Dysfonctions cérébrales et changements neuroanatomiques dans l’apnée obstructive du sommeil chez les personnes âgées

L’apnée obstructive du sommeil (AOS) est un trouble du sommeil particulièrement prévalent dans la population âgée, qui peut se présenter par différents niveaux de sévérité. Des études épidémiologiques récentes ont montré une association entre l’AOS et l’incidence de la démence. De plus, l’AOS a été identifiée de façon répétée comme un facteur de risque d’accident vasculaire cérébral. Ces conséquences potentielles de l’AOS sur le cerveau pourraient être dues à l’hypoxémie intermittente et à la fragmentation du sommeil causées par les obstructions respiratoires répétées. Bien que l’AOS soit impliquée dans l’incidence de conséquences graves sur le cerveau, son impact sur la fonction et la structure du cerveau vieillissant reste sous-évalué. Ainsi, l’objectif de cette thèse est d’évaluer l’association entre l’AOS et sa sévérité sur le fonctionnement cérébral et la structure neuroanatomique chez des personnes âgées de plus de 55 ans. Nous avons émis l’hypothèse que les changements cérébraux chez les personnes avec AOS pourraient s’apparenter aux profils observés dans les stades précurseurs de déclin cognitif. Pour ce faire, diverses méthodes de neuroimagerie ont été utilisées pour caractériser l’ensemble du cerveau des personnes avec AOS. Le fonctionnement cérébral au repos éveillé a été évalué par le biais de la tomographie par émission monophotonique en mesurant le flot sanguin cérébral régional. La structure anatomique de la matière grise et de la matière blanche a été évaluée en imagerie par résonance magnétique. La structure de la matière grise a été évaluée grâce à diverses techniques structurelles, mesurant le volume de la matière grise et l’épaisseur corticale. La structure de la matière blanche a été évaluée avec des méthodes d’imagerie de diffusion, mesurant la diffusivité des molécules d’eau dans la matière blanche. Dans notre premier article, nous observons que l’AOS sévère, ainsi que plusieurs marqueurs de sévérité de l’AOS sont associés avec des régions d’hypoperfusion au repos éveillé. Ces réductions régionales de la perfusion cérébrales pourraient être dues à un moins bon fonctionnement des neurones et cellules gliales. Dans notre deuxième article, nous montrons que le profil régional d’hypoperfusion cérébrale diurne diffère selon que l’AOS soit observée en sommeil paradoxal ou en sommeil lent. Chez des personnes avec une sévérité plus faible d’AOS, la présence d’évènements respiratoires en sommeil paradoxal était tout de même associée avec une réduction de la perfusion cérébrale. Ceci suggère que les évènements respiratoires en sommeil paradoxal pourraient être plus dommageables pour le cerveau que ceux en sommeil lent. Nos résultats suggèrent que l’AOS entraine une réduction du fonctionnement cérébral mesuré par une réduction régionale de la perfusion cérébrale. De plus, ces articles suggèrent également que plusieurs facteurs dans l’AOS contribuent différemment aux dysfonctions cérébrales. Dans notre troisième article, nous observons que les perturbations respiratoires, la fragmentation du sommeil, mais surtout l’hypoxémie contribuent à l’hypertrophie de la matière grise. Nous suggérons qu’un processus d’œdème, ou d’autres processus réactifs et aigus pourraient être en cause dans l’augmentation de la taille de la matière grise. Ce processus pourrait également expliquer nos résultats du dernier article. Dans notre quatrième et dernier article, nous montrons que l’AOS est associée avec une réduction de la diffusivité des molécules d’eau dans la matière blanche, surtout dans les cas légers d’AOS. Un processus d’œdème intracellulaire pourrait restreindre la diffusivité des molécules d’eau dans les cellules de la matière blanche. Les résultats de cette thèse clarifient les changements cérébraux observés dans la population vieillissante avec l’AOS. Ainsi, malgré des réductions de la perfusion cérébrale suggérant un dysfonctionnement cérébral, l’AOS est également associée avec des changements de structure de la matière grise et de la matière blanche suggérant des processus réactifs et aigus. Puisque ce profil a été rapporté dans les stades précurseurs de déclin cognitif et de démence, nos résultats soulèvent l’importance d’identifier les facteurs dans l’AOS qui sont associés avec les changements cérébraux, afin d’identifier les individus à risque de conséquences cérébrales négatives. De plus, nos résultats soulèvent également l’importance d’évaluer les effets du traitement de l’AOS pour éviter ou ralentir les conséquences de celle-ci sur la santé cérébrale et cognitive.Obstructive sleep apnea (OSA) is a sleep disorder especially common in the older population, which can present itself at different levels of severity. Recent epidemiological studies showed an association between OSA and incident dementia. In addition, AOS was repeatedly identified as a risk factor for stroke. These potential consequences of OSA on the brain could be caused by intermittent hypoxemia and sleep fragmentation, which is observed following repeated respiratory obstructions. Although OSA has been implicated in the incidence of serious consequences on brain health, its impact on the function and structure of the aging brain remains unclear. Thus, the objective of this thesis was to evaluate the association between OSA as well as its severity with cerebral functioning and structure in adults aged 55 years old and over. We hypothesized that cerebral changes in individuals with OSA would be similar to profiles observed in preclinical stages of cognitive decline. In order to achieve this goal, various neuroimaging methods were used to characterize the brain of individuals with OSA as a whole. Cerebral functioning during wakeful rest was evaluated with single-photon emission computed tomography by measuring regional cerebral blood flow. Grey matter and white matter structure were evaluated with magnetic resonance imaging. Grey matter structure was assessed with structural techniques that measure grey matter volume and cortical thickness. White matter was assessed with diffusion tensor imaging that measures water molecules diffusion. In our first study, we observed that severe OSA as well as many markers associated with OSA severity were correlated with hypoperfused regions during wakeful rest. These regions of reduced cerebral perfusion could present altered neuronal and glial functioning. In our second study, we showed that the daytime regional pattern of cerebral hypoperfusion was different whether apneas and hypopneas were observed during rapid eye movement sleep or non-rapid eye movement sleep. In individuals with a milder OSA severity, respiratory events during rapid eye movement sleep were still associated with regions of hypoperfusion. This suggests that respiratory events during rapid eye movement sleep may be more detrimental to brain health than those in non-rapid eye movement sleep. Overall, these results suggest that OSA leads to an altered cerebral functioning as evidenced by decreased regional cerebral perfusion. In addition, these studies also suggest that many factors contribute differently to cerebral dysfunction in OSA. In our third study, we observed that respiratory disturbances, sleep fragmentation, and mostly hypoxemia all contributed to grey matter hypertrophy. We suggest that oedema or other reactive or acute processes could cause these increased in grey matter structure. These processes may also explain our results observed in our last study. In that fourth study, we showed that OSA is associated with reduced white matter diffusivities, especially in milder OSA cases. An intracellular oedema process may restrict the diffusion of water molecules inside cells. The results of this thesis clarify the cerebral changes observed in the aging population with OSA. Although reduced regional brain perfusion suggests cerebral dysfunctions, OSA was also associated with grey and white matter structural changes that suggest reactive and acute processes. Because this pattern was reported previously in preclinical stages of cognitive decline and dementia, our results highlight the importance of identifying individuals at higher risk of negative outcomes to brain health. In addition, our results also emphasize the importance of understanding the efficiency of treating OSA in order to prevent or slow its impact on cerebral functioning and structure

