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The good news is that several of these symptoms and behaviors can be prevented or treated, experts noted.
Early onset (or young-onset) dementia occurs before the age of 65. Alcohol abuse was the most important risk factor found in the study, said lead researcher Peter Nordstrom. "In contrast, the influence of hereditary factors, that is dementia in the parents, was very small."
For the study, published online Aug. 12 in the journal JAMA Internal Medicine, Nordstrom's group collected data on men drafted into the Swedish military from mid-1969 through 1979. They were about 18 years old when they were drafted.
During the follow-up period of roughly 37 years, 487 men developed young-onset dementia at an average age of 54, the researchers found.
The risk factors identified "were multiplicative, most were potentially modifiable and could be traced to adolescence, suggesting excellent opportunities for early prevention," said Nordstrom, from the department of community medicine and rehabilitation at Umea University
Alcohol intoxication, stroke, use of antipsychotic drugs, depression, drug abuse, a father with dementia, poor mental function as a teen, being short and having high blood pressure were the risk factors they found.
Taken together, these accounted for 68 percent of the cases of young-onset dementia, the researchers said. Men with at least two risk factors and in the lowest third of overall mental ability had a 20-fold increased risk.
"Young-onset dementia, before age 65, is a devastating condition for patients and their families," said Dr. Deborah Levine, an assistant professor of medicine at the University of Michigan and author of an accompanying journal editorial.
One priority for young-onset dementia is research that identifies ways to prevent it, she said. Another is to improve care and access to long-term services for adults who are diagnosed with dementia before 65.
"This is urgent because adults with young-onset dementia and their families really need our help," she said. "More Americans may develop young-onset dementia because of increases in traumatic brain injury among young veterans and stroke among young African-Americans and middle-aged adults."
Dr. Sam Gandy, director of the Mount Sinai Center for Cognitive Health in New York City, said the new study is "extraordinary and deserves attention by others studying adolescent populations to see whether this Swedish experience can be confirmed elsewhere and independently.
"I always prefer to see replication of data -- especially 'game-changing' data like these -- before I sound any alarm, but given the treatability of these risks, I would say that these observations should prompt aggressive attention by physicians with access to adolescents," Gandy said.
Dementia affects an estimated 35 million people worldwide and is expected to rise sharply by 2050, according to study background information.
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SOURCES: Peter Nordstrom, Ph.D., department of community medicine and rehabilitation, Umea University, Sweden; Deborah Levine, M.D., M.P.H., assistant professor, medicine, University of Michigan, Ann Arbor; Sam Gandy, M.D., Ph.D., director, Mount Sinai Center for Cognitive Health, New York City; Aug. 12, 2013, JAMA Internal Medicine, online