What is the Wada test?

Answer: The Wada test is performed before surgery to identify hemispheric differences in speech / memory centers of the brain.

Wada test sodium amobarbital language surgery

Brain surgery is a dangerous procedure. It can have long term unintended consequences, such as the anterograde memory loss seen in Patient HM. But sometimes, risky surgical procedures must be undertaken for the sake of saving the life of the patient or dramatically improving their quality of life. For example, in the case of temporal lobe epilepsy, the removal of a small section of brain can help to prevent future seizures. Another brain surgery that might require the Wada test would be the resection of a brain tumor.

Before the brain surgery, however, the Wada test, or the intracarotid sodium amobarbital procedure (ISAP) can be performed. In the Wada test, the patient must be awake and behaving, so the process must be done before the patient in anesthetized. Sodium amobarbital, also called sodium amytal, is a derivative of a hypnotic barbiturate, and can cause depression of the CNS. During the Wada test, the dose of sodium amobarbital is delivered directly into the patient’s carotid artery. The drug is delivered unilaterally, so only one hemisphere of the brain receives the drug at a time.

Because the drug is a hypnotic sedative, one hemisphere of the brain essentially “shuts down” after infusion. A person may be asked to perform a simple task, such as repeated finger taps with both hands simultaneously. If the sodium amobarbital is delivered into the right hemisphere of the brain, then the left half of the body will stop being able to perform the finger taps while the right hand will continue unaffected. The drug effect washes out in minutes, then normal function will return to the affected half. The test is repeated with the other hemisphere.

One very important function that can be assessed with the Wada test before surgery is the lateralization of language. For most right handed people, language centers of the brain are localized to the left hemisphere. When this hemisphere is inactivated by the sodium amobarbital, they will be unable to perform simple speaking tasks. To assess language centers of the brain with the Wada test, a person will be asked to count while they are receiving the drug injection and finger tapping. When the left hemisphere of the brain is inactivated, the right hand will fail to tap and the patient will stop counting. This should be done before lesion surgeries, especially if the focus of the epilepsy is close to the language areas of the brain to minimize the chance of developing aphasia after the surgery.

The Wada test is often performed while the patient’s brain activity is being monitored via electroencephalogram (EEG). The EEG uses electrodes attached to the scalp that measures electrical activity of the underlying brain tissue. The EEG is an easily performed, noninvasive measure of brain activity which makes it a good method for studying brain activity.

It is generally a safe procedure, as the sodium amobarbital doesn't have long lasting effects. The risks involved are generally associated with the angiography process where a catheter is inserted into the carotid artery. For example, it is possible to make mistakes during the injection that can cause the veins to collapse.