Age-related hearing loss: is it a preventable condition?

Abstract

Numerous techniques have been tested to attempt to prevent the onset or progression of age-related hearing loss (ARHL): raising the animals in an augmented acoustic environment (used successfully in mouse and rat models), enhancing the antioxidant defenses with exogenous antioxidant treatments (used with mixed results in mouse and rat models), raising the animals with a calorie restricted diet (used successfully in mouse and rat models), restoring lost endocochlear potential voltage with exogenous electrical stimulation (used successfully in the Mongolian gerbil model), and hypothetical enhancement of outer hair cell electromotility with salicylate therapy. Studies of human ARHL have revealed a set of unique hearing loss configurations with unique underlying pathologies. Animal research has developed models for the different forms of age-related peripheral pathology. Using the animal models, different techniques for prevention of ARHL have been developed and tested. The current review discusses ARHL patterns in humans and animal models, followed by discussions of the different prevention techniques.

Figure 1

Audiograms that are representative of hearing loss patterns that are consistent with three different forms of Schuknecht’s category of ARHL: A) Sensory; B) Neural; C) Strial.

Figure 2

Comparison of the age-related ABR threshold in the F344/NHsd rat to the age-related hearing loss in humans as indexed by ISO (1999) Annex A.

Figure 3

Mean DPOAE amplitude input-output functions at 12 and 16 kHz for 2, 17, and 24 month-old F344/NHsd rats. Error bars are +/− 1 SEM.

Figure 4

Mean OHC cochleograms for F344/NHsd rats aged 3 and 20–27 months. Dying and missing OHC were grouped together under the label “deteriorated OHC” as presented on the y-axis. Reprinted with permission from Bielefeld et al., (2008) in Hearing Research.

Figure 5

Prestin changes in 2 month-old and 24 month-old F344NHsd rats. Panels A, B, and C are from three separate pairs of animals’ organ of Corti samples. Each pair includes on 2 month-old and one 24 month-old rat’s organ of Corti sample. The panels display much lower gray scale staining intensities for prestin in the 24 month-old rats (dark circles) than the 2 month-old rats (open circles). Panels D and E display representative organ of Corti sections for a 2 month-old (D) and 24 month-old (E) rat. The green fluorescence is prestin staining. Clear, consistent prestin staining can be seen in the outer walls of the OHCs in the young animal while very little staining in visible in the intact OHCs of the older animal. Asterisks in Panel E represent locations of missing OHCs. Most of the OHCs in the sample are present, but have reduced prestin staining intensity (From Chen et al., 2009).

Figure 6

Results of AAE treatment in aging F344/NHsd rats. Treatment was begun at 16 months of age. Panel A is ABR threshold shift after 2 weeks of treatment. Panel B is after 6 weeks. Panel C is after 9 weeks. Panel D is after 13 weeks. Dark circles are the mean threshold shifts in rats treated with AAE. Open circles are the control rats. The control rats underwent progressive ABR threshold shift over the 13-week period, while the AAE-treats did not show the threshold shift. Reprinted with permission from Tanaka et al. (2009) and Laryngoscope.