Alzheimers Disease Preceded by Years of Accelerating Cognitive Decline
By definition, dementia due to Alzheimer’s disease is preceded by a minimum of 6 months of cognitive decline. Scientists now know that the period of progressively accelerating decline in memory and thinking skills, clinically known as the prodromal phase, actually lasts about 5 to 7 years.
“The length of this prediagnosis period of decline was surprising,” said first study author Robert S. Wilson PhD of the Rush Alzheimer’s Disease Center in Chicago.
Semantic Memory and Working Memory Are First Cognitive Functions to Decline
Dr. Wilson and his team characterized the course of cognitive decline during the prodromal phases of Alzheimer’s disease and Mild Cognitive Impairment (MCI) in more than 2000 elderly participants enrolled in 2 ongoing studies – the Religious Orders Study and the Rush Memory and Aging Project. Both began in the 1990’s and the participants have received cognitive tests annually.
During the study period, 462 patients developed Alzheimer’s. Among these, the rate of global cognitive decline increased sharply about 5 to 6 years before diagnosis. After diagnosis, there was a further increase of about one-third in the annual rate of global cognitive decline.
The research indicates that semantic memory (the memory of meanings, understandings, and other concept-based knowledge) and working memory (the ability to actively hold information in the mind) were the first of the cognitive functions to decline.
"These results," Dr. Wilson added, "indicate that Alzheimer's disease has a much longer symptomatic course, and we think that this may be preceded by a period of similar length during which pathologic changes are accumulating in the brain but not yet producing symptoms.
Dr. Gustavo C. Roman MD, medical director of the Nantz National Alzheimer Center at the Methodist Neurological Institute in Houston, says that this is a very important finding because “by the time we put the label of Alzheimer's disease on a patient, you have lost 5 to 7 years of interventions and that's extremely important." Dr. Roman was not involved with the study.
It is believed that treatments for Alzheimer’s disease and MCI are most effective if started as early as possible, before significant cognitive problems set in. "Although we still don't have an actual cure or mechanism to stop the progression, I think there are enough vascular risk factors that can be controlled and need to be looked for in patients who complain of memory problems," Dr. Roman states.
It is estimated that 5.4 million Americans are currently living with Alzheimer’s and the number is expected to increase significantly over the next several decades.