Topography of neurovascular structures in relation to round window and how it relates to cochlear implantation.

PURPOSE: The purpose of this investigation was to evaluate the distances and angles on basal turn of cochlea in relation to round window at which the jugular bulb, internal carotid artery and facial nerve are at maximal risk and their implications in cochlear implantation (CI). METHODS: Fifty-four cadaveric temporal bones were microdissected to expose the basal turn of cochlea, the carotid canal, the facial canal and the jugular fossa. The points were marked on the basal turn of cochlea, where there was minimum distance of basal turn of cochlea from the roof of the jugular fossa (point a), carotid canal (point b) and facial canal (point c). The distances and angles of these points from the round window were measured. RESULTS: The points a, b and c were at mean (range) distances of 2.8 mm (1.3-4.1 mm), 8.4 mm (6.5-10.4 mm) and 16.4 mm (12.5-20.5 mm) and at mean angles of 30° (15°-45°), 111° (71°-136°) and 284° (255°-315°), respectively, from the round window. CONCLUSIONS: This study highlights that 2.8 ± 0.5 mm (30 ± 5.40), 8.4 ± 1 mm (111 ± 12.70) and 16.4 ± 1.7 mm (284 ± 13.5) from the round window are the high-risk points on the basal turn of the cochlea for the jugular bulb, internal carotid artery and facial nerve, respectively. A wide range found for each parameter indicates that it is mandatory to evaluate these distances in each CI patient on preoperative radiographs to avoid intraoperative injury to these vital structures.

Implanting straight into cochlea risks the facial nerve: a Cartesian coordinate study.

PURPOSE: To describe the straight-into-cochlea line that affords the best access for an electrode array to enter via the round window, and how this line relates to the facial nerve, the incus, and mastoid size. The straight-into-cochlea line is important to minimize the cochlear trauma and maximize the likelihood of placement into the scala tympani. METHODS: High-resolution CT scans were obtained for ten craniums with the extremes of large (N = 5) and small (N = 5) mastoid pneumatization; the specimens were from a series of 41 ear normal craniums. Using FIJI, a publicly available software program, the straight-into-cochlea insertion line was determined by defining the x-y-z coordinates of the middle of the round window and a point 6.0 mm into the cochlea on its centrifugal wall. Then, from the extended straight-into-cochlea insertion line, we determined the shortest perpendicular distance to the middle of the fallopian canal, and from that "fallopian point" to the apex of the posterior process of the incus. RESULTS: We found good repeatability of measurements. We found the extended straight-into-cochlea insertion lines routinely close to or in the midst of the fallopian canal (50 % ≤ 1.0 mm). We found the lines 4.7-7.8 mm from the apex of the posterior process of the incus. Line positions relative to "fallopian point" and incus showed no relation to mastoid pneumatization. For the distance "fallopian point" to incus, bilateral symmetry was suggested. CONCLUSIONS: Using landmarks registered in an x-y-z coordinate system, straight-into-cochlea insertion via the round window puts the facial nerve at risk.

Predicting the location of missing outer hair cells using the electrical signal recorded at the round window.

The electrical signal recorded at the round window was used to estimate the location of missing outer hair cells. The cochlear response was recorded to a low frequency tone embedded in high-pass filtered noise conditions. Cochlear damage was created by either overexposure to frequency-specific tones or laser light. In animals with continuous damage along the partition, the amplitude of the cochlear response increased as the high-pass cutoff frequency increased, eventually reaching a plateau. The cochlear distance at the onset of the plateau correlated with the anatomical onset of outer hair cell loss. A mathematical model replicated the physiologic data but was limited to cases with continuous hair cell loss in the middle and basal turns. The neural contribution to the cochlear response was determined by recording the response before and after application of Ouabain. Application of Ouabain eliminated or reduced auditory neural activity from approximately two turns of the cochlea. The amplitude of the cochlear response was reduced for moderate signal levels with a limited effect at higher levels, indicating that the cochlear response was dominated by outer hair cell currents at high signal levels and neural potentials at low to moderate signal levels.

