Neuroimaging in Epilepsy: Recent Developments in MR Imaging, Positron-Emission Tomography, and Single-Photon Emission Tomography

Association of Normative and Non-Normative Brain Networks With Cognitive Function in Patients With Temporal Lobe Epilepsy

BACKGROUND AND OBJECTIVES

Despite their temporal lobe pathology, a significant subgroup of patients with temporal lobe epilepsy (TLE) is able to maintain normative cognitive functioning. In this study, we identify patients with TLE with intact vs impaired neurocognitive profiles and interrogate for the presence of both normative and highly individual intrinsic connectivity networks (ICNs)-all toward understanding the transition from impaired to intact neurocognitive status.

METHODS

This retrospective cross-sectional study included patients with TLE and matched healthy controls (HCs) from the Thomas Jefferson Comprehensive Epilepsy Center. Functional MRI data were decomposed using independent component analysis to obtain individualized ICNs. In this article, we calculated the degree of match between individualized ICNs and canonical ICNs (e.g., 17 resting-state networks by Yeo et al.) and divided each participant's ICNs into normative or non-normative status based on the degree of match.

RESULTS

100 patients with TLE (mean age 42.0 [SD: 13.7] years, 47 women) and 92 HCs were included in this study. We found that the individualized networks matched to the canonical networks less well in the cognitively impaired (n = 24) compared with the cognitively intact (n = 63) patients with TLE by 2-way mixed-measures analysis of variance (impaired vs intact mean difference [MD] -0.165 [-0.317, -0.013], p = 0.028). The cognitively impaired patients showed significant abnormalities in the profiles of both normative (impaired vs intact MD -0.537 [-0.998, -0.076], p = 0.017, intact vs HC MD -0.221 [-0.536, 0.924], p = 0.220, and impaired vs HC MD -0.759 [-1.200, -0.319], p < 0.001) and non-normative networks (impaired vs intact MD 0.484 [0.030, 0.937], p = 0.033, intact vs HC MD 0.369 [0.059, 0.678], p = 0.014, and impaired vs HC MD 0.853 [0.419, 1.286], p < 0.001) while the intact patients showed abnormalities only in non-normative networks. At the same time, we found that normative networks held a strong, positive association with the neuropsychological measures, with this association negative in non-normative networks.

DISCUSSION

Our data demonstrated that significant cognitive deficits are associated with the status of both canonical and highly individual ICNs, making clear that the transition from intact to impaired cognitive status is not simply the result of disruption to normative brain networks.

Pre-surgical features of intrinsic brain networks predict single and joint epilepsy surgery outcomes

Despite the effectiveness of surgical interventions for the treatment of intractable focal temporal lobe epilepsy (TLE), the substrates that support good outcomes are poorly understood. While algorithms have been developed for the prediction of either seizure or cognitive/psychiatric outcomes alone, no study has reported on the functional and structural architecture that supports joint outcomes. We measured key aspects of pre-surgical whole brain functional/structural network architecture and evaluated their ability to predict post-operative seizure control in combination with cognitive/psychiatric outcomes. Pre-surgically, we identified the intrinsic connectivity networks (ICNs) unique to each person through independent component analysis (ICA), and computed: (1) the spatial-temporal match between each person's ICA components and established, canonical ICNs, (2) the connectivity strength within each identified person-specific ICN, (3) the gray matter (GM) volume underlying the person-specific ICNs, and (4) the amount of variance not explained by the canonical ICNs for each person. Post-surgical seizure control and reliable change indices of change (for language [naming, phonemic fluency], verbal episodic memory, and depression) served as binary outcome responses in random forest (RF) models. The above functional and structural measures served as input predictors. Our empirically derived ICN-based measures customized to the individual showed that good joint seizure and cognitive/psychiatric outcomes depended upon higher levels of brain reserve (GM volume) in specific networks. In contrast, singular outcomes relied on systematic, idiosyncratic variance in the case of seizure control, and the weakened pre-surgical presence of functional ICNs that encompassed the ictal temporal lobe in the case of cognitive/psychiatric outcomes. Our data made clear that the ICNs differed in their propensity to provide reserve for adaptive outcomes, with some providing structural (brain), and others functional (cognitive) reserve. Our customized methodology demonstrated that when substantial unique, patient-specific ICNs are present prior to surgery there is a reliable association with poor post-surgical seizure control. These ICNs are idiosyncratic in that they did not match the canonical, normative ICNs and, therefore, could not be defined functionally, with their location likely varying by patient. This important finding suggested the level of highly individualized ICN's in the epileptic brain may signal the emergence of epileptogenic activity after surgery.

