Abstracts

Rationale: Patients with epilepsy exhibit disturbances in sleep macro- and microstructure (1). However, the impact of epilepsy on specific sleep oscillations remains poorly understood. Sleep spindles are one of the hallmarks of NREM sleep stage N2, and play a key role in cognitive functioning, brain maturation and restorative sleep (2). Spindle frequency was found to be lower in patients with generalized compared to focal epilepsy (3), but the regional influence of the epileptogenic zone remains unclear. In this study, combined polysomnography high-density EEG (HD-EEG) was performed in consecutive patients with focal epilepsy to assess regional differences in rate and frequency of spindles.

Methods: Fourteen patients (8 women; 7 temporal lobe epilepsy; mean age: 32.8±10.7 years) with unilateral drug-resistant focal epilepsy underwent one night of combined sleep and 83-channel HD-EEG recording. Sleep was scored in 30 s epochs. For every patient, we selected the first 40 epochs of N2 sleep (early sleep). Electrodes were grouped into 11 regions (midline and bilateral frontal, central, temporal, parietal, occipital). Spindles were automatically detected in a combined mastoid montage and categorized into slow (10-12 Hz) and fast spindles (12-16 Hz). Spikes were marked by an epileptologist. Between region differences were assessed with a paired Wilcoxon signed-ranked test. To assess sleep homeostatic properties, we computed spindle rate and frequency in the last 40 N2 epochs (late sleep).

Results: There was no difference in spindle rates between the hemisphere ipsilateral to the epileptic focus versus the contralateral hemisphere (ipsilateral: median=5.2, range 3.7-7.2 and contralateral: median=5.5, range 3.8-7.3, p >0.05). On a regional level, spindle rates were reduced in the region with the epileptic focus (median 5.0, range 1.9-7.5) compared to the contralateral side (median=5.4, 1.5-7.4) (p=0.03; Cohen’s d=0.2, Figure 1). Fast spindle rates were found to be the driver for this reduction, as they were reduced in the region with the epileptic focus (median 1.9, range 0.5-5.6) compared to the contralateral side (median 2.1, range 0.4- 5.6) (p=0.02; Cohen’s d=0.2), while no difference was found for slow spindles (p >0.05). There was no difference in spindle rates for late N2 sleep (all p >0.05). Spike and spindle rates were not correlated (p >0.05).

Conclusions: The presence of an epileptic focus is associated with a regional decrease in spindle rate. This suggests that epilepsy locally disturbs the mechanisms involved in spindle generation.

1. Frauscher B, Gotman J. Sleep, oscillations, interictal discharges, and seizures in human focal epilepsy. Neurobiol Dis. 2019;127:545-53.
2. Fogel SM, Smith CT. The function of the sleep spindle: a physiological index of intelligence and a mechanism for sleep-dependent memory consolidation. Neurosci Biobehav Rev. 2011;35(5):1154-65.
3. Drake ME, Jr., Pakalnis A, Padamadan H, Weate SM, Cannon PA. Sleep spindles in epilepsy. Clin Electroencephalogr. 1991;22(3):144-9.

Funding: Please list any funding that was received in support of this abstract.: No funding.