Surgical anatomy of facial nerve for revision transmastoid surgery.

We analyze the relationships of the 3 segments of the facial nerve with respect to constant anatomic structures that can be identified during revision surgery via translabyrinthine approach. This study was conducted on 15 formalin-fixed cadavers whose facial nerves were dissected bilaterally under operative microscope via translabyrinthine approach. The distances between the round window niche and the midpoint of the tympanic segment and the beginning of the mastoid segment were 6.64 ± 1.79 mm and 3.99 ± 0.79 mm, respectively. The distances between the tympanic ostium of the eustachian tube and the first and the second genu were 7.02 ± 0.62 mm and 12.25 ± 1.24 mm, respectively. We used the superior semicircular canal, the tympanic ostium of the eustachian tube, and the round window niche as landmarks to identify the facial nerve during revision surgery. Our study also showed that the auricular branch may also be originated from the posterior surface of the facial nerve.

beta-Bungarotoxin application to the round window: an in vivo deafferentation model of the inner ear.

Hearing impairment can be caused by a primary lesion to the spiral ganglion neurons (SGNs) with the hair cells kept intact, for example via tumours, trauma or auditory neuropathy. To mimic these conditions in animal models various methods of inflicting damage to the inner ear have been used. However, only a few methods have a selective effect on the SGNs, which is of importance since it might be clinically more relevant to study hearing impairment with the hair cells undamaged. beta-Bungarotoxin is a venom of the Taiwan banded krait, which in vitro has been shown to induce apoptosis in neurons, leaving remaining cochlear cells intact. We wanted to create an in vivo rat model of selective damage to primary auditory neurons. Under deep anaesthesia, 41 rats received beta-Bungarotoxin or saline to the round window niche. At postoperative intervals between days 3 and 21 auditory brainstem response (ABR) measurement, immunohistochemistry, SGN quantification and cochlear surface preparation were performed. The results in the beta-Bungarotoxin-treated ears, as compared with sham-operated ears, show significantly increased ABR thresholds at all postoperative intervals, illustrating a severe to profound hearing loss at all tested frequencies (3.5, 7, 16 and 28 kHz). Quantification of the SGNs showed no obvious reduction in neuronal numbers until 14 days postoperatively. Between days 14 and 21 a significant reduction in SGN numbers was observed. Cochlear surface preparation and immunohistochemistry showed that the hair cells were intact. Our results illustrate that in vivo application of beta-Bungarotoxin to the round window niche is a feasible way of deafening rats by SGN reduction while the hair cells are kept intact.

Jacobson's nerve clues to the round window niche.

The round window niche is used as a primary guide to cochleostomy for cochlear implantation, but sometimes the niche is not visible through the dissected facial recess. In such cases, Jacobson's nerve may be a guide to the niche. The objectives of this study were to depict the distance from Jacobson's nerve to the lip of the round window niche, and how this distance may relate to orientation of the manubrium as viewed through the external ear canal. Also, are these distances related to mastoid pneumatisation size? The study involved post-mortem anatomic dissection of 41 bequeathed adult crania (82 temporal bones). Viewing with an operative microscope, distances were measured with a fenestrometer-type instrument. Mastoid sizes were determined radiographically. Jacobson's nerve was identifiable in 81 of 82 temporal bones. Distances from Jacobson's nerve to the lip of round window niche averaged 2.1 mm, range zero to 3.2 mm. Distances from Jacobson's nerve to the round window niche were not obviously associated with either mastoid size or orientation of the manubrium in the head. If the main trunk of Jacobson's nerve is visible on the promontory (as it is in > or =95% of cases), with 95 per cent certainty it is within 3.3 mm of the lip of the round window niche. Distances from Jacobson's nerve to the lip of the round window niche do not correlate with either manubrium orientation as viewed through the external ear canal, or the extent of mastoid pneumatisation.

