Spontaneous K-complexes in chronic psychophysiological insomnia

Insomnia nosology: a systematic review and critical appraisal of historical diagnostic categories and current phenotypes

Insomnia nosology has significantly evolved since the Diagnostic and Statistical Manual (DSM)-III-R first distinguished between 'primary' and 'secondary' insomnia. Prior International Classification of Sleep Disorders (ICSD) nosology 'split' diagnostic phenotypes to address insomnia's heterogeneity and the DSM nosology 'lumped' them into primary insomnia, while both systems assumed causality for insomnia secondary to health conditions. In this systematic review, we discuss the historical phenotypes in prior insomnia nosology, present findings for currently proposed insomnia phenotypes based on more robust approaches, and critically appraise the most relevant ones. Electronic databases PsychINFO, PubMED, Web of Science, and references of eligible articles, were accessed to find diagnostic manuals, literature on insomnia phenotypes, including systematic reviews or meta-analysis, and assessments of the reliability or validity of insomnia diagnoses, identifying 184 articles. The data show that previous insomnia diagnoses lacked reliability and validity, leading current DSM-5-TR and ICSD-3 nosology to 'lump' phenotypes into a single diagnosis comorbid with health conditions. However, at least two new, robust insomnia phenotyping approaches were identified. One approach is multidimensional-multimethod and provides evidence for self-reported insomnia with objective short versus normal sleep duration linked to clinically relevant outcomes, while the other is multidimensional and provides evidence for two to five clusters (phenotypes) based on self-reported trait, state, and/or life-history data. Some approaches still need replication to better support whether their findings identify true phenotypes or simply different patterns of symptomatology. Regardless, these phenotyping efforts aim at improving insomnia nosology both as a classification system and as a mechanism to guide treatment.

Exploring the functional role and neural correlates of K-complexes in isolated rapid eye movement sleep behavior disorder

Underlying neural mechanisms and cognitive implications of non-Rapid Eye Movement (NREM) sleep in isolated Rapid Eye Movement (REM) sleep behavior disorder (iRBD) are not yet fully elucidated. This study aims to evaluate brain metabolic connectivity of the anterior default mode network (ADMN) underlying a waveform that is an hallmark of NREM sleep, namely K-complex (KC) and their implication for neuropsychological functioning in iRBD patients. Combining polysomnographic and multivariate molecular imaging (FDG-PET) approaches may provide crucial insights regarding KCs role in the prodromal stages of synucleinopathies. We applied a seed-based interregional correlation analysis on FDG-PET data. iRBD patients with cognitive decline displayed a reduced KC density (KCd) in comparison to patients without cognitive impairments. KCd showed a significant positive correlation with global cognitive functioning, specifically with visuo-spatial and executive performances, two cognitive domains known to be relevant in predicting conversion into neurodegenerative disorders. Increased KCd was associated with a more preserved ADMN connectivity. Our study underlines the importance of NREM sleep in prodromal stages of synucleinopathies, and future investigations might clarify its role in iRBD.

Objective REM sleep characteristics of recurrent isolated sleep paralysis: a case-control study

STUDY OBJECTIVES

Recurrent isolated sleep paralysis (RISP) is a rapid eye movement (REM) parasomnia characterized by a dissociative state with characteristics of REM sleep and wakefulness. Pathophysiology has not yet been clarified and very little research has been performed using objective polysomnographic measures with inconsistent results. The main aim of our study was to find whether higher REM sleep fragmentation is consistent with the theory of state dissociation or whether signs of dissociation can be detected by spectral analysis.

METHODS

Nineteen participants in the RISP group and 19 age- and gender-matched participants in the control group underwent two consecutive full-night video-polysomnography recordings with 19-channel electroencephalography. Apart from sleep macrostructure, other REM sleep characteristics such as REM arousal index, percentage of wakefulness and stage shifts within REM sleep period were analyzed, as well as power spectral analysis during REM sleep.

