The selective amobarbital test in the anterior choroidal artery: Perfusion pattern assessed by intraarterial SPECT and prediction of postoperative verbal memory

Abstract

To screen for patients at risk for memory decline after temporal lobe epilepsy (TLE) surgery, selective amobarbital procedures, such as injection into the anterior choroidal artery (ACA-IAT), are sometimes used. We investigated the extent of the territory affected during ACA-IAT and its predictive value with respect to postoperative memory. Seventeen patients with TLE underwent ACA-IAT. In 9 of 17 patients, intraarterial SPECT co-registrated to MRI allowed delineation of amobarbital-perfused structures. Another subgroup of 9 of 17 patients underwent anterior temporal lobectomy. Verbal memory was tested pre- and postoperatively and during ACA-IAT. Major variations in the ACA-IAT perfusion pattern occurred and were not correlated with the verbal memory scores during ACA-IAT. Postoperatively, no patient experienced a severe verbal memory decline, but individual postoperative performance was not correlated with results during ACA-IAT. Our study suggests that ACA-IAT can be used to screen for severe postoperative amnesia in inconclusive cases, but cannot predict individual outcome, even when the perfusion pattern is taken into account.

Introduction

Temporal lobe epilepsy (TLE) is the most frequent cause of pharmacoresistant focal epilepsy in adults and adolescents. Resection of anterior and mesial temporal lobe structures can lead to complete seizure control in well-defined cases and has become an established treatment [1]. Postoperative impairment of memory, particularly verbal memory, remains a feared complication, and several methods have been developed to determine the risk of such an amnesic syndrome [2], [3]. The most recognized risk factors for significant postoperative verbal memory decline include dominant hemisphere surgery, intact preoperative interictal memory, and absence of hippocampal sclerosis on MRI [4], [5], [6]. Older age [4], [7], male gender [8], [9], extent of surgical resection [10], [11], low preoperative verbal IQ, as well as the presence of cortical dysgenesis [12], are other risk factors and should all be taken into account before surgery.

The intraarterial amobarbital test (IAT), initially developed to identify the language-dominant hemisphere [13], consists of injection of a short-acting barbiturate into a cerebral artery and produces transient deactivation of the corresponding vascularized structure. Behavioral testing of the patient during IAT reveals the functioning of the unanesthetized structures, that is, the integrity of the structures that will remain after the intervention. Intraarterial injection of amobarbital into the carotid artery is a validated procedure widely employed to screen for the risk of postoperative amnesia, to predict verbal memory decline after temporal lobe epilepsy (TLE) surgery, and to lateralize the seizure focus [14], [15]. It compares the cognitive performance after injection into the hemisphere ipsilateral and contralateral to the epileptogenic focus by using indices of hemispheric asymmetry. However, its role in memory evaluation may be hampered by the aphasia induced after injection into the dominant hemisphere or by confusion after injection into the nondominant hemisphere. Furthermore, although amobarbital injected into the carotid artery is known to produce hypoperfusion in the mesial temporal structures, as determined by intravenous SPECT [16], [17], [18], other reports using intraarterial injection of ^99mTc-hexamethylpropyleneamine oxime (^99mTc-HMPAO) during IAT have reported that the anesthetic agent did not perfuse the mesial temporal structures in up to 75% of the patients [16], [19], [20]. Repetition of the IAT in the same patients produces less reliable results for memory than for language testing [21].

These observations, combined with the development of more selective epilepsy surgery, led to the design of selective amobarbital tests targeting the mesial temporal structures. The mesial temporal lobe and, more specifically, the amygdaloid body and hippocampal formation are vascularized by the anterior choroidal artery (ACA) and the posterior choroidal artery (a branch of the posterior cerebral artery), with anastomoses between both vessels [22], [23]. IAT of the anterior choroidal artery (ACA-IAT) [24] and the posterior cerebral artery (PCA-IAT) [24], [25], [26], [27] have been found useful in this respect.

In the present case series, we were interested in evaluating the usefulness of ACA-IAT by determining exactly which brain regions are perfused by the test, how this affects memory performance (verbal and visual recognition), and, ultimately, whether the procedure results in accurate prediction of postoperative verbal memory decline.