Assessment of intracranial hemodynamics in sleep apnea syndrome

How does obstructive sleep apnea alter cerebral hemodynamics?

STUDY OBJECTIVES

We aimed to characterize the cerebral hemodynamic response to obstructive sleep apnea/hypopnea events, and evaluate their association to polysomnographic parameters. The characterization of the cerebral hemodynamics in obstructive sleep apnea (OSA) may add complementary information to further the understanding of the severity of the syndrome beyond the conventional polysomnography.

METHODS

Severe OSA patients were studied during night sleep while monitored by polysomnography. Transcranial, bed-side diffuse correlation spectroscopy (DCS) and frequency-domain near-infrared diffuse correlation spectroscopy (NIRS-DOS) were used to follow microvascular cerebral hemodynamics in the frontal lobes of the cerebral cortex. Changes in cerebral blood flow (CBF), total hemoglobin concentration (THC), and cerebral blood oxygen saturation (StO2) were analyzed.

RESULTS

We considered 3283 obstructive apnea/hypopnea events from sixteen OSA patients (Age (median, interquartile range) 57 (52-64.5); females 25%; AHI (apnea-hypopnea index) 84.4 (76.1-93.7)). A biphasic response (maximum/minimum followed by a minimum/maximum) was observed for each cerebral hemodynamic variable (CBF, THC, StO2), heart rate and peripheral arterial oxygen saturation (SpO2). Changes of the StO2 followed the dynamics of the SpO2, and were out of phase from the THC and CBF. Longer events were associated with larger CBF changes, faster responses and slower recoveries. Moreover, the extrema of the response to obstructive hypopneas were lower compared to apneas (p < .001).

CONCLUSIONS

Obstructive apneas/hypopneas cause profound, periodic changes in cerebral hemodynamics, including periods of hyper- and hypo-perfusion and intermittent cerebral hypoxia. The duration of the events is a strong determinant of the cerebral hemodynamic response, which is more pronounced in apnea than hypopnea events.

Sleep Duration/Quality With Health Outcomes: An Umbrella Review of Meta-Analyses of Prospective Studies

Background

To quantitatively evaluate the evidence of duration and quality of sleep as measured by multiple health outcomes.

Methods

This review is registered with PROSPERO, number CRD42021235587. We systematically searched three databases from inception until November 15, 2020. For each meta-analysis, the summary effect size using fixed and random effects models, the 95% confidence interval, and the 95% prediction interval were assessed; heterogeneity, evidence of small-study effects, and excess significance bias were also estimated. According to the above metrics, we evaluated the credibility of each association.

Results

A total of 85 meta-analyses with 36 health outcomes were included in the study. We observed highly suggestive evidence for an association between long sleep and an increased risk of all-cause mortality. Moreover, suggestive evidence supported the associations between long sleep and 5 increased risk of health outcomes (stroke, dyslipidaemia, mortality of coronary heart disease, stroke mortality, and the development or death of stroke); short sleep and increased risk of overweight and/or obesity; poor sleep quality and increased risk of diabetes mellitus and gestational diabetes mellitus.

Conclusions

Only the evidence of the association of long sleep with an increased risk of all-cause mortality was graded as highly suggestive. Additional studies are needed to be conducted.

Systematic Review Registration:https://www.crd.york.ac.uk/PROSPERO/, identifier: CRD42021235587

