Self-Reported Head Injury: Associated Risk in Mild Cognitive Impairment and Progression to Alzheimer Disease

Traumatic brain injury (TBI) has been associated with a higher risk for and earlier onset of neurodegenerative disorders, including Alzheimer disease (AD), but its mechanistic link is not well understood. TBI has been hypothesized to activate a progressive neurodegenerative process, accelerate an already present neurodegenerative disorder, or disrupt neuronal/cognitive reserve and interact with aging. Although previous research has investigated the link between TBI and dementia, little is known if TBI is also associated with development of mild cognitive impairment (MCI), a prodromal phase of AD, and progression from MCI to AD. This broad investigation consists of two studies devised to examine whether a history of TBI is a risk factor for MCI and progression from MCI to AD using a large, multicenter national database. The aim of Study 1 was to determine if a history of TBI with LOC was associated with an increased risk for and earlier onset of MCI. Results revealed that a history of TBI was associated with a 1.35 fold higher risk for a diagnosis of MCI, even after adjusting for well-known factors linked to cognitive decline. A history of TBI was also linked to a nearly 2 year earlier age of MCI diagnosis. Thus, a TBI history does appear to be associated with an earlier diagnosis of MCI and may be a risk factor for MCI. Study 2 was devised to investigate whether a history of TBI with LOC was associated with progression from MCI to AD. Results revealed that individuals with a history of TBI with LOC did not show faster progression from MCI to AD, higher annual rates of progression, or more rapid cognitive decline than those without a TBI history, suggesting that a history of TBI was not linked to progression from MCI to AD. This two-part investigation indicates that a history of TBI appears to be a risk factor for earlier development of MCI; however, once the neurodegenerative process for MCI to AD starts, a history of TBI appears unrelated to subsequent decline.