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They have no visible signs of injuries, no head wounds or burns. But soldiers who are nearby when an improvised explosive device detonates may suffer mild brain damage, new research suggests.

The shock wave produced in this type of explosion can travel through the brain and cause a concussion, also known as a mild traumatic brain injury, says Andrew Baker, a researcher at St. Michael's Hospital in Toronto.

A shock wave is a short, intense burst of energy, and Dr. Baker recently discovered that even a mild one causes damage in rats, in particular to the white matter that connects different parts of the brain and allows them to communicate with each other.

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"To stand by as a blast wave goes through your brain is not a good thing," he said.

Dr. Baker will describe the research at a news conference Thursday to celebrate the opening of the hospital's Keenan Research Centre, which will bring together clinician-researchers from a number of fields, including neuroscience.

The findings suggest that soldiers lucky enough to escape an obvious injury after an IED blast might still be hurt. Medics who assess them need to be on the lookout for such signs as dizziness, an inability to concentrate and sensitivity to sounds or light.

You can get a concussion without losing consciousness. And if a second brain injury occurs before the first one has healed, the result can be devastating and, in rare cases, deadly.

In the rats, the damage from the mild shock wave didn't occur instantly, but slowly, in the days that followed their exposure. This suggests it may be possible to intervene, says Dr. Baker, who published his findings earlier this spring. He and colleagues are looking at neuroprotective drugs that might limit the damage caused by this kind of injury.

Concussions usually occur when there is rapid acceleration or deceleration of the head and the brain moves or rotates inside the skull. Parts of the brain with different densities accelerate at different rates, causing damage as they move against each other. But in the case of shock waves, the injuries can occur at the junctions between different parts of the brain, Dr. Baker says.

"You could be standing perfectly still and have this burst of energy make its way through your head," he said.

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The lungs, bowel and inner ear are vulnerable to damage from shock waves, also known as blast waves. But in his research into shock waves' effect on the brain, Dr. Baker wants to know if the damage is the same as in concussions that occur after a blow to the head or a fall.

"One of the projects we might do next is, 'Is the physiology exactly the same?' " he said. "When you look at brains of people who have had repeated concussions, it is the white matter deterioration that is a problem."

.......

Blast injuries have four parts

Shock waves, flying debris, winds and heat or chemical exposure succeed one another

When an improvised explosive device detonates, it produces a short, intense burst of energy called a shock wave or blast wave. There are four components to blast injuries.

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Primary blast trauma: The shock wave causes injury to gas-filled organs like the lungs and bowels and can cause the eardrum to rupture. It may also cause brain damage.

Secondary blast trauma: These are injuries sustained when soldiers or civilians are hit with debris travelling at high speeds after the detonation.

Tertiary blast injuries: Blast winds trailing the primary shock wave can amputate limbs or throw people into the air.

Quaternary blast injuries: These result from high heat and exposure to noxious chemicals associated with the fire that follows the detonation.

Source: Andrew Baker, St. Michael's Hospital

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