A woman in China has stunned doctors after a trip to the emergency room for dizziness and nausea revealed that a key part of her brain was missing.

It appears the woman was born without a cerebellum, a small but major part of the brain that controls such things as movement and balance.

While many people survive being born without certain segments of their brain or can live after having parts of their brains removed, there have only been eight other documented of people surviving without a cerebellum. Most died quite young; this woman has already lived to 24 before receiving her diagnosis.

It's not clear how the woman had lived this long without a cerebellum, which is sometimes known as the "little brain." The brain segment, located underneath the two main hemispheres of the brain, represents only about 10 per cent of the brain's total volume, but it contains a full 50 per cent of its neurons.

The case, her doctors say, highlights just how well the brain can adapt to deficiencies.

The woman told doctors that she had had problems walking steadily for most of her life. Her mother reported the woman hadn't been able to stand without assistance until she was four, hadn't learned to walk until she was 7, and had never learned to run or jump. It has also been impossible to understand her speech until she was about six.

The discovery was made when the woman, a married mother of one, went to hospital in Shandong province complaining of intense dizziness and nausea that had lasted for a month. Doctors did a CT scan and immediately noticed that the space where the cerebellum should be was empty, filled instead with cerebrospinal fluid.

The woman's doctors suspect that the functions that the cerebellum normally performs had been taken over by the cerebral cortex.

The results of the woman's initial examination are published in the journal Brain.

As for the woman's prognosis, doctors are not able to say since her condition is so rare.