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The Wada test, also known as the "intracarotid sodium amobarbital procedure" (ISAP), is used to establish which cerebral functions are localised to which hemisphere.
Method
The test is conducted with the patient awake. Essentially, an anesthetic (which is usually sodium amobarbital) is introduced into one of the internal carotid arteries via a cannula or intra-arterial catheter from the femoral artery. The drug is injected into one hemisphere at a time. The effect is to shut down any language and/or memory function in that hemisphere in order to evaluate the other hemisphere ("half of the brain"). The patient then undergoes a neuropsychological assessment.
Uses
The test is usually performed prior to ablative surgery for epilepsy and sometimes prior to tumor resection. The aim is to determine which side of the brain is responsible for certain vital functions including speech and memory. The risk of damaging such structures during surgery can then be assessed, and the need for awake craniotomies can be determined as well.
The Wada test has several interesting side-effects. Drastic personality changes are rarely noted, but dysinhibition is common. Also, contralateral hemiplegia, hemineglect and shivering are often seen. During one injection, typically the left hemisphere, the patient will have impaired speech or be completely unable to express or understand language. Although the patient may not be able to talk, sometimes their ability to sing is preserved. Recovery from the anesthesia is rapid, and EEG recordings and distal grip strength are used to determine when the medication has worn off. Generally, recovery of speech is dysphasic (contains errors in speech or comprehension) after a dominant hemisphere injection. Although generally considered a safe procedure, there are at least minimal risks associated with the aniography procedure used to guide the catheter to the internal carotid artery. As such, efforts to utilize non-invasive means to determine language and memory laterality (e.g. fMRI) are being researched.
History
It is named after Dr. Juhn Wada, who first described the procedure.
See also
"Wada test"