A large, multi-institution study has found no connection between suffering a traumatic brain injury (TBI) with loss of consciousness and later development of Alzheimer’s disease or dementia. Even when participants reported loss of consciousness lasting more than an hour after sustaining such an injury, they were at no greater risk for dementia later in life.

The study, published July 11 in JAMA Neurology, was led by Paul Crane, a University of Washington professor of medicine, Division of General Internal Medicine, and adjunct professor of health services in the School of Public Health. Eric Larson, executive director of the Group Health Research Institute and clinical professor of health services at the UW School of Public Health, was senior author.

Researchers looked for connections between TBI and late-life development of neurodegenerative conditions — such as Alzheimer’s and Parkinson’s or parkinsonian features — and brain abnormalities discovered at autopsy. These can include plaques and tangles common in Alzheimer’s disease; protein clusters called Lewy bodies that are often seen in Parkinson’s disease; and small areas of brain cell death called microinfarcts.

“To our knowledge this is by far the largest study ever on this topic,” Crane said. “Several previous studies have suggested associations between TBI with loss of consciousness and Alzheimer’s disease. We had ample ability to find this same kind of connection, as hundreds of participants in this study ended up developing dementia or Alzheimer’s disease. But we didn’t find one.”

Interestingly, the new findings did suggest that TBI with loss of consciousness was linked with late-life development of Parkinson’s disease, the progression of parkinsonian features over time and the development of Lewy bodies. The researchers also found that people who were 25 years old or younger when they experienced TBI with loss of consciousness were more prone to developing Lewy bodies and microinfarcts.

These findings combined suggest an interesting “time capsule effect,” as Crane called it. This is where someone sustains a TBI and seemingly returns to normal functioning. Yet some 40-plus years later Parkinson’s or parkinsonian features emerge.

The researchers analyzed data from three separate studies that began in the 1990s. They followed participants until they developed dementia, dropped out, or died. Of the 7,130 people enrolled, 865 reported a TBI combined with loss of consciousness. Of these, 142 reported being unconscious longer than an hour.

Crane and Larson collaborated with University of Washington colleagues as well as researchers from Mount Sinai School of Medicine, Cleveland Clinic Foundation, University of Utah and Rush University Medical Center. The study was funded by the National Institutes for Health and the Paul Allen Family Foundation.