In contrast to age-related hearing loss, central hearing loss may share the same mechanism of neurodegeneration with cognitive decline, new research suggests.
The study showed that patients with central hearing loss, or central presbycusis, were twice as likely to have mild cognitive impairment (MCI) as those with no hearing loss, but there was no association between age-related hearing loss, or peripheral presbycusis, and cognitive impairment.
“Preventing hearing impairment with hearing aids early on could greatly reduce or delay the onset of cognitive neurodegeneration,” lead author, Rodolfo Sardone, AuD, EngD, MPh, research coordinator, The Great Age Study, National Institute of Health and University of Bari, Italy, told Medscape Medical News.
“To do this, we need to improve the assessment of hearing, improve hearing aids fitting, and decrease their costs.”
The study will be presented at the American Academy of Neurology (AAN) 2018 Annual Meeting in Los Angeles, California.
Central Deficit
An estimated one third of the elderly population worldwide has a disabling hearing loss. Many have peripheral presbycusis, which is caused by malfunctions of the inner ear or nerves.
Others may have central presbycusis, where the hearing threshold, as measured by tonal audiometry, is normal and therefore the cochlea is presumably intact.
“The central deficit manifests only in situations of competitive signals like another speaker or background noise,” explained Sardone. “It is a deficit linked to hearing perception rather than to hearing sensation.”
Mounting evidence supports a link between presbycusis and cognitive decline, but few studies have investigated the role of peripheral vs central presbycusis in MCI.
The Great Age Study is an ongoing population-based study on aging conducted in southern Italy. The study’s principal investigator is Giancarlo Logroscino, MD, PhD, director of the Neurodegenerative Diseases Unit and associate professor of neurology, University of Bari.
The current analysis included 1604 participants, mean age 73.7 years. Participants underwent numerous assessments, including neurologic, neuropsychological, and otolaryngology assessments.
Cognitive Assessment
To assess cognition, researchers used Petersen criteria (2004) for MCI. They targeted MCI, said Sardone, because “it’s considered a preclinical phase of dementia, and we’re interested in exploring the onset of neurodegeneration.”
All neurologic assessments were carried out by a clinical neuropsychologist and validated by a neurologist, said Sardone.
Peripheral and central presbycusis were identified by using Gates (2012) criteria, he said.
Peripheral hearing loss was an average pure tone audiometry (PTA) of 40 decibels hearing level (dBHL) or greater.
Central hearing loss was identified in participants with normal hearing threshold (<40 dBHL) who had difficulty hearing amid noise, as assessed by using the ipsilateral competitive message test.
Of the total study participants, 25.5% had peripheral presbycusis and 12.1% had central presbycusis.
This percentage of central hearing loss is “in line with” findings of other studies, including cohorts of the Framingham Heart Study, said Sardone.
About a third (33%) of study participants were diagnosed with MCI.
The analysis showed that compared with those with no hearing loss, participants with central presbycusis were at increased risk for MCI (odds ratio, 2.1, P < .0001).
The study found no association between peripheral presbycusis and cognitive impairment.
Of the 1604 participants in the study, 769 were male. There were no statistically significant differences in results between men and women.
The results suggest that central presbycusis may share the same progressive loss of functioning in brain cells that occurs in cognitive decline, rather than the sensory deprivation that happens with peripheral presbycusis, said Sardone.
Speech Discrimination
The researchers also looked at speech discrimination, or the ability to understand words at certain sound intensities.
Speech discrimination is assessed by using speech audiometry, using a score that ranges from 0% (worst) to 100% (best). The score is the number of words that a person is able to repeat from a list of 10, at an intensity higher than that of the person’s PTA, explained Sardone.
For this analysis, the cutoff for speech discrimination impairment was a percentage of repeated words less than 70% in the best ear at an intensity greater than 40 dBHL above its auditory threshold (PTA) in that ear, he said.
They found a positive correlation between the average speech discrimination score and the Mini-Mental State Examination (MMSE) score (P < .0001).
“We found that higher values of speech discrimination correlate with higher values of MMSE,” said Sardone. “Those who understand the words better have a better global cognitive function.”
Two possible hypotheses might explain the link between central presbycusis and MCI, said Sardone. The first is mechanistic — sensory deprivation leads to weakening of the neural structures of the brainstem and the temporal cortex.
This, he said, reflects the well-known neuroscientific concept of “use it or lose it.”
Another explanation is that hearing impairment starts with vascular degeneration, or some other common cause of neurodegeneration.
“This hypothesis better explains the presence of neuropathological changes, and the presence of speech discrimination disorders even in the absence of peripheral cochlear damage,” said Sardone.
All patients aged 65 years and older should have a hearing test once a year, said Sardone. He recommends that anyone who has difficulty understanding words, has concerns about memory, or has documented cognitive deficits should be tested, too.
Important Implications
Asked to comment, Pinky Agarwal, MD, Evergreen Health, Kirkland, Washington, professor of neurology, University of Washington, and a fellow of the American Academy of Neurology, said the study could have important implications for healthcare and for public policy.
She noted that while about 29% of the US population aged 60 to 69 years has hearing impairment, this increases to 63% among those aged 70 years or over.
Agarwal also pointed to the “high possibility” that cognitive impairment will progress to dementia.
“Prevention of cognitive impairment and dementia has become a public health priority due to its irreversibility and its burden upon individuals, families, and society.”
The current study tries to demonstrate the association between categories of hearing loss and cognitive impairment, but more research is warranted to provide additional insight, said Agarwal.
“Further studies should be conducted to generate conclusive evidence to elicit the causal link between cognitive impairment and hearing loss after controlling for variables such as age, gender, race, education, presence of diabetes, smoking history, and hypertension.”
And more research is needed to examine implications of environmental factors, such as loss of ability to communicate, social isolation, and cognitive impairment, among patients with central hearing loss, said Agarwal.
“If a causal relationship is established, screening for central hearing including speech discrimination may be added to annual wellness examinations.”
The study was supported by the Italian Ministry of Health and the Puglia Regional Government. Sardone and Agarwal have disclosed no relevant financial relationships.
American Academy of Neurology (AAN) 2018 Annual Meeting. Abstract 2928 (P2.179). To be presented April 23, 2018.
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