Head injury doubles the risk of Alzheimer's disease

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Young adults who experience a moderate or severe head injury have more than double the risk of developing Alzheimer's disease and other forms of dementia later in life, according to a new study.

Dr Brenda Plassman from Duke University Medical Center in Durham, North Carolina, and colleagues looked for an association between head injury and Alzheimer's disease in more than 7000 US navy or marine veterans from the second world war. In all, 548 veterans with head injury and 1228 without head injury completed all assigned stages of the study.

The researchers found that any medical history of head injury more than doubled both the risk of developing Alzheimer's disease and the chances of developing non-Alzheimer's dementia, even after adjustment for the effects of age (Neurology 2000;55:1158-66).

The worse the head injury, the higher the risk of Alzheimer's disease, according to the report. Moderate head injury was associated with a 2.3-fold increase in risk, whereas severe head injury more than quadrupled the risk.

A severe head injury was one in which an individual was admitted to hospital and remained unconscious or had amnesia for 24 hours or more; a moderate injury was considered to be a loss of consciousness or a bout of amnesia lasting no more than 30 minutes after the injury.

The researchers also tested for the presence of a form of the apolipoprotein E gene that has been linked to the development of Alzheimer's disease. Men who had the e4 form of the gene—one of three different versions of the gene—had a 14-fold higher risk of Alzheimer's disease and a 10-fold increase in the risk of dementia.

However, there did not seem to be any interaction between the type of apolipoprotein E gene present and head injury in predicting the risk of non-Alzheimer's dementia. Interest in an association between head injury and Alzheimer's disease increased after the publication of a letter suggesting an interaction between a specific allele of the apolipoprotein E gene and head injury as a risk factor for the disease (Neurology 1996;46:889-91).

Subsequent studies did not confirm this interaction but have instead reported that head injury and apolipoprotein E act additively to increase the risk of Alzheimer's disease. The current study shows that "moderate and severe head injuries in young men may be associated with increased risk of Alzheimer's disease and other dementias in late life."

Understanding the effects of head injury and the apolipoprotein E gene should lead to improved understanding of the causes of dementia, the authors said, and such knowledge "could well lead to interventions that retard or arrest the process early in its course."

As the veterans in this study sustained head injuries in early life, the increased risk of dementia 50 years later suggests that the pathogenesis of Alzheimer's disease may be traced to origins decades before the appearance of clinical symptoms. This is consistent with the perspective that the disease is a chronic one that unfolds over many decades, with an extended latent phase as well as a prodromal stage.

"While we may not fully understand what's going on, as a practical matter, it may be one more reason to wear that bike helmet instead of keeping it in a closet," said Dr Richard Havlik, one of the authors, from the National Institute on Aging in Bethesda, Maryland.
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Scott Gottlieb. Head injury doubles the risk of Alzheimer's disease. BMJ. 2000 November 4; 321(7269): 1100.

Articles from BMJ : British Medical Journal are provided here courtesy of BMJ Publishing Group