The Wada Test
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 2010)
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
Hello -
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
Brevital:
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
Henry A. Buchtel, Ph.D. gusb@umich.edu
Last modified: 07 January 2011 (HAB)
http://www-personal.umich.edu/~gusb/wadadesc.html