Tetsuji Sekiya

An in Vivo Quantifiable Model of Cochlear Neuronal Degeneration Induced by Central Process Injury

In the available in vivo experimental models for cochlear neuronal degeneration, the peripheral (hair cell side) process of the cochlear nerve has been injured in order to induce neuronal degeneration. However, there has been no dependable experimental model in which cochlear neuronal degeneration begins from the central (brain stem side) process. This lack of a central process injury model has probably been due to the experimental difficulties that had to be overcome in order to reproducibly and selectively injure the central process of the cochlear neurons while maintaining the patency of the internal auditory artery in small experimental animals such as rats. Using rats, we first developed a central process injury model in which the reduction of the spiral ganglion cells due to retrograde degeneration of cochlear neurons can be quantitatively evaluated. In our experimental model, the cochlear nerve was compressed and injured by a compression-recording (CR) electrode placed at the internal auditory meatus. First, the cochlear nerve was compressed until the compound action potentials of the cochlear nerve became flat, and then the CR electrode was advanced by various compression speeds (5, 10, or 200 micrometer/s) to reach the same depth (400 micrometer). In our model, therefore, the reduction of the spiral ganglion cells was caused compression speed dependently. This method made it possible to produce compression injury to the cochlear nerve without evidence of damage to the blood supply to the cochlea via the internal auditory artery. This model gives us the means to obtain knowledge that was previously impossible to derive from the peripheral process injury models.

Methylprednisolone ameliorates cochlear nerve degeneration following mechanical injury

We investigated whether methylprednisolone sodium succinate can ameliorate cochlear nerve degeneration following compression injury on the cerebellopontine angle portion of the cochlear nerve, using a quantitative animal experimental model that we have developed recently. In this model, cochlear nerve degeneration after compression could be quantitatively evaluated, while cochlear ischemia induced by the compression carefully maintained below the critical limit that causes irreversible damage to the cochlea. Eleven rats were treated with methylprednisolone during the pre- and post-compression period. Two weeks after compression, the numbers of SGC were compared between the rats that received the compression without and with methylprednisolone treatment. Methylprednisolone treatment improved the survival of SGC following cochlear nerve injury statistically highly significantly in the basal turn where the traumatic stress had been less than in the other cochlear turns in our experimental setting. Although it was not statistically significant, greater survival was also observed in the other cochlear turns. The results of this experimental study indicated that at least a portion of injured cochlear nerve had been potentially treatable, and that methylprednisolone might prevent such cochlear neurons from entering into the vicious process of irreversible damaging process.

Apoptosis of auditory neurons following central process injury

Although apoptotic changes in auditory neurons induced by injury to peripheral processes (dendrites) have been intensively studied, apoptotic changes in auditory neurons induced by injury to central processes (axons of spiral ganglion cells, SGCs) have not been reported previously, probably due to lack of an experimental model. The present study reports for the first time the appearance, extent, and time course of SGC apoptosis following injury to the central processes. Apoptosis was studied in a rat model that consisted of compression of the auditory nerve in the cerebellopontine (CP) angle cistern with intraoperative recordings of auditory nerve compound action potentials (CAPs) to ensure highly reproducible results. Rats were killed between day 0 and day 14 after compression and apoptosis of SGCs was evaluated quantitatively as well as qualitatively by terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) staining, anti-activated caspase-3 immunostaining, Hoechst 33342 staining, and electron microscopy. The average number of TUNEL-positive apoptotic SGCs in each cochlear turn increased from day 1 to day 5 and then decreased gradually to an undetectable level on day 14 after compression. The average proportion of apoptotic SGCs identified in any cochlear turn on any day was always lower than 10%. The results of our present study should be useful in determining the therapeutic time window for rescuing auditory neurons undergoing apoptosis due to injury during surgery in the CP angle.

