Starr, who -- with colleagues -- identified the disorder 15 years ago, determined that it is caused by a low cell count in the auditory nerve fibers that connect with, and are within, the ear's cochlea. The study appears in the July issue of Brain.
Starr and his research team's finding was the result of a forensic examination of the auditory nerve ganglion cells within the cochlea of a patient suffering from an inherited form of auditory neuropathy.
A healthy cochlea should have approximately 23,000 nerve cells. Starr's examination, however, found that the right ear's cochlea had only 1,161 nerves cells, and the left ear's cochlea had 1,548 -- each around 5 percent of a healthy cell count. He also confirmed that all other aspects of the ear's hearing mechanism were healthy, including the number of inner- and outer-hair cell receptors. He also confirmed that the auditory nerve connection between the brain and the ear was functioning normally.
"This lets us look into new ways of treating the problem," Starr said, "because it indicates that this disorder is clearly rooted in the auditory nervous system."
He pointed out that the current practice of treating auditory neuropathy is by removing the cochlea and replacing it with an implant that electrically activates the remaining nerve cells. "While an implant can be beneficial for restoring hearing and is fine for the moment, in the future, we may be able to find ways to keep these nerve cells from getting damaged or degenerating, and prevent the need for radical surgery," Starr said. "Through stem cell research, we may even find ways to regenerate these cells."
The study, which was funded by the National Institutes of Health, also links auditory neuropathy to similar neural disorders in the body, where lesions in nerve fibers inhibit sensory and motor functions. "The connection is so strong, many colleagues now question the legitimacy of even calling this condition auditory neuropathy, and I agree," Starr said. "My interest now is in advancing this research and seeing how it can be applied."
According to the Eaton-Peabody Laboratory of Auditory Physiology, which is affiliated with Harvard Medical School, approximately 250,000 people currently suffer from auditory neuropathy.
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