By James Kaltenbach, PhD

Hyperacusis is a condition characterized by a heightened sensitivity to sound that can result in patients having difficulty tolerating many sounds. Those with hyperacusis perceive environmental sounds as being louder and/or more annoying than what normal-hearing people perceive. Sounds that are particularly bothersome include automobile traffic, sirens and other background noise, but even the sounds of other people’s voices or music can create much discomfort. Patients with severe hyperacusis can have such a low tolerance for sound that they confine themselves to quiet rooms.

Our Study of Hyperacusis in a Rodent Model

As is the case with other hearing disorders, the development of effective therapies for hyperacusis will require a better understanding of its mechanisms. To achieve this, establishing an animal model of the chronic form of hyperacusis is necessary.  Recently, researchers in Cleveland Clinic’s Auditory Neurobiology Laboratory discovered that animals exposed to intense sound develop long-lasting hyperacusis-like “enhancements” in their responses to noise. These enhancements occurred in two ways:

• Animals that had been exposed to intense sound were still hypersensitive to noise a few weeks later. This hypersensitivity manifested as a heightened startle response when brief bursts of noise were presented. When we compared measures of the startle amplitude of the sound-exposed animals with those of unexposed control animals, we found a large increase in the amplitude of the startle in the exposed group when the sound pressure level exceeded 105 dB (Figure 1).

• The exposed animals exhibited an enhanced sensitivity to background noise. The addition of background noise resulted in a suppression of the startle response, as though the animals had become sensitized to background noise. In the normal-hearing animals, the addition of background noise to the startle stimulus had no significant effect on the startle response.

Both types of enhancements were observed over several months of testing, so it appears that the sensitization to noise in the exposed animals represents a chronic condition. Therefore, the question of whether these enhancements do indeed represent a chronic form of hyperacusis is of central importance. The answer may lie in some recent studies of hyperacusis and tinnitus in humans.

A Link to Tinnitus

The causes of hyperacusis are much the same as the causes of tinnitus, with which it is often associated. They include exposure to excessive noise or ototoxic drugs, in addition to other insults that induce injury to the inner ear. Two recent studies are of interest:

• Dauman and Bouscau-Faure found that the incidence of hyperacusis in patients with tinnitus was almost 80 percent. Based on the strong association, these two disorders are likely interrelated.

• Fournier and Hébert studied acoustic startle responses in tinnitus patients and found that most of them displayed enhanced startle responses, which were analogous to what we found in our study of sound-exposed animals.

Continued Quest for an Animal Model

Further work will be necessary to verify that an enhanced startle response is a characteristic of humans with hyperacusis, but for now, the parallel is quite striking and gives us reason to persevere with studies of startle in animals. Establishment of an animal model of chronic hyperacusis would move us a step closer to understanding its underlying mechanisms, which is ultimately the key to developing appropriate treatments.

Dr. Kaltenbach (kaltenj@ccf.org) is Director of Otology Research and Head of the Auditory Neurobiology Laboratory in the Head & Neck Institute.