As we age, our brains gradually get smaller. It can't be helped.
Human brain size peaks when we are in our 20s, and from there, it's steady, sustained shrinkage.
But for older adults with impaired hearing, brain tissue loss happens faster than it does for those with normal hearing, according to a recent study by researchers from Johns Hopkins and the National Institute on Aging. The findings, which will be published in an upcoming issue of Neuroimage, add to a growing list of health consequences associated with hearing loss, including increased risk of dementia, falls, hospitalizations, and diminished physical and mental health overall.
"Our results suggest that hearing loss could be another 'hit' on the brain in many ways," said study leader Frank Lin, an assistant professor at the Johns Hopkins schools of medicine and public health.
For the study, Lin and colleagues used information from the ongoing Baltimore Longitudinal Study of Aging—started in 1958 by the National Institute on Aging to track various health factors in thousands of men and women—to compare brain changes over time between adults with normal hearing and adults with hearing deficits.
Previous research from other studies had linked hearing loss with marked differences in brain structure compared to those with normal hearing, both in humans and animals. In particular, structures that process information from sound tended to be smaller in size in people and animals with impaired hearing.
It was unknown, however, whether these structural changes occurred before or after hearing loss set in.
In analyzing MRIs from participants in the longitudinal study, researchers found that those with impaired hearing lost more than an additional cubic centimeter of brain tissue each year compared with those with normal hearing. They also found a correlation between hearing loss and brain atrophy.
Those with impaired hearing had significantly more shrinkage in particular regions, including the superior, middle, and inferior temporal gyri, brain structures responsible for processing sound and speech. This wasn't a surprise, Lin said, but he added that 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.
Lin said he hopes to eventually examine whether treating hearing loss early can reduce the risk of associated health problems.
"If you want to address hearing loss well, you want to do it sooner rather than later," Lin said. "If hearing loss is potentially contributing to these differences we're seeing on MRI, you want to treat it before these brain structural changes take place."
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