Waking EEG functional connectivity in middle-aged and older adults with obstructive sleep apnea

Objectives: The present study aimed at investigating changes in waking electroencephalography (EEG), most specifically regarding spectral power and functional connectivity, in middle-aged and older adults with OSA. We also explored whether changes in spectral power or functional connectivity are associated with polysomnographic characteristics and/or neuropsychological performance. Methods: 19 OSA subjects (apnea-hypopnea index ≥ 20, age: 63.6 ± 6.4) and 22 controls (apneahypopnea index ≤ 10, age: 63.6 ± 6.7) underwent a full night of in-laboratory polysomnography followed by a waking EEG and a neuropsychological assessment. Waking EEG spectral power and imaginary coherence were compared between groups for all EEG frequency bands and scalp regions. Correlation analyses were performed between selected waking EEG variables, polysomnographic parameters and neuropsychological performance. Results: No group difference was observed for EEG spectral power for any frequency band. Regarding the imaginary coherence, when compared to controls, OSA subjects showed decreased EEG connectivity between frontal and temporal regions in theta and alpha bands as well as increased connectivity between frontal and parietal regions in delta and beta 1 bands. In the OSA group, these changes in connectivity correlated with lower sleep efficiency, lower total sleep time and higher apnea-hypopnea index. No relationship was found with neuropsychological performance. Conclusions: Contrary to spectral power, imaginary coherence was sensitive enough to detect changes in brain function in middle-aged and older subjects with OSA when compared to controls. Whether these changes in cerebral connectivity predict cognitive decline needs to be investigated longitudinally