Impact of intermittent hypoxia on human vascular responses during sleep

Exposure to intermittent hypoxia (IH) ≥15 times per hour is believed to be the primary mechanism for the increased risk of cerebrovascular and cardiovascular disease in patients with moderate to severe sleep apnea. Human experimental models of IH used to investigate this link have been predominantly employed during wakefulness, which limits extrapolation of findings to sleep apnea where IH occurs during sleep. Moreover, how IH impacts vascular regulation during sleep has not been measured quantitatively. Therefore, the objective of this study was to assess the impact sleep accompanied by IH on vascular responses to hypoxia and hypercapnia during sleep. Ten males performed two randomly scheduled 6-h overnight sleep studies. One sleep study was performed in room air (normoxia) and the other sleep study was performed during isocapnic IH (60 s hypoxia-60 s normoxia). On each night, cerebrovascular (peak blood velocity through the middle cerebral artery (V¯_P); transcranial Doppler ultrasound) and cardiovascular (blood pressure, heart rate) responses to hypoxia and hypercapnia were measured before sleep onset (PM-Awake), within the first 2 h of sleep (PM-Asleep), in the 5th (out of 6) hours of sleep (AM-Asleep) and after being awoken in the morning (AM-Awake). Sleep accompanied by IH had no impact on the V¯_P and blood pressure responses to hypoxia and hypercapnic at any timepoint (p ≥ 0.103 for all responses). However, the AM-Awake heart rate response to hypoxia was greater following sleep in IH compared to sleep in normoxia. Independent of the sleep environment, the V¯_P response to hypoxia and hypercapnia were reduced during sleep. In conclusion, cerebral blood flow responses are reduced during sleep compared to wakefulness, but 6 h of sleep accompanied by IH does not alter cerebrovascular and cardiovascular response to hypoxia and hypercapnia during wakefulness or sleep in healthy young humans. However, it is likely that longer exposure to IH during sleep (i.e., days-to-weeks) is required to better elucidate IH's impact on vascular regulation in humans.

Clinically Silent Small Vessel Disease of the Brain in Patients with Obstructive Sleep Apnea Hypopnea Syndrome

Obstructive sleep apnea hypopnea syndrome (OSAHS) is associated with increased risk of cerebrovascular disease. The aim of the present study was to investigate the association between the presence of the small vessel disease (SVD) of the brain in patients with OSAHS. The study included 24 patients with moderate to severe OSAHS and 34 healthy volunteers. All the subjects underwent magnetic resonance imaging (MRI) of the brain, in order to sought periventricular white matter (PVWM), deep white matter (DWM) and brainstem SVD. Among patients with OSAHS, 79.1% had SVD (grade 1-3, Fazekas score) in DWM and 91.7% in PVWM while 22.4% had brainstem-white matter hyperintensities (B-WMH). Patients with OSAHS had a much higher degree of SVD in the DWM and PVWM compared to the control group (p < 0.001). The multivariate analysis showed an independent significant association of OSAHS with SVD (DWM and PVWM) (p = 0.033, OR 95% CI: 8.66 (1.19-63.08) and: p = 0.002, OR 95% CI: 104.98 (5.15-2141)). The same analysis showed a moderate association of OSAHS with B-WMH (p = 0.050, OR 15.07 (0.97-234.65)). Our study demonstrated an independent significant association of OSAHS with SVD and a moderate association of OSAHS with B-WMH.

The role of circadian rhythms in Obstructive Sleep Apnea symptoms and novel targets for treatment

Obstructive Sleep Apnea (OSA) is a common disorder that is associated with disability, premature mortality and lost quality of life. Excessive daytime sleepiness and depressive symptoms confer a great portion of the disability and lost quality of life associated with the disorder. While showing robust rates of response and symptoms resolutions, current treatments aimed at correcting the respiratory disturbances are not universally successful and a non-negligible proportion of patients who are correctly using available therapies do not experience symptomatic relief, suggesting that mechanisms beyond the respiratory disturbances may be involved in the pathogenesis of symptoms. A growing body of literature concerning animal and human models suggests that the sleep and respiratory disturbances commonly seen in OSA, namely sleep fragmentation, partial sleep deprivation, intermittent hypoxia, can promote shifts in circadian rhythms ultimately leading to misalignment between sleep-wake rhythms and the internal clock, as well as desynchrony amongst peripheral clocks and peripheral and central clock. This manuscript reviews the current evidence in support of a circadian disturbance underlying OSA symptomatology and proposes new applications for existing chronotherapeutic interventions with the potential for improving symptoms and quality of life for those patients that do not find symptomatic relief with currently available treatments.

Cerebral blood flow, cerebrovascular reactivity and their influence on ventilatory sensitivity

NEW FINDINGS

What is the topic of this review? Cerebrovascular reactivity to CO₂ , which is a principal factor in determining ventilatory responses to CO₂ through the role reactivity plays in determining cerebral extra- and intracellular pH. What advances does it highlight? Recent animal evidence suggests central chemoreceptor vasculature may demonstrate regionally heterogeneous cerebrovascular reactivity to CO₂ , potentially as a protective mechanism against excessive CO₂ washout from the central chemoreceptors, thereby allowing ventilation to reflect the systemic acid-base balance needs (respiratory changes in P aC O 2 ) rather than solely the cerebral needs. Ventilation per se does not influence cerebrovascular reactivity independent of changes in P aC O 2 .

