Effects of sleep apnea events to hydrodynamic near-infrared spectroscopy signals

Abstract

The glymphatic system clears waste metabolites, such as Amyloid-$\beta$ from the brain via cerebrospinal fluid-interstitial fluid (CSF-ISF) exchange. Inefficient glymphatic system is linked with old age and neurodegenerative disease, such as Alzheimer's disease. The CSF-ISF exchange is enhanced during sleep and especially during slow-wave sleep (SWS). Sleep apnea is a common sleep disorder that could limit individual's time spent in SWS and thus limit the enhancing effect to the glymphatic system. Additionally, the CSF flow is coupled to cerebral hemodynamics, which are driven by cardiac, respiration, and vasomotor pulsations and these pulsations can be affected by sleep apnea events. Cerebral hemo- and hydrodynamics can be observed with near-infrared spectroscopy (NIRS) by measuring hemoglobin, water, and CSF concentrations below the skull.

A study was conducted utilizing NIRS and a home sleep apnea test to observe neural hydrodynamics during sleep in 9 sleep apnea patients and 10 controls. Standard deviation (SD) and spectral entropy (SE) were calculated for the whole night and sleep segments with sleep apnea events and without. The signals were also filtered to cardiac, respiration, and very low frequencies, to observe individual pulsation types. The results were compared with student's t-test for statistically significant differences.

Significant SD differences were mainly found between events of the groups and significant SE differences were mainly found in cardiac frequencies. Cardiac, respiration and very low frequencies showed unique changes. Deoxygenated hemoglobin exhibited most of the significant differences across the board.

The study concludes that sleep apnea patients and controls have differences in hydrodynamics during sleep apnea events and patient events have more severe changes and also that NIRS is a promising medical imaging tool for monitoring CSF dynamics during sleep, as it can detect CSF, hemoglobin and water concentration changes without sacrificing sleep quality.