Abstract
Hypersynchronous (HYP) and low voltage fast (LVF) activity are two separate ictal depth EEG onsets patterns often recorded in presurgical patients with MTLE. Evidence suggests the mechanisms generating HYP and LVF onset seizures are distinct, including differential involvement of hippocampal and extra-hippocampal sites. Yet the extent of extra-hippocampal structural alterations, which could support these two common seizures, is not known. In the current study, preoperative MRI from 24 patients with HYP or LVF onset seizures were analyzed to determine changes in cortical thickness and relate structural changes to spatiotemporal properties of the ictal EEG. Overall, onset and initial ipsilateral spread of HYP onset seizures involved mesial temporal structures, whereas LVF onset seizures involved mesial and lateral temporal as well as orbitofrontal cortex. MRI analysis found reduced cortical thickness correlated with longer duration of epilepsy. However, in patients with HYP onsets, the most affected areas were on the medial surface of each hemisphere, including parahippocampal regions and cingulate gyrus, whereas in patients with LVF onsets, the lateral surface of the anterior temporal lobe and orbitofrontal cortex showed the greatest effect. Most patients with HYP onset seizures were seizure-free after resective surgery, while a higher proportion of patients with LVF onset seizures had only worthwhile improvement. Our findings confirm the view that recurrent seizures cause progressive changes in cortical thickness, and provide information concerning the structural basis of two different epileptogenic networks responsible for MTLE. One, identified by HYP ictal onsets, chiefly involves hippocampus and is associated with excellent outcome after standardized anteromedial temporal resection, while the other also involves lateral temporal and orbitofrontal cortex and a seizure-free surgical outcome occurs less after this procedure. These results suggest that a more extensive tailored resection may be required for patients with the second type of MTLE.
Highlights
Evidence suggests there are subtypes of mesial temporal lobe epilepsy (MTLE) with or without hippocampal sclerosis (HS), and some of which could have less likelihood for postsurgical seizure-freedom [1,2,3,4]
Spatial distribution of seizure onset zone (SOZ) and initial spread of HYP and low voltage fast (LVF) onset seizures Manual review of depth electrode-recorded seizures found 10 patients who consistently had unilateral HYP onset seizures (Fig 1A) and 14 patients with seizures that regularly began with a LVF ictal onset pattern (Fig 1B)
Cluster analysis indicated site(s) of seizure onset (i.e., SOZ) and initial spread were consistent within each patient, but differences were found in the spatial distribution between HYP and LVF onset patterns
Summary
Evidence suggests there are subtypes of mesial temporal lobe epilepsy (MTLE) with or without hippocampal sclerosis (HS), and some of which could have less likelihood for postsurgical seizure-freedom [1,2,3,4]. The SOZ is more widespread and can include polar and lateral temporal lobe, peri-sylvian, insular or orbitofrontal cortex [8,9] These latter patients often have seizures that begin with a low voltage fast (LVF) EEG pattern [8,10,11]. Studies have shown a link between HYP onset seizures and HS [15], a classical pattern of HS with damage of dentate gyrus, CA3, CA1, and relative sparing of CA2 [16]. Based on neuroimaging and electrophysiological data, one hypothesis is that patients with HYP onset seizures have greater damage in hippocampal and related mesial limbic structures compared to patients with LVF onset seizures who have greater structural changes in anterolateral regions of temporal and possibly frontal lobes
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