Macrophage colony stimulating factor (M-CSF) protects spiral ganglion neurons following auditory nerve injury: morphological and functional evidence

Because hearing disturbance due to auditory nerve dysfunction imposes a formidable burden on human beings, intense efforts have been expended in experimental and clinical studies to discover ways to restore normal hearing. However, the great majority of these investigations have focused on the peripheral process side of bipolar auditory neurons, and very few trials have focused on ways to halt degenerative processes in auditory neurons from the central process side (in the cerebellopontine angle). In the present study, we investigated whether administration of macrophage colony-stimulating factor (M-CSF) could protect auditory neurons in a rat model of nerve injury. The electrophysiological and morphological results of our study indicated that M-CSF could ameliorate both anterograde (Wallerian) and retrograde degeneration in both the CNS and PNS portions of the auditory nerve. We attribute the success of M-CSF therapy to the reported functional dichotomy (having the potential to cause both neuroprotective and neurotoxic effects) of microglia and macrophages. Whether the activities of microglia/macrophages are neuroprotective or neurotoxic may depend upon the nature of the stimulus that activates the cells. In the present study, the neuroprotective effects of M-CSF that were observed could have been due to M-CSF we administered and to M-CSF released from endothelial cells, resident cells of the CNS parenchyma, or infiltrating macrophages. Another possibility is that M-CSF ameliorated apoptotic auditory neuronal death, although this hypothesis remains to be proved in future studies.

Changes in intracochlear and intracanalicular nerves after acoustic neurinoma excision confirmed by magnetic resonance imaging

Postoperative magnetic resonance imaging findings related to the vestibulocochlear and facial nerves within the internal auditory canal were analyzed in acoustic neurinomas. T1- and proton-weighted magnetic resonance images showed that the vestibulocochlear nerves distal to the internal auditory meatus increased in signal intensity after surgical intervention. These nerves were conspicuously enhanced after intravenous administration of gadolinium diethylene-triamine-pentaacetic acid. The preserved facial nerves were also markedly enhanced postoperatively. As a possible cause of these findings, we suggest operative disruption of the blood-nerve barrier with ensuing nerve edema, although the operative procedures were carefully carried out using a surgical microscope. The clinical significance of traumatic disruption of the blood-nerve barrier and subsequent nerve edema are discussed from the standpoint of preservation of cochlear nerve function.

Effect of compression on the cochlear nerve: a short- and long-term electrophysiological and histological study.

The short- and long-term effects of static compression of the cochlear nerve were studied in dogs. The nerve was exposed in the cerebellopontine angle and a modified aneurysm clip was applied to reduce the diameter of the nerve trunk to 50%, 40%, 30% or 20% of normal (designated respectively as 50%, 60%, 70%, and 80% compression). Brainstem auditory evoked potentials (BAEPs) were monitored intraoperatively and post-operatively. The animals were sacrificed between 5 and 119 days after nerve compression and temporal bones were examined histologically. In the 50% compression group, all peaks except peak I disappeared immediately after nerve compression. After release of the clip, however, peak II and subsequent components recovered and prolonged interpeak latency (IPL) between peaks I and IV normalized within 7 days. In the 60% compression group, recovery was incomplete for as long as 49 days after compression. Significant histological changes were not always reflected in the electrophysiological recordings, as shown by the finding of multiple cavitations at the compressed portion of the cochlear nerve in cases in which conduction block of cochlear nerve impulses was reversible. In the 70% compression group, peak IV did not reappear for more than 1 week, and histological examination revealed severe damage to all cochlear nerve fibers except those from the apical turn, which lie in the center of the cochlear nerve trunk. Severe injury occurred to the cochlear nerve fibers that are situated more superficially in the nerve, which are tonotopically responsible for the perception of high-frequency sound and the generation of BAEPs. This means that the BAEP changes due to cochlear nerve compression would be detectable by BAEP monitoring, although changes in the apical region of the cochlea are not fully detectable by BAEP monitoring. In the 80% compression group, all peaks except peak I were lost permanently and the amplitude of peak I, which had been preserved in the acute phase, gradually decreased. Reversibility of impaired cochlear nerve impulse conduction was related to the severity of compression, and at some level of compression between 70% and 80% the nerve fibers generating BAEPs permanently lost the ability to conduct electrical impulses proximal to the site of compression. In the 70% and 80% compression groups, the amplitude of peak I gradually decreased over the first 30 days after compression and did not change significantly thereafter. Histologically, the branches of the internal auditory artery were resilient to compression, although they are easily avulsed due to stretch force. Furthermore, retrograde degeneration of cochlear neurons triggered by compression at the cisternal portion of the cochlear nerve was apparent. Such slowly progressive degeneration of nerve fibers may play a part in development of the delayed postoperative hearing disturbance.

Changes of the auditory system after cerebellopontine angle manipulations.

