Cardiorespiratory Fitness Attenuates the Deleterious Effects of Sleep Apnea on Cerebral Structure and Perfusion in the Wisconsin Sleep Cohort Study

BACKGROUND

Emerging evidence suggests that age-related changes in cerebral health may be sensitive to vascular risk modifiers, such as physical activity and sleep.

OBJECTIVE

We examine whether cardiorespiratory fitness modifies the association of obstructive sleep apnea (OSA) severity with MRI-assessed measures of cerebral structure and perfusion.

METHODS

Using data from a cross-sectional sample of participants (n = 129, 51% female, age range 49.6-85.3 years) in the Wisconsin Sleep Cohort study, we estimated linear models of MRI-assessed total and regional gray matter (GM) and white matter (WM) volumes, WM hyperintensity (WMH:ICV ratio), total lesion volume, and arterial spin labeling (ASL) cerebral blood flow (CBF), using an estimated measure of cardiorespiratory fitness (CRF) and OSA severity as predictors. Participants' sleep was assessed using overnight in-laboratory polysomnography, and OSA severity was measured using the apnea-hypopnea index (AHI), or the mean number of recorded apnea and hypopnea events per hour of sleep. The mean±SD time difference between PSG data collection and MRI data collection was 1.7±1.5 years (range: [0, 4.9 years]).

RESULTS

OSA severity was associated with reduced total GM volume (β=-0.064; SE = 0.023; p = 0.007), greater total WM lesion volume (interaction p = 0.023), and greater WMHs (interaction p = 0.017) in less-fit subjects. Perfusion models revealed significant differences in the association of AHI and regional CBF between fitness groups (interaction ps < 0.05).

CONCLUSION

This work provides new evidence for the protective role of cardiorespiratory fitness against the deleterious effects of OSA on brain aging in late-middle age to older adults.

Age-related differences in white matter microstructure measured by advanced diffusion MRI in healthy older adults at risk for Alzheimer's disease

Neurite orientation dispersion and density imaging (NODDI) is an advanced diffusion imaging technique, which can detect more distinct microstructural features compared to conventional Diffusion Tensor Imaging (DTI). NODDI allows the signal to be divided into multiple water compartments and derive measures for orientation dispersion index (ODI), neurite density index (NDI) and volume fraction of isotropic diffusion compartment (FISO). This study aimed to investigate which diffusion metric-fractional anisotropy (FA), mean diffusivity (MD), NDI, ODI, or FISO-is most influenced by aging and reflects cognitive function in a population of healthy older adults at risk for Alzheimer's disease (AD). Age was significantly associated with all but one diffusion parameters and regions of interest. NDI and MD in the cingulate region adjacent to the cingulate cortex showed a significant association with a composite measure of Executive Function and was proven to partially mediate the relationship between aging and Executive Function decline. These results suggest that both DTI and NODDI parameters are sensitive to age-related differences in white matter regions vulnerable to aging, particularly among older adults at risk for AD.

Impact of sex and APOE ε4 on age-related cerebral perfusion trajectories in cognitively asymptomatic middle-aged and older adults: A longitudinal study

Cerebral hypoperfusion is thought to contribute to cognitive decline in Alzheimer's disease, but the natural trajectory of cerebral perfusion in cognitively healthy adults has not been well-studied. This longitudinal study is consisted of 950 participants (40-89 years), who were cognitively unimpaired at their first visit. We investigated the age-related changes in cerebral perfusion, and their associations with APOE-genotype, biological sex, and cardiometabolic measurements. During the follow-up period (range 0.13-8.24 years), increasing age was significantly associated with decreasing cerebral perfusion, in total gray-matter (β=-1.43), hippocampus (-1.25), superior frontal gyrus (-1.70), middle frontal gyrus (-1.99), posterior cingulate (-2.46), and precuneus (-2.14), with all P-values < 0.01. Compared with male-ɛ4 carriers, female-ɛ4 carriers showed a faster decline in global and regional cerebral perfusion with increasing age, whereas the age-related decline in cerebral perfusion was similar between male- and female-ɛ4 non-carriers. Worse cardiometabolic profile (i.e., increased blood pressure, body mass index, total cholesterol, and blood glucose) was associated with lower cerebral perfusion at all the visits. When time-varying cardiometabolic measurements were adjusted in the model, the synergistic effect of sex and APOE-ɛ4 on age-related cerebral perfusion-trajectories became largely attenuated. Our findings demonstrate that APOE-genotype and sex interactively impact cerebral perfusion-trajectories in mid- to late-life. This effect may be partially explained by cardiometabolic alterations.

