- Multiple impairments in sensory function of sight, hearing, smell, or taste, are a strong indicator of cognitive decline
- Among all the senses, the sense of smell, or olfaction, has a stronger association with dementia than touch, hearing or vision
- Decline in the sense of smell by 10 percent increases the chance of developing dementia by 19 percent
Sensory functions like sight, hearing, touch, and smell are closely linked to cognition. Aged people, who can identify smells like roses, turpentine, paint-thinner, and lemons, and have other senses retained, have only half the risk of developing dementia, compared to their peers suffering from sensory decline.
Sense of smell is a pre-clinical indicator of dementia, while hearing and vision may have more of a role in promoting dementia.
"Sensory impairments could be due to underlying neurodegeneration or the same disease processes as those affecting cognition, such as stroke," said first author Willa Brenowitz, PhD, of the UCSF Department of Psychiatry and Behavioral Sciences, and the Weill Institute for Neurosciences. "Alternatively, sensory impairments, particularly hearing and vision, may accelerate cognitive decline, either directly impacting cognition or indirectly by increasing social isolation, poor mobility, and adverse mental health."
For the study, researchers from the UC San Francisco, tracked the cognitive functions of 1,800 participants in their seventies for a period of up to 10 years. The participants were recruited from a random sample of Medicare-eligible adults in the Health, Aging and Body Composition study.
Cognitive testing at the start of the study showed that participants were dementia-free, but 328 participants (18 percent) developed the condition over the course of the study.
Dementia is defined in the study as a significant drop in the level of sensory impairments from the baseline score.
Multisensory testing was done in the third to fifth year, and it included hearing, contrast-sensitivity tests for vision, touch testing in which vibrations were measured in the big toe, and, smell testing that involved identifying odors like paint-thinner, roses, lemons, onions, and turpentine.
The results showed that sense of smell, or olfaction, has a stronger association with dementia than touch, hearing, or vision. Participants whose sense of smell declined by 10 percent had a 19 percent higher chance of developing dementia, compared to three percent increased risk for corresponding declines in vision, hearing, and touch.
"The olfactory bulb, which is critical for smell, is affected fairly early on in the course of the disease," said Brenowitz. "It's thought that smell may be a preclinical indicator of dementia, while hearing and vision may have more of a role in promoting dementia."
The participants who remained dementia-free at enrollment had higher cognition and no sensory impairments. Participants whose sensory levels ranked in the middle range, 141 of the 328 (19 percent), developed multiple mild impairments and dementia. Participants at higher risk had multiple moderate-to-severe impairments.
"We found that with deteriorating multisensory functioning, the risk of cognitive decline increased in a dose-response manner," said senior author Kristine Yaffe, MD, of the UCSF departments of Psychiatry and Behavioral Sciences, Epidemiology and Biostatistics, and Neurology, as well as the San Francisco VA Health Care System. "Even mild or moderate sensory impairments across multiple domains were associated with an increased risk of dementia, indicating that people with poor multisensory function are a high-risk population that could be targeted prior to dementia onset for intervention."
The study is published in Alzheimer's and Dementia: The Journal of the Alzheimer's Association.