Alzheimer’s disease (AD) not only dulls the mind, but it is also associated with a dulling of the senses that provide our brains with information about the world around us. Experts have found that any of the five senses—hearing, vision, taste, smell, and even touch—may show signs of negative changes in association with AD progression, experts have found.
“An association between sensory impairment and Alzheimer’s disease has been established in a number of research studies,” says MGH neurologist Mark W. Albers, MD, PhD, an expert in neurodegeneration and the olfactory system in AD. “There appears to be a perceptual aspect to many of these sensory problems that involves a deterioration in the brain’s ability to process the raw information provided to it by the sensory end organ—the eye, ear, nose, etc. But it is unclear whether sensory problems precede AD or the reverse, or whether both conditions are related to an underlying cause. Understanding the interrelationship between the two conditions may offer insights into brain functioning and perhaps lead to new therapies for AD.”
In a recent study examining the interconnection between cognitive decline and hearing loss—which affects as many as two-thirds of adults age 70 and older—researchers assessed hearing and cognitive performance in 1,984 older adults and followed participants over a six-year period. The scientists found that participants who had suffered hearing loss experienced 30 to 40 percent greater cognitive decline per year than those who had normal hearing. Rates of cognitive decline were directly related to the level of a participant’s hearing loss, according to a report on the research published in the February 25, 2013 issue of JAMA Internal Medicine. Earlier research found that AD risk in older adults increased by 20 percent for every additional loss of 10 decibels of hearing capacity.
“There are many plausible explanations for this synergy between hearing loss and AD,” says Dr. Albers. “For example, AD may affect nerve pathways that carry auditory information to the brain, or disturb a person’s ability to make sense of what he or she hears. Hearing loss may compromise mental processes because cognitive resources must be redirected from higher thinking such as decision-making and memory to the more elementary task of interpreting sounds. The cognitive decline may be associated with social isolation that often results from hearing impairment, or with the reduction in the input of sensory information from hearing that makes more complex thinking and memory possible.
“Whatever the link between hearing impairment and neurodegenerative disease, it’s clear that the two systems interact in ways that hasten cognitive decline. The research suggests that it may be possible to slow or prevent cognitive decline by taking advantage of hearing aids, telephone and television amplifiers, and other assistive devices to help counteract hearing loss so that it does not interfere with mental engagement.”
The Other Senses
Impairment of other sensory capacities is also associated with AD, and as with hearing, researchers are still working to understand the interaction between sensory impairment and disease development. Sensory impairment of any type limits the amount of information reaching the brain and reduces stimulation from the environment—a factor that could well contribute to the development of AD. On the other hand, the sensory impairment may be a symptom of disease, the result of neurodegeneraton that makes it more difficult to correctly process incoming sensory information.
- Vision: AD is known to cause increasing levels of difficulty with visual attention and memory, perception of color, depth, and motion, discriminating figures from their backgrounds, and in more serious disease, object and facial recognition and visual acuity. Some of these changes may show up years before AD is diagnosed. They are thought to be caused by disease-related deterioration of the parietal lobe and the medial temporal lobe.
- Olfaction (smell): There is clear evidence that tau fibrils and clumps of toxic beta-amyloid—the hallmarks of AD—build up and lead to neuronal loss in the nose and in the neural circuits involved in olfaction. A loss of the ability to smell certain odors is so closely associated with AD that a “smell test” has even been proposed as a diagnostic screen for the disease. “It is known that individuals with AD have olfactory deficits, and research has shown that people with mild cognitive impairment who also have olfactory deficits have a four-and-a-half times greater risk of developing AD,” Dr. Albers says. “We’re now trying to develop a test that identifies certain olfactory deficits to predict early AD in apparently healthy people, and initial results are promising.”
- Taste: About 70 percent of taste is based on the sense of smell, and is thus influenced by changes in the olfactory system related to AD. People with AD often undergo changes in how they experience flavors, resulting in a shift in food preferences.
- Touch: Not much is known about how AD affects the sense of touch, but anecdotal evidence suggests that individuals with advanced disease may lose their ability to interpret temperature changes and thus fail to react quickly to intense temperatures, such as heat from a radiator.
“It has been fairly well established that if a person has a sensory deficit combined with memory symptoms, the risk of developing Alzheimer’s disease is significantly increased,” says Dr. Albers. “But currently, much less is known about the ability of a sensory impairment alone to predict eventual development of AD. If we can establish that some or all of these changes represent the very early signs of AD, we can screen for them, begin treatment right away, and improve our chances of defeating this disease.
“Meanwhile, it’s important to ensure that your brain remains active and responsive to external stimuli by seeking help for sensory impairment as soon as possible, so that treatment can help prevent the isolation and extra demands on the brain that might lead to cognitive problems. Maximizing sensory input is essential at any age, but it’s especially important in older individuals.”