RESULTS

No difference was found in the macrostructural parameters of REM sleep (percentage of REM sleep and REM latency). Similarly, no significant difference was detected in REM sleep fragmentation (assessed by REM sleep arousal index, percentage of wakefulness and stage shifts within REM sleep). Power spectral analysis showed higher bifrontal beta activity in the RISP group during REM sleep.

CONCLUSIONS

The results showed an underlying persistent trait of higher cortical activity that may predispose patients with SP to be more likely to experience recurrent episodes, without any apparent macrostructural features including higher REM sleep fragmentation.

Enhanced Vigilance Stability during Daytime in Insomnia Disorder

Central nervous hyperarousal is as a key component of current pathophysiological concepts of chronic insomnia disorder. However, there are still open questions regarding its exact nature and the mechanisms linking hyperarousal to sleep disturbance. Here, we aimed at studying waking state hyperarousal in insomnia by the perspective of resting-state vigilance dynamics. The VIGALL (Vigilance Algorithm Leipzig) algorithm has been developed to investigate resting-state vigilance dynamics, and it revealed, for example, enhanced vigilance stability in depressive patients. We hypothesized that patients with insomnia also show a more stable vigilance regulation. Thirty-four unmedicated patients with chronic insomnia and 25 healthy controls participated in a twenty-minute resting-state electroencephalography (EEG) measurement following a night of polysomnography. Insomnia patients showed enhanced EEG vigilance stability as compared to controls. The pattern of vigilance hyperstability differed from that reported previously in depressive patients. Vigilance hyperstability was also present in insomnia patients showing only mildly reduced sleep efficiency. In this subgroup, vigilance hyperstability correlated with measures of disturbed sleep continuity and arousal. Our data indicate that insomnia disorder is characterized by hyperarousal at night as well as during daytime.

The heritability of the human K-complex: a twin study

Sleep electroencephalogram (EEG) has a trait-like nature. Several findings highlighted the heritability of spectral power in specific frequency ranges and sleep spindles during nonrapid eye movement (NREM) sleep. However, a genetic influence on the K-complex (KC), one of the electrophysiological hallmarks of NREM sleep, has never been assessed. Here, we investigated the heritability of the KC detected during NREM stage 2 comparing 10 monozygotic (MZ) and 10 dizygotic (DZ) twin pairs. Genetic variance analysis (GVA) and intraclass correlation coefficients (ICCs) were performed to assess the genetic effect and within-pair similarity for KC density, amplitude, and for the area under the curve (AUC) of the KC average waveform at Fz, Cz, and Pz scalp locations. Moreover, cluster analysis was performed on the KC average waveform profile. We observed a significant genetic effect on KC AUC at Cz and Pz, and on amplitude at Pz. Within-pair similarity (ICCs) was always significant for MZ twins except for KC density at Fz, whereas DZ twins always exhibited ICCs below the significance threshold, with the exception of density at Pz. The largest differences in within-pair similarity between MZ and DZ groups were observed again for AUC at Cz and Pz. MZ pairs accurately clustered for the KC average waveform with a higher frequency (successful clustering rate for MZ pairs: Fz = 60%; Cz = 80%; Pz = 90%) compared with DZ pairs (successful clustering rate for DZ pairs: Fz = 10%; Cz = 10%; Pz = none). Our results suggest the existence of a genetic influence on the human KC, particularly related to its morphology and maximally observable in central and parietal locations.

Sleep spindles, K-complexes, limb movements and sleep stage proportions may be biomarkers for amnestic mild cognitive impairment and Alzheimer's disease

PURPOSE

Sleep disturbances are common in Alzheimer's disease (AD) and mild cognitive impairment (MCI) patients. Non-rapid eye movement stage 3 (N3), rapid eye movement stage (REM), spindle density, and K-complex (KC) density are decreased in MCI and AD patients. Periodic limb movements in sleep (PLMS) are increased in other neurodegenerative diseases. We aimed to distinguish amnestic mild cognitive impairment (aMCI) patients from the overall population of MCI patients by comparing the N3 and REM proportions, the morphological characteristics of spindles and KCs and the periodic limb movement index (PLMI) among control, aMCI and AD subjects.

