Andreas Böhmer

Hydrostatic pressure in the inner ear fluid compartments and its effects on inner ear function

The present study summarizes the experimental findings obtained on the pressure in the inner ear fluids and on the effects of pressure changes on cochlear function in the guinea pig. Two types of pressures have to be distinguished in the inner ear fluid compartments: (i) hydrostatic fluid pressure and (ii) superimposed hydrodynamic high frequency (> 100 Hz) sound pressure oscillations. Hydrostatic pressure in the inner ear fluids in guinea pigs is in the order of 200 Pa (2 cm H2O) and shows slow (< 5 Hz) respiratory and pulsatory oscillations as well as considerable physiological variations in the range of -100 to +700 Pa. In normal ears, hydrostatic pressure in the perilymph equals pressure in the endolymph, and pressure changes applied to one compartment are immediately transmitted to the other one. A high compliance of Reissners membrane seems to be the cause of this endolymphatic-perilymphatic pressure equalization. In experimental endolymphatic hydrops, a unique animal model for Menieres disease, endolymphatic pressure is higher (100 Pa and above) than perilymphatic pressure. These pressure gradients occur only in late stages of hydrops, probably when Reissners membrane has lost its high compliance after long standing distension. Positive endolymphatic-perilymphatic pressure gradients are secondary to and not the primary cause of hydrops formation. Changes of hydrostatic pressure do not affect auditory function as long as they stay in the physiological range. This includes the sudden loss of positive inner ear pressure that occurs in perilymph fistulas. The rationale for surgical repair of perilymph fistulas in patients in order to restore the hearing function thus becomes questionable. Other aspects of surgical repair, however, as e.g. prevention of labyrinthitis due to permanently open fistula, could not be investigated in this model, because in guinea pigs even large fistulas heal spontaneously within a few days. In experimental endolymphatic hydrops, deterioration of auditory thresholds was partially correlated to the presence of positive endolymphatic-perilymphatic pressure gradients. A change in pressure, however, occurred later than the first deterioration in auditory function. Therefore positive endo-perilymphatic pressure gradients may contribute to, but are not the only cause of hearing impairment.

Pathomechanism of mammalian downbeat nystagmus due to cerebellar lesion: a simple hypothesis

Most of the various hypotheses on the pathomechanism of the slight ocular upward drift in normal mammals and on the prominent downbeat nystagmus following cerebellar lesions assume an inherent vertical asymmetry in the central vestibulo-ocular pathways. In this paper we propose that this vertical asymmetry is simply based on the anatomical orientation of the six semicircular canals in the head which is right-left symmetrical but lacks symmetry in the cranio-caudal direction. Presuming that each semicircular canal elicits eye movements in a direction roughly in its anatomical plane, vectorial addition of the tonic resting activity of all six canals leads to a cancellation of horizontal and torsional eye movement components but leaves an important vertical (slow phase) upward component. This peripheral vestibular bias is centrally cancelled by floccular and parafloccular inhibitory pathways which are related to the smooth pursuit system, but becomes disinhibited in the presence of posterior cerebellar lesions.

Vestibulo-ocular reflexes after selective plugging of the semicircular canals in the monkey--response plane determinations.

The contribution from different pairs of semicircular canals to the generation of horizontal vestibular nystagmus was examined in monkeys. Animals with different pairs of semicircular canals surgically plugged were accelerated sinusoidally at 1 Hz (a predictive stimulus) or with steps of angular velocity (a non-predictive stimulus) about an earth vertical axis while the head was placed in various static pitch positions. In normal animals, and in animals with only the lateral canals intact, horizontal nystagmus elicited with angular velocity steps is maximal at a static pitch angle of 15 degrees nose-down (relative to the horizontal stereotaxic plane). The response follows a cosine function of the pitch angle, approaches zero at an angle of 90 degrees to the optimal orientation, and finally reverses. In animals with only the vertical canals operating, direction specific horizontal nystagmus can still be elicited. Using velocity steps, a null plane at which nystagmus reverses can be determined. It is found at about 32 degrees nose-down and thus is different from the optimal plane of the lateral canals. Consequently, it is not possible to stimulate the lateral canals maximally without stimulating the vertical canals simultaneously. Using sinusoidal rotation, nystagmus is attenuated at the static pitch position of 32 degrees nose-down, but does not reverse direction with further pitching.

The measurement and manipulation of intralabyrinthine pressure in experimental endolymphatic hydrops

Three to four months after unilateral surgical ablation of the endolymphatic duct and sac, endolymphatic and perilymphatic pressures were measured in both the normal and hydropic ears of 11 guinea pigs. In normal ears, endolymphatic pressure always approximated perilymphatic pressure. Endolymphatic pressure exceeded perilymphatic pressure in all ears with hydrops, except one in which these pressures were equal. The effect of postural inversion on inner ear pressures were studied in both normal and hydropic inner ears. Normal ears showed endolymphatic and perilymphatic pressure to rise equally during this maneuver. In hydropic ears, the difference between endolymphatic and perilymphatic pressure was notably reduced from measurements obtained in the prone position. This study indicates that an alteration in pressure regulation within the inner ear may be important in the pathogenesis and manifestation of experimental endolymphatic hydrops in the guinea pig. Physiologic mechanisms and clinical implications of these results are described.

