New Study assessed that in contrast to age-related hearing loss, the central hearing loss might share the same mechanism of neurodegeneration with cognitive decline. The study showed that patients with the central hearing loss, or central presbycusis, were twice as likely to have the mild cognitive impairment (MCI) as those with no hearing loss, but there was no association between age-related hearing loss and cognitive impairment.
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. "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." The current analysis included 1604 participants, mean age 73.7 years. Participants underwent numerous assessments, including neurologic, neuropsychological, and otolaryngology assessments.
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.
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 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 can repeat from a list of 10, at an intensity higher than that of the person's PTA, explained Sardone.
They found a positive correlation between the average speech discrimination score and the Mini-Mental State Examination (MMSE) score (P < .0001). 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.
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.
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.
Further 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.