A few studies report that concussions or repetitive blows to the head associate with a higher risk of later dementia but this field of research is very new and the long-term effects for a given individual are still very unclear.
Concussions are a common form of mild traumatic brain injury (TBI) caused by head impacts. Concussion is currently defined as a head impact that results in loss of consciousness for less than 30 minutes and memory loss lasting less than 24 hours , although some organizations argue that concussions may have occurred even without a loss of consciousness (eg. MedlinePlus)
In general, a concussion results when the brain impacts against the inside of the skull; however, it is likely that no two concussions are exactly the same. The severity of a concussion depends on many factors, including the force and direction of impact as well as certain characteristics of the person who suffers the concussion, including neck strength and possibly their age and sex .
Common symptoms include headache, loss of memory, confusion, blurred vision and loss of balance. Around 15% of people may experience longer-lasting symptoms that can include anxiety, depression, moodiness or aggression. Here, we review the evidence that it might also raise the risk of dementia, worsen decline in people with cognitive impairment, or accelerate aging itself.
Moderate-to-severe TBI (from physical assaults or car accidents, for example) is beyond the scope of this report. Briefly, these more severe injuries are suspected to increase the risk of developing dementia. A recent meta-analysis of moderate to severe TBI, estimated a single TBI may raise dementia risk by 36% . However, many people with moderate to severe TBI never develop dementia because TBI is only one of many factors that influence a person’s risk of dementia.
While sports like football, hockey, soccer and lacrosse are commonly associated with concussions, falling is another common cause, particularly among older adults . Other activities like horse and bicycle riding, skiing, gymnastics and rollerblading are also sources of concussions. While it isn’t really practical (or fun) to abstain from all of these activities, taking common-sense precautions like wearing a helmet and protective gear may reduce some risk.
The elderly and other people at higher risk of falling can also take steps to ensure their safety such as putting hand rails on all stairs and using non-slip mats in the shower and bath tub. The AARP has more information and tips on preventing falls, as well as a “Preventing Falls Checklist” for caregivers.
Possibly, based on moderate evidence.
Growing evidence suggests that multiple concussions may raise dementia risk for adults (reviewed by ), implying that avoiding concussions probably reduces dementia risk. It is important to note that all of the clinical research is observational, which makes it challenging to understand cause and effect. Moreover, one person’s response to a concussion can be extremely different than another person’s.
In one recent retrospective study of over 112,000 patients, a single concussion was enough to increase dementia risk in older adults by 25% , confirming earlier findings of increased dementia risk . Another large study of nearly 100,000 patients also suggests concussion increases dementia risk by as much as 3-fold . The increased risk of dementia may not be limited to dementias of old age. A recent study of over 800,000 Swedish men suggested people who suffered two or more mild traumatic brain injuries (including concussions) were at much greater risk of developing “young onset” dementia than those who had one or no concussions .
Although these studies imply a profound increased risk from concussions, the risk to each individual is still very unclear. One person’s concussion may differ in profound unseen ways from another person’s concussion. Similarly, the vulnerability of someone to concussions may vary based on their age, genetics, and other factors.
An emerging area of concern is a rare type of dementia called chronic traumatic encephalopathy (CTE) that has been linked to multiple concussions or even repetitive blows to the head that, individually, are not as severe as a concussion. CTE is a dementia similar to Alzheimer’s but distinct in the way it harms the brain. So far, CTE has only been observed in people with a history of multiple head impacts, such as boxers and football, hockey and soccer players (from high school to professional levels) as well as military veterans who sustained blast injuries (reviewed by ).
In children and adolescents, concussions are never a good thing. However, very little is known about whether concussions in early life can raise the risk of dementia in late life. The majority of children and adolescents who suffer a concussion recover without lasting effects , although some percentage, estimated to be between 15 and 30%, experience symptoms that can last more than three months . These symptoms can include learning and memory problems, mood disorders like depression, anxiety, insomnia and problems in judgment.
Whether concussions in children will one day lead to a higher risk of dementia is unknown and understudied. The ADDF recently partnered with Safe Kids Worldwide and the Andrews Institute for Orthopaedics and Sports Medicine to lead the publication of an Expert Consensus Statement in Nature Reviews Neurology that discusses our current gaps in knowledge of pediatric concussions and outlines what is needed to accelerate research in this important field.
It is important to note that almost all studies of concussion are observational. Prospective observational research follows people over time and correlates their dementia risk with the number of concussions that they sustained and when they sustained them. Retrospective observational research (like the very large studies mentioned above) often uses medical records or personal questionnaires to determine injury history and incidence of dementia. Both types of observational research have their own drawbacks and biases. To learn more, check out our page on “Types of Evidence”.
APOE4 carriers: Much remains unknown about how someone’s genetics in general, and their APOE allele specifically, influences their risk of developing dementia following a concussion. One study suggests that APOE4 carriers with a history of TBI, including concussion, may be 5 times more likely to develop Alzheimer’s disease than APOE4 carriers who have never experienced a TBI [12,13]. These were small studies, however, and need to be replicated with larger numbers of people. Also, it remains unknown if the people who shared the APOE4 allele and were at greater risk of developing Alzheimer’s after concussion may also have shared other genetic traits that could account for their increased risk.
