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A sudden loss of smell has become a hallmark of Covid, but scientists are warning it may also be an early sign of dementia.
Studies had previously linked a gradual loss of smell to the memory-robbing disorder.
But new research suggests a rapid deterioration might be a better indicator.
US researchers monitored more than 500 elderly adults in the US for around 20 years.
Those who experienced anosmia over several years were almost twice as likely to develop Alzheimer’s compared to those who lost their smell over decades.
Senior study author Professor Jayant Pinto, from the University of Chicago, said it ‘provides another clue’ about the link between smell and dementia.
He suggested making smell tests as common as hearing and eyesight checkups for older people to screen for the disease.
US researchers, who monitored more than 500 older adults, found that those who experienced a sharp decline in their olfactory sense were nearly twice as likely to develop Alzheimer’s or dementia, compared to those who lost it more gradually
While the sense of smell is often considered less important than sight and hearing, it provides the brain with vital information.
Memory plays a critical role in the ability to recognise smells and researchers have long known of a link between the sense and cognitive decline.
Studies have shown that ‘tangles’ of protein amyloid in the brain — a telltale sign of dementia — often appear first in olfactory and memory-associated areas of the brain.
But it’s still unknown if this damage actually causes the decline in a person’s sense of smell.
Professor Pinto and his team wanted to investigate whether these alteration correlated with a person’s loss of smell and brain function over time.
Lead author Rachel Pacyna, a researcher at the university, said: ‘Our idea was that people with a rapidly declining sense of smell over time would be in worse shape — and more likely to have brain problems and even Alzheimer’s itself — than people who were slowly declining or maintaining a normal sense of smell.’
The researchers monitored 515 people in their seventies, who initially didn’t have dementia or cognitive issues, for 20 years.
The volunteers all lived in retirement homes and were tested annually for their ability to identify certain smells and signs of dementia. Some also underwent MRI scans.
Their decline in their sense of smell was measured by their scores in the olfactory tests, which were then mapped onto a graph. The downward trend in the slope was labelled ‘severe’, ‘decreased’, ‘unchanged’ or ‘improved’.
Around 100 of the cohort went on to be diagnosed with dementia or cognitive impairment.
Those who had no classic Alzheimer’s symptoms but experienced a rapid decline in their sense of smell were 89 per cent more likely to develop the memory-robbing conditions than those who lost their sense of smell more slowly.
A sharp loss of smell was also linked with a greater risk of going on to have smaller grey matter volume in parts of the brain related to smell and memory, compared to those who had a slower decline.
The changes were most obvious in the parts of the brain used for smell — including the amygdala and entorhinal cortex.
Their risk was similar to those who have the APOE-e4 gene, a known genetic risk factor for developing Alzheimer’s.
One in four people have the gene and are three times more likely to develop Alzheimer’s than those who don’t.
The researchers hope to eventually conduct autopsies on the volunteers, which is considered the gold standard for confirming whether a person has Alzheimer’s, to expand their findings.
And they want to trial using smell tests in clinics for older adults, in a similar way to eye and hearing tests, to screen and track for early signs of dementia. GOOD
They said scent tests are cheap, easy to use and involve smelling a series of sticks that look like felt-tip pens.
Each stick is infused with a distinct scent that individuals must identify from four choices.
Ms Pacyna said: ‘If we could identify people in their 40s, 50s and 60s who are at higher risk early on, we could potentially have enough information to enroll them into clinical trials and develop better medications.’
The team noted that only a fifth of participants underwent MRI scans and those who did only had one — meaning they lacked data to pinpoint when structural brain changes began.
And the majority of volunteers were white, so more research is needed to find out if other groups are similarly affected.
A loss or change to sense of smell or taste was one of the three key symptoms of Covid first identified by health chiefs when the virus swept the globe last year.
But as the virus has mutated and new variants have took hold, many infected people no longer report a change to their senses.