While the difference between larger-headed and smaller-headed people was significant from a statistical point of view, study co-author Farrer said it's impossible to pinpoint exactly what the difference means in terms of how the brain works overall.
The research doesn't confirm that brain size and the speed of the disease are directly connected. But if there is a connection, what's going on? "One possible explanation is that larger heads, and therefore larger brains, contain more nerve cells and connections between cells," reasoned study lead author Dr. Robert Perneczky, a researcher at the Technical University of Munich in Germany.
Therefore, he said, more brain cells have to die before "the threshold is crossed where brain damage leads to cognitive impairment and other symptoms of dementia."
Roe, the neurology instructor, said the study appears to be valid and useful, adding that it suggests that three things are connected: brain size, the shrinking of the brain and the progression of Alzheimer's disease.
Whatever your head size, she said, "the message is that the important thing is trying to keep your brain as healthy as possible throughout life, which hopefully will allow you to cope better with diseases like Alzheimer's if they occur."
There's much more on Alzheimer's disease at the Alzheimer's Association.
SOURCES: Lindsay Farrer, Ph.D., chief, genetics program, Boston University School of Medicine; Robert Perneczky, M.D., researcher, department of psychiatry and psychotherapy, Technical University of Munich, Germany; and Catherine Roe, Ph.D., research instructor in neurology, Washington University School of Medicine, St. Louis; July 13, 2010, Neurology
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