Hemispherectomy and hemispherotomy are major epilepsy operations used when severe drug-resistant seizures arise predominantly from one extensively abnormal cerebral hemisphere.
Modern surgery usually favours techniques that disconnect the diseased hemisphere from the rest of the brain while removing less tissue than historical anatomical hemispherectomy.
Why disconnect an entire hemisphere?
Some neurological disorders damage or malform such a large part of one hemisphere that smaller focal resections cannot control the seizure network.
If the opposite hemisphere can support essential function, hemispheric disconnection can offer substantial seizure control.
Conditions that may lead to hemispheric surgery
Examples include:
- hemimegalencephaly;
- Rasmussen encephalitis;
- large unilateral cortical malformations;
- major perinatal stroke or destructive brain injury;
- Sturge–Weber syndrome with extensive unilateral involvement;
- other severe unilateral epileptic encephalopathies.
Anatomical hemispherectomy
Historical anatomical hemispherectomy removes most of the cerebral hemisphere.
It achieved strong seizure control but was associated with important long-term complications, including superficial cerebral haemosiderosis and hydrocephalus.
Functional hemispherectomy
Functional hemispherectomy removes less tissue and combines resection with disconnection of the remaining hemisphere.
The goal is to interrupt seizure pathways while reducing some complications of complete anatomical removal.
Hemispherotomy
Hemispherotomy uses even smaller resections and strategically placed disconnections.
Vertical parasagittal and peri-insular techniques are among the established approaches.
What is actually disconnected?
The operation aims to disconnect corticocortical and subcortical pathways linking the diseased hemisphere to the healthy hemisphere and deeper structures.
Complete disconnection matters because residual connections can allow seizures to continue.
Existing weakness is common before surgery
Many candidates already have significant weakness or loss of fine hand function on the side opposite the affected hemisphere.
This is important because hemispheric surgery usually produces or confirms a permanent hemiparesis.
Vision
A contralateral homonymous visual-field defect is expected after complete hemispheric disconnection.
Some candidates already have visual-field impairment before surgery.
Language
Young brains can reorganise language extensively, especially when severe unilateral disease begins early.
Presurgical assessment examines whether critical language functions have shifted to the healthier hemisphere.
Age matters—but there is no single cut-off
Hemispheric surgery is most commonly performed in childhood because the relevant disorders often begin early and neuroplasticity is greater.
Selected adolescents and adults can also benefit.
Seizure outcomes
Large specialist-centre series show that hemispheric surgery can provide durable seizure freedom in a substantial proportion of carefully selected patients.
Outcome varies with underlying pathology, completeness of disconnection and whether there is significant disease in the opposite hemisphere.
2024 long-term experience
A 35-year paediatric series from Toronto reported the evolution of hemispherectomy practice across 146 children, reflecting the shift toward modern functional and disconnective techniques and broader understanding of candidacy and outcomes.
Incomplete disconnection
Persistent seizures after hemispherotomy may result from residual fibres that continue to connect the diseased hemisphere.
Repeat imaging and, in selected cases, repeat surgery may identify and complete the missing disconnection.
Hydrocephalus
Hydrocephalus is an important recognised complication and can occur after surgery, sometimes with delayed presentation.
Long-term follow-up therefore matters even when the immediate postoperative course is uncomplicated.
Other risks
Risks include haemorrhage, infection, stroke, electrolyte disturbance, venous complications and worsening neurological deficits.
These operations should be performed in experienced comprehensive epilepsy surgery centres.
Development and quality of life
The decision is not based solely on seizure counts.
Clinicians also consider developmental stagnation or regression, medication burden, repeated status epilepticus, injuries, hospital admissions and the possibility that seizure control could permit better development.
The central point
Modern hemispheric surgery is mainly a disconnection strategy for catastrophic epilepsy arising from one severely abnormal hemisphere. The neurological trade-offs are substantial, but many candidates already have major unilateral deficits and can gain meaningful seizure and developmental benefit.
Related reading
Hemimegalencephaly and Epilepsy →
Rasmussen Encephalitis →
Sturge–Weber Syndrome →
Epilepsy After Perinatal Brain Injury →
Sources
Evidence checked: September 2026.