histologically examined neuronal populations adjacent to the sites. We found that a portion of the ventral cochlear nucleus localized within the lateral recess might be the most appropriate location for placement of the central electroauditory prosthesis.     

Jugular foramen: microscopic anatomic features and implications for neural preservation with reference to glomus tumors involving the temporal bone

OBJECTIVE: Our goals were to study the normal histological features of the jugular foramen, compare them with the histopathological features of glomus tumors involving the temporal bone, and thus provide insight into the surgical management of these tumors with respect to cranial nerve function. METHODS: Ten jugular foramen blocks were obtained from five human cadavers after removal of the brain. Microscopic studies of these blocks were performed, with particular attention to fibrous or bony compartmentalization of the jugular foramen, the relationships of the caudal cranial nerves to the jugular bulb/jugular vein and internal carotid artery, and the fascicular structures of the nerves. In addition, we studied the histopathological features of 11 glomus tumors involving the temporal bone (10 patients), with respect to nerve invasion, associated fibrosis, and carotid artery adventitial invasion. RESULTS: A dural septum separating the IXth cranial nerve from the fascicles of Cranial Nerves X and XI, at the intracranial opening, was noted. Only two specimens, however, had a septum (one bony and one fibrous) producing internal compartmentalization of the jugular foramen. The cranial nerves remained fasciculated within the foramen, with the vagus nerve containing multiple fascicles and the glossopharyngeal and accessory nerves containing one and two fascicles, respectively. All of these nerve fascicles lay medial to the superior jugular bulb, with the IXth cranial nerve located anteriorly and the XIth cranial nerve posteriorly. All nerve fascicles had separate connective tissue sheaths. A dense connective tissue sheath was always present between the IXth cranial nerve and the internal carotid artery, at the level of the carotid canal. The inferior petrosal sinus was present between the IXth and Xth cranial nerves, as single or multiple venous channels. The glomus tumors infiltrated between the cranial nerve fascicles and inside the perineurium. They also produced reactive fibrosis. In one patient, in whom the internal carotid artery was also excised, the tumor invaded the adventitia. CONCLUSION: Within the jugular foramen, the cranial nerves lie anteromedial to the jugular bulb and maintain a multifascicular histoarchitecture (particularly the Xth cranial nerve). Glomus tumors of the temporal bone can invade the cranial nerve fascicles, and infiltration of these nerves can occur despite normal function. In these situations, total resection may not be possible without sacrifice of these nerves.     

Damage of the peripheral auditory system after operations in the cerebellopontine angle. A scanning electron-microscopic observation in dogs

Cochlear nerve injuries caused by manipulations in the cerebellopontine angle were electrophysiologically and morphologically investigated in dogs. Operative procedures similar to those performed in the cerebellopontine angle of humans were applied in dogs. Auditory-evoked potentials were recorded throughout the experiments. Postoperatively, the temporal bones were studied with a scanning electron microscope. The portions of the cochlear nerve fibers and branches of the internal auditory artery that exit from the bony cochlea into the internal auditory canal were susceptible to traction force derived from manipulations in the cerebellopontine angle. The cochlear nerve fibers from the basal turn of the cochlea were most easily pulled out from the fundus of the internal auditory canal. In some cases, massive hemorrhages and exudation of plasma were observed in the entire modiolus; these compressed the cochlear nerve trunk. The Schwann-glial junctions of the cochlear nerves were separated in some dogs, indicating this junction was one of the most vulnerable sites to operative manipulations in the cerebellopontine angle.     

Modified retrolabyrinthine approach with partial labyrinthectomy: anatomic study.

This study was undertaken to evaluate the feasability of the modified retrolabyrinthine approach (traditional retrolabyrinthine approach plus resection of the posterior semicircular canal) to expose the entire fundus of the internal auditory canal (IAC). This approach is advocated by its proponents to manage acoustic neuromas reaching the lateral IAC and with the preservation of hearing as the goal. Little anatomic data directly estimate the limitations of this exposure. Measurements were recorded from 25 cadaver temporal bones dissected with this modified approach. The distances were taken between the porus acousticus (inferior and superior portions), the dome of the jugular bulb, the midportion of the sigmoid sinus, and the fundus of the IAC (inferior and superior portions). All of the measurements were then compared with those of the translabyrithine approach. The current study shows that despite the sacrifice of the posterior semicircular canal, the superior lateral fundus cannot be completely visualized. There is a distance (on average 1.1 mm) that differentiates the superior area of the IAC accessible with translabyrithine and modified retrosigmoid techniques. This value is smaller than that observed in the classic retrosigmoid approach indicating that the modified technique affords a more adequate, even if not ideal, exposure to minimize the risk of recurrence. The modified retrolabyrinthine approach provided an optimal exposure of the inferior half of the IAC. A superior blind area, smaller than that of the traditional retrolabyrinthine technique, cannot be completely approached via this route. We believe that this approach can be considered as an alternative technique in selected cases especially for tumors involving the inferior vestibular nerve.     

