The largest part of the brain, the cerebrum, can be divided down the middle lengthwise into two halves, called hemispheres. A deep groove splits the left and right hemispheres, which communicate through a thick band of nerve fibers called the corpus callosum. Each hemisphere is further divided into four paired sections, called lobes -- the frontal, parietal, occipital, and temporal lobes.
The two different sides or hemispheres are responsible for different types of activities. The left side of the brain controls the right side of the body and vice versa. For most people, the ability to speak and understand the spoken word is a function of the left side of the brain. A functional hemispherectomy is a procedure in which portions of one hemisphere -- which are causing the seizures -- are removed, and the corpus callosum, which connects the two sides of the brain, is cut. This disconnects communication between the two hemispheres, preventing the spread of electrical seizures from one side of the brain to the other. As a result, the person usually has a marked reduction in physical seizures.
Seizures occur in girls and boys at an equal rate and are more common before the age of 15 and after age 65. For now, there is no way to screen for a seizure disorder before it develops. However, avoiding head injuries -- such as by wearing a helmet while riding a bicycle -- can reduce the risk of acquiring a seizure disorder.
Who Is a Candidate for a Functional Hemispherectomy?
This procedure generally is used only for people with epilepsy who do not experience improvement in their condition after taking many different medications and who have severe, uncontrollable seizures. This type of epilepsy is more likely to be seen in young children who have an underlying disease, such as Rasmussen's encephalitis or Sturge-Weber syndrome, that has damaged the hemisphere.
What Happens Before a Functional Hemispherectomy?
Candidates for functional hemispherectomy undergo an extensive pre-surgery evaluation -- including seizure monitoring, electroencephalography (EEG), and magnetic resonance imaging (MRI). These tests help the doctor identify the damaged parts of the brain and confirm that it is the source of the seizures. An intracarotid amobarbital test, also called a WADA test, is done to determine which hemisphere is dominant for critical functions such as speech and memory. During this test, each hemisphere is alternately injected with a medication to put it to sleep. While one side is asleep, the awake side is tested for memory, speech, and ability to understand speech.