Additionally, it remains unclear if APOE4 carriers experience, on average, poorer recovery outcomes following concussion. A 2008 meta-analysis suggests APOE4 carriers take longer to recover from concussions and experience more long-lasting symptoms than people without an APOE4 allele , but at least one later study found no association . To learn more about what APOE means to your health, visit our APOE information page.
Possibly, based on limited evidence.
Several lines of reasoning support the idea that concussions can accelerate decline in people who already have dementia. Concussions appear to increase the rate of brain atrophy (or shrinking) , a key feature of Alzheimer’s disease. A history of concussion within 10 years of dementia onset might also accelerate the rate of cognitive decline after dementia is diagnosed . However, more research is needed. The patients who are more likely to experience concussions due to a fall or other physical trauma may share other health factors that determine their rate of cognitive decline, including genetics and lifestyle factors. Also, the effects of a concussion vary extensively depending on the type of concussion and the individual who suffered it.
Possibly, based on limited evidence.
Proper and timely recognition and treatment of concussion is critical for improving recovery outcomes. Conversely, unrecognized or untreated concussions can result in death. However, the contribution of a single concussion or multiple concussions to the rate of aging or risk of death remains unclear.
A meta-analysis of 24 individual studies suggests mTBI increases the risk of death by 12% for older adults . A more recent large health database study of nearly 90,000 older adults suggests that those who suffered a concussion or mTBI were 25% more likely to die sooner than those who did not suffer mTBI . While these numbers sound scary, it is important to remember that that this concussions or mild TBI is only one of the many variables that can influence the risk of death. We also don’t know whether the injury led directly to the risk or whether people with a higher risk of head injury share other similarities that raise risk of dementia and death.
The largest short-term risk of death from concussion comes from “Second Impact Syndrome”, in which a second concussion is incurred while recovering from an earlier concussion. Although rare, these circumstances are characterized by brain swelling and can be fatal . Although many case studies involve youth athletes, “Second Impact Syndrome” can also affect adults and the elderly.
Our understanding of the biology of concussions, and exactly how they harm the brain, is still in its infancy. Some evidence suggests that concussions accelerate brain aging , causing the brain to deteriorate more quickly than in someone who never suffered a concussion .
A concussion begins a series of events that contributes to cellular and tissue damage in the brain, which in turn cause the common symptoms mentioned above. Neurons are particularly vulnerable to the forces of concussion because of a highly-specialized part of their cell body called the axon. The axon is an elongated tube-like structure that extends from the neuron cell body and connects the neuron to other neurons. Sheering forces of a blow to the head can twist, tear and break axons. This damage can set off a cascade of events, including neurotransmitter imbalance, inflammation and oxidative stress, all of which further damage brain cells .
Two of the hallmarks of Alzheimer’s disease, misfolded tau and beta-amyloid, are associated with the biology of concussions and CTE, a neurodegenerative disease thus far found only in people who suffered multiple concussions or relatively mild yet repetitive head impacts. Clinical research is currently underway to uncover the details of how they contribute to CTE and Alzheimer’s disease (see “What’s the future?” below).
Incurring another concussion while still recovering from a prior concussion can worsen recovery outcomes or even result in death, a phenomenon known as “Second Impact Syndrome” . Proper recognition of a concussion and limiting the chances of incurring another concussion within the recovery window are critical to improve the chances of full recovery.
Absolutely safe, based on strong evidence.
This one is a “no-brainer”.
Many people can reduce their risk of suffering a concussion by not engaging in behaviors that can result in concussions like motorcycle and horse riding, skateboarding, rollerblading and skiing. Avoiding high-impact contact sports like football and ice hockey (where most of the cases of CTE have been identified) may also be prudent. If you chose to engage in these activities, you may be able to reduce your risk of concussion by wearing appropriate protective gear, especially a helmet.
For many years, evidence was lacking that helmets actually reduced concussion risk, particularly in football (reviewed by ). However, recent innovations in football helmet design have been shown to reduce concussion risk among football players . There is some debate about whether helmets might actually increase concussion risk in sports players (because of an increased propensity for harder head impacts), but because of the difficulty in testing tackle football players who don’t wear helmets (there aren’t any), any evidence for this theory is largely anecdotal. Helmets or other protective head gear can also reduce the risk of more serious TBI which can result in death.
Older adults might reduce their concussion risk by taking steps to reduce their likelihood of falling, the biggest contributor to concussions in the elderly. The AARP provides information and tips on preventing falls, as well as a “Preventing Falls Checklist” for caregivers.
The potential lasting negative consequences of concussions are being extensively researched, including the risk of dementia. Several clinical studies are underway to discover how beta-amyloid and tau contribute to dementia risk in adults after concussion (NCT02134041, NCT01871610). The Alzheimer’s Drug Discovery Foundation is funding a clinical study by Dr. Sam Gandy at the Icahn School of Medicine at Mount Sinai, to learn how traumatic brain injury can alter biological markers of Alzheimer’s disease (NCT02266563). To explore more of the clinical studies and trials underway on concussions, see this list at ClinicalTrials.gov.
