Veterans who suffer from post-traumatic stress disorder, or PTSD, have a greater risk of developing dementia later in life, a new study suggests.

Higher rates of PTSD, which is caused by exposure to traumatic and life-threatening experiences, are found among veterans than in the general population. About 30 percent of Vietnam War veterans and 12 percent to 20 percent of Iraq War veterans have it, according to the National Center for Post Traumatic Stress Disorder.

The study included 10,481 veterans ages 65 or older who had visited the Michael E. DeBakey VA Medical Center in Texas at least twice between 1997 and 1999. Records on these veterans were followed though 2008.

Among those diagnosed with PTSD who did not also suffer injuries from combat, 11.1 percent had developed dementia. Among veterans without the disorder, 4.5 percent developed dementia.

It can't yet be said whether it's a matter of cause and effect, said study researcher Dr. Salah Qureshi, a psychiatrist at the center.

"We do not know if PTSD is the cause of higher rates of dementia," Qureshi told MyHealthNewsDaily, but "it will be interesting to see if treating PTSD successfully lowers the dementia rates in these individuals."

There were also differences in the prevalence of dementia among veterans who did suffer combat injuries: 7.2 percent of veterans with injuries and PTSD developed dementia, whereas 5.9 percent of those who had injuries but not PTSD did.

Researchers also accounted for added risk factors like heart disease, diabetes and stroke.

People with post-traumatic stress disorder can experience mood problems, difficulty sleeping, nightmares and extreme aversion to things or people that remind them of the traumatic experience.

Research still needs to be done to determine if the brains of people with PTSD are just more likely to develop dementia, or if the two illnesses have commonalities, according to the researchers.

"It is important to determine if treating PTSD effectively can reduce the risk of dementia," Qureshi said. "Returning soldiers from Afghanistan and Iraq should follow the recommendations made by their clinicians."

The study was published in the September issue of the Journal of the American Geriatrics Society.

This article was provided by MyHealthNewsDaily, a sister site to LiveScience.