A review of nearly 2.8 million patient cases in Denmark found that the risk of dementia in individuals with a history of TBI was 24% higher than those without a history of TBI, after accounting for other risk factors.
Risk was greater when the TBI was characterized as "severe" — 35% compared to an increased risk of 17% for a single "mild" TBI or concussion.
Moreover, dementia risk increased 33% higher for two or three TBIs, 61% higher for four TBIs, and 183% higher for five or more TBIs.
Additionally, the study found that if you have a brain injury in your 20s, the risk of developing dementia in your 50s increased by 60%.
The findings suggest that those with a history of TBI (particularly multiple injuries) should make an effort to reduce other risk factors, such as limiting alcohol and tobacco use, engaging in regular exercise, preventing obesity, and treating hypertension, diabetes, and depression.
Among the nearly 2.8 million people observed, 4.7% had at least one TBI diagnosis. Among first TBI diagnoses, 85% were characterized as mild.
Among men and women with TBI histories, men had slightly higher rate of developing dementia (30% vs. 19%).
Note that an earlier study (much smaller) found no evidence of a link between TBI and Alzheimer's.
This study involved 706 older adults from the National Alzheimer's Coordinating Center database, of whom 274 had Alzheimer’s. It found that there was no significant difference in the rate of cognitive decline between demographically and clinically similar participants with and without a history of TBI. This held regardless of APOE4 status.
However, researchers added the caveat that the findings should be interpreted cautiously due to the crude and limited assessment of TBI history available through the database.
Tripodis, Y. et al. 2017. The Effect of Traumatic Brain Injury History with Loss of Consciousness on Rate of Cognitive Decline Among Older Adults with Normal Cognition and Alzheimer’s Disease Dementia. Journal of Alzheimer's Disease, 59 (1), 251-263.