The Wada test (named for a neurologist, Juhn A. Wada) consists of behavioral testing after the injection of an anesthetic (such as sodium amobarbital or sodium methohexital*) into the right or left internal carotid artery. Depending on how the injection is made (and the quantity), we have a certain amount of time during which the activities of one of the cerebral hemispheres are suspended, so the abilities subserved by the other hemisphere can be tested in isolation. Typical uses of the test include the lateralization of language abilities (the surgeon wants to know if the hemisphere being operated on is the speech hemisphere or not), and a determination that the person will not be amnesic after surgery. Since epilepsy surgery is usually carried out for a non-life-threatening condition, this is an important consideration. Although the injection is commonly made into the internal carotid artery, there are occasions when a selective injection into the posterior cerebral artery has to be made. Usually this is because the patient can’t cooperate after the more widespread effects of the carotid injection, and is useful for assessing the risk to memory after surgery (it usually has little or no effect on language – a brief period of hemianopsia is sometimes the only obvious evidence that the drug is having an effect).
* Sodium amobarbital (Amytal) was in short supply in the summer of 1998 (no longer) so we started using sodium methohexital (Brevital). While it wears off very quickly (which might be a problem for those using lengthy memory or language testing) it has the advantage that complete recovery seems to be much faster than with sodium amobarbital, so one can do several tests in a short time, without having to wait the usual 40 minutes between injections (see Selwa et al., 1997, Epilepsia, 35:743-749). We’re in the process of writing up our experience with Brevital – contact me if you would like to know details. For an abstract of a paper on preliminary results that were presented at the 1999 meeting of the American Epilepsy Society, click here.
Added July 2003: Some people have contacted me about alternates and Propofol has been mentioned as a possible substitute. The shortage of Brevital has affected dental surgery procedures too, and the following is from someone in that setting:
“We are currently using Etomidate and Propofol [for dental work]. They work as an alternative but do not compare with the ease of Brevital. Etomidate would be a second choice and may end up being cheaper than Brevital when all is said and done. Propofol is expensive and needs to be refrigerated, but is a good alternative depending on the amount of use versus expiration of drug (20 cc vial to be used in 24 hrs.)” Note: (Updated January 2100) Etomidate is still not recommended by the manufacturer for intra-arterial injection. See http://www.bedfordlabs.com/BedfordLabsWeb/products/inserts/Div-ETMP02.pdf
With regard to the use of Propofol for the Wada Test, there are some unconvincing references in the literature. In addition, the following is a personal communication from an interventional neuroradiologist:
“Propofol is a very effective hypnotic when given intravenously, and very short-acting. However, it is not very water soluble and forms a suspension of little globules. I’m not sure of the safety profile for injecting it in the cerebral arteries. One would want to do some animal experiments first.”
On the availability of Amytal and Brevital:
From: Jensen, Valerie E
Sent: Friday, January 30, 2004 10:38 AM
Subject: RE: Amobarbital
Ranbaxy gave us an update on the Amytal release date – the expected date of availability is 2/16. We have let them know that procedures are scheduled and that hospitals will be in need of this product starting 2/1 when the current lot expires. We will keep in touch with Ranbaxy on this – they do not expect any further delays after this date but we will keep you all informed of any changes reported to us.
-Val Jensen, CDER Drug Shortage, FDA
From: “Jensen, Valerie E”
To: Henry Buchtel
Date: Monday – February 2, 2004 10:18 AM
Subject: RE: Methohexital
Hello Dr. Buchtel,
We have continued to follow the methohexital shortage and understand from King Pharmaceuticals (the manufacturer) that they expect availability in April 2004.
-Val Jensen, CDER Drug Shortage, FDA