It's one of the heart-wrenching images now etched into our minds: A Boston Marathon runner blown off his feet by a bomb blast -- one of two explosions on Monday that killed three people and injured more than 170 others.
"Shock waves just hit my whole body and my legs just started jittering around. I knew I was going down," 78-year-old Bill Iffrig of Lake Stevens, Wash., told CNN on Monday.
Iffrig walked away on his own. He later told reporters that he felt fine, with the exception of a scraped knee and ringing in his ears.
For runners and bystanders near the bombings, however, an absence of obvious physical injury may not guarantee an escape from bodily harm. Recent research on Gulf War veterans and NFL football players highlights the dangers posed by even mild brain trauma. Exposure to shock waves from the powerful explosions near the marathon finish line, experts warned, may result in initially silent yet potentially serious long-term effects on the brain.
A bomb blast's short-lived supersonic wave "packs a wallop," said Dr. Lee Goldstein, a biomedical engineer at Boston University. "It can knock you off your feet." He added that it's the second component of a blast that raises the biggest concern for the brain.
"Right behind that is this blast wind that goes back-and-forth, causing the head to swing back-and-forth on the neck, like a bobblehead, very quickly," Goldstein explained.
While health experts have long believed that one explosion exposure is unlikely to cause long-term damage, Goldstein pointed to emerging evidence that suggests otherwise. In a study published in May, Goldstein and his colleagues found that soldiers exposed to a single roadside bomb blast had brain trauma similar to football players who had suffered multiple concussions.
"One blast is really like getting multiple head injuries over a compressed period of time," Goldstein said, comparing each slug of alternating air to a hard-hitting tackle from a 300-pound linebacker.
Autopsies of both military veterans and former athletes showed signs of chronic traumatic encephalopathy, or CTE. Over time, the degenerative disease can resemble Alzheimer's, with symptoms that include irritability, memory and attention-span problems, dementia and suicidal thoughts. In the same study, Goldstein's team also found that animals developed evidence of the disease just two weeks after exposure to a single simulated blast.
Goldstein emphasized that much remains to be learned about CTE, and that we can't say whether anyone at the bomb site will actually develop the disease. The bombs used in Boston appear to have been made with relatively low-grade explosives. Still, he said, the possibility "certainly raises concern."
Dr. Linda C. Degutis, director of the Centers for Disease Control and Prevention Injury Center, also expressed concern.
"We know based on a lot of research done that with any explosive device there is a possibility of having some traumatic brain injury even if you're not actually hit with pieces of the bomb itself," said Degutis. Brain injuries frequently go undiagnosed after blasts, she said.
The CDC and the National Institute of Neurological Disorders and Stroke list potential early symptoms of a moderate or severe traumatic brain injury: repeated vomiting or nausea, convulsions or seizures, an inability to awaken from sleep, dilation of one or both pupils, slurred speech, weakness or numbness in the extremities, loss of coordination and increased confusion, restlessness, or other abnormal behavior.
The difficulty in recognizing milder injuries, generally accompanied by subtle symptoms such as headaches or a ringing in the ears, worry Degutis. Such minor damage can still lead to major problems in the future, particularly if the victim goes undiagnosed and untreated.
"Just like other injuries you might have, such as a sprained ankle, resting the brain is important," Degutis said, noting that a secondary injury while the brain is recovering may be more likely to cause devastation.
Dr. Jeff Bazarian, a brain injury expert at the University of Rochester Medical Center, referred to lessons from 9/11.
"People went to hospitals with fractures and internal organ injuries," Bazarian said. "But long after those bones healed and internal organs were fixed, it was the overlooked concussions that ultimately interrupted their ability to go back to what they were doing before.
"Concussions often get missed because other injuries are more life-threatening," added Bazarian, who, has studied impacts of head injuries on athletes and soldiers. Some people never go to the hospital, he said. They may feel fine at the time.
So just how many of these people, which Bazarian refers to as the "walking wounded," might there from the Boston bombings? That, of course, depends on how far the damaging waves travelled from the blast. And that, too, depends.
The U.S. military uses a bomb blast threshold of 50 meters as a rough radius within which it will classify someone as brain-injured. The military requires these soldiers to rest for 24 hours, regardless of symptoms.
While 50 meters is a good "rule of thumb," according to Bazarian, the strength and location of the blast is also key. A blast on the side of an urban street, where tall buildings can concentrate and reflect shock waves, would look very different than one detonated in the desert.
In fact, depending on the geometry of building walls and streets, it's possible that someone 10 feet from the blast may have greater injury than someone standing just three feet away, noted Goldstein.
Vulnerability to the blast also depends on the person. Young children, whose brains have not fully developed, and the elderly tend to be at greatest risk.
Bazarian noted that a pair of promising blood tests could one day help first responders identify people who have sustained brain injuries. The tests are not yet approved by the Food and Drug Administration.
For now, experts agreed that health professionals and victims should watch carefully for brain injury symptoms.
Of course, some of the symptoms can overlap with those of post-traumatic stress disorder, another serious concern after an emotionally-charged event like the Boston bombings.
"That's another reason for folks to see their doctor," said Bazarian. "If not recognized and treated early, PTSD can also become more of a long-term problem."