There are many surgical procedures which fall under the broad category of hemispherectomy. The main goal of each procedure is to stop the seizures by completely disconnecting one cerebral hemisphere from the other in order to prevent seizures from spreading to other parts of the brain.

What are the various hemispherectomy procedures?        

There are several different types of hemispherectomy procedures. They include the anatomical hemispherectomy, subtotal hemispherectomy, the functional hemispherectomies, and the hemispherotomies. There are also other procedures which fall under this category which include hemidecortication and other newer procedures.

Surgeons are not trained in all techniques and some techniques are more complicated than others. Most surgeons perform only the procedures they have been trained to perform and are familiar with.  This video explains the history of hemispherectomy surgery and why different techniques were developed:

In some cases, fever can be caused by meningitis after craniotomy caused by introduction of bacterial or virus (usually Staphylococcus) during placement of the extraventricular drain or from medical personnel, family, or visitors touching the drain after placement without appropropriate hand washing or glove wearing. The risk of infection increases with the length of time the drain is in place.

Differences In Hemispherectomy Procedures

Dr. Gary Mathern, Co-Director of the Pediatric Epilepsy Surgery Program at UCLA, discusses differences in hemispherectomy procedures, types of procedures and seizure recurrence, and differences in underlying conditions (etiologies) at the 2011 International Hemispherectomy Conference and Family Reunion sponsored by the Hemispherectomy Foundation.

Anatomical hemispherectomy

A true anatomical hemispherectomy is when all four lobes of one hemisphere of the cerebral cortex are removed in their entirety. They may be removed “en bloc” (all together) or in pieces. Because of where they are located within the brain, the amygdala and hippocampus on the affected side are removed. Sometimes the surgeon removes the thalamus and basal ganglia on the affected side as well. Technically, this is the least complicated of all the hemispherectomy procedures.

Anatomical hemispherectomy is often used today in many surgical facilities, usually for cases such as hemimegalencephaly where functional hemispherectomy can sometimes be unsuccessful.

There are significant short- and long-term complications that are associated with this procedure:

  • Hydrocephalus:  Hydrocephalus is a common side effect of anatomical hemispherectomy, with this study of 690 patients finding it in 30% of cases. Hydrocephalus is a risk throughout the lifespan, with 27% of children developing hydrocephalus three or more months after surgery;
  • Superficial hemosiderosis is an extremely rare condition caused by chronic iron deposits on brain tissue. This was reported as a late complication of anatomical hemispherectomy in the early research papers. Blood product in the cerebrospinal fluid, whether left behind from the surgery or caused by blood leaking from tiny rips and tears in capillaries that can occur over time due to brain shift or dislodging of the hemisphere, is one possible cause of this condition. Symptoms of superficial hemosiderosis include gait imbalance, progressive loss of hearing which sometimes begins as ringing of the ears (tinnitus), vertigo, and other symptoms. Because the delayed mortality rate was reported as high as 40% for those with superficial hemosiderosis after hemispherectomy, anatomical hemispherectomy was largely abandoned in the early 1970s by many facilities. It is now believed that the high mortality rate may have been due to untreated hydrocephalus rather than superficial hemosiderosis in many of the early cases.  Superficial hemosiderosis has not been reported in medical literature in over 30 years; however, there are anecdotal reports from parents of adults who had childhood anatomical hemispherectomy. It has also been reported many years after functional hemispherectomy;
  • Brain shift into the resection cavity, microvascular tearing, and dislodging of the remaining hemisphere. The incidence of these risks is unknown.

Functional hemispherectomy

Functional hemispherectomy is any procedure which disables the function of one cerebral hemisphere but does not remove the hemisphere itself.

Rasmussen was the first epilepsy surgeon to develop the functional hemispherectomy technique and is the most common technique today. The temporal lobe is removed but the frontal pole and occipital pole are preserved. This provides access to connections in the front and back of the hemisphere and the midbrain which the surgeon cuts and allows the surgeon to perform a complete corpus callosotomy. The brain that is left behind is living because the veins and arteries which provide its blood supply are not cut. For this reason, the remaining part of the brain may still seize, but because the axonal connections are severed, the seizures do not spread and have no effect.

Risk of this procedure include incomplete disconnection. Incomplete disconnection rates have been reported between 7 – 52%, thus requiring reoperation.


Although often classified as a type of functional hemispherectomy, hemispherotomies are technically different than functional hemispherectomy. As with functional hemispherectomy, living brain is left behind with an intact blood supply, but the diseased/affected hemisphere of the brain is disconnected from healthy brain. In the last 25 years, there has been a shift towards the functional hemispherectomy and hemispherotomy due to the significant risks of a true anatomical hemispherectomy.

