We have vastly underestimated the number of deaths due to Alzheimer's Disease

In a previous post I listed the top causes of death in the U.S. Number 1 was heart disease (~600,000/year). Number 2 was cancer (~567,000/year). Alzheimer's Disease (AD) ranked 6th with 79,000 deaths per year. There were about 2.4 million total deaths in 2009 according to the CDC.

A new study from doctors at Rush Medical Center suggests that the CDC has severely undercounted the AD toll, and puts the number of deaths attributable to Alzheimer's Disease at over 500,000 per year (more than 6x the current estimate), which would place it Number 3 in causes of death right behind heart disease and cancer.

First let's go through some of the numbers from the study:

1. 2,566 participants 65 years or older (mean age 78.1); none had Alzheimer's Disease at the start.
2. There were two groups, one was from a religious order (e.g. nuns) and the other from a nursing home.
3. They were watched over an average of 8 years and tested for Alzheimer's Disease (AD).
4. During the study 559 or 21.8% of subjects developed AD.
5. 1,090 (42.4%) died.
6. Median time from AD dementia diagnosis to death was 3.8 years.
7. In the AD group the mortality hazard ratio was approximately 3.5 compared to the non-AD group i.e. a person with AD had 3.5 times the chance of dying than a person without AD.
8. Population attributable risk was ~36% for those 75 and older i.e. AD contributed to 36% of deaths.
9. Extrapolated to the broader population, they calculate that 500,000 deaths per year for those 75 and older are attributable to Alzheimer's Disease.

There is a big difference between 500,000 and 79,000 (the official CDC number). What are the reasons for this discrepancy. First, people associate AD with dementia but not death:

"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. "Alzheimer's disease starts in the part of your brain that controls your memory and thinking, but over years it spreads to the parts of your brain that control more basic functions such as breathing and swallowing."

Second, AD may not be the immediate proximal cause of death, but may contribute significantly to death in a more indirect fashion:

"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."

There are some significant caveats of this study to keep in mind. First, the sample group was relatively small, only a few thousand subjects. Second, the sample group may not be representative of the broader population (i.e. less likely to get heart disease or cancer):

"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.

Nevertheless, it appears that AD deaths are being significantly undercounted, and this number will only increase in the future.  While heart disease and even cancer are starting to decrease in their incidence rates in the U.S., Alzheimer's Disease continues to rise, especially as people live longer.

A final point is that this study raises the issue that finding a single cause of death is too simplistic. In many cases death arises from a combination of factors (one of which may be AD):

 “Death certificates may not be the best way to measure how many people die from something that takes up to 10 years” to break down a person’s system, he said, adding that the disease leaves people more vulnerable to dying from infections and other problems. “We’re not saying they didn’t die of those things; we’re just saying, ‘Well, what put them in the hospital with that condition?’ ” For example, if Alzheimer’s compromises one’s ability to swallow and results in food repeatedly going down the windpipe, that can leave a person more vulnerable to dying of pneumonia, he said. “Trying to identify a single cause of death in elderly people is often not reflective of the real situation,” James said.

Figure 1. Alzheimer's Disease is a major killer.