Letter: Static acoustic impedance of the chinchilla middle ear: awake and sedated

The chinchilla animal model for hearing science and noise-induced hearing loss

The chinchilla animal model for noise-induced hearing loss has an extensive history spanning more than 50 years. Many behavioral, anatomical, and physiological characteristics of the chinchilla make it a valuable animal model for hearing science. These include similarities with human hearing frequency and intensity sensitivity, the ability to be trained behaviorally with acoustic stimuli relevant to human hearing, a docile nature that allows many physiological measures to be made in an awake state, physiological robustness that allows for data to be collected from all levels of the auditory system, and the ability to model various types of conductive and sensorineural hearing losses that mimic pathologies observed in humans. Given these attributes, chinchillas have been used repeatedly to study anatomical, physiological, and behavioral effects of continuous and impulse noise exposures that produce either temporary or permanent threshold shifts. Based on the mechanistic insights from noise-exposure studies, chinchillas have also been used in pre-clinical drug studies for the prevention and rescue of noise-induced hearing loss. This review paper highlights the role of the chinchilla model in hearing science, its important contributions, and its advantages and limitations.

Multifrequency tympanometry in chinchillas

Multifrequency tympanometry (MFT), using probe frequencies ranging from 226-2,000 Hz, was performed on normal chinchillas to obtain normative data against which to compare results from animals with middle ear pathology. A series of validating experiments was conducted to determine the effects of anatomical alterations of the middle ear on MFT. These included artificially extending the ear canal, opening the bulla, injecting saline into the middle ear, and disrupting the ossicular chain. The results indicate that MFT characteristics of chinchilla ears are qualitatively similar to those observed in normal humans and patients with middle ear disease, and MFT provides information that is not available from the 226-Hz tympanogram.

Different tympanometric procedures compared with direct pressure measurements in healthy ears

Different tympanometric procedures were compared regarding their reliability and systematic differences in middle ear pressure estimation in healthy adults. In a second part of the study the accuracy of measurement was judged by correlating tympanometric readings, obtained by using the different procedures, with known pressure levels applied after cannulating the mastoid air cell system. There were no significant differences in reliability between the different tympanometric procedures tested (p greater than 0.05). However, forward-backward tracing tympanometry and 'zero sweep rate' tympanometry gave smaller errors in the middle ear pressure estimates than the conventional decreasing pressure sweeps. Forward-backward tracing tympanometry at high sweep rate is recommended both for pressure measurements during physiological studies and in clinical practice.

Forward-backward tracing tympanometry in the diagnosis of middle ear pressure

Two tympanograms (TG) were routinely recorded on each ear by changing the pressure in the external auditory meatus (EAM), one in the decreasing (forward tracing: TG-F) followed by the other in the increasing direction (backward tracing: TG-B). Both TG were drawn on the same chart and the peak locations were compared. In a normal ear the TG-F peak tended to be formed in the negative pressure area and that of TG-B in the positive area. In a model whose middle ear pressure (MEP) was adjusted to the atmospheric pressure, the TG-F peak always indicated a negative pressure and that of TG-B a positive pressure value. As long as the same model was used, the magnitude of peak shift was identical irrespective of the middle ear pressure. The magnitude of the peak shift was influenced by the speed of EAM pressure change and a linear increase was observed up to the speed of 70 mmH2O/sec, both in a normal ear and a model. These findings seem to suggest that the peak location of a unidirectionally drawn TG can not be regarded as indicating the precise MEP. A valid MEP can better be estimated by averaging the peak pressure of TG-F and TG-B of forward-backward tracing tympanogram. In ears with pathology considerable variation was noted in the magnitude of the peak shift, so that relying solely on unidirectionally drawn TG could have lead to misdiagnosis in some cases. This was especially true with ears having small perforations covered with granulation or fluid.

The effects of experimentally-produced middle ear lesions on tympanometry in cats

Tympanometry was performed before and after producing specific lesions in the middle ears of cats. The lesions selected for study included stapes fixation, ossicular discontinuity, and scarred tympanic membranes. Stapes fixation resulted in marked increases in middle ear impedance, easily detected with tympanometry. Ossicular discontinuity resulted in complex tympanometric shapes which were easily accounted for by simple interactions between acoustic resistance and reactance. The complex shapes that occurred in normal and abnormal ears with pressure changing from negative to positive resulted from more complicated interactions. Large surgical incisions in the posterior-superior quadrant of the eardrum were quite visible at otoscopy but could not be detected tympanometrically one month after surgery.