When word got out that a soldier based at Joint Base Lewis-McChord was accused of killing multiple Afghan civilians, one of the first bits of information about him to emerge was that he had suffered a head injury on a previous tour of duty.

Robert Bales' attorney, John Henry Browne, has latched on to that detail, repeatedly mentioning traumatic brain injury (TBI) in conversations with reporters.

With facts about the incident, as well as Bales' history, so closely guarded, it's impossible to know what role, if any, TBI may have played in the case. There's been no official confirmation that he was, in fact, even diagnosed — and if he was, no word on how the injury manifested.

Army officials have said Bales was fit for combat and cleared for return to duty after the head injury he suffered during one of his three deployments to Iraq.

In fact, experts say TBI covers a very broad range of problems, and its symptoms can be anywhere from mild and temporary to life-altering and lifelong. At worst, a brain injury can lead to coma and death.

Harborview Medical Center neurosurgeon Randall Chesnut said TBI "is not a black-and-white thing. It's shades of gray." He studies and treats the disorder.

Most of us have had some type of head injury, he explained, although we might not have recognized it as such.

"Everybody's whacked their head a few times," Chesnut said. As a rugby player, he added, "I don't know how many concussions I've had." A concussion is a type of brain injury.

The Centers for Disease Control and Prevention estimates 1.7 million people sustain a TBI annually. Most are classified as mild.

In the military, head injuries are more common than in the general population, according to a 2008 report to the Army Surgeon General. They have been called the "signature injury" of the wars in Iraq and Afghanistan. Part of that is because head injuries are better recognized today. It's also because medical care has advanced, and soldiers are living with injuries that once might have killed them. But it's also part of the nature of wars in which roadside bombs are a weapon of choice.

Traumatic brain injury can be caused by a bump on the head, a penetrating wound or by the concussive force of explosions.

Symptoms can include changes in vision and the sense of smell, dizziness, loss of balance, irritability, sleeplessness, short-term memory lapses and problems maintaining attention.

Still, Chesnut said, "in 12 weeks, the vast majority of people are back to baseline."

Then there are those who never go back. Some patients may have physical symptoms, including such things as an inability to control a limb. Some may live with constant dizziness, as if the world is off-kilter. Others may have mood disorders that lead to problems at work and at home. They may need ongoing rehabilitation and counseling.

If someone like Bales had a serious head injury, he'd likely be exhibiting some pretty obvious symptoms.

"I think the military has become much more competent at diagnosing and treating these things," Chesnut said.

But can TBI lead someone to snap, to go crazy and commit mass murder? Out of the blue?

Chesnut, for one, doesn't see that as likely.

Maureen O'Hagan: 206-464-2562 or mohagan@seattletimes.com