A new U.S. study suggests that Alzheimer's disease and other dementias are markedly under-reported on death certificates and medical records. Death certificates tend to only provide an immediate cause, such as pneumonia, and don’t mention the underlying condition that provoked it.
The study involved 2,566 older adults (65+; mean age 78) who received annual testing for dementia. The death rate was more than four times higher for those aged 75-84 who had been diagnosed with Alzheimer’s, and nearly three times higher in those with Alzheimer’s aged 85+. More than one-third of all deaths in those age groups were attributable to Alzheimer's disease. Median time from Alzheimer’s diagnosis to death was 3.8 years.
All this translates into an estimated mortality rate from Alzheimer's that is five to six times higher than the accepted number (derived from death certificates), which has put Alzheimer’s as the 6th leading cause of death.
 Contribution of Alzheimer disease to mortality in the United States. Neurology. 82(12), 1045 - 1050.(2014).