The discovery was made when the woman was admitted to the Chinese PLA General Hospital of Jinan Military Area Command in Shandong Province complaining of dizziness and nausea.
She told doctors she'd had problems walking steadily for most of her life, and her mother reported that she hadn't walked until she was 7 and that her speech only became intelligible at the age of 6.
Doctors did a CAT scan and immediately identified the source of the problem – her entire cerebellum was missing (see scan, below left).
The space where it should be was empty of tissue. Instead it was filled with cerebrospinal fluid, which cushions the brain and provides defence against disease.
The cerebellum – sometimes known as the "little brain" – is located underneath the two hemispheres.
It looks different from the rest of the brain because it consists of much smaller and more compact folds of tissue. It represents about 10 per cent of the brain's total volume but contains 50 per cent of its neurons.
Although it is not unheard of to have part of your brain missing, either congenitally or from surgery, the woman joins an elite club of just nine people who are known to have lived without their entire cerebellum.
A detailed description of how the disorder affects a living adult is almost non-existent, say doctors from the Chinese hospital, because most people with the condition die at a young age and the problem is only discovered on autopsy (Brain, doi.org/vh7)
The cerebellum's main job is to control voluntary movements and balance, and it is also thought to be involved in our ability to learn specific motor actions and speak. Problems in the cerebellum can lead to severe mental impairment, movement disorders, epilepsy or a potentially fatal build-up of fluid in the brain.
However, in this woman, the missing cerebellum resulted in only mild to moderate motor deficiency, and mild speech problems such as slightly slurred pronunciation.
Her doctors describe these effects as "less than would be expected", and say her case highlights the remarkable plasticity of the brain.
"These rare cases are interesting to understand how the brain circuitry works and compensates for missing parts," says Mario Manto, who researches cerebellar disorders at the Free University of Brussels in Belgium.
The patient's doctors suggest that normal cerebellar function may have been taken over by the cortex – brain scans should reveal the answer.