Age-Related Hearing Loss: A Loss of Voltage, Not Hair Cells

ABSTRACT

One of the most significant results from animal and human studies of age-related hearing loss involves degeneration of the lateral wall (stria vascularis and spiral ligament) of the cochlea, which is responsible for generating electrochemical gradients and regulating ion homeostasis. Accompanying this degeneration is a decrease in the endocochlear potential (EP), which is an 80- to 100-mV dc resting potential located in scala media. Reductions in the magnitude of the EP affect the function of the cochlear amplifier, which provides a gain of ~20 dB at low frequencies, increasing to ~60 dB or greater as frequency increases from ~1 to 8 kHz. Indeed, age-related threshold shifts observed in animals and humans can be accounted for almost totally by age-related degeneration of the stria vascularis/spiral ligament with attendant reductions in the EP and in the gain of the cochlear amplifier. Although considerable support exists for the involvement of strial vasculature in degeneration of the stria vascularis and the associated declines in EP and cochlear amplifier, the question of what constitutes the initial injury remains unclear. It is tempting to speculate that atrophy of strial marginal or intermediate cells, or both, occurs secondarily to vascular insufficiency resulting from capillary necrosis; however, the reverse could also be true. In gerbil and human, age-related degeneration of the auditory nerve may be the second most distinctive feature of age-related hearing loss. Strikingly absent in gerbil data and in some human data are age-related losses of outer and inner hair cells, except in the most basal and apical regions of the cochlea. These results and others suggest that age-related hearing loss should be viewed as a vascular, metabolic, neural hearing loss rather than a sensory hearing loss.

Age-related hearing loss - presbyacusis

REFERENCES

• 1 Mills J H, Schmiedt R A, Kulish L F. Age-related changes in auditory potentials of Mongolian gerbil.  Hear Res. 1990;  46 301-310
• 2 Gates G A, Couropmitree N N, Myers R H. Genetic associations in age-related hearing thresholds.  Arch Otolaryngol Head Neck Surg. 1999;  125 654-659
• 3 Schuknecht H F. Pathology of the Ear. Cambridge, MA; Harvard University Press 1974
• 4 Schuknecht H F, Gacek M R. Cochlear pathology in presbycusis.  Ann Otol Rhinol Laryngol. 1993;  102(suppl 158) 1-16
• 5 Tarnowski B, Schmiedt R A, Hellstrom L, Lee F S, Adams J C. Age-related changes in cochleas of Mongolian gerbils.  Hear Res. 1991;  54 123-134
• 6 Schulte B A, Gratton M A, Smythe N. Morphometric analysis of spiral ganglion neurons in young and old gerbils raised in quiet. Nineteenth Association for Research in Otolaryngology Meeting 1996 Abs 640:160
• 7 Hellstrom L I, Schmiedt R A. Compound action potential input/output functions in young and quiet-aged gerbils.  Hear Res. 1990;  50 163-174
• 8 Boettcher F A, Mills J H, Norton B L. Age-related changes in auditory evoked potentials of gerbils. I. Response amplitudes.  Hear Res. 1993;  71 137-145
• 9 Mills J H, Matthews L J, Schmiedt R A, Boettcher F A, Dubno J R, Ahlstrom J B. Masking and aging: III. Human and gerbil neural input/output functions. Acoustical Society of America, 129th Meeting 1995 Washington, DC;
• 10 Schmiedt R A, Mills J H, Adams J C. Tuning and suppression in auditory nerve fibers of aged gerbils raised in quiet or noise.  Hear Res. 1990;  45 221-236
• 11 Boettcher F A, Gratton M A, Schmiedt R A. Effects of age and noise on hearing.  Occup Med. 1995;  10 577-591
• 12 Gates G A, Mills D M, Nam B-H, D'Agostino R, Rubel E W. Effects of age on the distortion product otoacoustic emission growth functions.  Hear Res. 2002;  163 53-60
• 13 Spicer S S, Schulte B A. Differentiation of inner ear fibrocytes according to their ion transport related activity.  Hear Res. 1991;  56 53-64
• 14 Schulte B A, Schmiedt R A. Na,K-ATPase and endocochlear potentials decline with age in quiet-reared gerbils.  Hear Res. 1992;  61 35-46
• 15 Gratton M A, Schmiedt R A, Schulte B A. Age-related decreases in endocochlear potential are associated with vascular abnormalities in the stria vascularis.  Hear Res. 1996;  102 181-190
• 16 Spicer S S, Schulte B A. The fine structure of spiral ligament cells relates to ion return to the stria and varies with place-frequency.  Hear Res. 1996;  100 80-100
• 17 Spicer S S, Gratton M A, Schulte B A. Expression patterns of ion transport enzymes in spiral ligament fibrocytes change in relation to strial atrophy in the aged gerbil cochlea.  Hear Res. 1997;  111 93-102
• 18 Schulte B A. Homeostasis of the inner ear. In: Handbook of the Senses. Philadelphia, PA; Elsevier In Press
• 19 Schmiedt R A. Cochlear potentials in quiet-aged gerbils: does the aging cochlea need a jump start?. In: Verrillo R Sensory Research: Multimodal Perspectives. Hillsdale, NJ; Erlbaum 1993: 91-104
• 20 Johnson T, Carson M, Spelman F, Corbett S, Clopton B, Schmiedt R A. Electrodes and stimulators for strial presbycusis. Presented at: the Neural Interfaces Workshop, NIDCD/NINDS November 2004 Washington, DC;
• 21 Evans E F, Klinke R. The effects of intracochlear and systematic furosemide on the properties of single cochlear fibers in the cat.  J Physiol. 1982;  331 409-428
• 22 Sewell W. The effects of furosemide on the endocochlear potential and auditory nerve-fiber tuning curves in cats.  Hear Res. 1984;  14 305-314
• 23 Mills D M, Rubel E W. Variation of distortion product otoacoustic emissions with furosemide injection.  Hear Res. 1994;  77 183-199
• 24 Schmiedt R A, Lang H, Okamura H, Schulte B A. Effects of furosemide applied chronically to the round window: a model of metabolic presbycusis.  J Neurosci. 2002;  22 9643-9650
• 25 Davis H. An active process in cochlear mechanics.  Hear Res. 1983;  9 79-90
• 26 Jerger J, Chmiel R, Stach B, Spretjnak M. Gender affects audiometric shape in presbyacusis.  J Am Acad Audiol. 1993;  4 42-49
• 27 Dubno J R, Mills J H, Ahlstrom J B, Matthews L J. Masking and aging: II. human behavioral and auditory brainstem response thresholds. Presented at: The Acoustical Society of America, 129th Meeting 1995 Washington, DC;