Intracranial EEG study of brain structures affected by internal carotid injection of amobarbital.

Hippocampal function, considered critical in memory processing, is supposedly tested in the intracarotid sodium amobarbital (ISA) procedure; however, since the hippocampus is not completely irrigated by the internal carotid artery, some believe the procedure may be invalid for memory testing. We quantified delta activity in intracerebral EEGs during ISA tests. There was increased delta in ipsilateral structures as follows: amygdala (6.4 minutes), anterior hippocampus (7.2), middle hippocampus (7.4), temporal neocortex (9.1), frontal lobe (8.4), central/parietal area (11.0), and occipital lobe (9.7). Contralateral structures usually (> 64%) showed increased delta lasting 4 to 5 minutes. The ipsilateral hippocampus had delta waves in over 90% of injections. We conclude that the hippocampus is clearly affected by the ISA injection. We argue that the slow waves may not be caused by a direct effect of the drug, but rather by a functional deafferentation due to the profound inactivation of structures surrounding the hippocampus. Similarly, slow waves contralateral to injection may be caused by sudden removal of neuronal input from the regions receiving the amobarbital.

EEG slow waves and memory performance during the intracarotid amobarbital test.

During intracarotid sodium amobarbital (ISA) testing, EEG slow waves appear in the injected hemisphere and usually also contralaterally. They are frequently used to estimate duration of drug effect and thus the window of valid memory testing, but the relationship between slow waves and memory performance is not established. In 50 tests, we examined recognition for stimuli presented after amobarbital injection in relation to the quantified EEG. Performance was compared for injection ipsilateral versus contralateral to epileptic focus, with slow waves present bilaterally, unilaterally, or dissipated. Results showed that memory was impaired after injections contralateral to the focus when slow waves were present on the side of the focus (contralateral to injection). Injection contralateral to focus did not interfere with memory if slow waves were not also present, and slow waves contralateral to injection did not interfere with memory if the focus was not also present. This result emphasizes the functional significance of slow waves, which probably reflect decreased neuronal functioning.

Duration of sodium amytal effect: behavioral and EEG measures.

In performance of an intracarotid sodium amytal test duration of action is a critical factor since measurements obtained after cessation of action are invalid. Duration of action is often monitored by measuring handstrength contralateral to injection or by visual inspection of amytal-induced EEG slow waves ipsilateral to injection. We describe new objective methods of monitoring drug effect: quantified EEG and sensory discrimination. In a first study of patients with scalp electrodes, the two traditional and the two new measures were compared in 40 injections. Results indicated that the two EEG measures did not differ significantly and estimated a shorter drug effect than the two behavioral measures, which were also not significantly different. The two new measures had a smaller SD, indicating that they may be less erratic. In a second study of 14 patients, using intracerebral electrodes, we showed that slow waves in the frontocentral region lasted as long as the motor and sensory effects, but also that slow waves in the medial temporal region dissipated earlier than the motor and sensory deficits. Our results suggest that motor and sensory measures may overestimate the time in which valid testing can occur if hippocampal function is at issue.

Amygdalar sclerosis: preoperative indicators and outcome after temporal lobectomy.

Isolated amygdalar sclerosis (AS) in the presence of an intact hippocampus has been described in a subset of patients who have undergone a temporal lobectomy for the relief of seizures. Clinical observation suggested that these patients might be distinguishable, before and after operation, from those with typical mesial temporal sclerosis, which implies combined amygdalar and hippocampal sclerosis (AHS). From a three year series, all 11 patients classified as having AS were included in this study. These patients were compared with a group of 20 randomly chosen patients with AHS. The groups were found to be well matched in duration of ongoing seizures, full scale IQ, and duration of follow up (mean 19 months). Compared with patients with AHS, patients in the AS group were less likely to have had a seizure in early childhood, a variety of auras, EEG abnormalities localised to one temporal lobe, or an abnormal MRI before operation. They also performed better on preoperative memory tests. At follow up, patients in the AS group were less likely to be seizure free and more likely to have a deterioration in memory after undergoing anterior temporal lobectomy, including part of the hippocampus. The results show that there are preoperative indicators of mesial temporal pathology that are also of prognostic importance given the differences in outcome between the two pathological groups.