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January 7, 2005

Beyond Animal Research

From: PCRM, DC - Jan 7, 2005

By Jonathan Balcombe, Ph.D.
January 2005

The Guinea Pigs Deserve a Hearing

Many years ago I remember seeing in a physiology textbook two photographs of a guinea pig's inner ear, taken under a microscope before and after exposure to destructively loud noise. The first image showed the sensitive cochlear hairs arranged in neat rows; in the second they looked like a stand of trees flattened by a violent hurricane.

Unfortunately, such experiments are still being done today. A Boolean search of "loud noise AND hearing loss" on the National Institutes of Health's PubMed database revealed about 35 animal studies between 1976 and 2004, most of which involved exposing the subjects to loud noise. Guinea pigs were the most often-used species (20 studies), then cats (8), monkeys (2), and one each of mice, chinchillas, pigeons, and lizards. A modified search would no doubt turn up other studies.

Here's a brief sampling from the animal protocols, spanning two decades:
â–ª Guinea pigs were exposed to loud pure tones. The effects on their inner ear structure and function were assessed at different times thereafter.1
â–ª Blood flow to the inner ear was measured in 15 guinea pigs using an implanted probe during delivery of sounds up to 120 dB intensity.2
â–ª The brains of 45 guinea pigs were electrically monitored during exposure to loud noise. The animals were then killed and their brain tissue analyzed for potential resistance to oxygen toxicity.3

The study of hearing loss and its treatment lends well to human clinical methods, because human patients can verbally report their hearing experiences to an experimenter. Here are three recent examples of clinical studies turned up by my search:
â–ª Three hearing aid circuits were compared for various parameters, including overall sound quality, using 360 hearing-impaired human volunteers.4
â–ª Nine human patients with deafness in one ear were evaluated for sound localization and speech perception, first with a conventional hearing aid, then with a bone-anchored hearing aid (BAHA) implanted in the deaf ear.5
â–ª Male aircraft factory workers (n=94) were screened to examine possible genotypic, demographic (age) and behavioral (smoking) predispositions to noise-induced hearing loss (NIHL).6

Other clinical approaches included preventive studies, individual case studies, and surgical interventions (including veterinary).

Overall, there appears to be little connection between the animal and human research, and it's unclear to what degree, if any, the animal studies inform clinicians. One thing is certain: the animal research—unlike the clinical—is not performed to the benefit of its subjects.

It makes far more sense to try to prevent hearing loss through public education. Several of the clinical studies I reviewed assessed the effects of loud music on party-goers and concert-goers and the failures of at-risk workers to deploy noise-protective equipment. Convincing people at rock concerts to wear earplugs, for instance, would be a good start.

1. Robertson D. Combined electrophysiology and ultrastructure of acoustic trauma in the guinea pig cochlea. Arch Otorhinolaryngol. 1981;230(3):257-63.
2. Okamoto A, Hasegawa M, Tamura T, Homma T, Komatsuzaki A. Effects of frequency and intensity of sound on cochlear blood flow. Acta Otolaryngol. 1992;112(1):59-64.
3. Shupak A, Tal D, Pratt H, Sharoni Z, Hochman A. Attenuation of cerebral oxygen toxicity by sound conditioning. Otol Neurotol. 2004 Mar;25(2):186-92.
4. Noffsinger D, Haskell GB, Larson VD, Williams DW, Wilson E, Plunkett S, Kenworthy D. uality rating test of hearing aid benefit in the NIDCD/VA Clinical Trial. Ear Hear. 2002 Aug;23(4):291-300.
5. Bosman AJ, Hol MK, Snik AF, Mylanus EA, Cremers CW. Bone-anchored hearing aids in unilateral inner ear deafness. Acta Otolaryngol. 2003 Jan;123(2):258-60.
6. Fortunato G, Marciano E, Zarrilli F, Mazzaccara C, Intrieri M, Calcagno G, Vitale DF, La Manna P, Saulino C, Marcelli V, Sacchetti L. Paraoxonase and superoxide dismutase gene polymorphisms and noise-induced hearing loss. Clin Chem. 2004 Nov;50(11):2012-8. Epub 2004 Sep 02.

© 2005 Physicians Committee for Responsible Medicine