You, Illuminated

Commonsense explanations of neuroscience

Brain Condition Saves Aurora Shooting Victim’s Life: Explained

Petra Anderson's brain "defect" may have saved her life.

Amid the tragedy and heartache in Aurora last week, a fascinating story of survival emerged. Petra Anderson was shot in the head, and after the bullet entered the front of her skull, it traveled completely through her brain before coming to rest at the back of her skull. I read her story in a blog written by her church pastor, Brad Strait, who has been with her in the hospital during her surgery and recovery. As Brad wrote:

“It seems as if the bullet traveled through Petra’s brain without hitting any significant brain areas. The doctor explains that Petra’s brain has had from birth a small ‘defect’ in it. It is a tiny channel of fluid running through her skull, like a tiny vein through marble, or a small hole in an oak board, winding from front to rear.

But in Petra’s case, the shotgun buck shot, maybe even the size used for deer hunting, enters her brain from the exact point of this defect. Like a marble through a small tube, the defect channels the bullet from Petra’s nose through her brain. It turns slightly several times, and comes to rest at the rear of her brain. And in the process, the bullet misses all the vital areas of the brain. In many ways, it almost misses the brain itself. Like a giant BB though a straw created in Petra’s brain before she was born, it follows the route of the defect. It is channeled in the least harmful way. A millimeter in any direction and the channel is missed. The brain is destroyed.”

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My curiosity was raised after hearing about this “defect,” but not given any information about it. To find out what it might be, I spoke with neurosurgeon Raymund Yong, MD, clinician and brain cancer researcher at the National Institutes of Health. He speculated that it was an arachnoid cyst, a condition present in one out of 50 people. It usually develops at birth but is typically harmless and undetectable without the visual aid of a CAT or MRI scan.

The arachnoid is one of three layers of tissue, along with the dura and pia mater, that surround and protect the brain. They form a cushion between the skull and the cortex, and they hold in the cerebro-spinal fluid that bathes the brain and provides nutrients.

The arachnoid, positioned beneath the thick, leathery dura, is the middle layer, and it has a porous structure that resembles a spider web, hence the name “arachno,” from the Greek for spider. Beneath the arachnoid layer, the pia mater hugs every curve of the cortex so small gaps are present between the arachnoid and pia layer.

An arachnoid cyst typically begins as a small bubble between the arachnoid and the pia layers that grows during development, pushing brain matter away from the location of the cyst. The cyst is simply a pocket of space filled with cerebro-spinal fluid.

While surviving bullet wounds to the brain is not uncommon, as evident by Congresswoman Gabby Giffords, there is usually a significant risk of brain damage. Yong called the likelihood of a bullet entering the skull and following the exact path of an arachnoid cyst a “one in a million probability.”

As Brad Strait put it, “Petra’s neurosurgeons used the words like "happily" and "wonderfully” to describe the chance of the bullet following the harmless path, but “I know a miracle when [I] see it.”

Joshua Gowin, Ph.D., earned his doctorate in behavioral neuroscience at the University of Texas Health Science Center in Houston.

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