METHODS

In 92 subjects (30 controls, 32 aMCI and 30 AD), sleep stages, spindles, KCs and PLMS were recorded during the second of two nights of polysomnography (PSG). We compared the above parameters among the three groups.

RESULTS

AD and aMCI subjects had lower proportions of N3 and REM, poorer spindle and KC activities and more frequent PLMS than controls. These alterations were associated with decreased Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA) scores. We determined cut-off values for distinguishing aMCI and AD using logistic regression and receiver operating characteristic (ROC) analyses.

CONCLUSIONS

AD and aMCI patients have abnormal sleep stage proportions, spindles, KCs and PLMS. The combination of the above alterations may distinguish aMCI and AD patients from controls with high specificity and sensitivity.

Spontaneous K-Complexes may be biomarkers of the progression of amnestic mild cognitive impairment

OBJECTIVE

Spontaneous K-complexes (SKCs), a hallmark of stage 2 sleep, have been reported to decrease in density in Alzheimer's disease (AD) patients. However, few former studies have explored the alterations in SKC characteristics in the pre-clinical phase of AD-amnestic mild cognitive impairment (aMCI). The aim of our prospective cohort study was to investigate the changing trend in SKC characteristics during the progression of aMCI.

METHODS

SKC density, amplitude and duration were measured in aMCI subjects and normal controls (NC) at two-year follow-up assessments by polysomnography (PSG). In sum, 22 NCs, 25 stable aMCI (sMCI) subjects and 20 progressive aMCI (pMCI) subjects finished the four follow-up PSG assessments, and their data were used for analysis.

RESULTS

SKC density and amplitude, but not duration, decreased during the follow-up assessments in both NCs and aMCI subjects, but the rate of decrease of these parameters was greater in aMCI subjects. With the progression of aMCI, significant differences in SKC density and amplitude among the three groups were observed, whereas SKC density showed no difference at the early stage of aMCI. The receiver operating characteristic (ROC) curve results demonstrated that SKC density and amplitude could distinguish aMCI subjects from NCs with high specificity and sensitivity.

CONCLUSION

Our results suggest that SKCs decrease with ageing and the progression of aMCI, and SKC characteristics may be potential biomarkers for diagnosing aMCI.

Data-Driven Analysis of EEG Reveals Concomitant Superficial Sleep During Deep Sleep in Insomnia Disorder

Study Objectives: The subjective suffering of people with Insomnia Disorder (ID) is insufficiently accounted for by traditional sleep classification, which presumes a strict sequential occurrence of global brain states. Recent studies challenged this presumption by showing concurrent sleep- and wake-type neuronal activity. We hypothesized enhanced co-occurrence of diverging EEG vigilance signatures during sleep in ID.

Methods: Electroencephalography (EEG) in 55 cases with ID and 64 controls without sleep complaints was subjected to a Latent Dirichlet Allocation topic model describing each 30 s epoch as a mixture of six vigilance states called Topics (T), ranked from N3-related T1 and T2 to wakefulness-related T6. For each stable epoch we determined topic dominance (the probability of the most likely topic), topic co-occurrence (the probability of the remaining topics), and epoch-to-epoch transition probabilities.

Results: In stable epochs where the N1-related T4 was dominant, T4 was more dominant in ID than in controls, and patients showed an almost doubled co-occurrence of T4 during epochs where the N3-related T1 was dominant. Furthermore, patients had a higher probability of switching from T1- to T4-dominated epochs, at the cost of switching to N3-related T2-dominated epochs, and a higher probability of switching from N2-related T3- to wakefulness-related T6-dominated epochs.