ABSTRACT

Alveolar ventilation and cerebral blood flow are both predominantly regulated by arterial blood gases, especially arterial P C O 2 , and so are intricately entwined. In this review, the fundamental mechanisms underlying cerebrovascular reactivity and central chemoreceptor control of breathing are covered. We discuss the interaction of cerebral blood flow and its reactivity with the control of ventilation and ventilatory responsiveness to changes in P C O 2 , as well as the lack of influence of ventilation itself on cerebrovascular reactivity. We briefly summarize the effects of arterial hypoxaemia on the relationship between ventilatory and cerebrovascular response to both P C O 2 and P O 2 . We then highlight key methodological considerations regarding the interaction of reactivity and ventilatory sensitivity, including the following: regional heterogeneity of cerebrovascular reactivity; a pharmacological approach for the reduction of cerebral blood flow; reactivity assessment techniques; the influence of mean arterial blood pressure; and sex-related differences. Finally, we discuss ventilatory and cerebrovascular control in the context of high altitude and congestive heart failure. Future research directions and pertinent questions of interest are highlighted throughout.

A critical analysis of the purported role of hypoxaemia in the comorbidity of obstructive sleep apnoea and epilepsy

Obstructive sleep apnoea (OSA) is a globally prevalent sleep disorder of significant health concern and confounded with several comorbidities resulting in adverse effect(s) on quality of life in patients afflicted with it. Of particular interest is the enigmatic high comorbidity of OSA with epilepsy, the exact underlying pathophysiology of which remains elusive despite a multitude of research performed in the last four decades. Hypoxaemia, which is an important characteristic feature found in OSA during apnoeic spells, has been implicated in the high comorbidity of OSA with epilepsy, the basis of which rests upon hypoxaemia-mediated brain damage, subcortical release phenomenon, oxidative stress and neuroinflammatory reactions. However, several studies present contradictory evidences that potentially refute the hypoxaemia-based mechanism. Additionally, the role of hypercapnia thatgenerally accompanies hypoxaemia during apnoeic spells, cannot be overlooked and is known to be potentially protective against neuronal hyperexcitability. Thus, hypoxaemia theory implicated in the high comorbidity of OSA and epilepsy appears weak and refutable. This brief paper studies and critically analyses the role of hypoxaemia in conjunction with hypercapnia in the underlying pathophysiology of the comorbidity.

Cerebral oxygenation in children with sleep-disordered breathing

Sleep-disordered breathing (SDB) is associated with neurocognitive and behavioral dysfunction, and structural brain abnormalities. Near infrared spectroscopy allows a continuous and non-invasive monitoring of brain tissue oxygenation, giving insight in some pathophysiological mechanisms potentially associated with SDB-related neurocognitive dysfunction. The present review summarizes the finding of studies describing brain tissue oxygenation in adults and children with SDB. Contrary to adults, mean nocturnal tissue oxygenation index (TOI) during sleep does not seem to be different in children with SDB as compared to healthy controls. During respiratory events such as apnoeas and hypopnoeas, the decrease in TOI precedes the peripheral, systemic desaturation. The decrease in TOI has been shown to be greater during apnoeas as compared to hypopnoeas, during rapid-eye movement sleep as compared to other sleep stages, in younger children as compared to their older counterparts, and in those with a high apnoea-hypopnoea index as compared with a low apnoea-hypopnoea index. Studies analyzing the association between repetitive changes in TOI and neurocognitive and behavioral dysfunction may help to decipher the pathophysiology of neurocognitive dysfunction associated with SDB in children.

Association of obstructive sleep apnea and cerebral small vessel disease: a systematic review and meta-analysis

Study Objectives

The objective of the present study was to investigate the association between obstructive sleep apnea (OSA) and the presence of various neuroimaging marker of cerebral small vessel disease (CSVD).

Methods

We systematically searched PubMed, Embase, Web of Science, Scopus, and Cochrane library (from inception to May 2019) for studies evaluating the association between OSA and CSVD, which included white matter hyperintensities (WMH), silent brain infarction (SBI), cerebral microbleeds (CMBs), and perivascular spaces (PVS). Pooled odds ratios (ORs) with 95% confidence interval (CIs) were estimated using random-effects meta-analysis.

