Sudden loss of smell has become a hallmark of the coronavirus, but scientists warn that it could also be an early sign of dementia, the Daily Mail reports.  U.S. researchers followed more than 500 elderly people in the United States for 20 years. Those who experienced loss of their sense of smell for several years were nearly twice as likely to develop Alzheimer's disease as those who lost their sense of smell for decades. Researchers followed 515 seventy-year-olds who did not initially have dementia or cognitive problems for 20 years.  About 100 of that cohort were later diagnosed with dementia or cognitive impairment.  And those who did not have the classic symptoms of Alzheimer's disease but experienced a rapid decline in their sense of smell were 89% more likely to develop the memory-destroying disease than those who lost their sense of smell more slowly. Abrupt loss of smell was also associated with a greater risk of decreased gray matter volume in brain regions related to smell and memory compared to those who lost their sense of smell more slowly. The changes were most obvious in those parts of the brain associated with olfaction, including the amygdala and entorhinal cortex. Their risk was similar to those with the APOE-e4 gene, a known genetic risk factor for Alzheimer's disease.