Topographical relationships among the facial nerve, chorda tympani nerve and round window with special reference to the approach route for cochlear implant surgery.

The topographical relationships among the facial nerve (FN), chorda tympani nerve (CT), and round window (RW) in 22 temporal bone specimens were analyzed morphometrically in order to examine which route is widest through the facial recess between the FN and CT during cochlear implant surgery and in order to establish some criteria to assist in the evaluation of the best surgical approach. Two lines, i.e., the FN-RW line and CT-RW line, were speculated as limitations of a visual field for this surgery. According to the relative position of these structures, including the posterior wall of the external auditory canal (EAC) and an inserted pin-gage that indicates the hypothetical widest approach route, the relationships were classified into five types. Most frequently, the widest approach route through the facial recess did not point directly at the RW, but at the basal turn at the promontory. Moreover, this approach route crossed the FN-RW line in a posterior to anterior direction and the CT-RW line frequently crossed the posterior wall of the EAC. The latter seemed to provide a critical landmark for avoiding damage to the CT during cochlear implant surgery. Therefore, we recommend inserting the electrode into the basal turn.

Surgical anatomy of the singular nerve.

Rare cases of chronic persistent positional vertigo do not respond to physiotherapy. For the treatment of these cases Gacek suggested singular nerve neurectomy as a new surgical procedure. The aim of the present study was to investigate the surgical anatomy of the singular nerve in order to evaluate the exact topography. In 25 cadaver temporal bones the posterior ampullary ("singular") nerve was prepared. The topographical correlation between singular nerve, posterior semicircular canal, and the round window membrane was evaluated. The average length of the nerve was 4.2 mm, its diameter 0.6 mm. The shortest distance between singular nerve and round window membrane was 0.7 mm in average, the nerve could be detected at a depth of 1.3 mm. The transtympanal access was impossible in 7 of 25 cases either because of its close relation to the round window or its direct course towards the ampulla. In only 5 of 25 cases was the nerve located sufficiently inferior in the round window niche for neurectomy. Thus, transtympanal singular nerve neurectomy can be performed only in selected cases. The presented data are helpful for preoperative and intraoperative decisions.

Posterior canal-activated excitatory vestibuloocular relay neurons participate in the vestibulocortical pathways in cats.

We have previously reported that axon collaterals of posterior canal-activated excitatory vestibular (PC) neurons project to the contralateral oculomotor nucleus, and rostrally to the thalamus. To elucidate the vestibulothalamocortical pathways we investigated the synaptic connections of the PC neurons with the thalamic neurons by post-spike averaging of compound potentials triggered by spikes of the PC neuron in anesthetized cats. The averaged field potential evoked in the ventrobasal complex (VBC) revealed a spike followed by a negative wave. Latencies of the wave ranged from 0.8 to 1.5 ms. Next, we examined the location and axonal projection of 36 thalamic neurons which were activated by nose-up head rotation and by contralateral labyrinth stimulation. Most of them were located in the VBC and some in the medial geniculate body. Thirteen of the 36 neurons were antidromically activated from the anterior suprasylvian sulcus or postcruciate dimple of the cortex. These results suggest that the PC neurons participate, at least in part, in the vestibulocortical pathways contributing to spatial orientation.

Effects of vestibular stimulation on acetylcholine release from rat hippocampus: an in vivo microdialysis study.

