Do quantified sleep architecture abnormalities underlie cognitive disturbances in amnestic mild cognitive impairment?

The study was designed to gauge association between occult sleep-related breathing disturbances and sleep architecture changes on cognitive trajectories in subjects with amnestic mild cognitive impairment (aMCI) relative to cognitively normal healthy controls, phenotyped by neuroimaging. Subjects with aMCI and normal cognition were prospectively recruited. Following standardized neuropsychological and sleep questionnaire assessment they underwent a single overnight polysomnography (PSG); multimodality MRI was used to ascertain age-corrected radiological differences between the 2 groups. The aMCI cohort was followed up longitudinally with serial cognitive assessments for the next 3 years. Thirty seven subjects with aMCI and 24 control subjects consented for evaluation. Although occult moderate to severe obstructive sleep apnea (OSA) was more prevalent in aMCI (43.6%) as opposed to controls (22.7%); higher median apnea-hypopnea index (AHI = 11.5) and total apnea-hypopnea time (26.6 min) were also noted in aMCI relative to controls (6.6 and 11.4 min respectively), the differences were not statistically significant. In the aMCI group, better sleep efficiency, longer duration of REM sleep correlated with higher associative learning, free-recall/recognition memory performance. Higher AHI had negative correlation with visual memory scores. However longitudinal cognitive trends in the aMCI group over 3 years reflected relative stability (only 5% progressed to AD) notwithstanding imaging differences from controls and appeared to be independent of sleep parameters. The study concluded that despite associations between sleep efficiency, REM sleep and sleep-related breathing variables with neuropsychological test-scores in aMCI, these appear to be comorbidities rather than causative factors for the degree of cognitive impairment or its longitudinal trajectory.