HeadWay
May 1, 2014

Lin and colleagues used information from the ongoing Baltimore Longitudinal Study of Aging to compare brain changes over time between adults with normal hearing and adults with impaired hearing. The Baltimore Longitudinal Study of Aging was started in 1958 by the National Institute on Aging to track various health factors in thousands of men and women.

Previous research from other studies had linked hearing loss with marked differences in brain structure compared with those with normal hearing, in both humans and animals. In particular, structures that process information from sound tended to be smaller in those with impaired hearing. It was unknown, however, whether these differences occurred before or after hearing loss.

As part of the Baltimore Longitudinal Study of Aging, 126 participants underwent yearly magnetic resonance imaging (MRI) to track brain changes for up to 10 years. Each also had complete physicals at the time of the first MRI in 1994, including hearing tests. At the starting point, 75 had normal hearing and 51 had impaired hearing, with at least a 25-decibel loss.

After analyzing their MRIs over the following years, Lin and his colleagues found that participants whose hearing was impaired at the start of the sub-study had accelerated rates of brain atrophy compared to those with normal hearing. Overall, the team showed that those with impaired hearing lost more than an additional cubic centimeter of brain tissue each year compared with those with normal hearing. Those with impaired hearing also had significantly more shrinkage in particular regions, including the superior, middle and inferior temporal gyri, brain structures responsible for processing sound and speech.

That such structures are affected in those with hearing loss wasn’t a surprise, says Lin—the shrinkage might simply be a consequence of an “impoverished” auditory cortex, which could become atrophied from lack of stimulation. However, he adds, these structures don’t work in isolation, and their responsibilities don’t end at sorting out sounds and language. The middle and inferior temporal gyri, for example, also play roles in memory and sensory integration and have been shown to be involved in the early stages of mild cognitive impairment and Alzheimer’s disease.

“Our results suggest that hearing loss could be another ‘hit’ on the brain in many ways,” Lin explains.

The study also gives some urgency to treating hearing loss. “If hearing loss is potentially contributing to these differences we’re seeing on MRI,” Lin says, “you want to treat it before these brain structural changes take place.”  

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