Even in the daily bedlam of Iraq, life for Jason Ehrhart and Larry Perry had a measure of clarity. But that was before roadside bombs blew them out of their Humvees and into a fog that has yet to lift.
It's not clear whether either suffered a direct blow to the head, but like many brain-injured comrades, they have lingering memory problems. What is clear is that invisible blast waves slammed into their skulls and shook their brains like gelatin.
As many as one in five combat troops in Iraq and Afghanistan have suffered traumatic brain injuries, and military medical experts believe the concussive force of blast waves has contributed to more than half of those. The damage can accumulate - the result of repeated exposure to explosions at a distance.
Long-term effects can include headaches, slow thinking, poor attention and difficulty carrying out plans.
Many of the injuries have multiple causes, such as blast waves combined with the impact of flying shrapnel or a crash landing.
But doctors have also treated patients who weren't hit by anything except blast forces. In some cases, they walked away without realizing they had suffered concussions.
"Blast injuries have unfortunately been a hallmark of the recent wars," said Dr. Paul Fishman, chief of neurology at the Baltimore Veterans Affairs Medical Center. "We now have a growing body of people who have traumatic brain injury but with no apparent head injury."
Although the heavy toll from improvised explosive devices (IEDs) in Iraq and Afghanistan has grabbed 21st-century headlines, blast injuries are not a new phenomenon. From the trenches of World War I to the jungles of Vietnam, combat troops have long been exposed to the brain-altering forces of exploding bombs.
The same is true for victims of suicide bombs in the Middle East, some of whom were standing far enough from explosions to escape obvious physical injury. Israeli doctors in particular have discovered serious and sometimes fatal damage to victims' brains and abdominal organs - the result of blast waves penetrating tissue.
"Blast is a bit difficult to understand," said Fishman, but he offered a simple analogy:
"With ultrasound, we can break up kidney stones. If you have energy directed into tissue and the tissue absorbs that energy, something's got to give."
Blast waves are accompanied by rapid changes in temperature and pressure that can damage the brain, spinal cord and other organs bathed in fluids, as well as air-filled organs like the ear and lungs.
