Half a Brain: The Variable Outcomes of Brain Surgery

Why removing half a brain can have opposite outcomes for different people.

Posted May 13, 2019

Vizualni/iStockphoto
Source: Vizualni/iStockphoto

The prospect of having any part of your brain removed is terrifying. If you are preparing for brain surgery, what are some outcomes you can expect? The truth is, it is hard for any doctor to predict with certainty what function would be lost permanently and what might be lost initially, only to be regained later. This is because the plasticity of the brain, or its ability to rewire, is immense, but also highly variable across individuals.

The Example of Hemispherectomies in Children

Having even a small part of your brain ablated or removed is cause for tremendous concern, but consider the prospect of having an entire lobe of your brain removed. This procedure, called a hemispherectomy, is performed only under dire circumstances. One such circumstance is Rasmussen’s encephalitis, a condition that can result in frequent and debilitating seizures, sometimes occurring several hundred times a day, leaving the person dangerously debilitated.

One such case was that of Cameron Mott. After her hemispherectomy, she awoke completely paralyzed on the left side of her body. This wasn’t a surprise because it was the right hemisphere that was removed, and it is well known that the right side of the brain controls the left side of the body and vice versa. The surprise came when, four weeks later, she walked out of physical therapy. No one had expected such a phenomenal result. She had weaker control of the left side of her body, but she had control. She also has a normal IQ and is able to function well in society, albeit with a huge chunk of her brain missing. The left side of her brain was able to compensate for many of the functions usually controlled by the right side. Her brain completely rewired itself.

Between 1994 and 2001, 31 children underwent hemispherectomies. They weren’t the only children to receive this procedure, but they were the ones tracked by a team of doctors. Years later, these children were given cognitive tests. Nine were too mentally compromised to participate. The remaining 22 had scores ranging from 45 to 82 on an IQ test. The average IQ score in normal populations is roughly 100. One would expect this kind of outcome from a group of children who, literally, had only half a brain. Yet in another 2013 study where 115 “hemi” children were tracked for six years after the surgery, nine were unable to walk, and 28 were either in school for the disabled or cared for at home. On the other hand, 96 of the children were able to walk independently, and five, like Cameron, were even attending regular schools without assistance.

Hemispherectomies in Adults

While children’s brains rewire more readily, the widely varying outcomes of neurosurgical procedures apply to adults as well. In a study of 25 adults undergoing brain surgery to relieve seizures, 92 percent were seizure-free post-surgery. Their functionality, however, varied wildly. Some 36 percent experienced impaired movement in the upper body, 20 percent had impairment in their lower limbs, and 28 percent had trouble speaking.

Another study, this time of 20 patients, found that half experienced little difference in their ability to walk post-surgery, while 30 percent improved, and 20 percent reported more trouble. Five patients reported better hand control, and three saw their speech improve.

Understanding Individual Variability

This variability extends beyond hemispherectomies and beyond brain surgery meant to cure seizures. Even cutting a small slice of brain tissue can result in very different outcomes for different patients. How can the same procedure leave one patient with a significantly better quality of life, and another worse off?  

The answer is that our brains are all different. The brain isn’t a machine that operates according to a precise blueprint. Rather it is a constantly evolving organ, wiring and rewiring and making and using different connections in response to its experiences. No two people have exactly the same experiences, so it stands to reason that no two people will have exactly the same wiring. Our brains are as diverse as we are. You can’t necessarily tell what functionality you’re cutting into by location alone. And in some brains, functions are better distributed across the brain than in others, with more redundancy built in, and some rewire more readily than others. Unfortunately, science doesn’t yet have the answers for why this is so. Consequently, every patient has to look at these widely spread statistics and decide whether to undergo surgery. Knowing there is a 30 percent chance some function will improve, a 20 percent chance it will deteriorate, and a 50 percent chance it will stay the same is not informative. In which group will you end up? Understanding and mapping your brain and its ability to rewire and learn new things under normal circumstances might help you know what outcome is most likely for you. Besides, the outcomes also tell us that there is always a reason for hope.