244 Physical fitness attenuates the deleterious association of sleep apnea with gray matter volume in the Wisconsin Sleep Cohort Study

Introduction

Recent evidence has illustrated that gray matter (GM) atrophy, a diagnostic hallmark of Alzheimer’s disease (AD), may be influenced by psychosocial risk modifiers such as physical exercise and sleep. Cardiorespiratory fitness, a measure of oxygen delivery and utilization during exercise, is positively associated with both sleep quality and gray matter volume in brain areas associated with age-related cognitive decline, such as the hippocampus. In contrast, sleep apnea has been linked to global and regional gray matter atrophy, which is thought to be driven in-part by the incomplete modulation of cardiovascular and respiratory control during sleep. This study examines whether cardiorespiratory fitness modifies the deleterious relationship between sleep apnea and GM volume in a sample of non-demented older participants from the Wisconsin Sleep Cohort (WSC).

Methods

Using data from a subset of WSC participants (n=129, 51% female, mean [range] age at baseline=68 [49-85] years), cardiorespiratory fitness was estimated using a Non-Exercise CardioRespiratory Fitness Measure (NECRFM; based on age, sex, BMI, self-reported physical activity, and resting heart rate). Sleep apnea severity was measured by overnight polysomnography and characterized by the base 10 logarithm of the apnea-hypopnea index, log10(AHI+1). We assembled cross-sectional linear models of MRI-measured total GM volume using NECRFM and log10(AHI+1) as predictors while controlling for age, sex, BMI, education, and hypertension. Regional volumetric changes in the hippocampus and amygdala were assessed using analogous linear models, adjusting both outcome volumetrics for total intracranial volume.

Results

While the interaction between fitness and apnea severity was not significant (p=0.50), results stratified at the median NECRFM illustrated that among the less fit individuals, higher log10(AHI+1) was associated with a significant reduction in total GM volume (B(SE)=-0.06 (0.02); p=0.007); this relationship was not significant among those who were more fit (B(SE)=-0.03 (0.02); p=0.11). There were no significant effects in the hippocampus or amygdala.

Conclusion

These results indicate that cardiovascular fitness may attenuate the effect of severe sleep apnea on GM volume in older adults, supporting the protective role of cardiovascular fitness in aging brain health.

Support (if any)

This work was supported by United States National Institutes of Health grants R01AG058680, R01HL62252, 1R01AG036838, 1UL1RR025011, and R01AG062167.

243 Adverse relationships between sleep apnea, white matter integrity and perfusion is attenuated in more physically fit older adults

Introduction

White matter (WM) integrity declines with age and is sensitive to vascular risk modifiers, such as exercise. Sleep apnea is believed to contribute to cerebral white matter change via intermittent hypoxia-induced alterations to cerebral blood flow resulting in cerebrovascular shearing. Existing literature highlights the relationships between poor sleep and numerous adverse health outcomes, including risk for cardiovascular disease and dementia. Conversely, exercise seems to have a positive effect on both brain structure and function. Here we examine a potential interaction between sleep apnea severity and cardiorespiratory fitness (characterized by a gender-specific Non-Exercise CardioRespiratory Fitness Measure, NECRFM) as predictors of MRI measures of brain WM integrity and cerebral blood flow (CBF) in a sample of non-demented older adult participants of the Wisconsin Sleep Cohort (WSC).

Methods

Cross-sectional linear models using data from a subset of 124 WSC participants (50% female; mean age[range]=67.6 [49.6, 85.3]) examined the relationship between sleep apnea severity (apnea-hypopnea index, AHI) and cardiorespiratory fitness in predicting MRI-assessed total WM and lesion volumes, WM hyperintensities (WMHs; a marker of small vessel disease in the brain) and Arterial Spin Labeling (ASL) CBF, controlling for age, sex, BMI, education, and hypertension.