1. The effects of electrical stimulation to the round window of the inner ear and caloric vestibular stimulation on the in vivo release of acetylcholine (ACh) from rat hippocampus were investigated, using brain microdialysis coupled with high performance liquid chromatography-electrochemical detection. 2. Hippocampal ACh release was increased to 152% of the basal release by 1-Hz, 200-ms, and 500-microA electrical stimulation to the round window. This effect was dependent on current intensity. 3. Microinjection of 6,7-dinitroquinoxaline-2,3-dione (DNQX) (10 nmol/0.2 microliter), an antagonist of the non-N-methyl-D-aspartate (non-NMDA) type of glutamate receptor, into the medial vestibular nucleus blocked the increase of hippocampal ACh release by electrical stimulation to the round window. In contrast, microinjection of DNQX (10 nmol/0.2 microliter) into the ventral cochlear nucleus did not inhibit the increase of hippocampal ACh release by electrical stimulation to the round window. 4. Electrical stimulation of the round window increased histamine release from the medial septum, which is the origin of the hippocampal cholinergic terminals. However, the depletion of neuronal histamine by alpha-fluoromethylhistidine (100 mg/kg ip) did not suppress the vestibular-evoked release of hippocampal ACh. 5. Although caloric stimulation with water at 37 degrees C did not affect the hippocampal ACh release, the release was increased to 131% of the basal release by caloric stimulation with hot water at 45 degrees C and was also increased to 160% of the basal release by caloric stimulation with ice water, suggesting that the changes in activities of vestibular afferents increased the hippocampal ACh release.(ABSTRACT TRUNCATED AT 400 WORDS)

The antidromic compound action potential of the auditory nerve.

1. The antidromic compound action potential (ACAP) of the auditory nerve was evoked by shocks to the auditory nerve root and recorded at the round window of the cochlea in anesthetized guinea pigs. The goal of this study was to determine the characteristics of the ACAP and compare these characteristics with those of the orthodromic, sound-evoked compound action potential (CAP). 2. The ACAP consists of an initial complex of a positive peak (p1) followed by a negative peak (n1). In contrast, the CAP consists of a negative peak (N1) followed by a positive peak (P1). These differences in waveform are likely due to the differences in conduction direction, antidromic for the ACAP vs. orthodromic for the CAP. 3. After the initial complex, the ACAP has a second complex of peaks (p2, n2) at a latency of approximately 1 ms; this complex is much smaller in amplitude than the initial complex (p1, n1). It is likely that the initial ACAP complex reflects firing of auditory-nerve fibers whereas the second complex reflects firing of neurons further centrally, perhaps in the cochlear nucleus, that are activated by orthodromic firing of auditory-nerve fibers. 4. Experiments with shock pairs are consistent with the idea that for auditory nerve fibers, the absolute refractory period is < 0.5 ms, and the relative refractory period is between 0.5 and at least 5 ms. 5. Experiments with click-shock pairs indicate that a shock interferes with the response to a click when the click and shock are given at about the same time.(ABSTRACT TRUNCATED AT 250 WORDS)

Surgical anatomy of the singular nerve.

The course of the posterior ampullary (singular) nerve has been studied in 30 preserved human temporal bones. In 17 dissections (57%), the nerve was readily accessible in the floor of the round window niche without undue risk to the round window membrane or the ampulla of the posterior semicircular canal. In 5 bones (16%), the nerve was closely related to the round window membrane and could not have been approached without significant risk of damage to the membrane. In 8 (27%), the nerve ran more or less directly medially to the internal auditory meatus without entering the round window niche. The ampulla of the posterior semicircular canal would therefore have been at risk if the nerve were approached surgically.

Threshold sensitivity and frequency selectivity measured with round window whole nerve action potentials in the awake, restrained chinchilla.

Chronic electrodes, placed on the round windows of four adult chinchillas, were used to measure whole-nerve action potential (AP) thresholds and tuning curves. The AP quiet thresholds were within 17 dB of thresholds measured by behavioral methods and within 3 dB of those measured by other evoked response procedures. The AP tuning curves, obtained by a simultaneous masking procedure, were also similar to previously measured tuning curves in the chinchilla. These results indicate that long-term indwelling electrodes can be successfully placed on the chinchilla round window and used to measure threshold sensitivity and frequency selectivity. Recordings from these electrodes could be used in a variety of situations and may be particularly useful in studying the effects of noise on the cochlea.

Effects of extracochlear direct current stimulation on the ensemble auditory nerve activity of cats.