99Tc-HmPAO SPECT in 13 patients with classic lissencephaly

In this study, technetium-99 ((99)Tc)-hexamethylpropyleneamine-oxine single-photon emission computed tomography (SPECT) was performed on 13 children with classic lissencephaly (nine with epileptic seizures, four without seizures). Focal or multifocal hypoperfusions were observed in 12 patients. The hypoperfused areas observed on SPECT scanning did not correlate with the localization of agyric-pachygyric regions in all patients. The distribution of perfusion abnormalities by SPECT and the localization of agyria-pachygyria as detected by magnetic resonance imaging did not correlate strongly. All nine patients with seizures and three of the four patients without seizures had focal or multifocal cerebral blood flow abnormalities on the SPECT scans. The presence of brain perfusion abnormalities detected by SPECT and the occurrence of epileptic seizures did not have a significant relationship. These results suggest that the role of SPECT studies in classic lissencephaly is not clearly defined. More sophisticated methods are needed to clarify the correlation between structural and functional abnormalities of patients diagnosed with lissencephaly.

Functional MRI and the Wada test provide complementary information for predicting post-operative seizure control

Prediction of post-surgical seizure relief and potential cognitive deficits secondary to anterior temporal lobectomy (ATL) are important to pre-surgical planning. Although the intracarotid amobarbital test (IAT) is predictive of post-ATL seizure outcome, development of non-invasive and more precise means for determining post-ATL seizure relief are needed. We previously reported on a technique utilizing functional MRI (fMRI) to evaluate the relative functional adequacy of mesial temporal lobe structures in preparation for ATL. In the present study, we report follow-up outcome data on eight temporal lobe epilepsy (TLE) patients 1-year post-ATL who were evaluated pre-surgically using IAT and fMRI. Functional memory lateralization using fMRI predicted post-ATL seizure outcome as effectively as the IAT. In general, asymmetry of functional mTL activation favouring the non-epileptic hemisphere was associated with seizure-free status at 1-year follow-up. Moreover, when combined, fMRI and IAT provided complementary data that resulted in improved prediction of post-operative seizure control compared with either procedure alone.

Neuroimaging in epilepsy

Neuroimaging techniques have improved the understanding, diagnosis, and management of epilepsy. By providing excellent structural information, MRI is the technique of choice in evaluating patients with epilepsy. Functional imaging techniques, including MR spectroscopy, functional MRI, positron emission tomography, and single photon emission CT, permit noninvasive assessment of the epileptic substrate, its functional status, and neuroreceptors. The MRI-based techniques will potentially assume a greater role in the cost-effective workup of the patient. Currently, newer techniques such as magnetoencephalography, magnetic source imaging, and optical imaging are research tools.

Localization of subclinical ictal activity by functional magnetic resonance imaging: correlation with invasive monitoring

Functional magnetic resonance imaging (fMRI) with susceptibility-based contrast was used to detect focal changes in cerebral blood flow and metabolism in a patient with focal epilepsy. The patient presented with frequent partial motor seizures involving his right lower face that spread to produce speech arrest and occasionally right arm jerking. Consciousness was never impaired during these events. A multislice echoplanar technique was used to acquire 16 contiguous axial slices every 4 seconds for 11 minutes. Although no overt seizures were observed or reported by the patient during the scanning, a time series analysis of the functional data revealed focal signal-intensity changes in the posterior left frontal lobe, which correlated well both in duration and spatial localization with ictal activity subsequently recorded by invasive electrophysiological monitoring. The spatial localization of fMRI was more accurate than electroencephalography recorded from a subdural grid in predicting the site of successful surgical therapy. These results illustrate the potential of functional MRI for localizing seizure foci with high spatial and temporal resolution. Such studies can be readily combined with high-resolution anatomical imaging, task-activation studies, and other magnetic resonance techniques.

Comparison of mesial versus neocortical onset temporal lobe seizures: neurodiagnostic findings and surgical outcome

We compared historical features, surface EEG findings, results of intracarotid sodium amobarbital memory testing (IAT), and outcome after anterotemporal lobectomy (ATL) in patients with mesiotemporal lobe seizure onset with those with more diffuse temporal lobe seizure onset (intracranial EEG). Forty-eight patients evaluated consecutively between July 1985 and October 1991 with both scalp/sphenoidal and intracranial EEG were shown to have seizures originating in one temporal lobe. No patients had temporal lobe tumor or vascular malformation. Thirty-seven of the 48 patients had seizure onset in the amygdala/hippocampus (amyg/hipp). Eleven of the 48 had either temporal neocortical onset or simultaneous amyg/hipp and neocortical onset. Patients with mesial onset seizures were more likely to have lateralized memory impairment on IAT (p = 0.05). We noted a trend toward a difference in age of first risk for epilepsy between the two groups (p = 0.09) but not for a difference in any specific risk factor. There were no significant differences in surface EEG interictal findings. Unlike in previous studies, comparison of outcome between the two groups showed no difference in seizure-free outcome. Sudden unexpected death (SUD) was more frequent in neocortical seizure patients who were not seizure-free (p < 0.05).