Horizontal and vertical vestibulo-ocular and cervico-ocular reflexes in the monkey during high frequency rotation

In the alert monkey the horizontal vestibulo-ocular reflex (VOR) is basically compensatory over the range of 0.5 to 6 Hz with a gain near unity, and with the phase of the compensatory eye position having a minimal lag with respect to head position. Typical frequency-dependent eye movement patterns were observed. Vertical VOR is also compensatory having the same phase relations but with a reduced gain (-2.5 to -3.7 dB). In this range, vestibular input appears to be the predominant sensory influence on reflex eye movements. Additional optokinetic reflexes do not improve the VOR above 0.5 Hz. The horizontal cervico-ocular reflex (COR) is minimal or absent in normal monkeys.

The Preyer reflex--an easy estimate of hearing function in guinea pigs.

Preyer reflex thresholds elicited monaurally by tone bursts from 0.25 to 8 kHz were determined in more than 150 guinea pig ears. Normal reflex thresholds were between 85 and 95 dB SPL in the low and middle frequencies, decreasing to 75 dB in the higher frequencies. The range of measurements at single frequencies usually did not exceed 10 dB. In unselected populations of adult guinea pigs, about half of the animals had increased reflex thresholds--at least unilaterally, in most cases due to middle ear infections. The Preyer reflex showed parallel threshold increase with compound action potential thresholds in conductive loss and recruitment in cochlear hearing loss. Normal Preyer reflex thresholds do not necessarily mean normal hearing, but increased thresholds do indicate hearing impairment. This technique is valuable in the selection and monitoring of animals for otologic experiments.

Three-dimensional analysis of spontaneous nystagmus in peripheral vestibular lesions

The direction of spontaneous nystagmus was recorded in three dimensions with scleral dual search coils in three patients after vestibular neurectomy and in seven patients with vestibular neuritis. The rotation vectors of the spontaneous nystagmus clustered along the sensitivity vector of the lateral semicircular canal (SCC). The direction of the spontaneous nystagmus after resection of the whole eighth nerve was not different from that after resection of only the superior branch of the vestibular nerve. Deviations from this direction were observed only after resection of the inferior vestibular nerve and in one patient with vestibular neuritis. The absence of nystagmus components in direction of the vertical SCC reflects an anisotropy of oculomotor efferents of the vestibulo—ocular reflex arc rather than alesion limited to the lateral SCC afferents. Therefore, the three-dimensional analysis of spontaneous nystagmus does not permit accurate localization of a peripheral vestibular lesion.

Short Latency Vestibular Evoked Responses to Linear Acceleration Stimuli in Small Mammals: Masking Effects and Experimental Applications

Different potential were recorded from a site close to the 8th nerve in chinchillas in response to linear acceleration pulses. Acoustic masking allowed us to distinguish between an early response (within 1 ms after initiation of the acceleration) of probable vestibular origin and later responses of probable cochlear origin. The latter were abolished by intense acoustic masking and by surgical ablation of the cochlea. The early potential was slightly reduced by simultaneous acoustic masking with white noise above 65 dB SPL and was most sensitive to 1 kHz narrow band masking. Vestibular neurons seem to be stimulated by high frequency movements of their hair cell cilia, and vestibular compound action potentials can be recorded as soon as a sufficient number of neurons are brought to a synchronized response. These vestibular evoked potentials may provide a tool for experimental studies on vestibular function in laboratory animals.

Three-dimensional analysis of caloric nystagmus in the rhesus monkey.

The aim of this study was to investigate whether caloric nystagmus contains response components that can be attributed to a stimulation of the vertical semicircular canals. Three dimensional eye movement recordings with a dual search coil technique revealed important horizontal, vertical and torsional nystagmus components following irrigation of the external ear canal with cold water in various head positions relative to gravity. Horizontal nystagmus components, i.e. lateral semicircular canal vectors, followed a cosine function of both the pitch and yaw angle of the head relative to gravity, confirming a mainly thermovective mechanism for stimulation of the horizontal canals. Vertical and torsional nystagmus components behaved differently following left and right ear irrigations. Right-left symmetrical vectors emerged only when the vertical and torsional components were transposed into vectors of single semicircular canal directions. The intensity of these vertical semicircular canal vectors as a function of the position of the corresponding canal relative to gravity, however, excludes important thermovective mechanisms acting at the vertical canals. It remains an open question whether these vertical canal vectors represent a non-thermovective caloric stimulation of vertical canal afferents.

Contributions of Single Semicircular Canals to Caloric Nystagmus as Revealed by Canal Plugging in Rhesus Monkeys

We demonstrated specific responses from the anterior and the posterior semicircular canal to irrigation of the outer ear canal with cold water in the Rhesus monkey. This required i) three-dimensional analysis of the evoked eye movements in the planes of the semicircular canals (canal plane vectors, CPV); ii) assessing these CPV responses in eight different head positions relative to gravity; iii) comparing the responses in 6 normal animals (12 ears) with responses after selective plugging of pairs of semicircular canals (all, both lateral, and right anterior + left posterior). The results showed: i) Irrigation of the outer ear canal with cold water induces thermoconvection also in the posterior and anterior semicircular canals. This can be inferred from the sinusoidal modulation of eye movement components with changes in position of the corresponding semicircular canal plane relative to gravity; ii) Specific vertical canal responses occur exclusively in the direction of the corresponding semicircular canal, though they are superimposed with response components of other origin, one probably related to endolymph shift in the lateral semicircular canal; iii) before possible clinical application, these different response components of vertical canals will need to be determined in humans.