Diagnostic yield of high-density versus low-density EEG: The effect of spatial sampling, timing and duration of recording.

OBJECTIVE: To investigate the effect of spatial sampling and of recording duration on the diagnostic yield of EEG for identification of interictal epileptiform discharges (IEDs). Previous studies demonstrated that high-density (HD) recordings increased accuracy of localization compared to low-density (LD) recordings. METHODS: We have prospectively evaluated the effect of spatial sampling and of recording duration in patients who had short-term (ST) recordings with a HD array of 256 electrodes following long-term (LT) recordings with a LD array consisting of the standard IFCN array of 25 electrodes. IED clusters were identified in four datasets: LT-LD, ST-LD (spatially down-sampled to the standard IFCN array), ST-HD and a shortened (90 minutes) epoch of LT-LD. RESULTS: Sixty consecutive patients were recruited. We identified 89 IED clusters totally. Two clusters were found by increasing spatial sampling from 25 to 256 electrodes. This modest increase was not statistically significant. Eight clusters were missed by reducing the recording duration to 90 minutes, as compared with the LT recordings (p = 0.003). CONCLUSIONS: Recording duration is more important for the diagnostic yield of EEGs than increasing spatial sampling beyond the standard IFCN electrode array. SIGNIFICANCE: The standard IFCN electrode array provides sufficient spatial sampling for identification of the IEDs.

The slow-wave component of the interictal epileptiform EEG discharges.

The interictal epileptiform discharges (IEDs) consist of a fast component (FC; spike or sharp-wave) followed by a slow-wave component (SC). Our purpose was to assess the intra-individual variance, the diagnostic significance and the effect of sleep on the SC. Ninety-nine EEG recordings from 50 consecutive patients with IEDs were analysed. We measured the duration (ms) of the SC (SC-duration), while the amplitude of the SC was divided by the amplitude of the FC yielding a normalized value (SC/FC amplitude-ratio). Intra-individual, intra- and inter-recording coefficients of variation (CV) were calculated for the SC-duration and SC/FC amplitude-ratio. The correlation with the diagnosis, and the effect of sleep was analysed. The SC-duration and the SC/FC amplitude-ratio had low CV (<27%). The SC-duration was not correlated with the diagnosis. The SC/FC amplitude-ratio was significantly higher in the patients with generalized epilepsies as compared with the localization-related ones, and it was higher in the patients with idiopathic epilepsies as compared with the symptomatic ones. These predictors were independent. The SC/FC amplitude-ratio of the patients with idiopathic epilepsy increased significantly during sleep. We conclude that the SC-duration and SC/FC amplitude-ratio are stable parameters. The amplitude of the SC in relation to the fast component is larger in patients with generalized and idiopathic epilepsies, suggesting higher degree of cortical inhibition in these patients, possibly corresponding to specific protective mechanisms.