Results

Greater sleep apnea severity was associated more strongly with both total lesion and WMH loads in less fit compared to more fit persons (p’s&lt;0.05) in the absence of significant differences in total WM volume. Regional perfusion revealed higher CBF in the angular gyrus, middle frontal cortex, and superior frontal gyrus and lower CBF in the anterior cingulate gyrus and the hippocampus of more fit compared to less fit persons (p’s&lt; 0.05), in the absence of differences in mean global perfusion.

Conclusion

Overall, our results suggest that better general fitness may attenuate negative brain health outcomes related to poor sleep. We highlight an important relationship between brain health and modifiable behavioral factors, namely sleep and fitness, that have the potential to help maintain or improve brain integrity with age.

Support (if any)

This work was supported by United States National Institutes of Health grants R01AG062167, R01AG058680, R01HL62252, 1R01AG036838 and 1UL1RR025011.

539 Interactions Between Cardiorespiratory Fitness and Sleep Apnea in Predicting Risk of Alzheimer’s Disease

Introduction

Several studies suggest a link between obstructive sleep apnea (OSA) and Alzheimer’s disease (AD). Additionally, frequent exercise is associated with more favorable AD biomarker profiles, and emerging evidence suggests greater cardiorespiratory fitness levels may be associated with lower risk of cognitive decline. We investigated whether cardiorespiratory fitness modifies the association of OSA and risk of AD.

Methods

A subset of the Wisconsin Sleep Cohort study participants with study visits starting in 2000 (n=1182, 46% female, mean [range] age at baseline=57 [37–82] years) completed multiple [range, 1–5] in-laboratory protocols that included overnight polysomnography, anthropometric measurements, and questionnaires. Additionally, the National Death Index was searched to determine cause of death among decedents. Cox proportional hazards models estimated relative hazards of AD (self-reported physician diagnosis or indication of AD on the death certificate) associated with the joint effects and the interaction of OSA – characterized by the base 10 logarithm of the apnea-hypopnea index (log10(AHI+1)) – and cardiorespiratory fitness (an index based on age, sex, BMI, self-reported physical activity, and resting heart rate). Additionally, the sample was stratified by fitness level at the 3rd quartile (&gt;75th percentile compared to &lt;75th percentile) and the hazard ratio for log10(AHI+1) was estimated for the lower and higher fitness-level groups. Results were adjusted for age, sex, BMI, and education.

Results

There were 10 incident cases of AD. The mean [range] fitness level was 7.1 [0–12.3]. 28% of the sample had moderate OSA (AHI 5–15); and 26% had severe OSA (AHI&gt;15). Higher log10(AHI+1) was associated with greater hazards (p=0.03) of AD and there was a significant interaction between log10(AHI) and fitness (p=0.04), such that at greater fitness levels, the effect of log10(AHI) on AD was mitigated. In stratified analysis, among the less fit, the hazard ratio for an increment of 1 in log10(AHI) was 12.8 (95% CI, 1.1–153.8, p=0.04); among those who were more fit, the hazard ratio was not significant.

Conclusion

More severe OSA is associated with higher risk of AD, and this risk is greater among those with lower levels of cardiorespiratory fitness.

Support (if any)

This work was supported by US NIH grants R01AG058680, R01AG062167, R01HL62252, 1R01AG036838 and 1UL1RR025011.

Cardiorespiratory Fitness Associates with Cerebral Vessel Pulsatility in a Cohort Enriched with Risk for Alzheimer's Disease

BACKGROUND

There is increasing evidence that vascular disease risk factors contribute to evolution of the dementia syndrome of Alzheimer's disease (AD). One important measure of cerebrovascular health is pulsatility index (PI) which is thought to represent distal vascular resistance, and has previously been reported to be elevated in AD clinical syndrome. Physical inactivity has emerged as an independent risk factor for cardiovascular disease.

OBJECTIVE

This study aims to examine the relationship between a measure of habitual physical activity, cardiorespiratory fitness (CRF), and PI in the large cerebral vessels.