Regional cerebral blood flow during wakeful rest in older subjects with mild to severe obstructive sleep apnea

Objectives: To evaluate changes in regional cerebral blood flow (rCBF) during wakeful rest in older subjects with mild to severe obstructive sleep apnea (OSA) and healthy controls, and to identify markers of OSA severity that predict altered rCBF. Design: High-resolution 99mTc-HMPAO SPECT images during wakeful rest. Setting: Research sleep laboratory affiliated with a University hospital. Participants: Fifty untreated OSA patients aged between 55 and 85 years divided into mild, moderate and severe OSA and 20 age-matched healthy controls. Interventions: N/A Measurements: Using statistical parametrical mapping, rCBF was compared between groups and correlated with clinical, respiratory and sleep variables. Results: Whereas no rCBF change was observed in mild and moderate groups, participants with severe OSA had reduced rCBF compared to controls in the left parietal lobules, precentral gyrus, bilateral postcentral gyri, and right precuneus. Reduced rCBF in these regions and in areas of the bilateral frontal and left temporal cortex was associated with more hypopneas, snoring, hypoxemia, and sleepiness. Higher apnea, micro-arousal, and body mass indexes were correlated to increased rCBF in the basal ganglia, insula, and limbic system. Conclusions: While older individuals with severe OSA had hypoperfusions in the sensorimotor and parietal areas, respiratory variables and subjective sleepiness were correlated with extended regions of hypoperfusion in the lateral cortex. Interestingly, OSA severity, sleep fragmentation and obesity correlated with increased perfusion in subcortical and medial cortical regions. Anomalies with such a distribution could result in cognitive deficits and reflect impaired vascular regulation, altered neuronal integrity, and/or undergoing neurodegenerative processes

Gray matter hypertrophy and thickening with obstructive sleep apnea in middle-aged and older adults

Rationale: Obstructive sleep apnea causes intermittent hypoxemia, hemodynamic fluctuations, and sleep fragmentation, all of which could damage cerebral gray matter that can be indirectly assessed with neuroimaging. Objectives: To investigate whether markers of obstructive sleep apnea severity are associated with gray matter changes among middle-aged and older individuals. Methods: Seventy-one subjects (ages: 55 to 76; apnea–hypopnea index: 0.2 to 96.6 events/h) were evaluated with magnetic resonance imaging. Two techniques were used: 1) voxel-based morphometry, which measures gray matter volume and concentration; 2) FreeSurfer automated segmentation, which estimates the volume of predefined cortical/subcortical regions and cortical thickness. Regression analyses were performed between gray matter characteristics and markers of obstructive sleep apnea severity (hypoxemia, respiratory disturbances, sleep fragmentation). Measurements and Main Results: Subjects had few symptoms, i.e. sleepiness, depression, anxiety and cognitive deficits. While no association was found with voxel-based morphometry, FreeSurfer revealed increased gray matter with obstructive sleep apnea. Higher levels of hypoxemia correlated with increased volume and thickness of the left lateral prefrontal cortex as well as increased thickness of the right frontal pole, the right lateral parietal lobules, and the left posterior cingulate cortex. Respiratory disturbances positively correlated with right amygdala volume while more severe sleep fragmentation was associated with increased thickness of the inferior frontal gyrus. Conclusions: Gray matter hypertrophy and thickening were associated with hypoxemia, respiratory disturbances, and sleep fragmentation. These structural changes in a group of middle-aged and older individuals may represent adaptive/reactive brain mechanisms attributed to a presymptomatic stage of obstructive sleep apnea