Hamartomas of the internal auditory canal: report of two cases

OBJECTIVE AND IMPORTANCE: To highlight the clinical, radiological, and surgical findings and therapeutic options for this rare entity, which may mimic a purely intrameatal vestibular schwannoma, and to define the particular aspects of preoperative differential diagnosis and surgical management. CLINICAL PRESENTATION: Two patients presented with clinical findings typical of vestibular schwannomas, i.e., tinnitus, hearing loss of 30 dB, and an intrameatal contrast-enhancing lesion on magnetic resonance imaging studies. TECHNIQUE: The lesions were exposed via a suboccipital transmeatal approach, and tumor infiltration of the cochlear and/or facial cranial nerves was identified. In view of the unclear intraoperative histology, surgical management was based on criteria of cranial nerve function. In Patient 1, after nerve decompression by subtotal tumor removal, preserved auditory brainstem responses and facial nerve electromyography indicated functional nerve preservation and facilitated the decision for partial resection. In Patient 2, minimal tumor dissection resulted in complete loss of auditory brainstem response without reversibility. Therefore, a radical tumor removal was performed that sacrificed the cochlear but preserved the facial nerve. CONCLUSION: Symptoms and signs of internal auditory canal hamartomas are congruent with other typical pathological lesions of the internal auditory canal and cerebellopontine angle. Accurate preoperative diagnosis by radiological means is not possible, but careful evaluation of the different signal intensities on magnetic resonance imaging studies may indicate this rare pathological condition. Intraoperative surgical findings of tumor infiltration of the faciocochlear cranial nerve complex may support simple observation. In view of the nonneoplastic characteristic of these lesions, a more conservative approach is justified. The decision should be based on the functional status of the cranial nerves, for which reliable electrophysiological monitoring is indispensable.     

Infralabyrinthine approach to the internal auditory canal

Multiple surgical procedures have been developed to expose the contents of the internal auditory canal. These include the middle fossa approach, the posterior fossa approach, the retrolabyrinthine approach, and the retrosigmoid approach. Each has its own unique benefits, as well as disadvantages. A new posterior-inferior approach to the internal auditory canal (the infralabyrinthine approach) has been developed. This approach affords exposure of at least the medial half of the internal auditory canal while remaining extradural and extralabyrinthine. By dissecting below the posterior semicircular canal, and remaining extradural, hearing is preserved. The dissection can proceed far enough laterally in the internal auditory canal to separate the cochlear and vestibular nerve divisions. Further anatomic considerations, as well as clinical applicability, will be discussed.     

Failure to preserve hearing in acoustic neuroma surgery: experiences after introduction of MRI

This report elucidates our experiences on acoustic neuroma surgery, in which auditory function was lost postoperatively, although conservation of hearing had been intended preoperatively. Five ears from four patients (two ears: unilateral, three ears from bilateral acoustic neuromas) were operated on via standard retromastoid route, with monitoring of auditory evoked potentials. Abolition of ABR occurred when surgical manipulations were performed within the internal auditory canal. Pulling tumor tissue away from the cochlea toward the brain stem has proved to be a hazardous procedure. Recognizing the condition of tumor tissue within the internal auditory canal is prerequisite for hearing preservation. For this purpose, MRI is very useful because tumor tissue within the internal auditory canal can be clearly visualized. The preoperative criteria in selecting candidates for hearing preservation operations should be more strict because most acoustic neuromas with a diameter of more than 10mm cannot be resected without causing loss of hearing, and, even in such small tumors, the cochlear nerves are infiltrated by tumor cells. Most ABR changes during operations seem to be explicable from avulsion of the cochlear nerve fibers and/or the internal auditory artery from the fundus of the internal auditory canal-the tractus spiralis foraminosus. Postoperative recordings of ABR indicated that progression of degeneration of the cochlear nerve fibers occurred after surgery. This phenomenon may explain postoperative delayed hearing loss observed clinically.     

