Abstract
Monitoring epileptic activity in the absence of interictal discharges is a major need given the well-established lack of reliability of patients’ reports of their seizures. Up to now, there are no other tools than reviewing the seizure diary; however, seizures may not be remembered or dismissed voluntarily. In the present study, we set out to determine if EEG voltage maps of epileptogenic activity in individual patients can help to identify disease activity, even if their scalp EEG appears normal. Twenty-five patients with pharmacoresistant focal epilepsy were included. For each patient, 6 min of EEG with spikes (yes-spike) and without visually detectable epileptogenic discharges (no-spike) were selected from long-term monitoring recordings (EEG 31–37 channels). For each patient, we identified typical discharges, calculated their average and the corresponding scalp voltage map (‘spike-map’). We then fitted the spike-map for each patient on their (i) EEG epochs with visible spikes, (ii) epochs without any visible spike and (iii) EEGs of 48 controls. The global explained variance was used to estimate the presence of the spike-maps. The individual spike-map occurred more often in the spike-free EEGs of patients compared to EEGs of healthy controls (P = 0.001). Not surprisingly, this difference was higher if the EEGs contained spikes (P < 0.001). In patients, spike-maps were more frequent per second (P < 0.001) but with a shorter mean duration (P < 0.001) than in controls, for both no-spike and yes-spike EEGs. The amount of spike-maps was unrelated to clinical variables, like epilepsy severity, drug load or vigilance state. Voltage maps of spike activity are present very frequently in the scalp EEG of patients, even in presumably normal EEG. We conclude that spike-maps are a robust and potentially powerful marker to monitor subtle epileptogenic activity.
Highlights
Epilepsy is one of the most frequent chronic neurological disorders, with a prevalence of 0.5–1% in the general population (Sander, 2003)
This study strongly suggests the presence of spike-specific maps in the scalp EEG even in the absence of visible spikes, which could be a marker of ongoing activity in the epileptogenic focus area
We investigated whether spikemaps, obtained in a clinical setting, can be detected in the EEG of patients with known active uncontrolled focal epilepsy but who showed no visible spikes during the EEG epochs
Summary
Epilepsy is one of the most frequent chronic neurological disorders, with a prevalence of 0.5–1% in the general population (Sander, 2003). A challenging problem in managing patients is to determine if their seizures are, controlled by the prescribed medical treatment given the unreliability of patient self-report and the absence of devices that reliably monitor seizure frequency (Cook et al, 2013) To close this gap, an objective marker of ongoing epileptogenic activity is needed. In a combined EEG-functional magnetic resonance imaging study using caps with 32–94 electrodes, Grouiller et al (2011) showed that specific voltage maps of epileptogenic discharges—‘spike-maps’—have haemodynamic correlates that are concordant with the epileptic focus (Grouiller et al, 2011) They used long-term EEG recording results of patients with focal epilepsy to define the topographic voltage map of the spikes and fitted the patient’s map to the ongoing EEG recorded in the scanner (without any visually detectable spikes). This study strongly suggests the presence of spike-specific maps in the scalp EEG even in the absence of visible spikes, which could be a marker of ongoing activity in the epileptogenic focus area
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