In Search of Sleep Biomarkers of Alzheimer’s Disease: K-Complexes Do Not Discriminate between Patients with Mild Cognitive Impairment and Healthy Controls

Abstract

The K-complex (KC) is one of the hallmarks of Non-Rapid Eye Movement (NREM) sleep. Recent observations point to a drastic decrease of spontaneous KCs in Alzheimer’s disease (AD). However, no study has investigated when, in the development of AD, this phenomenon starts. The assessment of KC density in mild cognitive impairment (MCI), a clinical condition considered a possible transitional stage between normal cognitive function and probable AD, is still lacking. The aim of the present study was to compare KC density in AD/MCI patients and healthy controls (HCs), also assessing the relationship between KC density and cognitive decline. Twenty amnesic MCI patients underwent a polysomnographic recording of a nocturnal sleep. Their data were compared to those of previously recorded 20 HCs and 20 AD patients. KCs during stage 2 NREM sleep were visually identified and KC densities of the three groups were compared. AD patients showed a significant KC density decrease compared with MCI patients and HCs, while no differences were observed between MCI patients and HCs. KC density was positively correlated with Mini-Mental State Examination (MMSE) scores. Our results point to the existence of an alteration of KC density only in a full-blown phase of AD, which was not observable in the early stage of the pathology (MCI), but linked with cognitive deterioration.

Characteristics of Sleep Structure Assessed by Objective Measurements in Patients With Amnestic Mild Cognitive Impairment: A Meta-Analysis

“…Information of the eight included studies (22)(23)(24)(25)(26)(27)(28)(29), such as publication time, countries, major criteria of aMCI, and characteristics of the aMCI and control samples are listed ( Table 2).…”

“…Different measurements: five (22)(23)(24)(25)27) studies provided detailed PSG data, while two (26,28) provided actigraphy data. In the actigraphy subgroup, no significant difference was observed between the aMCI and NC groups [SMD = −0.00, 95%CI (−0.62∼0.62), P = 1.00].…”

“…Seven (22)(23)(24)(25)(26)(27)29) included studies showed that there was no significant difference between the aMCI and NC groups [SMD = 0.18, 95%CI (−1.04∼1.40), P = 0.77].…”

“…The result showed that the NC group had great SWS than the aMCI group [SMD = −3.52, 95%CI (−4.96∼2.08), P < 0.00001]. Four (22)(23)(24)27) included studies showed that N1% in the aMCI group was greater than that in the NC group [SMD = 0.95, 95%CI (0.07∼1.82), P = 0.03] ( Figure 5).…”

Sleep efficiency and neurophysiological patterns in middle‐aged men are associated with cognitive change over their adult life course

“…() explained this apparent contradiction by the partial frequency overlap between delta EEG activity and K‐complexes that have been associated with memory consolidation. The K‐complex number and shape are altered in the course of neurodegeneration but seem to be unaltered in earlier stages of cognitive impairment (Reda et al., ). The other N2‐defining EEG events, sleep spindles, are a product of thalamocortical innervation and mainly occur in the sigma frequency range between 11 and 15 Hz.…”