Pathophysiology of cochlear nerve injury incurred through surgical manipulation in the cerebellopontine angle. Scanning electron microscopic observations

Cochlear nerve injuries caused by surgical manipulation in the cerebellopontine (CP) angle were electrophysiologically and morphologically investigated in dogs. Operative procedures similar to those performed in the CP angle in humans were performed. Lateral-to-medial retraction of the cerebellar hemispheres applied traction force to the cochlear nerve. Brainstem auditory evoked potentials and compound action potentials from the intracranial portions of the cochlear nerves were recorded during the procedures. As a result of the traction force produced by manipulations in the CP angle, the Schwann-glial junctions of the cochlear nerve were separated in some dogs. The exit portions of the cochlear nerve fibers and the branches of the internal auditory artery from the tractus spiralis foraminosus at the fundus of the internal auditory canal. This finding may explain occasional occurrence of postoperative high frequency hearing loss among patients who undergo surgical manipulation in the CP angle. In some cases, massive hemorrhage and exudation of plasma were observed in the deep portion of the modiolus, where they compressed the cochlear nerve trunk. This is apparently one of the causes of intraoperative failure of cochlear function. In this study, no correlations between electrophysiological and morphological findings were established.     

Adjunctive use of endoscopy during posterior fossa surgery to treat cranial neuropathies.

OBJECTIVE: The objective of this study was to determine the utility and safety of rigid endoscopy as an adjunct during posterior fossa surgery to treat cranial neuropathies. METHODS: A suboccipital craniotomy was performed for 19 patients with non-neoplastic processes involving the Vth, VIIth, and/or VIIIth cranial nerves. Ten patients with trigeminal neuralgia (n = 8), hemifacial spasm (n = 1), or intractable tinnitus (n = 1) underwent primarily microvascular decompression procedures. One patient with geniculate neuralgia underwent nervus intermedius sectioning combined with microvascular decompression. Eight patients underwent unilateral vestibular nerve neurectomies for treatment of Meniere's disease. A 0- or 30-degree rigid endoscope was used in conjunction with the standard microscopic approach for all procedures. RESULTS: All patients experienced resolution or significant improvement of their preoperative symptoms after posterior fossa surgery. The endoscope allowed improved definition of anatomic neurovascular relationships without the need for significant cerebellar or brainstem retraction. Cleavage planes between the cochlear and vestibular nerves entering the internal auditory canal and sites of vascular compression could not be microscopically observed for several patients; however, endoscopic identification was possible for all patients. There were no complications related to the use of the endoscope. CONCLUSION: The rigid endoscope can be used safely during posterior fossa surgery to treat cranial neuropathies, and it allows improved observation of the cranial nerves, nerve cleavage planes, and vascular anatomic features without significant cerebellar or brainstem retraction.     

Fascicular anatomy and surgical access of the human pudendal nerve

The ability to access selectively distal nerve branches at the level of the compound pudendal nerve (PN) would allow control of multiple neural pathways and genitourinary functions at a single location. Nerve cuff electrodes can selectively stimulate individual fascicles; however the PN fascicular anatomy is unknown. The fascicular representation of distal branches was identified and traced proximally to create fascicle maps of 12 compound PNs in seven cadavers. Distal nerves were represented as groups of individual fascicles in the PN. Fascicle maps were consistent between specimens and along the PN within specimens. PN branch free length was 26±7.7 mm. PN cross-sections were relatively flat with major and minor diameters of 4.3±0.90 and 1.7±0.45 mm, respectively. Placing a nerve cuff on the PN is anatomically and surgically feasible. The PN fascicular anatomy, branch free length, and cross-section geometry are conducive to selective stimulation of distal nerves with a single nerve cuff electrode.     

Cochlear Schwannomas

Schwannomas arising solely from the cochlear nerve and limited to the internal auditory canal are rare. Only three prior cases have been specifically described in the literature. We report a 38-year-old male with and asymmetric audiogram and poor discrimination. Magnetic resonance imaging revealed a 3-mm mass occupying the inferior portion of the internal auditory canal. Discrimination improved following a course of steroids, but thresholds did not. The patient underwent a translabyrinthine removal of the tumor, which was confirmed to involve only the cochlear nerve. It is important to obtain evidence regarding the possibility that a tumor confined to the internal auditory canal is a cochlear schwannoma. If so, surgery can be deferred until hearing is no longer functional or tumor growth mandates removal.     

Cochlear schwannomas

Schwannomas arising solely from the cochlear nerve and limited to the internal auditory canal are rare. Only three prior cases have been specifically described in the literature. We report a 38-year-old male with and asymmetric audiogram and poor discrimination. Magnetic resonance imaging revealed a 3-mm mass occupying the inferior portion of the internal auditory canal. Discrimination improved following a course of steroids, but thresholds did not. The patient underwent a translabyrinthine removal of the tumor, which was confirmed to involve only the cochlear nerve. It is important to obtain evidence regarding the possibility that a tumor confined to the internal auditory canal is a cochlear schwannoma. If so, surgery can be deferred until hearing is no longer functional or tumor growth mandates removal.     

