Nearly half a million elderly Americans likely died from Alzheimer's disease in 2010, a figure almost six times higher than previous estimates of annual deaths, according to a new study released on Wednesday.
The Centers for Disease Control and Prevention has estimated that approximately 5 million people are living with Alzheimer's disease in the United States, and that 83,000 die from the condition each year.
"Many people do not realize that Alzheimer's is a fatal disease," said lead author Bryan D. James of the Rush Alzheimer's Disease Center in Chicago.
Current national estimates are based on death certificates, which tend to underestimate deaths from dementia, he and his colleagues write in the journal Neurology.
They analyzed data from two existing studies that followed people age 65 and older, starting at a time when they did not have Alzheimer's. The participants were tracked for an average of eight years, with annual checkups and brain donation in the case of death.
One study followed religious orders of nuns and priests and the other followed people in retirement communities and senior housing facilities. In all, the studies tracked 2,566 people.
Over the course of the two studies, 559 participants developed Alzheimer's disease and 1,090 participants died.
People diagnosed with Alzheimer's were more than three times as likely to die as those without it. The risk was more than four times as high among participants aged 75 to 84.
Applying these figures to U.S. deaths in 2010, when the data in the two studies were collected, the authors estimate that about 500,000 people over age 75 died from Alzheimer's disease that year.
"There's no doubt that if you have Alzheimer's disease, it increases mortality risk," said Dr. James Leverenz of the Cleveland Clinic Lou Ruvo Center for Brain Health in Ohio.
But although current national estimates for Alzheimer's deaths are definitely low, he's not sure the true number is as high as the one found in this study.
"The two groups are pretty highly educated," said Leverenz, who was not involved in the new research. "They were in generally a little bit better health than the general population."
That means people in these studies could have been less likely to die from heart disease or other conditions, so a higher proportion might have died from Alzheimer's, he explained.
One of the reasons it is so hard to estimate the number of deaths from Alzheimer's is that dementia can be the underlying reason for a number of more immediate causes of death, Leverenz said. For instance, severe dementia can lead to problems swallowing, which leads to malnutrition, which can lead to pneumonia, the study authors write.
Death certificates tend to list the immediate cause of death, in this case pneumonia, and leave out dementia.
"Understanding that AD may contribute to almost as many deaths as the two leading killers in America, heart disease and cancer, is an eye-opening figure that may convince the public and policy makers that AD funding should be increased," James said.
In the study, participants lived an average of four years after an Alzheimer's diagnosis, but Leverenz said he has seen patients live with the condition for much longer - 10 or even 20 years for those with an earlier onset of disease.
"The aging of the baby boomer population means more people living with Alzheimer's disease, which in turn means more people dying from Alzheimer's disease since no effective treatment or cure exists," James said.