More research is needed to determine if concussions and mTBI can accelerate the rate of cognitive decline in people who already have dementia. Additionally, research efforts should focus on the potential link between childhood concussions and the risk of later-life dementia (reviewed in this scientific Consensus Statement freely available at Nature Reviews Neurology, an effort co-led by the Alzheimer’s Drug Discovery Foundation ).
1. Levin, H.S. and R.R. Diaz-Arrastia, Diagnosis, prognosis, and clinical management of mild traumatic brain injury. Lancet Neurol, 2015. 14(5): p. 506-17.
2. Voss, J.D., et al., Update on the Epidemiology of Concussion/Mild Traumatic Brain Injury. Curr Pain Headache Rep, 2015. 19(7): p. 32.
3. Perry, D.C., et al., Association of traumatic brain injury with subsequent neurological and psychiatric disease: a meta-analysis. J Neurosurg, 2015: p. 1-16.
4. Haring, R.S., et al., Traumatic brain injury in the elderly: morbidity and mortality trends and risk factors. J Surg Res, 2015. 195(1): p. 1-9.
5. Carman, A.J., et al., Expert consensus document: Mind the gaps-advancing research into short-term and long-term neuropsychological outcomes of youth sports-related concussions. Nat Rev Neurol, 2015. 11(4): p. 230-44.
6. Gardner, R.C., et al., Dementia risk after traumatic brain injury vs nonbrain trauma: the role of age and severity. JAMA Neurol, 2014. 71(12): p. 1490-7.
7. Wang, H.K., et al., Population based study on patients with traumatic brain injury suggests increased risk of dementia. J Neurol Neurosurg Psychiatry, 2012. 83(11): p. 1080-5.
8. Lee, Y.K., et al., Increased risk of dementia in patients with mild traumatic brain injury: a nationwide cohort study. PLoS One, 2013. 8(5): p. e62422.
9. Nordstrom, P., et al., Traumatic brain injury and young onset dementia: a nationwide cohort study. Ann Neurol, 2014. 75(3): p. 374-81.
10. Guskiewicz, K.M., et al., Cumulative effects associated with recurrent concussion in collegiate football players: the NCAA Concussion Study. JAMA, 2003. 290(19): p. 2549-55.
11. Eisenberg, M.A., et al., Time interval between concussions and symptom duration. Pediatrics, 2013. 132(1): p. 8-17.
12. Mayeux, R., et al., Synergistic effects of traumatic head injury and apolipoprotein-epsilon 4 in patients with Alzheimer's disease. Neurology, 1995. 45(3 Pt 1): p. 555-7.
13. Sundstrom, A., et al., Increased risk of dementia following mild head injury for carriers but not for non-carriers of the APOE epsilon4 allele. Int Psychogeriatr, 2007. 19(1): p. 159-65.
14. Zhou, W., et al., Meta-analysis of APOE4 allele and outcome after traumatic brain injury. J Neurotrauma, 2008. 25(4): p. 279-90.
15. Hiekkanen, H., et al., Association of injury severity, MRI-results and ApoE genotype with 1-year outcome in mainly mild TBI: a preliminary study. Brain Inj, 2009. 23(5): p. 396-402.
16. Cole, J.H., et al., Prediction of brain age suggests accelerated atrophy after traumatic brain injury. Ann Neurol, 2015. 77(4): p. 571-81.
17. Gilbert, M., et al., The association of traumatic brain injury with rate of progression of cognitive and functional impairment in a population-based cohort of Alzheimer's disease: the Cache County Dementia Progression Study. Int Psychogeriatr, 2014. 26(10): p. 1593-601.
18. McIntyre, A., et al., Mortality among older adults after a traumatic brain injury: a meta-analysis. Brain Inj, 2013. 27(1): p. 31-40.
19. Cheng, P.L., et al., Higher mortality rates among the elderly with mild traumatic brain injury: a nationwide cohort study. Scand J Trauma Resusc Emerg Med, 2014. 22: p. 7.
20. Wetjen, N.M., M.A. Pichelmann, and J.L. Atkinson, Second impact syndrome: concussion and second injury brain complications. J Am Coll Surg, 2010. 211(4): p. 553-7.
21. Broglio, S.P., et al., Cognitive decline and aging: the role of concussive and subconcussive impacts. Exerc Sport Sci Rev, 2012. 40(3): p. 138-44.
22. Dashnaw, M.L., A.L. Petraglia, and J.E. Bailes, An overview of the basic science of concussion and subconcussion: where we are and where we are going. Neurosurg Focus, 2012. 33(6): p. E5: 1-9.
23. Harmon, K.G., et al., American Medical Society for Sports Medicine position statement: concussion in sport. Br J Sports Med, 2013. 47(1): p. 15-26.
24. Rowson, S., et al., Can helmet design reduce the risk of concussion in football? J Neurosurg, 2014. 120(4): p. 919-22.
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