Eye movements evoked by selective saccular nerve stimulation in cats

OBJECTIVE: Because of technical obstacles in controlling current spread to adjacent peripheral nerve, eye movements evoked by activation of the otolith organs have not been investigated in detail compared to eye movements evoked by activation of the canal organs. We attempted to solve this problem by applying more sensitive methods using fine needle and strictly controlling stimulus current intensity compare with filed potential for selective stimulation. METHODS: Eye movements evoked by selective, unilateral saccular (SAC) nerve stimulation were investigated using both electrooculography (EOG) and video recording in decerebrated cats in the presence or absence of anesthesia. Electrical stimulation was applied to the SAC nerve through implanted acupuncture needles. RESULTS: In the absence of anesthesia and with stimulus intensities less than (3.1 +/- 2.7) x N(1)T, we found supraduction in both eyes or in either the ipsilateral or contralateral eye of different cats. We observed downward eye movements using a stronger stimulus intensity ((6.2 +/- 2.9)) x N(1)T). The threshold for downward eye movements was significantly greater than that for upward eye movements (P < 0.05). In anesthetized cats, only downward eye movements were observed when stimulus intensities less than 10 x N(1)T ((7.8 +/- 2.3) x N(1)T) were used. CONCLUSION: These results confirm the known sacculo-ocular anatomical connections, which are involved predominantly in vertical eye movements. Because the sacculo-ocular connections are relatively weak, the normal supraduction evoked by SAC activation can be easily modified by factors such as level of anesthesia and the method of stimulation.     

Surgical section of the posterior ampullary nerve

Surgical section of the posterior ampullary nerve (PAN) has been used as a form of therapy for persistent benign paroxysmal positional vertigo (BPPV). Eighteen patients were reviewed. The PAN was identified in 16 patients, 15 of whom were completely cured and one of whom had improvement of symptoms. Severe sensorineural hearing loss (SNHL) occurred in three patients. This study suggests that surgical section of the PAN is effective. The procedure is facilitated by a postauricular incision and canalplasty.     

Eye movements evoked by the selective stimulation of the utricular nerve in cats

Objective: Eye movements evoked by otolith organ are not well-investigated compare with canal related eye movements due to the technical difficulties. We try to solve this problem by means of our methods. Methods: Eye movements evoked by selective utricular (UT) nerve stimulation were investigated using both electrooculography (EOG) and video recording in decerebrated cats in the presence or absence of anesthesia. Electrical stimulation was applied to the UT nerve through implanted acupuncture needles. Results: In the absence of anesthesia and with stimulus intensities less than 2.6±0.7×N₁T, we found ipsilaterally directed horizontal eye movements in both eyes in one cat, abduction in the ipsilateral eye in two cats, and adduction in the contralateral eye in another cat. Other types of eye movements (e.g., supraduction or diagonal eye movements) were observed in both eyes of cats in the absence of anesthesia at a stimulus intensity of 12.2±7.6×N₁T, an intensity in which current spread to the adjacent nerve could not be ruled out. In the presence of anesthesia, UT nerve stimulation alone failed to evoke horizontal eye movements, but with an intensity 13.8±6.4×N₁T, supraduction or diagonal eye movements were evoked. UT nerve stimulation at 2–3×N₁T facilitated horizontal eye movements induced by ipsilateral abducens (AB) nucleus stimulation or contralateral horizontal canal nerve stimulation. Conclusion: This is the first report to our knowledge in which UT nerve-evoked horizontal eye movements are documented. These results confirm the known monosynaptic and disynaptic anatomical connections from utricular primary afferents to the ipsilateral AB nucleus neurons.     

Analysis of field potentials in the postcruciate dimple evoked by stimulation of the ampullary nerve of the posterior semicircular canal in cats

Field potentials evoked by stimulation of the posterior semicircular canal nerve (PCN) were studied in the postcruciate dimple (PCD) in anesthetized cats. When weak stimulation was provided, field potentials composed of 3 components of small positive, large positive and large negative waves were observed on the surface of the PCD. The field potentials could be observed at the stimulus intensity weaker than 1.5 x N1T. The latencies of initiation of the 3 components were 2.32 +/- 0.75 (mean +/- S.D.; n = 37), 3.75 +/- 0.92 (mean +/- S.D.; n = 99) and 9.34 +/- 2.81 (mean +/- S.D.; n = 101), respectively. The large positive wave of the maximum amplitude was recorded in the lateral part of the PCD. In this point, the large and small positive waves reversed their polarity at the layer III and V, respectively, which located about 500 microns and 1000 microns deep from the surface. Stimulations of the whole vestibular nerve (WVN) and the deep radial nerve (DR) shared response areas of field potentials. These findings agreed with the previous results as to a neuronal convergence of the somatic and vestibular inputs in the cat.     