Conclusion: Even during their deepest sleep, the EEG of people with ID express more N1-related vigilance signatures than good sleepers do. People with ID are moreover more likely to switch from deep to light sleep and from N2 sleep to wakefulness. The findings suggest that hyperarousal never rests in ID.

Sleep Stage Transition Dynamics Reveal Specific Stage 2 Vulnerability in Insomnia

Study Objectives

Objective sleep impairments in insomnia disorder (ID) are insufficiently understood. The present study evaluated whether whole-night sleep stage dynamics derived from polysomnography (PSG) differ between people with ID and matched controls and whether sleep stage dynamic features discriminate them better than conventional sleep parameters.

Methods

Eighty-eight participants aged 21-70 years, including 46 with ID and 42 age- and sex-matched controls without sleep complaints, were recruited through www.sleepregistry.nl and completed two nights of laboratory PSG. Data of 100 people with ID and 100 age- and sex-matched controls from a previously reported study were used to validate the generalizability of findings. The second night was used to obtain, in addition to conventional sleep parameters, probabilities of transitions between stages and bout duration distributions of each stage. Group differences were evaluated with nonparametric tests.

Results

People with ID showed higher empirical probabilities to transition from stage N2 to the lighter sleep stage N1 or wakefulness and a faster decaying stage N2 bout survival function. The increased transition probability from stage N2 to stage N1 discriminated people with ID better than any of their deviations in conventional sleep parameters, including less total sleep time, less sleep efficiency, more stage N1, and more wake after sleep onset. Moreover, adding this transition probability significantly improved the discriminating power of a multiple logistic regression model based on conventional sleep parameters.

Conclusions

Quantification of sleep stage dynamics revealed a particular vulnerability of stage N2 in insomnia. The feature characterizes insomnia better than-and independently of-any conventional sleep parameter.

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

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.

The Fall of Sleep K-Complex in Alzheimer Disease

Although a slowing of electroencephalographic (EEG) activity during wakefulness and -to some extent- sleep of Alzheimer disease (AD) patients (i.e., increased slow-frequency activity) was documented, recent findings in healthy elderly show a decreased 0.6-1 Hz slow wave activity (SWA) during NREM, which was associated to β-amyloid deposition and impaired hippocampal memory consolidation. We hypothesize that the apparent contradiction may be explained by the partial overlap between 0.6-1 Hz EEG activity and K-Complex (KC). According to this view, we studied both frontal KCs and SWA in 20 AD patients and 20 healthy age-matched controls (HC) during nightly sleep, under the hypothesis that KCs better discriminate patients from healthy elderly than ≤1 Hz SWA. A drastic decrease of KC density during stage 2 NREM was found in AD compared to HC. Patients show more than 40% reduction of the KC density, allowing a correct classification of 80%. On the other hand, ≤1 Hz SWA of AD patients is slightly (not significantly) higher in most cortical areas compared to HC. Although no significant changes of ≤1 Hz SWA are detectable over frontal areas in AD, KC density decreases over the same location, and its decrease is related to the cognitive decline.

Insomnia is Associated with Cortical Hyperarousal as Early as Adolescence

STUDY OBJECTIVES

To examine whether insomnia is associated with spectral electroencephalographic (EEG) dynamics in the beta (15-35Hz) range during sleep in an adolescent general population sample.

METHODS

A case-control sample of 44 adolescents from the Penn State Child Cohort underwent a 9-h polysomnography, clinical history and physical examination. We examined low-beta (15-25 Hz) and high-beta (25-35 Hz) relative power at central EEG derivations during sleep onset latency (SOL), sleep onset (SO), non-rapid eye movement (NREM) sleep, and wake after sleep onset (WASO).

RESULTS

Compared to controls (n = 21), individuals with insomnia (n = 23) showed increased SOL and WASO and decreased sleep duration and efficiency, while no differences in sleep architecture were found. Insomniacs showed increased low-beta and high-beta relative power during SOL, SO, and NREM sleep as compared to controls. High-beta relative power was greater during all sleep and wake states in insomniacs with short sleep duration as compared to individuals with insomnia with normal sleep duration.

