People with small head size and the gene variant apolipoprotein E e4 (ApoE e4) are 14 times more likely to develop Alzheimer's disease than people without that combination. Having a small head size without the gene did not significantly increase the risk of developing Alzheimer's.
This result adds weight to the theory that having a large "brain reserve" protects against Alzheimer's. Head circumference is one way to measure brain reserve.
"The theory is that the symptoms appear when the loss of brain cells goes below a critical threshold of brain reserve," said Amy Borenstein Graves, PhD, of the University of South Florida in Tampa, who conducted the study with researchers at the University of Washington in Seattle. "People with ApoE e4 are likely to have more rapid brain cell damage. Those with large brain reserves may have the same changes in their brains, but they don't show symptoms of the disease until much later."
The study involved 1,869 healthy Japanese Americans age 65 and older in King County, Washington. The study participants were followed for an average of 3.8 years. During that time, 59 people developed Alzheimer's disease.
The people who developed Alzheimer's were also older, less educated, shorter, lighter and had lower estimated verbal IQ (intelligence quotient) scores than those who did not develop Alzheimer's during the study period.
Borenstein Graves said that while brain growth is controlled in part by genetics, it may also be influenced by factors during the first 10 years of life such as malnutrition, poverty, infection, parental occupation and educational status, family size and birth order.
Later in life, factors such as higher education and income and mental and physical exercise may play a role in delaying the onset of Alzheimer's disease.
"It's interesting to speculate on whether we could prevent this disease if we could systematically boost our brain reserves throughout life," Borenstein Graves said. "In our study 18 percent of the risk of Alzheimer's was attributable solely to small head size. So if it were possible to increase brain reserve through prevention of brain damage that occurs across the life span, nearly 20 percent of the disease among these individuals might be preventable."
Study participants were divided into three groups, based on head circumference. Only the group with the smallest head circumference (less than 21.4 inches), and ApoE e-4, had a significantly greater risk of Alzheimer's.
Information on ApoE e-4 status was available for 59 percent of the study participants.
Borenstein Graves said a limitation of the study is its sample population of only Japanese Americans, which makes it difficult to apply the results to the general population.
"However, several studies with other populations have shown a relationship between head circumference and cognition and rates of developing Alzheimer's," she said.
For a copy of the study, contact Cheryl Alementi, 651-695-2737 firstname.lastname@example.org
The study was supported by the National Institute on Aging.
The American Academy of Neurology, an association of more than 17,500 neurologists and neuroscience professionals, is dedicated to improving patient care through education and research. For more information about the American Academy of Neurology, visit its web site at www.aan.com.