Plasticity during recovery: How young brains adapt to hemispheric surgeries

According to the CDC, about 456,000 people in the US under the age of 17 have epilepsy. This equates to approximately one in 160 children who suffer from seizures. Some individuals can be treated with medication, but others may be drug–resistant, in which case doctors can recommend a hemispheric surgery. This type of operation stops seizures by disconnecting the brain’s two hemispheres so that seizures can’t spread. Although it sounds heavy–handed, hemispheric surgeries work because they remove the epileptogenic zone or the amount of tissue needed to generate seizures. At the same time, the brain is extremely plastic, or adaptive; it has built–in mechanisms to compensate for its missing connection.

There are multiple types of hemispheric surgeries, which are differentiated by the amount of tissue that is removed and what is disconnected in the brain. The overarching procedure is referred to as a hemispherectomy. In anatomical hemispherectomies, the entirety of one hemisphere is removed. This includes all four lobes of the cortex, the amygdala and hippocampus, and sometimes the thalamus and basal ganglia. In functional or disconnective hemispherectomies, only the temporal lobe (which is largely responsible for processing auditory information) is removed. This disables the damaged hemisphere and separates it from the unaffected hemisphere, but doesn’t remove it from the body entirely. Children can also have a hemispherectomy, where the surgeon makes tiny holes in the affected hemisphere rather than removing pieces of tissue.

Each type of hemispherectomy has its own benefits and risks. Functional hemispherectomies are generally preferred because there is less blood loss. Additionally, even if seizures return after surgery, they won’t spread due to the complete disconnection of the epileptogenic zone from the rest of the healthy brain. A significant risk of functional hemispherectomies, however, is that if the surgeon is unable to completely disconnect the damaged hemisphere, seizures are likely to return and the patient would have to undergo another operation. The most common side effect of both types of hemispherectomies is hydrocephalus — a buildup of cerebrospinal fluid that causes harmful pressure to brain tissue.

There are two additional subtypes of hemispherectomies, which differ in what part of the brain is disconnected. Therefore, they also have characteristic benefits and risks that must be weighed depending on the individual child. A peri–insular hemispherectomy disconnects the damaged hemisphere through the ventricles — hollow cavities that produce cerebrospinal fluid to maintain the nervous system. This operation requires less time in surgery and results in less blood loss. However, there is a higher risk of incomplete disconnection and, subsequently, reoperation. On the other hand, in a modified lateral hemispherectomy, the surgeon severs the middle cerebral artery to limit blood loss, but there is more tissue loss and a higher risk of developing hydrocephalus compared to peri–insular methods. 

Success rates of these surgeries can depend on how old children are when seizures first appear and whether they have any preexisting conditions such as Rasmussen’s encephalitis or Sturge–Weber syndrome. In general, numerous evaluations have found that across all surgery types, the seizure control rate was about 71% five years after the operation. However, there are cognitive and sensorimotor side effects of hemispheric surgeries that influence the quality of life for almost all children. Patients will likely suffer vision and hearing impairments, and they may also have reduced sensitivity to temperature and pain. One of the most severe motor impairments is hemiparesis, which is a significant weakness or even paralysis on the opposite side. Fine motor movements such as grasping, writing, and using utensils are severely hampered, but some children are able to recover gross motor movements such as walking. Others are even able to use both hands for a particular task. Additionally, kids may have trouble with producing speech. Intellectual abilities can widely vary; some may need intensive reading help while others may participate in mainstream schooling. Longitudinal studies have illustrated that the extent of recovery depends on seizure etiology, whether children remained seizure–free after surgery, and how long they dealt with epilepsy before the operation. 

So how do children overcome these post–op difficulties and return to a stable quality of life? The remarkable long–term recoveries are due to a process called neuroplasticity, or the idea that the brain is constantly changing and adapting both structure and function based on new experiences and conditions. Hemispheric surgeries leave children with disabled brain tissue or a lack of an entire hemisphere. A study led by Dorit Kliemann at the California Institute of Technology investigated how the brain may compensate for this extreme loss of neural tissue by examining functional connectivity, or how different brain regions interact with each other, in six adults who had an anatomical hemispherectomy. Different types of connectivity are responsible for different brain functions. For example, local communication, or connectivity within a neural network, is thought to control motor execution. Integrative communication, or connectivity between neural networks, is responsible for cognitive functions like working memory. The researchers found that after hemispheric surgery, functional reorganization was characterized by stronger interactions between networks. By adapting to increase connectivity between networks, the surviving pieces of the brain could maintain overall cognitive function despite tissue loss.

The plasticity of the human brain, especially during childhood, enables extraordinary growth and adaptation to abnormal circumstances. Studies like these reveal how intricate neural mechanisms can drive prolonged recovery in children who have undergone hemispheric surgery. 

“The plasticity of the human brain, especially during childhood, enables extraordinary growth and adaptation to abnormal circumstances.”