A study involving over 180,000 older veterans (average age 68.8 at study start), of whom 29% had PTSD, has revealed that those with PTSD had a significantly greater risk of developing dementia. Over the seven years of the study, 10.6% of the veterans with PTSD developed dementia compared to 6.6% of those without PTSD. When age was used as the time scale, the risk for those with PTSD was more than double. Results were similar when those with a history of head injury, substance abuse, or clinical depression, were excluded.
One possibility for the link is that the stress induced by PTSD contributes to the development of dementia.
(2010). Posttraumatic Stress Disorder and Risk of Dementia Among US Veterans.
Arch Gen Psychiatry. 67(6), 608 - 613.