Sudden loss of smell might not be Covid-19

A sudden loss of smell could be an early warning sign of dementia, scientists have warned. It comes as the symptom has become linked to the coronavirus pandemic, which could lead to an incorrect or overlooked diagnosis.

In the past, studies have found a gradual loss of smell to be an indication of dementia risk, but now they believe rapid deterioration could also be a sign. Research by a team at the University of Chicago in the US involved 500 adults aged over 70 and monitored them for 20 years.

Professor Jayant Pinto, senior author of the study, pointed out that the work is another clue that indicates a link between smell and dementia. He is calling for smell tests to be routine for older people in the same way they have hearing and eye exams.

Scent tests are a cheap and effective way of evaluating a patient’s olfactory capacity at a clinic. The sticks used resemble felt tip pens, but each one is infused with a distinct scent and the individual is asked to identify it from four choices.

Smell is often an overlooked sense that is not considered as important as hearing or sight. It performs a vital role, however, delivering information to the brain in a different way to other senses. Since memory enables individuals to recognise senses, it makes sense there’s a link to cognitive decline.

There are a number of theories about what may cause dementia and among them is tangles of amyloid protein in the brain. Studies have seen these signs appearing first in areas of the brain that are responsible for olfactory and memory tasks.

Rachel Pacyna, lead author of the study, 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.”

They went on to investigate this theory with participants living in retirement homes, who were tested each year on their abilities to identify smells. They were also assessed for signs of dementia, with some undergoing MRI scans.

Scores from the olfactory tests were plotted on a graph for analysis and to identify trends. It was found that participants with no classic Alzheimer's symptoms but who experienced a rapid loss of smell were 89 per cent more likely to develop cognitive conditions than those who gradually lost their sense of smell.

There were also found to be links between a sharp loss of smell and a reduced volume of grey matter in areas of the brain associated with memory, when compared with those who lost the sense more slowly. The risk in these individuals is similar to those with the APOE-e4 gene, which makes people predisposed to Alzheimer’s.