The current tendency in acoustic neuroma surgery to attempt the preservation of hearing function and the problem of accidental hearing loss caused by microsurgical neurovascular decompression operations for hemifacial spasm or trigeminal neuralgia prompted us to study the exact surgical vulnerability of the auditory system. The surgical procedures for operation on the cerebellopontine angle of dogs were carried out according to the sequence of the posterior fossa transmeatal operation for acoustic neuroma. The operation was tentatively divided into three stages: (a) craniectomy and dural opening, (b) cerebellar retraction, and (c) identification of the cochlear nerve in the unroofed internal auditory canal (IAC). The postoperative behavior of the auditory system was evaluated electrocochleographically (EcochG) and histologically. Overzealous retraction of the cerebellar hemisphere caused transient disturbance of the EcochG pattern. Mechanical stretching of both the cochlear nerve and the internal auditory artery may cause a disturbance in the synchronized discharge of the cochlear neurons. Various manipulations at the porus acusticus internus or the IAC (such as pinching the nerve with forceps or electrocoagulation) produced thoroughly distorted EcochG patterns. From the histological findings, the main causative factor for these labyrinthine damages was considered to be vascular insufficiency. The current need for neurosurgical operations to preserve hearing is discussed in the light of these findings.

Macrophage invasion into injured cochlear nerve and its modification by methylprednisolone.

Post-traumatic invasion of macrophages into the cochlear nerve of the rat and measurement of how their invasion was modified by the administration of methylprednisolone were investigated for the first time by using a reproducible and quantifiable experimental model of cochlear nerve injury. Two weeks after precise cochlear nerve compression, a massive invasion of ED1 immunostained macrophages was observed at the compressed portion of the cochlear nerve, and this invasion of macrophages was markedly reduced in the rats to which methylprednisolone had been administered during the pre- and post-compression period. Concomitantly, the residual number of spiral ganglion cells was found to be greater in the compression+methylprednisolone group than in the control compression group. The tissue loss observed in the lesion epicenter was also significantly less in the compression+methylprednisolone group than in the control compression group. The results of our present study demonstrated the effectiveness of methylprednisolone treatment to ameliorate trauma induced cochlear nerve degeneration in the acute phase. However, these results may reflect the sum effects of methylprednisolone on macrophages, including both its beneficial effect by inhibiting the negative aspects of macrophages through attenuating macrophage recruitment to the lesion site, and at the same time an undesirable effect by sacrificing the positive aspects of macrophage function. Moreover, one reservation should be added that the protective effects of steroid to injured cochlear nerve may have operated via a pathway not related to macrophage function. Besides macrophages, various cells and factors participate in the process of CNS injury, and their effects may potentially work either positively or negatively with respect to CNS protection and regeneration at each particular time during the on-going process of CNS injury. Therefore, future investigation in CNS injury should be directed toward understanding such complex mechanisms involved in this process.

A ring electrode to record extraocular muscle activities during skull base surgery

SummaryA ring-shaped electrode was developed and used in 20 patients to record evoked electromyographic responses directly from the extraocular muscles during skull base surgery. Intra-operative monitoring with this electrode helps the surgeon to localize the nerves that innervate the extraocular muscles precisely and to refrain from disturbing important neural structures during operations. Such monitoring also provides some insight into the pathophysiology of the dysfunction of these nerves resulting from skull base lesions.

Nimodipine ameliorates trauma-induced cochlear neuronal death.

Abstract Excessive entry of Ca2+ into injured cochlear neurons activates various Ca2+ -activated enzymes and subsequent spiral ganglion cell death. Therefore, preventing intracellular calcium overload by using Ca2+ channel antagonists may become an important countermeasure to spiral ganglion cell death. We experimentally investigated whether an L-type Ca2+ channel blocker (nimodipine) can rescue traumatized cochlear neurons from degeneration. A group of rats (n = 6) was pre-operatively treated with nimodipine for one week and compression injury was applied to the cerebellopontine angle portion of the cochlear nerve in a highly quantitative fashion. The rats from the compression with nimodipine treatment groups were post-operatively treated with nimodipine for 10 days and killed for histological examination. The histological analysis of the temporal bones revealed that the spiral ganglion cells in the basal turn of the cochlea where the magnitude of traumatic impact had been the least in our experimental condition were rescued in a statistically significant fashion in the compression with nimodipine treatment group. The results of the present study indicate that nimodipine may become an intra- and post-operative important adjunct to raise the rate of hearing preservation in vestibular schwannoma excision or other cerebellopontine angle surgical interventions.