George Turrentine

Anatomical correlates of impulse noise-induced mechanical damage in the cochlea

Changes in the surface morphology of the organ of Corti in the chinchilla were studied following exposure to blast waves at 160 dB peak SPL. The generation and development of a mechanically induced lesion on the organ of Corti was followed over a period of 30 days using scanning electron microscopy. The most prominent feature of the lesion was the complete separation of a 5-7 mm strip of the sensory epithelia consisting of outer hair cells, Deiter cells and Hensen cells from the reticular lamina and the basilar membrane. The inner hair cells in this same area survived for several days in a remarkably normal condition. A spectrum of ciliary changes was observed on the inner and outer hair cells that differ somewhat from those commonly reported following continuous noise exposure. Some of the observed changes in morphology can be related to a variety of inferred mechanical events on the basilar membrane.

Damage of the auditory system associated with acute blast trauma.

This paper reviews the results of several studies on the effects of blast wave exposure on the auditory system of the chinchilla, the pig, and the sheep. The chinchillas were exposed at peak sound pressure levels of approximately 160 dB under well-controlled laboratory conditions. A modified shock tube was used to generate the blast waves. The pigs and sheep were exposed under field conditions in an instrumented hard-walled enclosure. Blast trauma was induced by the impact of a single explosive projectile. The peak sound pressure levels varied between 178 and 209 dB. All animals were killed immediately following exposure, and their temporal bones were removed for fixation and histologic analysis using light microscopy and scanning electron microscopy. Middle ears were examined visually for damage to the conductive system. There were well-defined differences in susceptibility to acoustic trauma among species. However, common findings in each species were the acute mechanical fracture and separation of the organ of Corti from the basilar membrane, and tympanic membrane and ossicular failure.

Surface morphology of the inner sulcus and related epithelial cells of the cochlea following acoustic trauma.

When the organ of Corti is severely traumatized by intense (160 dB) blast waves, such that as much as 7 mm of Cortis organ is torn loose from the basilar membrane, the cells of the inner sulcus respond to the altered biochemical milieu of the endolymph by a profile elaboration of surface membrane, zeiosis and the development of numerous pseudopodia and microvilli. On the basis of our longitudinal study, this surface reaction appears to peak at approximately 10 days after trauma and may indicate that the inner sulcus cells are extremely active in the endocytosis of cellular debris. Signs of active changes on the surface of the inner sulcus cells occur immediately following trauma, and activity continues for as long as 30 days after exposure. The cells of Claudius, as well as other epithelial cells on the basilar membrane, are also capable of extreme membrane proliferation and mobility. Possible mechanisms for the unusual behavior and the role of the inner sulcus cells in the normal functioning cochlea are discussed.

The Role of Peak Pressure in Determining the Auditory Hazard of Impulse Noise

Most current Damage Risk Criteria (DRC) for human exposure to impulse noise are written in terms of peak pressure as the primary index of the traumatic potential or hazard associated with exposure to an impulse noise. Since the peak pressure is only one of many parameters of an impulse, there is a question whether a DRC based on peak pressure can reflect accurately the hazard to hearing posed by impulse noise.

Mechanically-Induced Morphological Changes in the Organ of Corti

Acute exposures to high-level noise impulses damage the cochlea via mechanical mechanisms that are associated with excessive displacements and stresses developed in the delicate epithelial tissues of the organ of Corti. Such damage has been discussed in the literaure a number of times, and an especially clear description was provided by Davis [1]. Davis and his colleagues used continuous noise at levels of nearly 150 dB SPL at the eardrum. They noted that the Hensen cell attachments represent a mechanically weak link in the structural organization of the organ of Corti. This result was confirmed by Beagley [2], who illustrated the separation of cell junctions between the Deiter and Hensen cells following overstimulation. Since then, others (notably Spoendlin [3] and Voldrich [4]), also using high levels of continuous noise, have demonstrated lesions on the basilar membrane of an equivocal mechanical origin, including rupture of the basilar membrane and Reissners membrane. Spoendlin suggested intensities of around 125 dB SPL as the threshold for mechanically-induced lesions as opposed to metabolically-induced damage. However, the dependance of this rms sound pressure on the exposure duration is not clear. Spoendlin is in agreement with Davis and Beagley concerning the susceptibility to acoustic trauma of the Hensen cell attachments, but he further implicates the pillar cells and the medial attachments of the inner hair cell cuticular area as, “weak spots.” This paper attempts to provide a clear documentation of the morphological sequence of events which is eventually responsible for producing massive structural damage to the organ of Corti. Using blast waves as a vehicle, we will further attempt to qualitatively illustrate a fundamental difference in the way in which continuous and impulse noise may need to be evaluated when assessing the potential for producing trauma.

Frequency selectivity in noise‐exposed chinchillas

Evoked‐potential tuning curves (TCs) were recorded from chronic electrodes in the inferior colliculus of 102 chinchillas before and after acoustic overstimulation in order to relate the effects of changes in frequency selectivity to the condition of the cochlea. Pre‐ and post‐exposure measures of auditory threshold and masked thresholds (simultaneous tone‐on‐tone paradigm) were obtained at 0.5, 1, 2, 4, 8, and 11.2 kHz. The three TC variables Q 10 dB, and the low‐ and high‐frequency slopes were compared in two different groups of chinchillas: (1) animals (N = 54) with cochleas that showed distinctly normal and lesioned regions, and (2) animals (N = 48) that had near‐normal thresholds, but either had lesions that were not manifested in the threshold measures or had nearly normal sensory cell populations. In the first group, the TCs measured from normal regions of the cochlea, in general, showed no statistically (t test) significant decreases in the group averaged low‐ or high‐frequency slope, i.e., upward ...

Kurtosis measurements: Implications for noise exposure criteria

Two conventional measures of an industrial noise environment are the overall sound‐pressure level (SPL) and the power spectrum. While these two measures are necessary, they may not be sufficient to describe a noise environment for the purposes of evaluating the hazard to hearing. This presentation will describe the results of an experiment designed to test the hypothesis that for equal SPL and power spectrum a high‐kurtosis noise exposure is more hazardous to hearing than is a low‐kurtosis noise. Two groups of chinchillas, with two animals per group, were exposed continuously for 5 days, to one of two classes of noise at a 90 dB SPL. The power spectrum of both noises was identical but the noises had different values of kurtosis. The results clearly showed that there was up to a 20 dB greater permanent hearing loss for the animals exposed to the high‐kurtosis noise. Detailed results of asymptotic and permanent threshold shifts and sensory cell losses will be presented. These results would appear to be a cl...

Scanning electron microscopic study of impulse noise—Induced mechanical damage in the cochlea

Binaural chinchillas were exposed at normal incidence to 160 dB peak SPL impulse noise at the rate of 2 impulses per min for 50 min. Animals were sacrificed at post‐exposure times varying from t = 0 through 30 days. The cochleas were prepared for SEM observation using a standard protocol. Immediately following exposure, a large (6‐mm) area of the organ of Corti was separated from its attachments to the basilar membrane along a fracture line that follows the outer pillar cells. The separated portion of the organ of Corti is left floating in the scala media with both ends attached to viable portions of the remaining sensory epithelia. Surprisingly, in the denuded areas of the basilar membrane, the inner hair cell surface structure remains comparatively normal during the early post‐exposure times while outer hair cells in the region bordering the main lesion show considerable changes in cilia structure and in the appearance of the reticular lamina‐cuticular plate complex. Scar formation and the absorption/ph...