Amobarbital Effects and Lateralized Brain Function
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The case report by Loddenkemper et al 1 is most important as it poses the question of whether unilateral Wada testing is sufficient for language lateralisation. It is certainly possible that similar complications resulting from epilepsy surgery based on Wada tests are underreported. The implications of erroneous language lateralisation by unilateral amobarbital injection, which is increasingly used in several epilepsy centres including our own, would be far reaching.
There are, however, some caveats regarding the interpretation of the Wada test results reported. The first two refer to the assessment of cortical inactivation in this patient with pre-existing damage to the motor cortex; the third refers to the extent of brain areas inactivated by the intracarotid Wada procedure.
(1) Little is known about the relative susceptibility to the effect of amobarbital of language areas compared with damaged motor cortex. Thus it may be that a low dose of amytal (which 100 mg is) does impair a partially damaged motor cortex when language areas still maintain sufficient function to solve the tasks tested during the Wada procedure. In our experience, higher doses (140 to 200 mg) are often needed in adults to induce a complete contralateral hemiplegia. In order to avoid a possible risk of incomplete inactivation of the middle cerebral artery (MCA) territory and of language areas in particular, it may be advantageous to choose higher injection dosages.
(2) There is a striking discrepancy between well preserved proximal arm power (4-/5) after hemispherectomy as compared with …
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The Wada test is used to determine lateralization of cerebral speech dominance and to assess memory functions in preoperative evaluation of neurosurgeries in which language or memory areas are to be operated. Amobarbital, a barbiturate, is injected into one of the internal carotid arteries. During the induced hemiplegia, patient speech abilities are tested, and/or memory items are presented to be assessed after return to neurological baseline. In France, amobarbital use is limited to hospitals. An authorisation from the French drug agency is requiered prior to buy and use amobarbital.
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Intracarotid injections of amobarbital have been performed for clinical purposes since 1949, when Wada described a method for determination of hemispheric language dominance. It was noted that the intracarotid injection of amobarbital, performed in an attempt to investigate the interhemispheric spread of epileptiform discharges, produced a transient ipsilateral paralysis of hemispheric function without eliciting unacceptable sedation or interruption of vital functions. It was reasoned that this method would be useful for determination of hemispheric language dominance in patients who were to undergo neurosurgical procedures on the language dominant hemisphere (Wada, 1949). Eighty patients were evaluated by Wada between 1948 and 1954 without major complications (Wada and Rasmussen, 1960).
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Eighty-eight patients had bilateral intracarotid amobarbital (Wada) testing to determine hemispheric dominance for language in preparation for epilepsy surgery, as well as unilateral extraoperative cortical electrical stimulation using subdural electrode arrays. In none of the patients with left dominance by Wada testing were language areas found with right-sided stimulation, but two patients with right dominance by Wada testing had language areas mapped on the left side. These findings suggest that left dominance by Wada testing is strong evidence for exclusive lateralization of language function in the left hemisphere, but there is concern about the ability of the Wada test to exclude the possibility of some left-sided language function despite apparent right-sided dominance. Patients with left dominance on Wada testing do not need cortical stimulation before extensive right temporal lobectomy, but we believe that patients with right or bilateral dominance on Wada testing should have cortical stimulation for localization of language areas if extensive left or right temporal or frontal resection is planned.
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