CONCLUSIONS

Adolescent insomnia is associated with increased beta EEG power during sleep, which suggests that cortical hyperarousal is present in individuals with insomnia as early as adolescence. Interestingly, cortical hyperarousal is greatest in individuals with insomnia with short sleep duration and may explain the sleep complaints of those with normal sleep duration. Disturbed cortical networks may be a shared mechanism putting individuals with insomnia at risk of psychiatric disorders.

Enhanced frontoparietal synchronized activation during the wake-sleep transition in patients with primary insomnia

INTRODUCTION

Cognitive and brain hyperactivation have been associated with trouble falling asleep and sleep misperception in patients with primary insomnia (PI). Activation and synchronization/temporal coupling in frontal and frontoparietal regions involved in executive control and endogenous attention might be implicated in these symptoms.

METHODS

Standard polysomnography (PSG) and electroencephalogram (EEG) were recorded in 10 unmedicated young patients (age 19-34 yr) with PI with no other sleep/medical condition, and in 10 matched control subjects. Absolute power, temporal coupling, and topographic source distribution (variable resolution electromagnetic tomography or VARETA) were obtained for all time spent in waking, Stage 1 and Stage 2 of the wake-sleep transition period (WSTP), and the first 3 consecutive min of N3. Subjective sleep quality and continuity were evaluated.

RESULTS

In comparison with control subjects, patients with PI exhibited significantly higher frontal beta power and current density, and beta and gamma frontoparietal temporal coupling during waking and Stage 1.

CONCLUSION

These findings suggest that frontal deactivation and disengagement of brain regions involved in executive control, attention, and self-awareness are impaired in patients with PI. The persistence of this activated and coherent network during the wake-sleep transition period (WSTP) may contribute to a better understanding of underlying mechanisms involved in difficulty in falling asleep, in sleep misperception, and in the lighter, poorer, and nonrefreshing sleep experienced by some patients with PI.

The role of the spontaneous and evoked k-complex in good-sleeper controls and in individuals with insomnia

STUDY OBJECTIVES

Spontaneous and evoked K-complexes are believed to reflect a mechanism of sleep protection, yet their functional role remains a subject of debate. Studying the K-complex in primary insomnia, for which sleep protection appears to be of central importance, may provide further insight on its role. The purpose of the present study was to determine the role of spontaneous and evoked K-complexes in good sleepers and to investigate its role in primary insomnia.

PARTICIPANTS

Twelve individuals with primary insomnia (mean age = 45.4 years) and 12 good-sleeper controls (mean age = 44.3 years).

METHODS AND PROCEDURE

Participants underwent 4 consecutive nights of polysomnographic recordings. Spontaneous K-complexes (SKC) were analyzed during continuous stage 2 sleep on the third night, whereas evoked K-complexes (EKC) were elicited by auditory stimuli on the fourth night. Relative spectral power was measured on 1-second electroencephalographic segments prior to and following SKC, EKC, and trials in which auditory stimuli did not elicit a K-complex (EKC-). K-complex amplitudes were also measured.

RESULTS

Measures of relative spectral power showed significant changes following EKC (increase in activity in the delta frequency band and decrease in activity in the theta, sigma, beta, and gamma frequency bands) and SKC (increase in activity in delta frequency band and decrease in activity in theta, alpha, sigma, and beta frequency bands) for good sleepers, whereas there was no such marked change in electroencephalographic activity surrounding EKC-. A similar pattern of electroencephalographic activity was also observed for the insomnia group on trials of each different K-complex. Auditory stimuli elicited the same proportion of EKC for both groups, whereas SKC density was higher for the insomnia group (1.52 SKC/min) than for good sleepers (1.06 SKC/minute)

CONCLUSION

These results suggest that the SKC and the EKC both promote deeper sleep, further supporting a sleep-protection role.