Study population
52 community-dwelling cognitively healthy older adults were enrolled in the study, following approval from the Institutional Review Board on Human and Animal Research at Stanford University. This research was performed in accordance with the Declaration of Helsinki. Informed consent was obtained from all participants. The inclusion criteria were the following: 60 years of age or older, a score of ≥26 in the Mini-Mental State Exam (MMSE), absence of a diagnosis of possible or probable dementia or a profile on the cognitive battery indicative of dementia, lack of any serious medical illness, and any axis I disorder assessed using the Structured Clinical Interview for DSM-5 within the last two years or major unstable medical diseases or symptoms.
The mean and standard deviation (SD) of participants’ ages and years of education were 72.0 (7.1) and 17.5 (2.8), respectively. Of the 52 participants, 28 were women, 43 identified as Caucasians, and nine identified as Asians.
All participants underwent overnight ambulatory PSG coupled with NIRS recordings at their residences. Adherence to participants’ natural sleep-wake schedules was emphasized throughout the study protocol.
Procedures
Cognitive battery
Participants underwent a comprehensive cognitive assessment utilizing a battery of tests designed to detect mild cognitive impairment (MCI) in older adults 33. The cognitive domains examined included: 1) delayed verbal recall on the Rey Auditory Verbal Learning Test 20-minute delay trial (RAVLT) 34, 2) information processing speed and attention on the Stroop Color and Word Test (STRCW) 35, 3) phonemic fluency and executive function on Controlled Oral Word Association (COWA) 36, 4) verbal naming on the Boston Naming Test (BNT) 37, and 5) visuospatial ability on the Judgment of Line Orientation (JLO) 38. Notably, participants were screened for color blindness prior to undertaking the STRCW, with none being identified as such. Prior research has consistently associated older age with diminished performance on delayed recall measures 39,40, while deficits in the remaining cognitive domains are prevalent in older adults and indicative of heightened risk for cognitive decline and dementia 41. Additionally, the MMSE was administered to provide a succinct assessment of overall cognitive functioning. All selected measures have demonstrated validity and widespread applicability in older populations 34,35,37,38.
PSG and NIRS recording
For overnight NIRS recordings during sleep, we utilized the Octamon® NIRS system equipped with flat light emitting diode (LED) optodes to mitigate sleep disturbances. To optimize battery usage and minimize interference from hair during overnight recording, data collection was restricted to a single NIRS channel (optode-receiver pair) positioned on the left frontal region. Firm attachment of the probe to the forehead was ensured using a headband. Simultaneous PSG recordings were conducted using the Nox A1 PSG monitoring system, which captures EEG, electrooculogram, submental electromyogram, nasal airway pressure, nasal thermal airflow sensor, finger pulse oximetry, electrocardiogram, rib cage and abdominal movements, snoring, and body position. Following the application of PSG electrodes and NIRS probes, participants adhered to their natural sleep-wake schedules. Data was recorded continuously throughout the night, retrieved the subsequent day, and stored digitally. Sleep stages were manually and visually scored by a board-certified sleep technician or physician according to established standard conventions 42. Respiratory events were identified and scored using standard definitions, with related sleep parameters derived from established methodologies 43.
Analytical approach: First, PSG data were visually scored based on standard AASM criteria 44. Subsequent quality checks and preprocessing of NIRS data were conducted using an in-house package and Homer 2 in MATLAB 45-47. Motion artifacts in the optical density data were corrected using a wavelet motion correction procedure, followed by band-pass filtering with cutoff frequencies of 0.008 Hz and 0.5 Hz before conversion to oxy-Hb using the modified Beers-Lambert law 48. Next, SDB desaturation events, characterized by a 3% or greater reduction in oxygen saturation from baseline, were scored and visually confirmed. For each SDB desaturation event, the reduction in oxy-Hb was quantified as the maximum drop in oxy-Hb signal normalized by baseline oxy-Hb. Coherence between extracted oxy-Hb and oxygen saturation (SpO2) signals was quantified using wavelet coherence for each SDB event 49,50. The mean wavelet coherence values were averaged across the SDB events. Subsequently, the association between the mean coherence of SDB events and cognitive measures was examined. A lower coherence between oxy-Hb and SpO2 signal would suggest higher decoupling and stronger compensation against peripheral oxygen drops at the cortical level (Figure 1).
Primary analysis: We examined whether the mean coherence of oxy-Hb and SpO2 during SDB desaturation events could predict cognition. We performed a linear regression analysis using the RAVLT delayed recall, STRCW, COWA, BNT, and JLO scores as dependent variables. For each primary outcome measure, we conducted linear regression with mean coherence, while adjusting for age, sex, and education.
Secondary analysis: We examined if changes in the NIRS parameters, specifically the reduction of oxy-Hb during desaturation events, could serve as predictors of cognition.Similar to the primary analysis, we performed a linear regression analysis, considering the RAVLT delayed recall, STRCW, COWA, BNT, and JLO scores as dependent variables. For each secondary outcome measure, we conducted linear regression with the change in oxy-Hb during SDB, while adjusting for age, sex, and education.
Exploratory analysis: We further investigated associations between conventional SDB-related sleep parameters and cognitive measures by performing Spearman’s bivariate correlations. This analysis encompassed parameters from both SDB-related sleep (including AHI, ODI 3%, ODI 4%, duration of desaturation less than 90%, and lowest SpO2) and cognitive measures (comprising RAVLT, STRCW, COWA, BNT, and JLO scores) across all participants.
Adjustment for multiple comparisons: To account for multiple comparisons across the primary, secondary, and exploratory analyses involving five cognitive measures (RAVLT delayed recall, STRCW, COWA, BNT, and JLO), we implemented Bonferroni correction. With a corrected alpha of .05/5, equating to .01, this correction ensures the maintenance of statistical rigor.