How Much of the Brain Can You Live Without?

Author: Mary Bates


One year ago, a 24-year-old Chinese woman walked into a hospital complaining of dizziness and nausea. A brain scan revealed she was missing her entire cerebellum, a part of the brain involved in motor control and balance. The empty space where it should have been was filled with cerebrospinal fluid.

How can someone not realize a huge chunk of their brain is missing? In fact, surviving, and even thriving, without major parts of the brain is possible. The woman's doctors believe that other parts of her brain, like the cortex, took over the cerebellum's usual responsibilities as her incomplete brain developed.

Stories like these, of people born with brain regions missing or who have had large portions of their brains removed surgically, reveal a lot about how the brain works and compensates for missing parts.

"The little brain"

Although the Chinese woman with no cerebellum started walking late (at age 7) and walks unsteadily as an adult, it's remarkable that she can walk at all.

The cerebellum, also known as the "little brain," looks like a cauliflower hanging off the back of the brain's two cerebral hemispheres. It takes up about 10 percent of the brain's total volume, but contains nearly half of the brain's neurons.

The cerebellum's main job is to control voluntary movements, coordination, and balance, although recent research also points to a role in language, emotion, memory, and attention. People who suffer illnesses or injuries to the cerebellum as adults usually have severe impairments in movement and speech.

But the Chinese woman, one of just nine people who are known to have lived without their entire cerebellums, had only mild to moderate motor problems and slightly slurred speech. Her case highlights the amazing capacity of the brain to rewire itself to cope with new demands — a feature called neuroplasticity.

Removing half the brain

There are a number of cases of people missing half of their brains. A teenage girl in Germany, for instance, was born without the right hemisphere. The issue wasn't discovered until she was three years old. According to her doctors, the girl has normal psychological function and is living a normal life.

Sometimes, to treat seizures or remove a tumor, doctors have to remove brain hemisphere in an operation known as hemispherectomy. The surgery was first performed on a dog in 1888 by German physiologist Friedrich Goltz as proof of concept. Walter Dandy pioneered the surgery on a human patient in 1923 to remove a brain tumor. Fifteen years later, neurosurgeon Kenneth McKenzie performed a hemispherectomy on a 16-year-old girl, resulting in the elimination of her seizures.

Today, hemispherectomy is a last resort for children who suffer intractable seizures. Persistent seizures can damage the developing brain if not treated, and surgery is an option for those patients with severe seizures that don't respond to medications.

Gary Mathern, a neurosurgeon at the University of California, Los Angeles, says surgery is usually performed on children younger than ten years old, and sometimes as young as a few months old.

"It's a fairly rare procedure, but in the U.S. there are probably in the neighborhood of a few hundred that are done every year," he says.

The surgery can be very successful, with 75-80 percent of children becoming seizure-free after the operation. Incredibly, memory and personality develop normally after hemispherectomies, and children's academic performances often improve after the surgery.

Patients with hemispherectomies are left with some disabilities. "Primarily, they have a loss of sensory and motor function on the opposite side of the body, often walking with a limp or experiencing loss of use of the hand opposite of the hemisphere that was removed," Mathern says.

"But if you perform the operation at a young age, the remaining side of the brain takes over many functions, compensating for the loss through neuroplasticity. You'd be surprised at how effective these kids are over time."

Missing more than half a brain

It happens rarely, but as a recent case shows, even people missing the majority of their brains can live normal lives.

A 44-year-old man in France went to the doctor because of weakness in his left leg. Doctors were shocked when they looked at MRI scans of the man's head. Most of his skull cavity was filled with cerebrospinal fluid, with just a thin sheet of brain tissue lining the inside of the skull.

The man's IQ was below average, but he was not mentally disabled. He was a married father of two and worked as a civil servant in a local tax office.

It turns out that as an infant, the man had a condition called hydrocephalus — water in the brain — and had a shunt inserted into his head to drain the fluid. The shunt was removed when he was 14 years old, but it seems that the excess fluid accumulated and squished his brain toward his skull. So much fluid built up that the ventricles, usually small chambers that hold cerebrospinal fluid, greatly expanded and pushed his brain aside.

Until the problem with his leg popped up, the man had no idea that his head was essentially filled with fluid. A doctor inserted another shunt and within weeks the man's neurological problems subsided and he was back at work.

Plasticity and redundancy

The French man's case, like the others, reveals the enormous potential of the brain to reorganize and adapt to early brain damage. If parts of the brain are missing from birth, or removed at an early age, different parts can take up the functions that would normally be done by the missing parts.

Besides brain's plasticity, the good outcomes in these cases are also due to another factor: the ability of multiple different brain structures to support a single function, a phenomenon known as degeneracy. Many of the important functions of our brains are not the sole province of single, distinct brain regions, but are supported by multiple regions working together. If one region can't perform, the others can pick up the slack.

The brain's capacity to make up for lost parts is good news for hemispherectomy patients and people born without certain brain structures. With seemingly even the bare minimum amount of brain, some people are still able to live normal, fulfilling lives — sometimes without even realizing what is missing.