The Hearing Review Cross-Currents are staff-reviewed articles, features and news items that relate to hearing issues from a variety of sources and disciplines. If you are interested in a particular item, we encourage you to obtain a copy of the cited publication. —KES

New ion channel may yield clues to hearing
M Barinaga
While neuroscience and research on hair cell function has progressed substantially in the last decade, there are still a great number of questions about the functioning of the cilia that move in response to vibrations within the cochlea. One of the most intriguing questions—taken up by this excellent article by Maria Barinaga—is how hair cells operate and specifically the nature of the hair cell ion channel.

Hair cell channels are particularly interesting because they have few similar counterparts in nature. Their movements in response to the motion of the cilia is incredibly fast—opening and closing within microseconds compared to the milliseconds of biochemically activated channels. Hair cells can also detect incredibly tiny movements, deflections as small as an atom’s width. To put that in perspective, movements of the cilia that can be detected are by the hair cell are “akin to bending the tip of the Eiffel Tower by the width of your thumb.” Hair cells are also able to detect the direction of the stimulus, and the channels can also somehow register impulses in addition to constant deflections—in other words, they can “acknowledge” a constant stimulus like background noise and, at the same time, pick up and focus on other fluctuations (for example, meaningful impulses like speech) in the incoming vibrations.

Charles Zuker and researchers at the Univ. of California-San Diego have been conducting experiments using cloned ion channel from the neurons that underlie the bristles of fruit flies. These bristles respond to mechanical forces that the fly encounters in its environment, and Zuker’s team has demonstrated that the bristle neurons convert movement into electrical impulses much like hair cells in the cochlea. If the bristles do function similarly and their ion channels are also structurally similar, the cloned gene could provide insights into the ion channels of human hair cells, which in turn could shed light on the causes of hereditary deafness, hearing loss and other hearing-related problems.

Seven years ago, Zuker and Maurice Kernan (now at State Univ. of New York-Stony Brook) screened flies that were defective in their sense of touch, with the hope that these flies might have mutations specific to the touch response itself. In separate research, Richard Walker of Zuker’s lab conducted experiments to see if the bristle system was a good model for human hair cell function. Walker found that the cells showed fast responses to the smallest movements, as well as directional sensitivity—very similar to human hair cells. Walker then applied his research methods to Zuker and Kernan’s mutant flies to uncover a gene called nompC (short for “no mechanicoreceptor potential C”). Mutations in this gene either stopped the channel from opening and closing or attenuated operation of the ion channel, allowing less electrical current through than normal. The genetic sequence of nonpC has been discovered to encode structural proteins that form ion channels, adding further credance to its link with hair cells.

The nompC gene also provides clues to how ion channels might function. The associated protein has an exceptional ability to anchor the channel to the hair cell “skeleton.” This lends further support to the importance of nompC, because the opening of the channel and its changing of shape in response to cilia movement is dependent on the channel being anchored to the cell.

There is a long way to go, however, before a direct relationship between the fly’s bristle function can be directly equated with the function of hair cells in the human cochlea. A group headed by James Hudspeth of The Rockefeller Univ. in New York City is using the Zuker group’s cloned gene to research expression of similar genes in chickens. The ultimate hope is that a link can be established, or eventually a link can be found between a human version of the nompC gene and hereditary deafness or other genetically related hearing losses.

March 24, 20000; V. 287: 2132-33

Hear, Hear
N Seppa
Margaretha Casselbrant, an otolaryngologist with the Children’s Hospital of Pittsburgh, has been collecting data on otitis media (OM) in fraternal and identical twins for 17 years. Along with her colleagues, she is now analyzing this data which encompasses 168 sets of twins and seven sets of triplets who were examined for OM at least every two months and whenever they had an earache. After analyzing 14 years of the data, they have determined that identical twins and triplets are twice as likely to get ear infections simultaneously than fraternal ones. They also found that a baby whose identical twin/triplet has an ear infection is six to eight times as likely to contract OM as a child of the same age outside their home.

Other studies have been made relative to OM and genetics, with some showing a connection and others showing no connection. The only other major study similar to that of Casselbrant et al.’s concluded that genetics accounted for half to three-quarters of a child’s susceptibility. Casselbrant’s team estimates that three-quarters of susceptibility can be attributed to genetics.

Armed with this information, physicians may be able to treat some of their patients more effectively. Peter Casano, an otolaryngologist at the Univ. of Mississippi Medical Center in Jackson says, “Now, if you see someone whose older brother or sister needed tubes, you may not have to wait quite as long [to decide] that they need aggressive treatment.” The article points out that physicians may also be able to recommend positive strategies for parents of high-risk children, including breast-feeding, avoidance of cigarette smoke and keeping the children out of large day-care centers where they can be exposed to viruses. Additionally, genetic screening in the future may be able to predict if a child is at greater risk for ear infections.

Science News
Feb. 26, 2000, V. 157: 136

Technology aids hearing impaired and Hearing aids: New technology comes to the aid of hearing impaired
C Tevis
Does your office/practice see its fair share of farmers who come in with hearing problems? If so, this issue of Successful Farming is one that you should obtain.