Results

After screening 7290 publications, 20 studies were finally included involving 6036 subjects. The sample size ranged from 27 to 1763 (median 158, interquartile range: 67–393). The meta-analysis showed that moderate to severe OSA was positively associated with WMH (13 studies, n = 4412, OR = 2.23, 95% CI = 1.53 to 3.25, I2 = 80.3%) and SBI (12 studies, n = 3353, OR 1.54, 95% CI = 1.06 to 2.23, I2 = 52%). There was no association with CMBs (three studies, n = 342, OR = 2.17, 95% CI = 0.61 to 7.73, I2 = 60.2%) or PVS (two studies, n = 267, OR = 1.56, 95% CI = 0.28 to 8.57, I2 = 69.5%). There was no relationship between mild OSA and CSVD.

Conclusion

Current evidence suggests that moderate to severe sleep apnea is positively related to WMH and SBI, but not CMBs or PVS, which suggests that OSA may contribute to the pathogenesis of CSVD. Further large cohort studies should be prioritized to confirm the findings.

Current understanding of the effects of inspiratory resistance on the interactions between systemic blood pressure, cerebral perfusion, intracranial pressure, and cerebrospinal fluid dynamics

Negative intrathoracic pressure (nITP) is generated by the respiratory muscles during inspiration to overcome inspiratory resistance, thus enabling lung ventilation. Recently developed noninvasive techniques have made it possible to assess the effects of nITP in real time in several physiological aspects such as systemic blood pressure (BP), intracranial pressure (ICP), and cerebral blood flow (CBF). It has been shown that nITP from 0 to −20 cmH2O elevates BP and diminishes ICP, which facilitates brain perfusion. The effects of nITP from −20 to −40 cmH2O on BP, ICP, and CBF remain largely unrecognized, yet even nITP at −40 cmH2O may facilitate CBF by diminishing ICP. Importantly, nITP from −20 to −40 cmH2O has been documented in adults in commonly encountered obstructive sleep apnea, which justifies research in this area. Recent revelations about interactions between ICP and BP have opened up new fields of research in physiological regulation and the pathophysiology of common diseases, such as hypertension, brain injury, and respiratory disorders. A better understanding of these interactions may translate directly into new therapies in various fields of clinical medicine.

Microvascular cerebral blood flow fluctuations in association with apneas and hypopneas in acute ischemic stroke

In a pilot study on acute ischemic stroke (AIS) patients, unexpected periodic fluctuations in microvascular cerebral blood flow (CBF) had been observed. Motivated by the relative lack of information about the impact of the emergence of breathing disorders in association with stroke on cerebral hemodynamics, we hypothesized that these fluctuations are due to apneic and hypopneic events. A total of 28 patients were screened within the first week after stroke with a pulse oximeter. Five (18%) showed fluctuations of arterial blood oxygen saturation ( ≥ 3 % ) and were included in the study. Near-infrared diffuse correlation spectroscopy (DCS) was utilized bilaterally to measure the frontal lobe CBF alongside respiratory polygraphy. Biphasic CBF fluctuations were observed with a bilateral increase of 27.1 % ± 17.7 % and 29.0 % ± 17.4 % for the ipsilesional and contralesional hemispheres, respectively, and a decrease of - 19.3 % ± 9.1 % and - 21.0 % ± 8.9 % for the ipsilesional and contralesional hemispheres, respectively. The polygraph revealed that, in general, the fluctuations were associated with apneic and hypopneic events. This study motivates us to investigate whether the impact of altered respiratory patterns on cerebral hemodynamics can be detrimental in AIS patients.

Continuous positive airway pressure alters brain microstructure and perfusion patterns in patients with obstructive sleep apnea

OBJECTIVES

To assess the effects of continuous positive airway pressure (CPAP) treatment on brain structure and function in patients with obstructive sleep apnea (OSA).

METHODS

A prospective study of seven OSA patients recruited from the sleep center at our institution was carried out. Patients were treated with six weeks of CPAP treatment. Pre-treatment and post-treatment magnetic resonance imaging (MRI) perfusion scans were obtained and compared to assess for treatment-induced changes. Microstructural changes were quantified using functional anistrophy (FA) and mean diffusivity (MD), and brain perfusion was quantified using cerebral blood flow (CBF) and cerebral blood volume (CBV).

