Army researchers, whose findings were published in the journal Headache, examined nearly 1,000 soldiers with a history of deployment-related concussion and found 20 percent had suffered frequent headaches diagnosed as "chronic daily headache" for three months or more.

Of those, a quarter had the headaches every day. More soldiers with chronic headaches had symptoms of post traumatic stress disorder (PTSD) than those who did not suffer frequent headaches.

Concussion is considered a mild traumatic brain injury and is commonly followed by headaches. But little was understood about how many military personnel were experiencing the intense head pain daily, or close to it, for months on end.

"In general we know that chronic daily headache is itself one of the most debilitating forms of headache ... and can sometimes be difficult to treat," said Major Brett Theeler, the study's lead author.

To gauge how widespread the problem is, Theeler - a doctor with the AMEDD Student Detachment, 187th Medical Battalion, Fort Sam Houston, Texas - and his colleagues surveyed 978 soldiers who had been deployed in Iraq or Afghanistan.

Each of the soldiers suffered a concussion while abroad, and 98 percent of them reported headaches afterward.

Twenty percent of the soldiers screened positive for chronic daily headache, while the rest had headaches on occasion.

Those whose headaches started within a week of their concussion were at greater risk for developing chronic daily headache as opposed to less frequent "episodic" headaches. A little more than half the soldiers with chronic headaches reported that they started within a week of the concussion.

The chronic headache group was also more likely to score higher on a test for signs of PTSD. Nearly twice as many with chronic headache - 41 percent - screened positive for PTSD, compared with 18 percent who didn't have headaches as often.

The finding that more soldiers with chronic daily headache also had PTSD symptoms supports the idea that the headaches could be related to the actual physical brain injury or to the psychological trauma of the event that caused the concussion.

"Head injury is actually a stressful event, independent of what happens to the brain," said Richard Lipton at the Albert Einstein School of Medicine in New York, who was not involved in the study.

Lipton said he'd like to see whether the rates of chronic daily headache among members of the military are similar to those among civilians who have experienced a concussion.

"I don't know, but I would think, of people who live in a military context, in a heightened level of vigilance, it wouldn't be surprising if rates of both episodic and chronic headaches were higher than in the civilian population," he added.

Although the headaches after concussion can be very debilitating for some people, the good news is that they usually clear up with time. Lipton said studies of civilians show that in most cases, they resolve on their own after a year or two.

(Reporting from New York by Kerry Grens at Reuters Health; Editing by Elaine Lies and Paul Tait)

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