Changes in the Spatial Structure of Synchronization Connections in EEG During Nocturnal Sleep Apnea

Abstract

This study involved 72 volunteers divided into two groups according to the apnea-hypopnea index (AHI): AHI>15 episodes per hour (ep/h) (main group, n=39, including 28 men, median AHI 44.15, median age 47), 0≤AHI≤15ep/h (control group, n=33, including 12 men, median AHI 2, median age 28). Each participant underwent polysomnography with a recording of 19 EEG channels. Based on wavelet bicoherence (WB), the magnitude of connectivity between all pairs of EEG channels in six bands was estimated: Df1 0.25;1, Df2 1;4, Df3 4;8, Df4 8;12, Df5 12;20, Df6 20;30 Hz. In all six bands considered, we noted a significant decrease in symmetrical interhemispheric connections in OSA patients. Also, in the main group for slow oscillatory activity Df1 and Df2, we observe a decrease in connection values in the EEG channels associated with the central interhemispheric sulcus. In addition, patients with AHI>15 show an increase in intrahemispheric connectivity, in particular, forming a left hemisphere high-degree synchronization node (connections PzT3, PzF3, PzFp1) in the Df2 band. When considering high-frequency EEG oscillations, connectivity in OSA patients again shows a significant increase within the cerebral hemispheres. The revealed differences in functional connectivity in patients with different levels of AHI are quite stable, remaining when averaging the full nocturnal EEG recording, including both the entire sleep duration and night awakenings. The increase in the number of hypoxia episodes correlates with the violation of the symmetry of interhemispheric functional connections. Maximum absolute values of correlation between the apnea-hypopnea index, AHI, and the WB synchronization strength are observed for the Df2 band in symmetrical EEG channels C3C4 (-0.81) and P3P4 (-0.77). The conducted studies demonstrate the possibility of developing diagnostic systems for obstructive sleep apnea syndrome without using signals from the cardiovascular system and respiratory activity.

Keywords: connections; polysomnography; sleep apnea; spatial structure; synchronization.

Figure 1

(a) diagram of the arrangement of six EEG electrodes used in polysomnographic studies, (b) diagram of the arrangement of nineteen EEG electrodes according to “10–20” EEG scheme, (c) diagram of the arrangement of polysomnographic equipment during a night study of the patient’s sleep, (d) diagram of the presentation of the results of the assessments of synchronization between EEG channels.

Figure 2

(a–f) color maps of the spatial structure of synchronization for the frequency bands $D{f}_1\left[0.25;1\right]$, $D{f}_2\left[1;4\right]$, $D{f}_3\left[4;8\right]$ Hz. The left column, marked with a green stripe in the center of the figure, is constructed for the EEG of healthy subjects during night sleep, and the right column (red stripe) corresponds to the OSA data of patients. Below is a color scale demonstrating the correspondence of each color to a certain value of the strength of the average WB connection between each pair of EEG channels.

Figure 3

(a–f) color maps of the spatial structure of synchronization for the bands $D{f}_4\left[8;12\right]$, $D{f}_5\left[12;20\right]$, $D{f}_6\left[20;30\right]$ Hz. The left column, marked with a green stripe in the center of the figure, is constructed for the EEG of healthy subjects during night sleep, and the right column (red stripe) corresponds to the OSA data of patients. Below is a color scale demonstrating the correspondence of each color to a certain value of the strength of the average WB connection between each pair of EEG channels.

Figure 4

(a–f) Difference schemes of changes in synchronization measures calculated from EEG signals recorded in groups of healthy subjects and OSA patients in bands $D{f}_1$ [0.25; 1], $D{f}_2$ [1; 4], $D{f}_3$ [4; 8], $D{f}_4$ [8; 12], $D{f}_5$ [12; 20], $D{f}_6$ [20; 30] Hz. On the right is a color scale showing the correspondence between the color designation and the magnitude of the difference between the coupling strengths for each pair of EEG channels.