Non-linear EEG analysis in children with epilepsy and electrical status epilepticus during slow-wave sleep (ESES)

Objective: The objective of this work was to study the non-linear aspects of electroencephalography (EEG) in children with epilepsy and electrical status epilepticus during slow-wave sleep (ESES). Methods: In this study, we recorded the sleep EEG in 5 subjects with ESES (4 males and one female, aged 6.5–10 years) who were also mentally retarded and affected by cerebral palsy (3 subjects) and hydrocephalus (two subjects). The signals were sampled at 128 Hz and stored on hard disk. All the subsequent computational steps were performed on EEG epochs (4096 data points) selected from wakefulness and non-rapid eye movement (non-REM) (with ESES) or REM sleep. The dynamic properties of the EEG were assessed by means of the non-linear cross prediction (NLCP) test which uses 3 different ‘model’ time series in order to predict non-linearly the original data set (Pred, Ama and Tir). Pred is a measure of the predictability of the time series and Ama and Tir are measures of asymmetry, indicating non-linear structure. Moreover, the correlation dimension (D2) was estimated by means of the algorithm by Grassberger and Procaccia (1983) for the epochs showing non-linear nature. Results: The NLCP test provided evidence of significant non-linear dynamics in all epochs of non-REM sleep, when ESES was evident. Only during this stage, the possible presence of low-dimensional chaos could also be suspected (average D2=4.02; range 3.16–6.21). EEG without ESES could not be distinguished from linearly filtered noise. Conclusions: The results of the present study seem to indicate that subjects with ESES show a profound modification of their EEG dynamics with the occurrence, during sleep, of long periods characterized by non-linear dynamics and, probably, low-dimensional chaotic structure able to modify in a substantial way their brain functioning during sleep.