Brain white matter damage and its association with neuronal synchrony during sleep

The restorative function of sleep partly relies on its ability to deeply synchronize cerebral networks to create large slow oscillations observable with EEG. However, whether a brain can properly synchronize and produce a restorative sleep when it undergoes massive and widespread white matter damage is unknown. Here, we answer this question by testing 23 patients with various levels of white matter damage secondary to moderate to severe traumatic brain injuries (ages 18–56; 17 males, six females, 11–39 months post-injury) and compared them to 27 healthy subjects of similar age and sex. We used MRI and diffusion tensor imaging metrics (e.g. fractional anisotropy as well as mean, axial and radial diffusivities) to characterize voxel-wise white matter damage. We measured the following slow wave characteristics for all slow waves detected in N2 and N3 sleep stages: peak-to-peak amplitude, negative-to-positive slope, negative and positive phase durations, oscillation frequency, and slow wave density. Correlation analyses were performed in traumatic brain injury and control participants separately, with age as a covariate. Contrary to our hypotheses, we found that greater white matter damage mainly over the frontal and temporal brain regions was strongly correlated with a pattern of higher neuronal synchrony characterized by slow waves of larger amplitudes and steeper negative-to-positive slopes during non-rapid eye movement sleep. The same pattern of associations with white matter damage was also observed with markers of high homeostatic sleep pressure. More specifically, higher white matter damage was associated with higher slow-wave activity power, as well as with more severe complaints of cognitive fatigue. These associations between white matter damage and sleep were found only in our traumatic brain injured participants, with no such correlation in controls. Our results suggest that, contrary to previous observations in healthy controls, white matter damage does not prevent the expected high cerebral synchrony during sleep. Moreover, our observations challenge the current line of hypotheses that white matter microstructure deterioration reduces cerebral synchrony during sleep. Our results showed that the relationship between white matter and the brain’s ability to synchronize during sleep is neither linear nor simple

BDNF Val66Met polymorphism interacts with sleep consolidation to predict ability to create new declarative memories

It is hypothesized that a fundamental function of sleep is to restore an individual's day-to-day ability to learn and to constantly adapt to a changing environment through brain plasticity. Brain-derived neurotrophic factor (BDNF) is among the key regulators that shape brain plasticity. However, advancing age and carrying the BDNF Met allele were both identified as factors that potentially reduce BDNF secretion, brain plasticity, and memory. Here, we investigated the moderating role of BDNF polymorphism on sleep and next-morning learning ability in 107 nondemented individuals who were between 55 and 84 years of age. All subjects were tested with 1 night of in-laboratory polysomnography followed by a cognitive evaluation the next morning. We found that in subjects carrying the BDNF Val66Val polymorphism, consolidated sleep was associated with significantly better performance on hippocampus-dependent episodic memory tasks the next morning (β-values from 0.290 to 0.434, p ≤ 0.01). In subjects carrying at least one copy of the BDNF Met allele, a more consolidated sleep was not associated with better memory performance in most memory tests (β-values from -0.309 to -0.392, p values from 0.06 to 0.15). Strikingly, increased sleep consolidation was associated with poorer performance in learning a short story presented verbally in Met allele carriers (β = -0.585, p = 0.005). This study provides new evidence regarding the interacting roles of consolidated sleep and BDNF polymorphism in the ability to learn and stresses the importance of considering BDNF polymorphism when studying how sleep affects cognition

Obstructive sleep apnea during REM sleep and daytime cerebral functioning : a regional cerebral blood flow study using high-resolution SPECT

Obstructive sleep apnea (OSA) predominantly during rapid eye movement (REM) sleep may have impacts on brain health, even in milder OSA cases. Here, we evaluated whether REM sleep OSA is associated with abnormal daytime cerebral functioning using high-resolution single-photon emission computed tomography (SPECT). We tested 96 subjects (25 F, age: 65.2 ± 6.4) with a wide range of OSA severity from no OSA to severe OSA (apnea–hypopnea index: 0–97 events/h). More respiratory events during REM sleep were associated with reduced daytime regional cerebral blood flow (rCBF) in the bilateral ventromedial prefrontal cortex and in the right insula extending to the frontal cortex. More respiratory events during non-REM (NREM) sleep were associated with reduced daytime rCBF in the left sensorimotor and temporal cortex. In subjects with a lower overall OSA severity (apnea–hypopnea index<15), more respiratory events during REM sleep were also associated with reduced daytime rCBF in the insula and extending to the frontal cortex. Respiratory events that characterized OSA during NREM versus REM sleep are associated with distinct patterns of daytime cerebral perfusion. REM sleep OSA could be more detrimental to brain health, as evidenced by reduced daytime rCBF in milder forms of OSA

