CAROTID SODIUM AMOBARBITAL INJECTION: GENERAL INFORMATION
- Jun Wada pioneered the technique of injecting sodium amobarbital into the carotid artery to produce a brief period of anesthesia of the ipsilateral hemisphere
- Injections are now normally made through a catheter inserted into the femoral artery
- This procedure results in an unequivocal localization of speech, because injection into the speech hemisphere results in an arrest of speech lasting up to several minutes; as speech returns, it is characterized by aphasic errors
- Injection into the nonspeaking hemisphere may produce no speech arrest or only brief arrest.
- The amobarbital procedure has the advantage that each hemisphere can be studied separately in the functional absence of the other, anesthetized one (Some institutions do both hemispheres on the same day; others do it on separate days)
- Memory testing is also performed for each hemisphere. Again, there is no standard procedure, and different institutions perform it differently.
- In a typical Wada test, a patient is given a “dry run” to become familiar with the tests that will be done during and after the drug injection. This dry run establishes a baseline performance level against which to compare the postinjection performance
THE WADA PROCEDURE
- The Wada procedure starts with the supine patient raising both arms and wiggling the fingers and toes
- The patient is asked to start counting from 1, and, without warning, the neurosurgeon injects the drug through the catheter for 2 to 3 seconds.
- Within seconds, the contralateral arm falls to the bed with a flaccid paralysis, and there is no response whatsoever to a firm pinch of the skin of the affected limbs
- If the injected hemisphere is nondominant for speech, the patient may continue to count and carry out the verbal tasks while the temporary hemiparesis is present, although often the patient appears confused and is silent for as long as 20 to 30 seconds but can typically resume speech with urging
- When the injected hemisphere is dominant for speech, the patient typically stops talking and remains completely aphasic until recovery from the hemiparesis is well along, usually in 4 to 10 minutes
- Speech is tested by asking the patient to name a number of common objects presented in quick succession, to count and recite the days of the week forward and backward, and to perform simple object naming and spelling
- In addition to aphasia and paresis, patients with anesthesia of either hemisphere are totally nonresponsive to visual stimulation in the contralateral visual field. For example, there is no reflexive blinking or orientation toward suddenly looming objects
RESEARCH DATA (Milner, et al.)
- In a series of studies, Brenda Milner and her colleagues demonstrated that about 98% of righthanders and 70% of left-handers show speech disturbance after sodium amobarbital injection into the left hemisphere and not after injection into the right
- Curiously, roughly 2% of right-handers have their speech functions lateralized to the right cerebral hemisphere, which is roughly the proportion of righthanded people who show aphasia from right-hemisphere lesions. This finding reminds us that speech is sometimes found in the right hemisphere of righthanded people
- The results for left-handed patients support the view that the pattern of speech representation is less predictable in left-handed and ambidextrous subjects than in right-handers but that the majority of left-handers do have speech represented in the left hemisphere
- In these studies, none of the right-handers showed evidence of bilateral speech organization, but 15% of the non-right-handers displayed some significant speech disturbance subsequent to the injection of either side
- These patients probably did not have a symmetrical duplication of language functions in the two hemispheres; the injection of one hemisphere tended to disrupt naming (for example, names of the days of the week), whereas the injection of the other hemisphere disrupted serial ordering (for example, ordering the days of the week)