Low-frequency modulation of compound action potential in experimental perilymphatic fistula and endolymphatic hydrops

A comparison of three methods of using transtympanic electrocochleography for the diagnosis of Meniere's disease: click summating potential measurements, tone burst summating potential amplitude measurements, and biasing of the summating potential using a low frequency tone

CONCLUSION

Stimulus biasing modulated the amplitude of the tone burst evoked summating potential (SP) in ears affected by Meniere's disease less than in normal ears. A reduced SP bias ratio added diagnostic accuracy for the diagnosis of Meniere's disease.

OBJECTIVES

To evaluate the effect of stimulus biasing on the human tone burst SP, and to determine if stimulus biasing could contribute to the electrocochleography as a means of confirming the diagnosis of Meniere's disease.

PATIENTS AND METHODS

Patients referred for transtympanic electrocochleography (TT ECochG) were assessed prospectively on clinical grounds according to the AAO-HNS criteria and a scale devised by one of the authors. A Meniere's group of ears and a non-Meniere's group of ears was determined. The ears opposite a Meniere's ear were not included in the analysis. The ratio of the click SP amplitude and the action potential (AP) amplitude (SP/AP ratio), the tone burst SP amplitude at 500 Hz,1 kHz, 2 kHz and 8 kHz, and the effect of stimulus biasing on the tone burst SP were measured.

RESULTS

A unipolar stimulus biasing ratio established for the modulation of the 1 kHz tone burst SP separated the Meniere's ears from the non-Meniere's ears with a sensitivity of 85% at a specificity of 80.6% and the difference between groups reached statistical significance (p=0.016). The 1 kHz SP amplitude measurements and the stimulus biasing measurements were superior to the click SP/AP ratio for identifying the Meniere's group. A combination of 1 kHz SP amplitude measurements and SP bias ratio separated the Meniere's ears from the non-Meniere's ears with an accuracy of 85%.

Effects of experimental round window membrane laceration on the free amino acid profile of perilymph

To evaluate biochemical changes of inner ear fluid following perilymphatic fistula (PLF), free amino acid (FAA) profiles of perilymph in experimental PLF were determined using high performance liquid chromatography (HPLC). Thirty-five guinea pigs were anesthetized and prepared as PLF models by perforating the round window membrane (RWM) of the left ear. Right ears served as controls. Samples (2 microliters) were aspirated from scala tympani through a RWM perforation. Animals were divided into two groups according to time of sampling following PLF induction: 2-week group (n = 17) and 4-week group (n = 18). Compound action potential (CAP) evoked by 1, 2, 4 and 8 kHz tone bursts were measured using a round window electrode from the left ear before PLF induction and from both ears before final sampling. RWM perforations were completely closed at the time of the final sampling in 8 of 17 animals from the 2-week group, and 15 of 18 animals from the 4-week group. In comparison with that in control ears, concentrations of FAA throughout all profiles was dramatically elevated in the PLF ears with a healed RWM perforation. Most PLF ears with persistent RWM perforation showed minimal differences between 2-week and 4-week groups. No remarkable CAP threshold changes were found at any frequencies tested following PLF induction in both the 2-week and 4-week groups. The unchanged 8 kHz threshold suggests that FAA concentration increases only at the basal end of the cochlea. FAAs accumulate within the basal end of scala tympani in ears with a healed RWM.(ABSTRACT TRUNCATED AT 250 WORDS)

Low-frequency biasing of round window responses in guinea pigs and chinchillas

The acoustic biasing technique using low-frequency sound is of increasing interest to investigators, not only as a means of studying cochlear transduction but also as a promising tool for assessing cochlear pathology such as endolymphatic hydrops. We compared normal modulation patterns of round window responses in guinea pigs and chinchillas, whose low-frequency auditory characteristics are known to be different. A 50-Hz sine wave (90 dB SPL for guinea pigs and 80 dB SPL for chinchillas), which evoked an equivalent magnitude of cochlear microphonics (CM) in both species, was used to modulate the compound action potential (CAP) and the summating potential (SP) elicited by 8-kHz tone bursts. Overall patterns of CAP and SP modulation were almost identical between the two species except for a difference in the phase of 50 Hz CM. The phase of maximum SP enlargement was in accord with that of maximum CAP suppression, which led to inferred basilar membrane (BM) position at maximum scala tympani displacement by between 45 and 90 degrees. More complex or hysteresis effects seemed to be involved in the modulation of CAP and SP magnitude, in addition to the biasing effect owing solely to BM displacement.

Hearing loss after neurosurgery. The influence of low cerebrospinal fluid pressure

A prospective study was performed to investigate the effect of neurosurgery on hearing. Thirty-two patients underwent neurosurgery while 32 patients who had surgical procedures not involving puncture or drainage of the subdural space, served as a control group. In the neurosurgical group, a significant loss of hearing was observed in the immediate post-operative period, with recovery over one week. No average threshold shift was observed in the control group. It is suggested that following neurosurgery the mechanism of hearing loss results directly from a decrease in pressure and/or volume of the cerebrospinal fluid, which is reflected within the perilymphatic fluid, comparable to a transitory endolymphatic hydrops.

Functional changes associated with experimentally induced endolymphatic hydrops

A guinea pig model with surgically induced endolymphatic hydrops of the inner ear has been developed and studied over the past thirty years. The aim of such studies is to obtain insight into physiological processes associated with endolymphatic hydrops in man and in particular in Menière's disease where endolymphatic hydrops is systematically encountered at post-mortem examination of the temporal bones. The present review attempts to draw together the data pertaining to functional modifications of inner ear function in the animal model. For simplicity the data are categorised under five main titles: electrochemical modifications, electrophysiological modifications, pressure and hydrops, sensitivity to other insults and vestibular dysfunction. One of the most striking observations that can be made is that the data originating from different authors are very variable. There is, however, some evidence suggesting that the evolution of the auditory dysfunction could be considered as consisting of a series of different phases. This kind of information could serve as a basic framework for future research on the animal model.