RESULTS

Of the seven patients included the in study, six (85.7%) were male, and the mean age was 51 years (standard deviation = 13.14). Increased FA and decreased MD were found in the hippocampus, temporal lobes, fusiform gyrus, and occipital lobes. Decreased FA and increased MD were found in frontal regions for all patients (p < 0.05). Increased CBF and CBV were also observed following treatment (p < 0.05).

CONCLUSION

In addition to symptom resolution, CPAP treatment may allow for healing of OSA-induced brain damage as seen by restoration of brain structure and perfusion.

Characterization of the microvascular cerebral blood flow response to obstructive apneic events during night sleep

Obstructive apnea causes periodic changes in cerebral and systemic hemodynamics, which may contribute to the increased risk of cerebrovascular disease of patients with obstructive sleep apnea (OSA) syndrome. The improved understanding of the consequences of an apneic event on the brain perfusion may improve our knowledge of these consequences and then allow for the development of preventive strategies. Our aim was to characterize the typical microvascular, cortical cerebral blood flow (CBF) changes in an OSA population during an apneic event. Sixteen patients (age 58 ± 8    years , 75% male) with a high risk of severe OSA were measured with a polysomnography device and with diffuse correlation spectroscopy (DCS) during one night of sleep with 1365 obstructive apneic events detected. All patients were later confirmed to suffer from severe OSA syndrome with a mean of 83 ± 15 apneas and hypopneas per hour. DCS has been shown to be able to characterize the microvascular CBF response to each event with a sufficient contrast-to-noise ratio to reveal its dynamics. It has also revealed that an apnea causes a peak increase of microvascular CBF ( 30 ± 17 % ) at the end of the event followed by a drop ( - 20 ± 12 % ) similar to what was observed in macrovascular CBF velocity of the middle cerebral artery. This study paves the way for the utilization of DCS for further studies on these populations.

[The dynamics of cerebral vascular reserve in sleep apnea patients during continuous positive airway pressure therapy]

AIM

To explore the possibility of cerebral hemodynamic reserve (CHR) normalization in patients with obstructive sleep apnea syndrome (OSAS), aged over fifty, who received continuous positive airway pressure (CPAP) therapy.

MATERIAL AND METHODS

The main group (20 patients, aged 50-65 years, with severe OSAS) and the control group (20 volunteers without snoring and OSAS matched for age and physical status) were studied. Diagnosis included cardio-respiratory monitoring of nocturnal sleep and ultrasound examination of cerebral blood flow with functional tests (breath holding and hyperventilation) and calculating the coefficient of vasomotor reactivity (CVMR) before the start of CPAP-therapy and after one month therapy.

RESULTS

The significant CVMR decrease in vertebral (VA) and basilar arteries (BA) in patients with OSAS compared to the control group was identified: 38.9±8.5 and 55.7±11.3 for VA, 36.8±15.7 and 54.0±5.4 for BA. After one month CPAP-therapy, no significant coefficient gains was observed: 38.9±8.5 and 42.3±9.1 for VA, 36.8±15.7 and 37.2±7.7 for BA.

CONCLUSION

The reduction of CVMR in patients older than 50 years with severe OSAS suggests the disturbance of CHR. The absence of its recovery within one month of CPAP-therapy may be indicative of the irreversible damage of the cerebral blood flow regulation mechanisms. Continued observations for a longer period are needed.

Human cerebral blood flow control during hypoxia: focus on chronic pulmonary obstructive disease and obstructive sleep apnea

The brain is a vital organ that relies on a constant and adequate blood flow to match oxygen and glucose delivery with the local metabolic demands of active neurons. Thus exquisite regulation of cerebral blood flow (CBF) is particularly important under hypoxic conditions to prevent a detrimental decrease in the partial pressure of oxygen within the brain tissues. Cerebrovascular sensitivity to hypoxia, assessed as the change in CBF during a hypoxic challenge, represents the capacity of cerebral vessels to respond to, and compensate for, a reduced oxygen supply, and has been shown to be impaired or blunted in a number of conditions. For instance, this is observed with aging, and in clinical conditions such as untreated obstructive sleep apnea (OSA) and in healthy humans exposed to intermittent hypoxia. This review will 1) provide a brief overview of cerebral blood flow regulation and results of pharmacological intervention studies which we have performed to better elucidate the basic mechanisms of cerebrovascular regulation in humans; and 2) present data from studies in clinical and healthy populations, using a translational physiology approach, to investigate human CBF control during hypoxia. Results from studies in patients with chronic obstructive pulmonary disease and OSA will be presented to identify the effects of the disease processes on cerebrovascular sensitivity to hypoxia. Data emerging from experimental human models of intermittent hypoxia during wakefulness will also be reviewed to highlight the effects of intermittent hypoxia on the brain.

