MRI scans showed no recognizable cerebellar structure, with a pocket of dark fluid in the back of the brain where the cerebellum should be. Feng Yu et al, Brain Feng Yu et al, Brain

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A 24-YEAR-OLD WOMAN walked into a Chinese hospital with some relatively routine complaints.

She said she had been feeling dizzy and had been nauseous and vomiting for a month. But some of her problems were more peculiar: For more than 20 years, she said, she had been unable to walk steadily.

“According to her mother, she was 4 years old before she could stand unassisted, and did not begin to walk unassisted until the age of 7,” her doctors wrote in the journal Brain. “She never ran or jumped. Her speech was not intelligible until 6 years of age and she did not enter school.”

As an adult, her language was mostly normal, though her speech showed a “mild voice tremor with slurred pronunciation.”

At 24, she had “mild mental retardation” and was married with a daughter, living a relatively ordinary life. But when the doctors scanned her brain, they were stunned: Near the back of her brain, where her cerebellum should have been, there was an empty space. In place of brain tissue, the gap had filled with fluid.

The condition is almost mind-bogglingly rare. As New Scientist, which reported on the case, notes: “Just nine people … are known to have lived without their entire cerebellum.”

F, Braieng Yu et al F, Braieng Yu et al

The cerebellum, Latin for “little brain,” is a small but densely packed structure. Neuroscience students typically learn that while “it accounts for approximately 10% of the brain’s volume, it contains over 50% of the total number of neurons in the brain.”

The cerebellum is known to be important to the ability to move smoothly — especially our coordination, balance, posture, and motor learning — but recent research suggests it may also be at least nominally involved in a huge number of cognitive functions.

“The range of tasks associated with cerebellar activation is remarkable and includes tasks designed to assess attention, executive control, language, working memory, learning, pain, emotion, and addiction,” a team of scientists, led by neurobiologist Peter Strick, wrote in 2009.

That’s why it seems remarkable that a woman could survive until 24 without anyone ever realizing that this crucial part of her brain was completely absent.

CT scans showed a dark abyss instead of a cerebellum. Feng Yu et al, Brain Feng Yu et al, Brain

The case, the doctors suspect, is just more evidence of the brain’s plasticity: its extraordinary ability to adapt and grow, even in the face of severe deficiencies. Additional testing may reveal a clearer picture of exactly how this woman’s brain has compensated for its missing piece.

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