Does asymmetric basal ganglia or thalamic activation aid in seizure foci lateralization on ictal SPECT studies?

Basal ganglia or thalamic activation has been reported in ictal SPECT studies of patients with intractable epilepsy. We hypothesized that lateralization of activation of these subcortical structures may aid in the lateralization of seizure foci in patients in whom the cortical focus is subtle or equivocal.

This was a retrospective analysis of 72 ictal (99m)Tc-ethylcysteinate dimer SPECT studies in 43 patients with intractable epilepsy in whom seizure laterality could be eventually determined. All patients underwent video-electroencephalography (EEG) monitoring, MRI, and one or more ictal SPECT scans as well as an interictal SPECT scan. Intracranial electrode EEG monitoring and surgery were performed as clinically indicated. Ictal and interictal studies were coregistered with patients' MRI scans using automated software, and ictal minus interictal subtraction images were obtained. The presence of asymmetric basal ganglia or thalamic activation was determined by 2 experienced observers who were unaware of clinical information. The final seizure focus was determined by surgical cure in 37 patients. In patients in whom surgery was not indicated or initial surgery was performed at another institution (n = 6), a consistent focus detected by intracranial electrode monitoring or repeated stereotypical seizures all originating from the same site on video-surface EEG monitoring was considered to indicate the final seizure focus.

Thirty-five patients had neocortical seizures and 8 had mesial temporal lobe seizures. Asymmetric basal ganglia activation was seen in 22 (30.6%) studies. This activation was ipsilateral to the final determined seizure focus in 17 of 22 of these studies (77.3%) and contralateral in 5 of 22 (21.7%). Asymmetric thalamic activation was seen in 15 studies (20.8%), of which 12 of 15 (80%) were ipsilateral to the final seizure focus, whereas 3 of 15 (20%) were contralateral. In 3 of 5 studies with contralateral basal ganglia activation and 1 of 3 studies with contralateral thalamic activation, the SPECT study as a whole was found to be falsely localizing. In another 2 cases of contralateral subcortical activation, the SPECT study as a whole was considered nonlocalizing. Worse outcome was not observed in patients with false ictal SPECT subcortical lateralization; however, the presence of asymmetric subcortical uptake, regardless of relationship to seizure focus, was associated with decreased incidence of seizures at 1 y after surgery.

Although asymmetric basal ganglia or thalamic activation is common, it is rarely the sole indicator of seizure localization. However, it may be a useful confirmatory sign in subtle cases of cortical localization. In cases of false ictal SPECT subcortical lateralization, the basal ganglia appear to follow cortical activation pattern. Furthermore, there appears to be a correlation between lateralizing uptake in subcortical structures on ictal SPECT and postsurgical outcome in intractable epilepsy patients.