The influence of direct current applied by round window stimulation on the whole nerve response of the auditory nerve of the cat has been studied. Effects on acoustically driven activity (CAP) and on the ensemble spontaneous activity of the nerve were observed. Stimulation with positive current suppressed driven and spontaneous activity. The strength and spread of suppressive effects was a function of the applied current level. After a period of positive electrical stimulation, driven and spontaneous activity rapidly returned to normal values. A rebound effect was sometimes observed, marked by a brief increase in spontaneous activity above the normal level. Negative current initially produced an increase in the amplitude of driven and spontaneous responses. Prolonged stimulation with negative current (greater than 30 s) resulted in a subsequent, graded reduction of neural activity, until a profound suppression of spontaneous and evoked neural activity was attained. The amplitude/latency relationship of CAPs was altered during passing of negative currents but not during passing of positive currents. Recovery from the suppression generated by negative currents was commonly prolonged for anything from a few seconds to many minutes; prolongation was dependent on stimulus amplitude, duration and duty cycles.

Effects of sympathetic stimulation on the round window compound action potential in the rat.

The effect on the ear of stimulating the sympathetic nervous system was studied in rats by recording the compound action potentials (N1N2) in response to 2 kHz tonebursts presented to anesthetized rats before, during, and after electrical stimulation of the superior cervical ganglion, and evaluating the changes in N1 latency which resulted. Stimulation of the superior cervical ganglion was found to cause an increase in the N1 latency which was more pronounced at low stimulus intensities (mean value 0.09 +/- 0.04 ms (S.E.) at 5 dB above threshold) than at moderate stimulation intensities (0.08 +/- 0.04 ms at 15 dB above threshold), with little change in latency occurring at the highest intensity tested (0.02 +/- 0.01 ms at approximately 25-30 dB above threshold). In addition, individual animals varied in their responses to stimulation of the superior cervical ganglion, with some animals evidencing a great change in the latency of the response (0.4 ms increase at 10 dB above threshold) and others showing very little change in latency. This variability could not be related to the condition of the animal at the time of observation of the response. In one of the twelve animals there was a slight decrease in latency as a result of sympathetic stimulation (0.07 ms at 5 dB above threshold), and although not studied systematically, low frequencies seemed to be affected more than high frequencies. Further, the change in the amplitude of N1 was not systematically related to sympathetic stimulation. After the administration of hexamethonium (which blocks transmission in autonomic ganglia) in three rats there was no effect on the latency of the N1 potential from sympathetic stimulation as recorded before sympathetic stimulation.

Vestibular nuclear neuron activity in chronically hemilabyrinthectomized cats.

The activity of central vestibular neurons (Vn) of the horizontal canal system was recorded in chronically hemilabyrinthectomized cats and compared with that of labyrinth intact animals. In both groups the cerebellar vermis was removed in order to assess the efficacy of the vestibular brainstem commissure alone by means of polarizing currents applied to the labyrinths. Experiments were carried out under Ketamine anaesthesia. In control animals the mean resting rates of type I and type II Vn measured 22.4 +/- 14.0 and 27.5 +/- 14.6 imp/s respectively, and the type I responses occurred ca. 3 X more frequently than type II. In the lesioned animals a drastic reduction of the number of type I responses was found on the deafferented side, while that on the intact side remained normal. The resting rates of type I Vn on the two sides did not differ significantly from each other but were significantly lower than those of control animals. In contrast, type II responses were present on the deafferented side, but almost completely missing on the intact side. Applying polarizing stimuli in control animals, it was found that both labyrinths have similar weight in driving Vn. In lesioned animals, no major changes in the efficacy of the commissural path were found when polarizing stimuli were applied to the intact side. It is concluded that vestibular nerve section causes a severe loss of type I responses in the vestibular nuclei on the side of the lesion which apparently is not compensated by an adaptive change in the commissural path and, therefore, may be mainly responsible for the VOR asymmetry observed concomitantly.

