Abstract
Aims: Intracranially recorded high-frequency oscillations (>80 Hz) are considered a candidate epilepsy biomarker. Recent studies claimed their detectability on the scalp surface. We aimed to investigate the applicability of high-frequency oscillation analysis to routine surface EEG obtained at an epilepsy monitoring unit.Methods: We retrospectively analyzed surface EEGs of 18 patients with focal epilepsy and six controls, recorded during sleep under maximal medication withdrawal. As a proof of principle, the occurrence of motor task-related events during wakefulness was analyzed in a subsample of six patients with seizure- or syncope-related motor symptoms. Ripples (80–250 Hz) and fast ripples (>250 Hz) were identified by semi-automatic detection. Using semi-parametric statistics, differences in spontaneous and task-related occurrence rates were examined within subjects and between diagnostic groups considering the factors diagnosis, brain region, ripple type, and task condition.Results: We detected high-frequency oscillations in 17 out of 18 patients and in four out of six controls. Results did not show statistically significant differences in the mean rates of event occurrences, neither regarding the laterality of the epileptic focus, nor with respect to active and inactive task conditions, or the moving hand laterality. Significant differences in general spontaneous incidence [WTS(1) = 9.594; p = 0.005] that indicated higher rates of fast ripples compared to ripples, notably in patients with epilepsy compared to the control group, may be explained by variations in data quality.Conclusion: The current analysis methods are prone to biases. A common agreement on a standard operating procedure is needed to ensure reliable and economic detection of high-frequency oscillations.
Highlights
High-frequency oscillations (HFOs) in the EEG are fast local oscillatory field potentials, commonly sub-classified as ripples (80–250 Hz) and fast ripples (>250 Hz) [1]
HFOs have been claimed to be detectable on the surface and scalp HFO analysis has been discussed as a potential tool for epilepsy screening [3, 4]
Spontaneous HFOs were detected across the overall ROI Sleep with an average rate of 0.017 events/min (SD = 0.016, N = 24) and found to be increased in patients with epilepsy (M = 0.021, SD = 0.016, n = 18) compared to the control group (M = 0.007, SD = 0.008, n = 6)
Summary
High-frequency oscillations (HFOs) in the EEG are fast local oscillatory field potentials, commonly sub-classified as ripples (80–250 Hz) and fast ripples (>250 Hz) [1]. Scalp HFOs were predominantly detected interictally during non-rapid eye movement sleep in patients with focal epilepsy associated with traditional markers and the seizure-generating zone [3, 6]. Findings appear ambiguous, as HFOs, primarily suggested to be a specific marker of the epileptic focus, did not differentiate between focal and generalized epilepsy [7]. The relationship of HFOs with the epileptic focus and traditional markers must not be exclusive. Intracranial HFOs preceding ictal spasm have been shown to not exclusively map the seizure-onset zone and reflect the occurrence of motor symptoms in neighboring areas [8]; scalp HFOs were more closely correlated with seizure frequency than traditional markers [9]. The differentiation of pathological and physiological HFOs within subject is largely unexplored in scalp EEG
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