Posted on June 19, 2011, under Epilepsy.
Speech is usually located on the left side of the brain, in the posterior temporal lobe (see Chapter 6). However, in 10 to 15 percent of left-handed people speech is on the right side. It is vital to know where it is before proceeding with surgery.
The Wada test, named after the neurosurgeon Dr. Juhn Wada, is designed to localize speech and memory. A catheter is threaded from the groin of the awake patient up to the internal carotid artery, the main artery supplying one side of the brain. After a small injection of a dye, which can be seen on x-ray, a small amount of barbiturate is injected and that side of the brain is briefly “put to sleep.”
As the test begins, the patient is asked to hold his arms up in the air and to count. If the injection is done on the left side of the brain, the right arm becomes weak as the left side of the brain goes “to sleep.” If speech is on that same side, the child will simultaneously, but briefly, lose the ability to speak or count. Memory is also tested by showing objects and pictures. When the medication wears off, the patient will be asked to recall the objects he has seen. If lost, speech and memory quickly return when the drug wears off. In this crude fashion the laterality (side) of speech is determined. The same procedure may also be carried out on the other side of the brain, because occasionally speech is located on both sides.
If injecting the right side produces no alteration in speech or memory, then it can be assumed that it is safe to operate on that side. If speech is on the left and the surgery is to be done near that area, then far more careful evaluation of speech, language, and the epileptic focus must precede a decision about surgery.
Detailed neuropsychological testing may be performed prior to surgery to assess the person’s intellectual function and personality. This may help in understanding if certain parts of the brain previously have been damaged.