A 24-year-old woman complaining of dizziness and nausea was admitted to a hospital in Shandong Province recently, where she told doctors she had struggled with balance all her life. When doctors performed a brain scan, they immediately noticed the problem: The woman was missing her cerebellum.

Above: An MRI of the woman's head reveals "no recognizable cerebellar structure." The posterior fossa, write the researchers, is filled instead by cerebrospinal fluid | Image Credit: Feng Yu et al.

The cerebellum, which is Latin for "little brain," is a small lump of brain matter situated below and toward the rear of the brain's two cerebral hemispheres. So densely packed are its neurons, that despite accounting for just 10% of the brain's volume, the cerebellum manages to cram in more than half of the brain's total neurons. To go about one's life without a cerebellum, it should go without saying, is far from common; in the August 22 issue of Brain, doctors led by neurosurgeon Feng Yu report the woman is one of just nine people known to have done so. That Yu's team documented her condition while she was alive makes the discovery all the more exceptional.

The woman's case presents a fascinating example of neuroplasticity, the process by which one or more regions of the brain adapt to compensate for damage to a different area of the brain, or a loss of some bodily function. If you lose a finger, for example, the neural representations of the neighboring fingers get bigger. Sever someone's optic nerve, and the neurons devoted to vision will be co-opted by neurons associated with other cognitive functions. This is one reason blind people tend to have excellent audio acuity.



In this woman's case, however, the missing body part is not a finger, nor an optic nerve, but a sizable chunk of the brain, itself. The cerebellum plays an important role in motor control. Timing, coordination, fine movement – all of these things rely in large part on this small, sub-hemispheric brain. Yu's team calls the woman's condition a rare example of complete primary cerebellar agenesis. "This surprising phenomenon," the authors write, "supports the concept of extracerebellar motor system plasticity, especially cerebellum loss, occurring early in life."

According to Mitchell Glickstein, emeritus professor of neuroscience at University College London, "the claim that people with complete cerebellar agenesis can be entirely symptom free is widespread," and kept alive by an "oral tradition [that] people who are born without a cerebellum may have no observable symptoms at all." In fact, he says, every documented case of the condition has been linked to "a profound deficit in the development of normal movement." This certainly appears to be the case with this 24-year-old woman, whose mother reports she was four years old before she could stand on her own, and seven before she could walk unassisted, "with a persistently unsteady gait." Her speech was also reportedly unintelligible until the age of 6 (difficulty articulating is a symptom of cerebellar disfunction). [Pictured Here: "Human Brain Without Cerebellum," on display in the Anatomy Department of the University of Cambridge, Cambridge, UK | Via Glickstein]

Today, however, the 24-year-old woman's symptoms are not characterized as debilitating, but as "mild to moderate" – her movements "slowed," or "slightly irregular." Yu's team notes that she is married with a daughter, and describe her pregnancy and delivery as "uneventful." The authors describe her neurological exam in detail:


A neurological examination revealed she could cooperate and fully orientate. A verbal analysis test revealed her word comprehension and expression remained intact and she had no sign of aphasia, but mild to moderate signs of cerebellar dysarthria. The patient has mild voice tremor with slurred pronunciation and her voice quality is slightly harsh. Cerebellar ataxia including Romberg's sign, and there is evidence of heel-knee-tibia impairment. The patient experienced mild to moderate dysmetria in reaching the nose when administered the finger-to-nose test. Pronation-supination alternating movements were slightly irregular and slowed. While she is able to walk unsteadily without support, her gait is moderately unsteady. The patient has evidence of tandem gait and moderately reduced gait speed. There is no focal paresis but the muscle tone is mildly increased. Evaluation of the sensory system showed no abnormalities, no deformities of the fingers and toes were observed, and her complete blood count and urinalysis were normal.

Sometimes brain activities associated with a certain area are redistributed to other locations not following an injury, but in response to the area never having formed at all. Read more about this rare case study in the latest issue of Brain. See also: Helen Thomson's coverage of the study in New Scientist.

H/t To everyone who e-mailed us on this, but to Karl, especially, who was the first to do so!