The human brain continues to mystify. Small injuries can be lethal, yet some people manage surprisingly well with large portions of their brains missing. An extraordinarily rare case of a woman with a missing cerebellum was presented last month in the journal Brain.

The 24-year old woman was only recently diagnosed as having congenital cerebellar agenesis, or no cerebellum from birth. Her condition is only the ninth documented case in the world. It is not impossible for individuals to survive partial loss of the cerebellum as a result of some injury or disease, but living and functioning without ever having one is extraordinary.

The term “cerebellum” means “little brain.” It is located on the back of the brain stem, just under the rear-most lobes of the cerebrum, or “big brain.” The cerebellum is responsible for fine motor control, balance, posture, motor learning of activities such as walking, and the motor aspects of speech. It comprises only about ten percent of the total human brain volume but houses nearly half of all brain neurons.

Doctors discovered the missing cerebellum in the young married mother of one child after the woman sought medical consultation for persistent nausea and vertigo. Imagining revealed a fluid-filled space where the woman’s cerebellum should have been. The woman has had life-long difficulty walking without support and producing speech.

The physicians were surprised at how the woman’s deficits were not more severe. Researchers have identified approximately 30 mutations associated with cerebellum structural disruption. However, no one knows what leads to complete failure of a cerebellum to form during development.