Mild Traumatic Brain Injury
MEDICAL ANIMATION TRANSCRIPT: The brain is the most complex part of the human body. This three-pound organ is the seat of intelligence, database of memories, interpreter of the senses, and the director of all movement. Lying in its bony shell and washed by protective fluid, the brain is also the most fragile organ in the body, with the same texture and consistency as gelatin. Within the brain are over 100 billion nerve cells, called neurons, sending electrical and chemical signals to and from the body. Each neuron has a cell body, a long nerve fiber called an axon, and projections of the cell body called dendrites. Dendrites extend out from the cell body to receive messages from other nerve cells. Axons in the brain connect neurons with each other, which in turn provide extensive interconnections with other brain areas. Because the brain and its nerve cells are so fragile, sudden rapid movements of the head can cause injuries. During one such injury, called coup-contrecoup or acceleration-deceleration injury, the brain bounces back and forth against the bony interior wall of the skull. In high-speed coup-contrecoup injuries, the impact may be violent enough to cause swelling and bruising of the brain tissue called a contusion. However, in cases involving low-speed coup-contrecoup injuries, the resulting damage may not be visible to the naked eye. As the brain moves back and forth within the skull, areas of varying density in the brain slide over each other at different speeds. Axons crossing these junctions experience tremendous shearing forces, causing them to stretch and tear from the cell body. This event is called axonal shearing or diffuse axonal injury. Brain damage can continue to occur for hours or days after the initial injury. Damage to the axons can lead to a breakdown of communication among neurons in the brain. The torn axons quickly degenerate, releasing toxic levels of chemicals called neurotransmitters into the extracellular space. In turn, many of the surrounding neurons begin to die over the next 24 to 48 hours, worsening the initial effects of the injury. Mild to moderate cases of diffuse axonal injury, or DAI, may result in symptoms such as brief loss of consciousness, impaired long-term memory, reduced problem-solving ability, lower social inhibition, and problems with attention and perception. Severe cases of diffuse axonal may result in a coma or a persistent vegetative state. In the United States, over 1 million cases of mild traumatic brain injuries, including diffuse axonal injury, are reported each year. Of this number, over 300,000 patients suffer long-term effects from the damage. Computed tomography, or CT, and magnetic resonance imaging, or MRI, are tests that can be performed to check for mild traumatic brain injury. The results of these tests usually show a normal reading. Therefore, doctors must rely on patient history and a clinical exam to diagnose mild traumatic brain injury.