Iatrogenic impairment of hearing during surgery for acoustic neuroma

Isolated or combined labyrinthine, neural, and vascular damage account for failure to preserve hearing during removal of acoustic neuromas. However, the specific mechanisms of auditory impairment remain unclear unless surgical maneuvers can be related to peri- and postoperative hearing on the basis of intraoperative monitoring of auditory function.Among the different auditory monitoring techniques, recording of cochlear nerve action potentials (CNAPs) from the intracranial portion of the nerve has proven particularly useful for identifying the mechanisms of iatrogenic auditory injury.The present investigation analyzes intra- and postoperative auditory impairment in relation to surgical steps in a group of 38 subjects with acoustic neuroma (size ranging from 5 to 24 mm) undergoing removal via a retrosigmoid approach.Coagulation close to the cochlear nerve, drilling of the internal auditory canal, and removal of the intrameatal portion of the acoustic neuroma have prove to be the most critical surgical steps in hearing preservation.Changes were correlated with intra- and extrameatal tumor size, the relationship between the internal auditory canal and vestibule, and internal auditory canal enlargement, anatomic involvement of the cochlear nerve, preoperative auditory level, and ABR and ENG test findings.Changes in CNAP morphology and latency are detailed, and mechanisms of injury are analyzed and discussed as a function of these variables.     

Magnetic resonance cisternography used to determine precise topography of the facial nerve and three components of the eighth cranial nerve in the internal auditory canal and cerebellopontine cistern

OBJECT: The detailed anatomy of intracranial structures has been studied mainly in cadavers, but the absence of cerebrospinal fluid and blood pressure in these models distorts normal spatial relationships. The authors investigated the rotation of the facial nerve (FN), superior vestibular nerve (SVN), inferior vestibular nerve (IVN), and cochlear nerve (CN) in the internal auditory canal (IAC) and cerebellopontine cistern in human volunteers and compared their results with those reported in cadaver studies. METHODS: The IACs and cerebellopontine cisterns of 30 normal adults (34 sides) were examined using magnetic resonance (MR) cisternography with a heavily T2-weighted two-dimensional fast spin-echo technique. The positions of the four components were unaffected by the presence of the meatal loop of the anterior inferior cerebellar artery in the IAC. The spatial relationship between the FN and SVN was quite constant, but the spatial relationship between the CN and SVN was quite variable: the former changed position, mainly in the IAC, on nine (26.5%) of 34 sides, and in the cerebellopontine cistern on the other sides (73.5%), conflicting with findings in cadaver studies. CONCLUSIONS: It is more accurate to describe the CN and IVN as coursing beneath the SVN in either the IAC or cerebellopontine cistern, rather than stating that the three components rotate, as reported in cadaver studies. The MR cisternography studies provided quite detailed information about the topography of the four components and the relationship between the blood vessels and cranial nerves in the IAC and the cerebellopontine cistern.     

Evolution of a cochlear schwannoma on clinical and neuroimaging studies. Case report

The authors report on a patient with a rare schwannoma that arose from the cochlear division of the vestibulocochlear nerve. Distinctively, the lesion appeared to arise from the cochlea itself and was monitored with clinical and neuroimaging studies for 12 years before it was diagnosed and treated. The atypical occurrence of schwannomas of the vestibulocochlear nerve originating in the inner ear structures underscores the high level of clinical suspicion required for the diagnosis of these lesions in patients presenting with persistent auditory and vestibular symptoms.     

Middle fossa surgery. Report of 153 cases

The principal indications for the middle cranial fossa approach to the petrous apex and internal auditory canal are section of the vestibular nerves in vertigo, management of lesions of the labyrinthine segment of the facial nerve, and removal of mass lesions of the internal auditory canal. We report 153 cases of pathosis of the temporal bone and related structures for which this approach was used.     

Cochlear and vestibular gross and histologic anatomy (as seen from postauricular approach)

The otologic surgeon must have a clear understanding of the anatomy of the seventh and eighth cranial nerves from the labyrinth to the brain stem, as seen from the postauricular approach. The surgical anatomy of the seventh and eighth cranial nerves was studied in 64 transcochlear eighth-nerve sections and 33 retrolabyrinthine vestibular neurectomies. Analysis indicates the nerves rotate 90 degrees in their course from the ear to the brain. The key relationship is that the cochlear nerve is always the most inferior, rotating from anterior (medial) near the labyrinth to posterior (lateral) near the brain stem. The seventh (facial) nerve rotates from anterosuperior (medial superior) near the labyrinth to anteroinferior (medial inferior) near the brain stem. The seventh nerve is easily seen in the transcochlear approach and hidden from view in the retrolabyrinthine approach. Twenty-seven fixed nerve specimens were examined with an operating microscope before being prepared for sectioning. In 73% (19 of 26) a cleavage plane was seen on the lateral aspect of the eighth nerve (that portion of the nerve facing the surgeon in the retrolabyrinthine approach).