Visual fixation in the intensive care unit: a strong predictor of post-traumatic amnesia and long-term recovery after moderate-to-severe traumatic brain injury

Objective: We examined whether visual fixation at 24h of intensive care unit (ICU) admission is superior to the initial Glasgow Coma Scale (GCS) score to predict PTA duration and long-term TBI recovery. Design: Two-phase cohort study. Setting: Level I trauma ICU. Patients: Moderate-to-severe TBI discharged alive between 2010-2013. Interventions: None. Measurements and Main Results: Presence/absence of visual fixation at 24h of ICU-admission was determined through standard behavioral assessments in 181 TBI patients and compared to the GCS score to predict PTA duration during hospitalization (Phase 1) and performance on the Glasgow Outcome Scale-Extended (GOS-E) 10-40 months after (n=144; Phase 2a). A subgroup also completed a visual attention task (n=35; Phase 2b) and brain magnetic resonance imaging post-TBI (n=23; Phase 2c). Presence/absence of visual fixation at 24h of ICU-admission showed a sensitivity of 84%, a specificity of 82% and an AUC of 0.87 for the prediction of PTA duration. Visual fixation (AUC=0.85) was also found as performant as PTA (AUC=0.81; difference-between-AUC=0.04; 95%CI:-0.03-0.116; p=0.28) for the prediction of GOS-E scores. Conversely, the GCS score was a poor predictor of both PTA and GOS-E. Even when controlling for age/medication/CT scan findings, fixation remained a significant predictor of GOS-E scores (=-0.29, p<0.05). Poorer attention performance and increased regional brain volume deficits were also observed in participants who could not fixate 24h following ICU-admission versus those who could. Conclusions: Visual fixation within 24h of ICU-admission could be as performant as PTA for predicting TBI recovery, introducing a new variable of interest in TBI outcome research

Sleep spindles are resilient to extensive white matter deterioration

Sleep spindles are an essential part of non-rapid eye movement sleep, notably involved in sleep consolidation, cognition, learning and memory. These oscillatory waves depend on an interaction loop between the thalamus and the cortex, which relies on a structural backbone of thalamo-cortical white matter tracts. It is still largely unknown if the brain can properly produce sleep spindles when it underwent extensive white matter deterioration in these tracts, and we hypothesized that it would affect sleep spindle generation and morphology. We tested this hypothesis with chronic moderate to severe traumatic brain injury (n ¼ 23; 30.5 6 11.1 years old; 17 m/6f), a unique human model of extensive white matter deterioration, and a healthy control group (n ¼ 27; 30.3 6 13.4 years old; 21m/6f). Sleep spindles were analysed on a full night of polysomnography over the frontal, central and parietal brain regions, and we measured their density, morphology and sigma-band power. White matter deterioration was quantified using diffusion-weighted MRI, with which we performed both whole-brain voxel-wise analysis (Tract-Based Spatial Statistics) and probabilistic tractography (with High Angular Resolution Diffusion Imaging) to target the thalamo-cortical tracts. Group differences were assessed for all variables and correlations were performed separately in each group, corrected for age and multiple comparisons. Surprisingly, although extensive white matter damage across the brain including all thalamo-cortical tracts was evident in the brain-injured group, sleep spindles remained completely undisrupted when compared to a healthy control group. In addition, almost all sleep spindle characteristics were not associated with the degree of white matter deterioration in the braininjured group, except that more white matter deterioration correlated with lower spindle frequency over the frontal regions. This study highlights the resilience of sleep spindles to the deterioration of all white matter tracts critical to their existence, as they conserve normal density during non-rapid eye movement sleep with mostly unaltered morphology. We show that even with such a severe traumatic event, the brain has the ability to adapt or to withstand alterations in order to conserve normal sleep spindles