More than half of U.S. combat troops in Afghanistan have been exposed to bomb blasts in the last year, but only about 1 in 5 of them said they were examined for concussions, according to a draft of a recent military survey.

Medical officials failed to screen about 80 percent of soldiers and Marines who reported being within 50 meters of a roadside blast during their tour of duty, according to combat troops surveyed in July and August of last year.

The survey noted, however, that the troops were quizzed before full implementation of a new military policy in June mandating screening for troops exposed to such bombs.

The survey, which has not been finalized, but was obtained by ProPublica, NPR and USA Today, is conducted to assess the mental health and morale of America's troops. Part of the survey examines the military's efforts to treat traumatic brain injuries, also known as concussions.

Screenings for such wounds are important because concussions caused by blast waves are difficult to detect, yet may cause lasting cognitive issues, especially when soldiers absorb multiple injuries. Most soldiers recover within two weeks. But civilian and military studies have suggested that a minority of concussion victims, between 5 percent to 15 percent, go on to suffer cognitive problems, such as having difficulty reading or following instructions. Multiple concussions over a long period have been linked in athletes to a condition called chronic traumatic encephalopathy, which has dementia-like symptoms.

Official military figures show that more than 155,000 troops have suffered concussions since the beginning of the wars in Afghanistan and Iraq, many of them caused by blasts from roadside bombs, a common insurgent weapon. Nearly 50,000 others have suffered more severe brain injuries. Previous ProPublica and NPR stories cited studies showing that as many as 40 percent of mild traumatic brain injuries go undiagnosed.

Overall, the survey presented a bleak picture of an increasingly dangerous war in Afghanistan. One extraordinary statistic: Near the peak of violence in Iraq in 2006, from 12 percent to 15 percent of troops responding to a similar survey reported killing an enemy. In Afghanistan last year, 48 percent to 56 percent of combat troops surveyed reported being "directly responsible" for killing a combatant—a more than threefold increase.

In addition, about 50 percent to 60 percent of soldiers and Marines in Iraq in 2006 reported that a comrade had suffered a casualty. In Afghanistan, 73 percent of soldiers and almost 80 percent of Marines reported having a buddy who was wounded or died.

The Army has struggled to keep up with flood of soldiers suffering from so-called invisible wounds of war, such as traumatic brain injury and post-traumatic stress disorder. Last month, ProPublica and NPR reported the Army is facing a "critical" shortage of neurologists to implement its new initiative to improve diagnosis and treatment of mild traumatic brain injuries.

At a hearing last June, Gen. Peter Chiarelli, the Army's vice chief of staff, told Congress that the Army had a total of 52 neurologists, though only 40 were practicing—a figure, he said, that included child neurologists. "We're an Army that's in uncharted territory here," Chiarelli recently told USA Today. "We have never fought for this long with an all-volunteer force that's 1 percent of the population."