TUESDAY, Dec. 9 (HealthDay News) -- New research suggests that medical officials often fail to record severe dementia as a cause of death in patients with the condition.

This finding not only points to a lack of knowledge about how dementia -- a hallmark of Alzheimer's disease -- can be a deadly illness, it suggests that deaths due to Alzheimer's and dementia may be severely underestimated, said report co-author Dr. Susan Mitchell, an associate professor of medicine at Harvard Medical School.

"With dementia so underrepresented on death certificates, it further exacerbates the issue of dementia not being a terminal illness," Mitchell said. In addition, she said, bad assumptions about dementia can lead doctors and relatives to make ill-informed decisions about patients at the end of life.

Alzheimer's disease is the fifth leading cause of death among people 65 and older in the United States, according to 2004 federal statistics. Those figures were based on information from death certificates, Mitchell said, and some studies have suggested the numbers are too low.

In the new study, Mitchell and her colleagues examined the medical records and death certificates of 165 patients with advanced dementia who died between 2003 and 2007. They all lived in Boston-area nursing homes.

Thirty-seven percent of the death certificates didn't list dementia as the main cause of death or a contributing factor. Just 16 percent listed dementia as the main cause of death.

In patients with diagnosed Alzheimer's disease, one-third didn't mention the condition as a cause of death or contributing factor, the study found.

The findings were published in a letter in the Dec. 10 issue of the Journal of the American Medical Association.

Mitchell said the failure to recognize dementia as a cause of death doesn't appear to be intentional. "There has been a general under-recognition of dementia as a cause of death. People have trouble getting their head around that."