New York Consortium for Alzheimer Research and Education
The first evidence of a link between history of head injury and risk for developing Alzheimer’s disease (AD) was a case report of AD pathology in the brain of a 38 year-old man who had suffered a single episode of a head injury 16 years earlier. This finding led to the idea that head injury is a risk factor for AD. A number of research studies investigated the link between head injury and AD, with conflicting findings. Some studies showed that loss of consciousness following head injury increased the risk of developing AD. Other studies suggested that it was not loss of consciousness, per se, but amnesia following the head injury, such as forgetting the ambulance ride to the emergency department, that increased risk of AD. One study, the Rochester Epidemiology Project, examined medical records of all head injury cases from 1935-1984 in Minnesota residents, and found that while the number of cases of AD among individuals with a history of head injury was the same as in the general Rochester population, individuals with a history of head injury were diagnosed with AD at a young age at twice the rate of the general population.
There are several biological explanations for the possible increased risk of AD after head injury. First we know that the ApoE e4 allele increases both the risk of AD and the risk of cognitive deficit after a head injury. Also, we know the brains of people with AD and those who have suffered a head injury have amyloid plaques and neurofibrillary tangles. However, while the brains of boxers diagnosed with dementia pugilistica have a significant number of tangles in some of the same areas where tangles are found in AD patients, tangles are also found in these boxers in brain regions that are involved with emotions. This has led to the idea that head injury results in a type of dementia called chronic traumatic encephalopathy, or CTE, rather than AD.
The traditional view of head injury, more accurately referred to as traumatic brain injury (TBI), suggested that there was an initial period of recovery followed by life-long stable cognitive functioning. Recently, we have become aware of individuals with TBI who were stable for some years, but then began to demonstrate cognitive, functional and behavioral deterioration resulting in an earlier onset of dementia.
Stories of former boxers who are diagnosed with dementia pugilistica and more recently, former NFL players who are diagnosed with dementia at younger ages suggest that even mild head injuries may cause early onset AD. The findings in former boxers and football players lead us to wonder about players of other types of sports that are associated with concussion. Will those players be at risk, too?
Mild TBI (MTBI) is the most common form of head injury and is often the result of a concussion from motor vehicle accidents, falls, sports-related activities or repeated bumps to the head. Concussion can result in temporary loss of awareness, dizziness, nausea, vomiting or brief loss of consciousness. All age groups are at risk for these injuries. While younger people may be at risk of an MTBI from a sports-related incident, older adults are at risk of concussion from falls. While there is overlap in cognitive symptoms between AD and MTBI such as memory loss (more profound in AD versus MTBI) and attention problems, there are differences as well. For example, the memory problems in AD are characterized as a profound difficulty in learning and retention of new information. In MTBI, memory can often be aided by cuing. In addition, executive function (thinking abilities required for planning, problem solving, self-regulation of behavior, and judgment) may be impaired early in MTBI while in AD, executive dysfunction becomes more prominent later, as the disease progresses. Complicating the diagnostic scenario is the fact that depression is frequently found in both AD and MTBI patients.
To this day, how and whether MTBI triggers progressive brain changes leading to AD remains controversial, but there is a growing body of research trying to understand this important issue. Brain imaging techniques offer a glimpse into the living human brain, allowing us to identify and track the disease process. Current studies are using a relatively new brain imaging technique called diffusion tensor imaging (DTI) to examine the integrity of nerve cell connections in the brain, which are sheared and torn following TBI, which may shed light on the underlying disease process. Imaging in combination with neuropsychological assessment, which is the use of objective measures (i.e., paper/pencil tests) to assess brain experiactivity, will hopefully clarify the diagnostic picture.
While researchers are working to better understand the link between head injury and AD, it is important for people both young and old to follow some safety precautions to avoid head injuries. When riding a bicycle a helmet should always be worn. Because motor vehicle accidents occur so frequently and increase the risk of a head injury, people of all ages should wear a seat belt.
Some simple precautions for older adults to take to avoid falling include using a steady step ladder and wearing sensible shoes. Do not trust your eyes to see in dark places if you know you have vision problems. You could easily trip over that lost shoe, fall, and hit your head on a piece of furniture. Be sure to obey signs about wet floors. Caution leading to prevention is the best way to avoid falls and may reduce your risk of dementia.