FRIDAY, Oct. 28 (HealthDay News) -- Headaches are a leading reason for medical evacuation of U.S. troops from Iraq and Afghanistan and for the falling number of active-duty troops in those countries, a new study says.

Neurological illness is one of the top three causes of non-combat-related loss of unit strength among U.S. troops in Iraq and Afghanistan, and headaches are the most common neurological problem, according to the Johns Hopkins-led team of researchers.

Post-concussion headaches and migraines are the most common forms of headache requiring evacuation, and physical trauma was the cause of nearly half of the debilitating headaches.

Only one-third of troops who are sent home because of headaches return to duty in either place, the researchers found.

They reviewed the medical records of all 985 military personnel who were medically evacuated from Iraq and Afghanistan between 2004 and 2009 with a primary diagnosis of headache (about one in 1,000 of the soldiers deployed in the regions).

The study did not include those who were medically evacuated primarily for other reasons (such as trauma) that caused headaches.

About 67 percent of troops who were evacuated because of headaches never returned to the war zone. Those most likely to return were officers, whose jobs are less physically strenuous, and women, who are much less likely to have combat roles.

Nearly half of those with tension headaches returned to the war zone, compared with 20 percent of those whose headaches were associated with physical trauma, such as post-concussion headaches.

The lowest rates of return were among headache sufferers who also had a diagnosis of a psychiatric illness or traumatic brain injury and those whose headaches were treated with narcotic pain relievers.

Many of the headaches suffered by troops were the result of damage to, or pressure on, the occipital nerve located at the back of the head. This can be caused by the heavy Kevlar helmets soldiers often have to wear for long periods of time.

"Everyone who goes on patrol wears a Kevlar helmet. They are heavy. They are hard to wear. But if you get a headache from your helmet, you still must wear it. If you can't tolerate your helmet, you can't do your job. It would be too dangerous. So these folks end up being evacuated and not returning to duty," study leader Dr. Steven P. Cohen, an associate professor of anesthesiology and critical care medicine at the Johns Hopkins University School of Medicine and a colonel in the U.S. Army Reserves, said in a Hopkins news release.

The study was published online Oct. 12 in the journal Cephalalgia.

More information

The U.S. National Institute of Neurological Disorders and Stroke has more about headache.