The cerebellum is a portion of the brain that is responsible for fine motor movements including posture, balance, motor learning (like learning to kick a ball), and speech. Located at the base of the skull, the cerebellum contains about half of all neurons in the brain, though it represents only 10% of the volume. Losing partial function due to injury or disease isn’t completely unprecedented, though lacking a cerebellum from birth is exceedingly rare. Physicians in China discovered a 24-year-old woman who is only the ninth known case of a living person with cerebellar agenesis. Her condition was described in the journal Brain.
The woman’s condition was discovered after she sought medical attention due to nausea and vertigo. CT scans and MRI images revealed the missing cerebellum, which readily explains why those symptoms would be present. It also explains why she wasn’t able to speak until she was six and wasn’t able to walk until age seven. She had never been able to play and jump like normal kids due to this defect.