The first of the two articles explains the results of the National Council on the Aging (NCOA) study and its implications on a person’s quality of life, social well-being and relationships with loved ones (see the article by Sergei Kochkin and Carole Rogin in the Jan. 2000 issue of HR). It also interviews audiologist Angela Johnson of Ames, IA, who says, “Many farmers have a noticeable loss in their left ear. But if they have a loss in both ears, one hearing aid won’t solve the problems of noise, clarity and sound location.” The article contains a chart on the reasons people don’t use hearing aids and explains that “farmers need to keep a supply of earplugs nearby, and ask their physicians to add a hearing test to their annual exams.”

The second part of the feature is a product guide for purchasing hearing instruments that includes products from Starkey, Argosy, ReSound, Micro-Tech, Phonak, Danavox, Siemens and Telex—with instruments of all styles (CICs to BTEs), technologies (conventional to DSP) and price ranges ($395-$4750). It describes the potential benefits of digital instruments at length, the different styles of instruments and makes a good case for binaural fittings. Tips provided for making purchase decisions include the assessment of qualifications and years of experience of the hearing care provider; location and cost of after-sale services; warranty period coverage and terms of the transaction; right of return and the professional services being defined in the sale.

The article also offers a “Glossary of Terms” and side-by-side photos with descriptions of the hearing instruments mentioned above.

Successful Farming
December 1999: 51.

The Big Bang!
J Carmichel & E Carmichel
Jim Carmichel, a long-time writer on sport shooting, opens this article with a description of his own hearing loss and tinnitus—how he can’t hear in noisy environments, his loss of high frequency sound perception and his over-sensitivity to loud noises. He then goes on to detail what led to it: a .22 rifle as a farm boy, a stint on the Army Rifle Team and his university rifle team, and a lifetime full of shooting guns, including “one month-long marathon of ammo testing [that] I blasted some 30,000 rounds of .45 ACP ammo. That’s about when the occasional ringing in my ears became a steady hum.” His story is essentially the urban legend horror stories that hair cells would tell around a campfire if they could.

What follows is an entertaining and informative account of how Carmichel and his son, Eric, tested numerous rifles using a sound level meter and a special formula that Eric (an engineer) devised. It does a good job of pointing out the threat of rifles to hearing health and looks at the effect that a BOSS muzzle brake has on the sound. It has to be said, however, from a hearing health standpoint the two men spend a lot of time figuring out why their sound level meters didn’t show a distinction between the muzzled and unmuzzled conditions—a difference of 4 dB (156 dB vs. 160 dB). While this is of interest to the rifle enthusiast, the message should be that 156 dB is still just plain loud and damaging to a person’s ears.

The father and son team also tested personal protection devices via probe mic measurements. Using a peak pressure level (PPL) of 134 dB outside the ear (i.e., the unattenuated rifle shot), foam insert earplugs yielded 128 dB PPL, ear muffs 111 dB, and the combined protection of the two was 29 dB. Electronic shooting protectors were not tested.

The use of both foam insert earplugs and earmuffs are strongly recommended by the authors for both shooting and for any noisy occupational situations. Jim Carmichel also attributes much of what shooting enthusiasts call “recoil” as pain from muzzle blasts: “Whenever someone asks me how to reduce the recoil of their shotguns and high-powered rifles, my unfailing response is, ‘With the best hearing protection you can get.’ This may sound strange, but the pain of muzzle blasts tends to get confused with the pain of recoil. Flinching, trigger-jerking and other bad shooting habits that are usually attributed to recoil may just as likely be caused by painful exposure to muzzle blasts.”

He concludes by espousing some shooting etiquette: “A shooter who fires a gun at a public range without first determining that all persons nearby are using ear protection may in fact inflict lasting damage to the hearing of an innocent bystander. That is not only rude but actually unsafe, and such yahoos should be kicked off the range. Likewise, bystanders who don’t wear ear protection when advised to do so are too dumb to be allowed around guns. It’s grand to see parents taking their children to the shooting range, but not when the kiddies put their fingers in their ears and grimace every time Daddy’s gun goes bang. That’s why I always carry an extra supply of ear protectors and pass them around to everyone who needs them…”

Outdoor Life
April 2000: 68-73, 80

Moviegoers get a fair hearing
This news item relates to the Americans with Disabilities Act (ADA) and provision of assistive listening devices (ALDs) in movie theaters. The ADA requires public assembly places built or renovated after 1992 with seating for 50 or more people to provide ALDs for at least 4% of the people (it excludes places of worship).

The article details the use and function of a typical ALD, and also explains telecoils for those movie-goers who wear hearing instruments. Brenda Battat (now executive director of Self Help for Hard of Hearing People) and Donna Sorkin (now executive director of the A.G. Bell Assn.) are quoted within the article. They advocate that hearing-impaired people should utilize these services and make sure that theaters use and maintain these systems for their patrons: “It’s wise to call or visit the theater in advance to arrange for a test run of the device. If the device isn’t working properly, you have the right to ask that it be fixed. If no device exists, let the manager know the theater is not in compliance with the law,” says Sorkin.

Consumer Reports
February 1999: 10