Results of singular neurectomy in the posterior ampullary recess

OBJECTIVE: To determine the effect on hearing and balance symptoms following singular neurectomy (SN) for benign paroxysmal positional vertigo (BPPV) in the ampullary recess of the posterior semicircular canal. RESEARCH DESIGN: The charts of 242 patients with chronic disabling BPPV who were treated with SN over a 29-year period (1972-2001) were reviewed. The results on relief of BPPV and hearing function were recorded. A subset of 16 patients where the posterior ampullary recess was entered to expose the SN is described in detail with regard to an effect on hearing and balance. RESULTS: A total of 252 SN were performed in 242 patients. Ten patients underwent bilateral SN sequentially; the remaining 232 patients had unilateral SN. The ages of the patients ranged from 21 to 86 years, with a mean at 57 years. The female:male ratio was 174:68. Complete relief of BPPV was achieved in 244 patients (96.8%), incomplete relief in 3 (1%), and no relief in 5 (2%). Sensorineural hearing loss (SNHL) occurred in 9 patients (3.7%). A subset of 16 patients in whom the ampullary recess was opened during SN ranged in age from 21 to 79 years, with a mean at 56 years. The female:male ratio was 12:4, with right and left sides divided almost equally. Relief of BPPV was achieved in all 16 patients with no loss of hearing function. Five patients complained of a fistula response postoperatively (31%). The fistula response resolved by 6 months postoperatively in all 5 patients. CONCLUSIONS: SN is effective in relief of BPPV with little risk of SNHL (3.7%). The risk of SNHL is not increased when the posterior ampullary recess must be entered in order to transect the singular nerve. A positive fistula response may be present temporarily in almost one third of these patients.     

Superior laryngeal nerve stimulation effects on respiratory movements of the vocal cords

An animal model was designed to reproduce conflicting effects of the superior laryngeal nerve (SLN) irritation and hypoventilation found in a clinical situation produced by an upper airway foreign body. The effects of continuous stimulation of the SLN on changes in glottic pressure were studied under conditions of a) normoxia and normokapnia , b) hypoxia and hyperkapnia , and c) normoxia and hyperkapnia . Recorded fluctuations of glottic pressure were found to correspond closely to activity of the muscles of respiration. Condition of combined hypoxia and hyperkapnia seemed to protect animals from disruption of breathing caused by the SLN stimulation, while hyperkapnia itself did not influence significantly inhibitory effects of the SLN stimulation. It has been observed that irritation of the SLN may produce reflex swallowing. In certain animals single shocks delivered to SLN in expiratory phase induced additional breaths. The authors emphasize the importance of the larynx as a receptor organ and a trigger zone for a number of clinically important reflexes.     

Spontaneous and electrically induced movements of ampullary kinocilia and stereovilli

In the transparent vestibular organ of young eels, isolated in toto, movements of individual kinocilia and hair bundles of the frontal ampulla were recorded by photodiodes and a video system. Flagella-like oscillations of kinocilia occurred spontaneously when preparations deteriorated and could be induced regularly in fresh preparations by pressing onto the tip of the cilium. Upon step-like electrical polarization of the epithelium hair bundles deflected in a tonic, pointer-like manner. When the apical membrane was hyperpolarized the hair bundles deflected towards the kinocilium (positive deflection) amounting to about 0.6 degrees when the polarization was made strong enough to cause saturating responses in the ampullary nerve. In response to sinusoidal voltage the amplitude of the hair bundle deflection declined by -4 dB/octave for frequencies above 1.3 Hz. When the kinocilium was disconnected from the bundle of stereovilli by transient reduction of divalent cations, voltage induced deflections occurred, of both the kinocilium and the stereovilli. Reducing the extracellular Ca-activity seemed to destabilize the electrically induced deflections; blocking the oxidative metabolism (CN-) had no effect. The induced deflections only disappeared upon chemical fixation by glutaraldehyde or treatment with triton X-100. Surface tension and electrostriction of the cell membrane are discussed as possible force generators.     

