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David & Goliath stone hits head of Goliath

A loss of consciousness. Christopher Giza, professor of pediatric neurology and neurosurgery at UCLA, explains what happens when you’re knocked out, how it’s different from the average concussion, and the best course of action following a knock-out blow.

What does it mean to be knocked unconscious?

Your brain has three big parts. There’s the left hemisphere, the right hemisphere, and the brainstem at the bottom. You can lose consciousness if both hemispheres are turned off at once — though if only one is affected, the other can pick up some of the slack. You can also lose consciousness if part of the brainstem is knocked offline. Brain activity can be affected by a number of things — oxygen can be cut off to certain parts, or a blood vessel might burst. But a blow to the head can also cause a disruption that could lead to a loss of consciousness.

Think about the brain as firm jello. The two hemispheres are heavy, and the brainstem connecting the two hemispheres to the rest of the nervous system is narrow, like the stem of a flower. When the head is moved violently, the brain moves around in the skull. The heaviest part of the brain puts a lot of pressure on the brainstem, which can be twisted and pulled during the blow as the rest of the brain moves out of place. That twisting and pulling can cause brain circuits to break, or lose their insulation, or get kinked up, and that shuts off parts of the brain. If the part of the brainstem responsible for consciousness is affected, then you would be knocked out.

What is the difference between being knocked out and suffering a concussion?

They’re related, but not the same. People used to think that if you didn’t lose consciousness, you didn’t have to worry about being concussed. Thanks to further research into concussions, we now know that, in about 90 percent of diagnosed concussions, there is no loss of consciousness.

During any severe blow to the head, the same thing is happening. The brain twists, the circuits can break, and the damage sends the brain into crisis. But different parts of the brain can be affected. Concussions typically cause vision problems, disorientation, memory loss, headaches, balance issues, and a host of other ailments as the circuits that undergird these functions go down. But as long as the part of the brain responsible for consciousness is minimally affected, there’s no guarantee that you’ll be knocked out.

What are the long term effects of being knocked unconscious?

It depends on the severity of the injury. If you lose consciousness briefly, and suffer a concussion, 75 to 90 percent of people will fully recover in a few months. But severe damage to the brain can cause unconsciousness for days, weeks, or even longer. If there is internal bleeding or swelling in the brain, surgery may be necessary to relieve pressure on the brain. Severe injuries can also cause lasting effects that vary — including memory loss, paralysis, seizures, and lasting behavioral or cognitive changes — depending on the areas of the brain affected. But in those cases, unconsciousness is a symptom of the injury, not a cause of long term deficits.

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