FRIDAY, Jan. 20 (HealthDay News) -- British troops in Iraq and Afghanistan have lower rates of minimal traumatic brain injury than American troops, new research finds.

In addition, psychological factors present before troops are deployed to combat zones have a major effect on lasting symptoms in military personnel with this condition.

The findings appear in the January edition of the Journal of Head Trauma Rehabilitation, a special issue on minimal traumatic brain injury.

The journal notes that the minimal traumatic brain injury -- injuries with short-term loss of consciousness or changes in mental status, sometimes followed by long-term physical, cognitive and behavioral symptoms -- has been called the "signature injury" of the wars in Iraq and Afghanistan.

In one study, researchers looked at more than 4,600 U.K. troops sent to Iraq or Afghanistan and found that the overall rate of minimal traumatic brain injury was 4.4 percent, increasing to 9.5 percent among those involved in combat. Blast injuries and vehicle crashes were the most common causes.

Previous studies have found that rates of minimal traumatic brain injury among U.S. troops range from 12 percent to more than 22 percent, according to the journal news release.

In another study, the same team of researchers at King's College London found that the difference in minimal traumatic brain injury rates between U.K. and U.S. troops is partly related to deployment length, which is longer for U.S. troops.

However, deployment time could not completely account for the difference between the U.K. and U.S. troops, the researchers added.

They also found that psychological factors such as previous mental health symptoms, alcohol abuse and post-traumatic stress disorder were risk factors for persistent symptoms of minimal traumatic brain injury.

In another study, U.S. researchers at Walter Reed Army Medical Center found that symptoms related to minimal traumatic brain injury were lower in veterans with more severe physical injuries. This may be because the symptoms go unnoticed or develop later in these patients, the researchers said.

Other studies found that caring for someone with minimal traumatic brain injury places a heavy burden on family caregivers, and reported on a promising new treatment approach for veterans who minimal traumatic brain injury as well as post-traumatic stress disorder.

More information

The Defense and Veterans Brain Injury Center has more about traumatic brain injury.