brain trauma myelin
July 15, 2015

Why kids’ recovery times vary widely after brain injury

Traumatic brain injury (TBI) is the single most common cause of death and disability in children, and while some recover from it quickly, others suffer from often devastating side effects for years—especially deficits in cognitive function. Scientists had no explanations for the difference until now, when a study out of UCLA/USC determined the problem came down to myelin, a fatty substance coating parts of the brain.

Short circuits

The neurons of the brain communicate to each other via connecting “wires” known as axons. And like real-life wires, axons are often coated to make them more effective.

“Think of myelin around axons as the insulation on electrical wires,” Dr. Christopher Giza, a professor of pediatrics and neurosurgery at UCLA’s David Geffen School of Medicine and Mattel Children’s Hospital, explained to redOrbit via email. “Without it, the wire still conducts electricity, but not as well and it is more likely to short circuit.”

The researchers suspected that myelin was playing a key role in traumatic brain injury severity. “Although myelin is like insulation, it is made mostly of lipids (fat), and it is wrapped around the axon like an onion skin (many layers). When the axon (wire) is stretched by trauma (TBI), the wrapping is also stretched and unloosened. Then the wire/axon is more likely to short circuit.”

To determine if myelin was indeed to blame, the researchers evaluated 31 healthy children and 32 children, ages 8 to 19, who had suffered from a TBI in the past five months. The children were given tests to evaluate their processing speed, short-term memory, verbal learning, and cognitive flexibility.

Then, the electrical activity of their brains was measured to determine how quickly their nerve fibers could transmit information. Images were also taken to assess their brains’ structural soundness—the first time two such tests were combined in a study of TBI.

The outcome

Half of the 32 children studied with brain injuries showed widespread damage to the myelin in their brains. In this half, performance on cognitive tests dropped 14% compared to the control group, and their writing was three times slower.

The other 16 children had myelin that was nearly intact, and performed nearly as well as the control group; on the cognitive tests, they scored 9% better than the 16 children with more myelin damage, indicating that damaged myelin led to greater deficits.

By identifying the cause of cognitive issues in traumatic brain injury, the researchers have identified biomarkers that physicians will be able to use. “Our research suggests that imaging the brain’s wiring to evaluate both its structure and function could help predict a patient’s prognosis after a traumatic brain injury,” said author Emily Dennis, a postdoctoral researcher at USC’s Keck School of Medicine.

What about adults?

“Myelin injury would/does have an effect on cognitive function in adulthood, but not necessarily a greater effect,” wrote Giza. “Brain fibers (axons) with myelin are more resistant to biomechanical injury like trauma. Young brains are not completely myelinated, and so the brain fibers in kids more vulnerable to traumatic brain injury (TBI) than in adults, which have more myelin. Younger is not always better! Also, brain networks (which are made up of different brain regions connected by myelinated axons, like the 'internet' for the brain) are not fully developed in young brains...so if the networks are disrupted before they are fully formed, there is a greater chance for lasting injury.”

Recovery is still possible for the myelin-damaged children, but inflammation caused by brain trauma can often complicate matters. “TBI induces a whole response of inflammation, and inflammatory cells in the brain like to clean up damaged parts. So inflammation induced by trauma can result in inflammatory cells (microglia) cleaning up bits of damaged myelin and often incidentally cleaning up bits of axon and neurons that are damaged.”

Inflammation cannot only harm neural connections, but can stop the regrowth of myelin as well. “Myelin regenerates slowly but cannot grow back if myelin-producing cells (oligodendrocytes) are damaged or destroyed. Also, if inflammatory cells (like microglia) are blocking the way to the axon, then even if the oligos survive they cannot reach out and wrap their myelin around damaged axons.”

(Image credit: Thinkstock)

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