The study in the NIA-sponsored Baltimore Longitudinal Study of Aging was one of Dr. Lin’s first to show such a strong link between hearing loss and dementia (Arch Neurol. 2011;68:214-220). The study included 639 participants, most of whom were between 60 and 80 years of age. At baseline, none of the participants had any evidence of cognitive impairment, although some had hearing loss. During a median follow-up of 11.9 years, 58 cases of incident all-cause dementia were diagnosed, of which 37 were considered to be Alzheimer’s disease. The risk for participants developing dementia “increased linearly with the severity of hearing loss,” Dr. Lin said. Compared with normal hearing, the hazard ratio (risk) for dementia was 1.89 (1.00–3.58) for mild hearing loss, 3.00 (1.43–6.30) for moderate hearing loss and 4.94 (1.09–22.40) for severe hearing loss. These results were supported by a subsequent study of nearly 2,000 patients (JAMA Intern Med. 2013;173:293-299), and in another study by an independent study group in the U.K. (Neurology. 2012;1583-1590).
An argument could be made that some other, yet-to-be-determined physiologic factor may serve as a common cause for both hearing loss and dementia. But Dr. Lin believes that the strength of the epidemiologic evidence suggests otherwise. “If we saw a relatively small effect, it would be logical to dismiss the link between hearing and cognition as just being part of some natural aging process or from unaccounted cardiovascular risk factors. But when you see that strong of a link, it’s unlikely that any degree of confounding by other physiologic risk factors could fully argue away that effect,” Dr. Lin said. “Clearly, something else is going on.”
What Interventions Work?
Dr. Lin acknowledged that a key question remains: Can cognitive declines be reduced by hearing aids and other rehabilitative interventions? “Scientifically, we just have no clue,” he said. “It may be that those interventions are too little, too late. However, from a clinical standpoint—based on my own practice experience—these interventions, whether a hearing aid or cochlear implant, do seem to make a difference.”