PRIMARY BLAST INJURY
An explosion generates a blast wave traveling faster than sound and creating a surge of high pressure followed by a vacuum. Studies show that the blast wave shoots through armor and soldiers' skulls and brains, even if it doesn't draw blood. While the exact mechanisms by which it damages the brain's cells and circuits are still being studied, the blast wave's pressure has been shown to compress the torso, impacting blood vessels, which send damaging energy pulses into the brain. The pressure can also be transferred partially through the skull, interacting with the brain.
SECONDARY BLAST INJURY
Shrapnel and debris propelled by the blast can strike a soldier's head, causing either a closed-head injury through blunt force or a penetrating head injury that damages brain tissue.
TERTIARY BLAST INJURY
The kinetic energy generated and released by an explosion can accelerate a soldier's body through the air and into the ground or nearby solid object. Once the body stops, the brain continues to move in the direction of the force, hitting the interior of the skull and then bouncing back into the opposite side, causing a coup-contrecoup injury.
Sources: Interviews with Dr. Ibolja Cernak, M.D., M.E., Ph.D., of the Johns Hopkins University Applied Physics Laboratory and Dr. Michael R. Yochelson, M.D., of the National Rehabilitation Hospital; "Traumatic Brain Injury: An Overview of Pathobiology With Emphasis on Military Populations" by Ibolja Cernak and Linda J. Noble-Haeusslein in the Journal of Cerebral Blood Flow & Metabolism; brainline.org; Defense and Veterans Brain Injury Center