Temporal response patterns of auditory nerve fibers to electrical stimulation in deafened squirrel monkeys

Electroneural response patterns of single auditory-nerve neurons were studied in aminoglycoside-deafened squirrel monkeys. The electrical stimuli were delivered through bipolar electrodes implanted in the scala tympani. The effects of pulse width, shape, frequency, and intensity on neural adaptation, phase locking, and spectral content were evaluated. Our results did not demonstrate the characteristic adaptation seen in auditory-nerve neurons in response to acoustic stimulation. Phase locking to a broad stimulus pulse (3200 microseconds/phase) was found to a very restricted phase angle of the electrical stimulus which was broader for square wave than for sine wave stimulation. The latency of the phase locked response varied inversely with stimulus intensity with greater variation for square wave stimulation than for sine wave stimulation. Auditory neurons were capable of a very high degree of phase locking to a 200-microseconds/phase pulse presented at 156 pulses per second (PPS) and to the first pulse of a 2500-Hz pulse burst. Phase locking was much poorer for the subsequent 200-microseconds/phase pulses comprising the 2500-Hz pulse burst where the neuron's response was determined by its relative recovery status. These findings can be explained by an interaction between the neuron's relative refractory status and its integration of charge over the stimulatory half cycle of the electrical stimulus. These two factors also appear to determine the interspike interval of the neural response. This interval decreased monotonically with increasing stimulus intensity. The neural spike rate (150-500 Hz) producing this interval increased with intensity and may be a source of periodicity information which the central auditory nervous system could interpret as pitch. This may account for the proportional relationship between pitch and stimulus intensity seen in some cochlear implant patients. Our study demonstrates that auditory-nerve neurons comply with basic neurophysiological principles in their responses to electrical stimulation. These principles should be incorporated into the cochlear prosthesis stimulator if more normal neural response patterns are desired in the cochlear prosthesis patient.     

Responses from the brainstem at the entrance of the eighth nerve in human to contralateral stimulation

Recordings were made from the exposed eighth nerve (N VIII) and the lateral brainstem near the root entry zone of N VIII to contralateral click stimulation in patients undergoing microvascular decompression operations to relieve hemifacial spasm or trigeminal neuralgia. Similar recordings were made in patients undergoing operations to remove acoustic tumors using a retromastoid approach to the cerebellopontine angle. The waveform of the response that was recorded using a monopolar electrode placed on the intracranial portion of the eighth nerve was similar to the potentials recorded from the lateral surface of the brainstem near the entrance of the eighth nerve, and consisted of a positive deflection with a latency of about 4 ms, and sometimes a second and smaller positive peak with a slightly shorter latency than that of peak V in the BAEP could be seen in such recordings. It was concluded that the potentials are generated in brainstem structures, most likely the cochlear nucleus. It is assumed that when the potentials are recorded from the exposed eighth nerve, the nerve passively conducts the activity from the active structure to the recording site. The latency of the main positive peak in these potentials does not bear any direct relationship to any identifiable component of the farfield evoked potentials when these potentials are recorded from the vertex or the earlobe of the stimulated ear and a noncephalic reference is used. It seems likely that the potential is generated by fibers that originate in cells in the cochlear nucleus on the stimulated side, and that the initial positivity reflects the termination of these fibers in the cochlear nucleus on the side from which the recording is made.     

Eye movements and imaging in vestibular migraine

Background and objectiveMigrainous dizziness is one of the most frequent complaints. Dizziness associated with migraine may be the result of abnormal eye movements. Brain imaging and changes in eye movements may explain the dizziness and highlight possible pathophysiological substrates in migraine dizziness. Our aim is to evaluate eye movement using videonystagmography (VNG) and video head impulse test (vHIT) and to study the occipital lobe metabolic profile in vestibular migraine patients (VM).Materials and methodsThere were 2 groups enrolled in the study; the first group consisted of 25 vestibular migraine patients (VM) according to the recent criteria of Barany society. The second group consisted of 20 age matched healthy subjects. Both groups underwent the following: (1) A detailed history, VNG test protocol, vHIT in three planes. (2) Magnetic resonance imaging (MRI) for the brain and inner ear using 1.5 T magnet and proton magnetic resonance spectroscopy (H1-MRS).ResultsSixty eight percent of the patients complained of spontaneous vertigo and 28% complained of positional vertigo. Non-paroxysmal positional nystagmus was recorded in 92% during their dizzy spell. The brain MRI was unremarkable in 72% of the cases. Chemical shift in the occipital lobe was found in 92% of VM. Lactate peaks were statistically significant related with the presence of non-paroxysmal positional nystagmus.ConclusionsA statistically significant relationship exists between non-paroxysmal positional nystagmus and presence of lactate peaks in the occipital lobe in VM patients.     

Eye movements in patients with speech dyspraxia

Saccades, smooth pursuit and angular acceleration induced nystagmus were analysed quantitatively in 10 patients with speech dyspraxia. The saccades were less accurate, had a prolonged reaction time and showed a tendency to reduced peak velocity, though only contralateral to the lesion. Smooth pursuit was imparied, with a reduction in maximum velocity gain. The vestibular responses tended to be hyperactive, indicating facilitated brain-stem reflexes. The findings show that a lesion in the frontal eye field can produce various oculomotor disturbances, in which the triggering of eye movements and their control, and pacing of the various movement sequences are disturbed. In addition, anticipation of a movement pattern requiring volition may be greatly impaired.