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.
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.