A hemispherectomy is a brain surgery that removes or disconnects one entire hemisphere of the brain to stop severe, uncontrollable seizures. It sounds radical, and it is. But for children with catastrophic epilepsy that doesn’t respond to medication, it is one of the most effective surgical treatments available, with 73% to 85% of patients achieving seizure freedom or major seizure reduction in long-term follow-up studies.
Why Anyone Would Need This Surgery
Hemispherectomy is reserved for people, almost always children, whose seizures originate from widespread damage across one hemisphere of the brain and cannot be controlled with medication. The seizures are typically so frequent and severe that they threaten development, cognitive function, and in some cases life itself. Before this surgery is considered, the damaged hemisphere has usually already lost most of its useful function, and thorough testing must confirm that the opposite hemisphere is healthy.
The conditions that lead to hemispherectomy fall into three broad categories. Acquired injuries include perinatal strokes, where a blood vessel blockage during or around birth damages one side of the brain. Congenital conditions include hemimegalencephaly, where one hemisphere develops abnormally large, and other malformations of cortical development like focal cortical dysplasia. Progressive diseases include Rasmussen encephalitis, an inflammatory condition that slowly destroys one hemisphere, and Sturge-Weber syndrome, a condition involving abnormal blood vessels in the brain. In all of these, the common thread is the same: one hemisphere is extensively damaged and generating relentless seizures that medications cannot stop.
Anatomical vs. Functional Approaches
There are two main types of hemispherectomy, and the distinction matters because it affects both the risk of complications and the recovery process.
An anatomical hemispherectomy physically removes the frontal, parietal, temporal, and occipital lobes of the affected hemisphere. First reported as an epilepsy treatment in 1938, this was the original approach. It achieves the most complete disconnection possible, but it carries higher risks of blood loss and fluid buildup in the skull afterward.
A functional hemispherectomy (also called hemispherotomy) takes a different approach. Instead of removing the entire hemisphere, the surgeon removes a smaller portion of tissue and then carefully severs all the nerve fiber connections between the damaged hemisphere and the rest of the brain. The key disconnections include cutting through the corpus callosum (the thick band of fibers connecting the two hemispheres), severing the pathways that run between the cortex and deeper brain structures, and removing the inner temporal lobe structures including the hippocampus and amygdala. Developed in the 1990s by several surgical teams working independently, this technique achieves the same electrical isolation as full removal but through smaller openings and with fewer complications.
A functional hemispherectomy is now the preferred approach in most cases. Anatomical removal is typically reserved for patients whose seizures persist after a functional disconnection.
How Effective It Is
For a surgery this dramatic, the outcomes are remarkably good. In a study following 186 patients who underwent the procedure, 136 (73%) achieved seizure freedom or major reductions in seizure frequency. Another study of 92 pediatric patients found that 85% were seizure-free at their last follow-up. In one long-term series tracked for an average of nearly 13 years, two-thirds of patients remained completely seizure-free, and 75% were either seizure-free or had a greater than 90% reduction in seizure frequency. Some patients were eventually able to stop taking anti-seizure medication entirely.
Risks and Complications
The most common complication requiring further surgery is hydrocephalus, a buildup of cerebrospinal fluid in the brain. A large meta-analysis found this occurs in about 16% of patients, who then need a shunt (a small tube placed surgically to drain excess fluid). The anatomical approach carries a higher risk of significant blood loss and fluid-related problems compared to the functional technique, which is one reason the functional approach has largely replaced it.
How the Brain Adapts
One of the most remarkable aspects of hemispherectomy is how well the remaining hemisphere compensates for the lost one, particularly in young children. The brain’s ability to rewire itself is at the heart of why this surgery works, especially when performed early in life.
Language is the function people worry about most, particularly when the left hemisphere (the dominant language side in most people) is the one being removed. Research shows that in children under age 3, the right hemisphere can take over language processing with no obvious deficits. The brain regions that handle language in the remaining hemisphere typically mirror the locations where language would normally reside on the opposite side, and patients often recruit additional brain areas to support language tasks, suggesting they develop alternative processing strategies.
For children who suffer left-hemisphere damage before or around birth, right-sided language reorganization frequently happens on its own, even before surgery. Studies using specialized brain imaging have shown that these children often already have language represented in their right hemisphere by the time surgery is considered. The younger a child is when the brain injury occurs, the more likely language will successfully shift to the other side.
What to Expect Physically
Because each hemisphere controls movement on the opposite side of the body, hemispherectomy results in permanent weakness or paralysis on the side opposite to the removed or disconnected hemisphere. Children who undergo this surgery will typically lose fine motor control in the affected hand and have weakness in the affected leg, though many learn to walk with a limp. They will also lose vision on one side (the visual field opposite the surgery). These deficits are significant, but in most cases the affected hemisphere was already so damaged before surgery that the child had limited use of that side to begin with.
Rehabilitation after the procedure involves intensive physical and occupational therapy to help the child maximize function on the affected side and strengthen compensatory skills on the unaffected side. The recovery timeline varies depending on the child’s age, the underlying condition, and whether the approach was anatomical or functional. The functional approach generally involves less surgical trauma and a shorter initial recovery.
Why Age Matters
Timing is one of the most important factors in hemispherectomy outcomes. Younger brains have far greater plasticity, meaning they can reorganize functions more completely. Children under 3 show the strongest capacity for transferring language and other cognitive functions to the remaining hemisphere. Operating early also limits the developmental damage caused by constant, uncontrolled seizures during critical periods of brain growth.
That said, the surgery is not exclusively for very young children. Older children and, in rare cases, adults can benefit as well, particularly when the diseased hemisphere has been deteriorating over time, as in Rasmussen encephalitis. The trade-off is that the remaining hemisphere has less capacity to absorb new functions the older the patient is at the time of surgery.