METHODS

Ninety-two cognitively-healthy adults (age = 65.34±5.95, 72% female) enrolled in the Wisconsin Registry for Alzheimer's Prevention participated in this study. Participants underwent 4D flow brain MRI to measure PI in the internal carotid artery (ICA), basilar artery, middle cerebral artery (MCA), and superior sagittal sinus. Participants also completed a self-report physical activity questionnaire. CRF was calculated using a previously-validated equation that incorporates sex, age, body-mass index, resting heart rate, and self-reported physical activity. A series of linear regression models adjusted for age, sex, APOE4 status, and 10-year atherosclerotic cardiovascular disease risk were used to analyze the relationship between CRF and PI.

RESULTS

Inverse associations were found between CRF and mean PI in the inferior ICA (p = .001), superior ICA (p = .035), and basilar artery (p = .040). No other cerebral vessels revealed significant associations between CRF and PI (p≥.228).

CONCLUSIONS

Higher CRF was associated with lower PI in several large cerebral vessels. Since increased pulsatility has been associated with poor brain health and reported in persons with AD, this suggests that aerobic fitness might provide protection against cerebrovascular changes related to the progression of AD clinical syndrome.

Evolution of Magnetic Resonance Imaging as Predictors and Correlates of Functional Outcome after Spinal Cord Contusion Injury in the Rat

Clinical methods for determining the severity of traumatic spinal cord injury (SCI) and long-term functional outcome in the acute setting are limited in their prognostic accuracy because of the heterogeneity of injury and dynamic injury progression. The aim of this study was to evaluate the time course and sensitivity of advanced magnetic resonance imaging (MRI) methods to neurological function after SCI in a rat contusion model. Rats received a graded contusion injury at T10 using a weight-drop apparatus. MRI consisted of morphological measures from T₂-weighted imaging, quantitative T₂ imaging, and diffusion-weighted imaging (DWI) at 1, 30, and 90 days post-injury (dpi). The derived metrics were compared with neurological function assessed using weekly Basso, Beattie, and Bresnahan (BBB) locomotor scoring and return of reflexive micturition function. At the acute time point (1 dpi), diffusion metrics sensitive to axonal injury at the injury epicenter had the strongest correlation with time-matched BBB scores and best predicted 90-dpi BBB scores. At 30 dpi, axonal water fraction derived from DWI and T₂ values were both correlated with time-matched locomotor scores. At the chronic time point (90 dpi), cross-sectional area was most closely correlated to BBB. Overall, the results demonstrate differential sensitivity of MRI metrics at different time points after injury, but the metrics follow the expected pathology of acute axonal injury followed by continued degeneration and finally a terminal level of atrophy. Specificity of DWI in the acute setting may make it impactful as a prognostic tool while T₂ imaging provided the most information about injury severity in chronic injury.

Longitudinal In Vivo Diffusion Magnetic Resonance Imaging Remote from the Lesion Site in Rat Spinal Cord Injury

Diffusion tensor imaging (DTI) has demonstrated success as a biomarker of spinal cord injury (SCI) severity as shown from numerous pre-clinical studies. However, artifacts from stabilization hardware at the lesion have precluded its use for longitudinal assessments. Previous research has documented ex vivo diffusion changes in the spinal cord both caudal and cranial to the injury epicenter. The aim of this study was to use a rat contusion model of SCI to evaluate the utility of in vivo cervical DTI after a thoracic injury. Forty Sprague-Dawley rats underwent a thoracic contusion (T8) of mild, moderate, severe, or sham severity. Magnetic resonance imaging (MRI) of the cervical cord was performed at 2, 30, and 90 days post-injury, and locomotor performance was assessed weekly using the Basso, Bresnahan, and Beattie (BBB) scoring scale. The relationships between BBB scores and MRI were assessed using region of interest analysis and voxel-wise linear regression of DTI, and free water elimination (FWE) modeling to reduce partial volume effects. At 90 days, axial diffusivity (AD_FWE), mean diffusivity (MD_FWE), and free water fraction (FWF_FWE) using the FWE model were found to be significantly correlated with BBB score. FWE was found to be more predictive of injury severity than conventional DTI, specifically at later time-points. This study validated the use of FWE technique in spinal cord and demonstrated its sensitivity to injury remotely.