POINT/COUNTER-POINT-Beyond the headlines: the actual evidence that traumatic brain injury is a risk factor for later-in-life dementia.

Traumatic brain injury (TBI) as a risk factor for developing dementia later in life has been a subject of debate and controversy. TBI has been found to be associated with an increased likelihood for developing dementia 10-30 years later in several retrospective studies using population records. However, understanding the link between TBI and dementia requires looking beyond calculated risk estimates and delving into the association TBI has with pathological changes seen in Alzheimer's disease and related conditions, as well as those seen in normal aging. Some individuals with TBI, notably those with more serious injuries, show evidence of AD-related pathological changes, such as tau aggregates, at a much earlier age than healthy older individuals without a history of TBI. This would suggest that some people may be more susceptible to the effects of TBI, accumulating additional pathological changes seen in Alzheimer disease and related conditions, which may synergistically and/or cumulatively interact with factors associated with aging. The strongest support to date suggests that TBI may confer an increased risk for earlier onset of neurodegenerative changes in some individuals, possibly as a function of an accumulation of additional pathological changes. While there appears to be a link between TBI and the development of dementia in group studies, the evidence to date does not suggest an association between TBI and progressive cognitive decline during normal aging nor a greater rate of decline in those with dementia. Thus, there remains much to be learned about the pathophysiology of this apparent relationship.