The distinction between hemispherotomy and functional hemispherectomy is that in hemispherotomy less brain tissue is removed than in functional hemispherectomy in order to reduce the chances of excessive bleeding during surgery, hydrocephalus, and tearing of very small blood vessels and veins over time which can lead to superficial hemosiderosis. Because so little brain tissue is removed, the surgeon is really making a hole (or holes) in the hemisphere (-otomy) rather than removing large sections of brain (-ectomy).

The different hemispherotomy techniques were introduced in the 1990s and include the modified lateral hemispherotomy, the vertical parasagittal hemispherotomy, and the peri-insular hemispherotomy. These techniques are continually refined today and are the main technique used at most epilepsy centers in the 21st century. It is important to note that techniques may vary from center to center.

Peri-Insular Hemispherotomy

First described in 1995, there have been several modifications to this technique over the years. In this technique, the surgeon will disconnect the affected hemisphere through the ventricles of the brain (the areas in red below). Because the ventricles are very deep within the brain, the surgeon must create several “windows” or holes through the brain so the ventricles can be accessed. These holes are cut through the affected hemisphere above and below the insular cortex (thus the name peri (around) insular hemispherotomy.) Very little brain matter is removed during surgery.

What are the benefits of peri-insular hemispherotomy?

Generally, the benefits of this procedure are shorter time in surgery and less intraoperative blood loss than other hemispherectomy techniques. Some children, however, may require blood transfusion during surgery.

What are the risks of peri-insular hemispherotomy?

The greatest risk of peri-insular hemispherotomy is incomplete disconnection.

Post-operative fevers, meningitis, and irritability are reported as less after peri-insular hemispherotomy than other techniques. Some research shows that the incidence of hydrocephalus after peri-insular hemispherotomy is as low as 0 – 4%; however, a recent large study of  690 children and adults who have had hemispherectomy shows the rate as high as 23% for functional hemispherectomy in general. Parents should continue to look out for symptoms of hydrocephalus throughout the lifespan.

Death is extremely rare after peri-insular hemispherotomy. Only two are reported in the literature.  One was due to brain swelling from stroke in the disconnected hemisphere or bleeding. One case is believed to have been caused by brain shift.

Seizure control rates are reported as high as 90% after peri-insular hemispherotomy. The highest seizure control rates are reported where the condition which causes the seizures is stroke or Rasmussen’s encephalitis; the lowest seizure control rates are with cortical dysplasia and hemimegalencephaly

Modified Lateral Hemispherotomy

Modified lateral hemispherotomy is similar to peri-insular hemispherotomy, but differs because 1) the middle cerebral artery is severed to limit blood loss and 2) a central block of cortex (the operculum) is removed to allow the surgeon access to the ventricles, remove the insula, and portions of the basal ganglia and thalamus. The anterior temporal lobe is also removed.

Hydrocephalus is also a post-operative risk, with 23% of children developing hydrocephalus after functional hemispherectomy.

What are the chances of total seizure control after hemispherectomy?

A recent systemic review of 15 studies which address seizure outcomes across several different hemispherectomy procedures shows that the long-term seizure control rate at five years or more after surgery is 71%.

What factors affect seizure control?

Children whose seizures began after 3 1/2 months of age have a higher chance of long-term seizure control. Also, children who have abnormal MRI findings prior to surgery also have higher rates of control. Sturge-Weber syndrome, Rasmussen’s encephalitis, and pediatric stroke syndrome have the highest seizure control rate of approximately 80%.

Children with hemimegalencephaly have the lowest at seizure control rate at approximately 66%.

What are the surgical risks of hemispherectomy?

Hemispherectomy is an extremely delicate procedure which should only be performed by an experienced pediatric epilepsy surgeon.

Hydrocephalus is the most significant risk after hemispherectomy surgery. Approximately 23% of patients will develop hydrocephalus, with 27% of those children developing hydrocephalus 90 days or more after surgery. The risk of developing hydrocephalus is 20% after functional hemispherectomy and 30% after anatomical hemispherectomy. Children with hemimegalencephaly have a higher chance of developing hydrocephalus – about 40%.

This risk is across the lifespanwith cases reported ten or more years after surgery.

If your child is having or has had hemispherectomy surgery, we strongly encourage you to familiarize yourself with hydrocephalus and its symptoms. Learn more about the risk of hydrocephalus here. We also have a downloadable guide titled Hydrocephalus After Hemispherectomy for you to share with your child’s care team, teachers, and other aligned professionals.

Blood loss requiring transfusion is also risk, especially for babies and infants. In very rare cases, stroke, brain swelling (cerebral edema), or other complications can occur.

Post-operative fevers are common after surgery and may occur for several months later. This is because blood product and protein in the cerebrospinal fluid from the surgery can irritate the brain, causing the body to respond as though there is an infection. Most surgeons use an external ventricular drain to evacuate blood product and other matter from the cerebrospinal fluid, as well as to monitor intracranial pressure. The use of this drain can also reduce the incidence of fevers after surgery.

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