Efferent effects elicited by electrical stimulation at the round window of the guinea pig.

We report a technique for activating the efferent nerve fibres to the cochlea by electrical stimulation at the round window. Such electrical stimulation caused a reduction in the amplitude of the gross nerve response (N1) to a click presented after the electrical stimulus but did not alter the latency of the response. The reduction increased with increasing current strength above 200 microA and increasing rate of electrical pulses above 50 Hz. The effect was also dependent on the duration of the shock train and the pulse width. The reduction in N1 was most pronounced at low click intensities. Recovery of the N1 was almost complete about 80 ms after the end of the electrical stimulus. The effect of electrical stimulation in reducing the N1 amplitude could almost always be blocked by intraperitoneal injections of strychnine. Recovery from the strychnine block was observed when animals were maintained for periods of more than 60 min after the administration of strychnine. The ease of this technique allows it to be used to examine the effects of efferent stimulation on various aspects of cochlear function in the guinea pig.

Singular neurectomy update.

The results of a poll revealed that 96 singular neurectomies have been performed by ten surgeons in this country. Eighty-eight of these (91.7%) resulted in complete relief of benign paroxysmal positional vertigo (BPPV). In seven patients (7.3%) there was a sensorineural hearing loss as a result of this procedure. The average hospital stay for patients having this procedure ranged from 2-6 days, and the return to work time ranged from 1 to 3 weeks. These results indicate that singular neurectomy is an effective selective vestibular ablation procedure for BPPV and that the risks and disability are comparable to other routine middle ear procedures. The causes for failure of this procedure to relieve positional vertigo are a) misdiagnosis of BPPV and b) failure to recognize the singular nerve in the middle ear. Knowledge of the pathophysiology of the disorder and of the anatomical variation in the location of the singular canal will reduce these causes of failure.

Electrophysiological study of the maturation of auditory responses from the inner ear of the chick.

Three electrophysiological functions of the chick basilar papilla were studied during development by recording the compound actin potential (AP) at the round window. The auditory thresholds showed a continuous maturation between the fifteenth day of incubation (E15) and the first post-hatching day (P1), when they attained adult values. Responses matured first to low frequencies and later to high frequency stimuli. The input-output (intensity-amplitude) functions matured regularly and never demonstrated the classical two slopes seen in mammals. The tuning properties, studied by tone-on-tone masking of the AP, achieved mature values before the thresholds; the Q10s reached adult values at E17 for a 500-Hz probe tone and at E19 for a 1000-Hz probe tone. The fact that a low-to-high frequency development trend was found with the embryonic middle ear cleared of fluids further suggests that this property of auditory ontogeny may be a function of changes in the transduction properties of the cochlea.

Development of the cat peripheral auditory system: input-output functions of cochlear potentials.

Compound auditory nerve action potentials (APs) and cochlear microphonics (CMs) were recorded from the round-window of kittens aged 3-9 weeks and of adult cats. Animals were anaesthetized and pure tone stimuli were delivered via calibrated, sealed, transducer systems. AP and CM amplitude and AP latency were measured over a wide range of stimulus intensities (up to 80 dB SPL) and at 5 octave-interval stimulus frequencies (1-16 kHZ). At low stimulus intensity levels, AP amplitude had attained adult levels to low and high frequency stimuli by 6 1/2 weeks of age and to mid-frequency stimuli by 9 weeks. As stimulus intensity levels were increased, the kitten input-output functions diverged progressively from those of the adults. At these higher intensity levels, AP amplitude maturation in even the 9 week animals was incomplete. AP latencies to stimuli of all frequencies shortened between the third and fourth weeks but remained stable thereafter. CM amplitude also reached maturity by the fourth week. These findings suggest that the development of AP after the fourth week consists of an increase in the synchrony of auditory nerve fibre responses, since both the fine structure of the cochlea and the responses of single nerve fibres are known to be mature by the end of the first postnatal month.