Repercusiones neurológicas del síndrome de apnea-hipopnea obstructiva del sueño (SAHOS)

El síndrome de apnea-hipopnea obstructiva del sueño (SAHOS) es una patología con repercusiones sistémicas frecuentes. Uno de los órganos más afectados es el cerebro, así como el sistema nervioso central, convirtiéndose en un factor de riesgo cerebrovascular independiente de otros factores; este síndrome puede incrementar la frecuencia y severidad de enfermedades primarias del sistema nervioso central como epilepsia y síndromes demenciales.Un síntoma cardinal del SAHOS es la cefalea, que presenta características específicas; además, existen diferentes mecanismos fisiopatológicos identificados involucrados en su desarrollo como microdespertares frecuentes con interrupción del sueño, somnolencia diurna secundaria, alteración de la capacidad de atención, concentración, reacción motora, afecto, etc. En conjunto, estos mecanismos afectan otras funciones mentales superiores como el juicio, el raciocinio o la memoria. Factores como hipoxemia, hipercapnia, incremento de la frecuencia cardíaca y presión arterial, secreción de adrenalina, noradrenalina o cortisol durante el sueño contribuyen a la alteración endotelial responsable de enfermedad cerebrovascular.

A study of apnea induced covariations of cerebral blood flow and exhaled CO2

Sleep apnea is identified by repetitive reduction or complete cessation of breathing during sleep. Sleep apnea affects cerebral hemodynamics and it is important to study this effect. Measuring cerebral blood flow during sleep is challenging due to the need to maintain a contact between the flow probe and the skull. It is hypothesized that there exists a relationship between the variations in the exhaled CO2 and Cerebral Blood Flow during sleep apnea. To test this hypothesis, the present study was conducted in two parts: simulated and nocturnal sleep study. 9 volunteer subjects (6 Male and 3 Female Age: 23.11±1.59 years BMI: 21.9±2.409kg/m2) participated in the simulated study and 10 volunteer subjects (9 Male and 1 Female Age: 50.2±7.48 years BMI: 31.541±4.56 kg/m2 AHI: 62.84±20.44) participated in a nocturnal sleep study. Analyzing full waveforms of cerebral blood flow velocity (CBFV) and exhaled CO2, the relationship between 4 metrics from CBFV and 2 metrics from exhaled CO2 were investigated. Although one metric pair showed statistically significant and relatively high correlation (ρ= 0.68 p=7.96×10-7) during the simulated study, the same was not observed during the nocturnal study. Therefore, the proposed hypothesis could not be proven.

Obstructive Sleep Apnea is Linked to Depression and Cognitive Impairment: Evidence and Potential Mechanisms

Obstructive sleep apnea (OSA) is highly prevalent but very frequently undiagnosed. OSA is an independent risk factor for depression and cognitive impairment/dementia. Herein the authors review studies in the literature pertinent to the effects of OSA on the cerebral microvascular and neurovascular systems and present a model to describe the key pathophysiologic mechanisms that may underlie the associations, including hypoperfusion, endothelial dysfunction, and neuroinflammation. Intermittent hypoxia plays a critical role in initiating and amplifying these pathologic processes. Hypoperfusion and impaired cerebral vasomotor reactivity lead to the development or progression of cerebral small vessel disease (C-SVD). Hypoxemia exacerbates these processes, resulting in white matter lesions, white matter integrity abnormalities, and gray matter loss. Blood-brain barrier (BBB) hyperpermeability and neuroinflammation lead to altered synaptic plasticity, neuronal damage, and worsening C-SVD. Thus, OSA may initiate or amplify the pathologic processes of C-SVD and BBB dysfunction, resulting in the development or exacerbation of depressive symptoms and cognitive deficits. Given the evidence that adequate treatment of OSA with continuous positive airway pressure improves depression and neurocognitive functions, it is important to identify OSA when assessing patients with depression or cognitive impairment. Whether treatment of OSA changes the deteriorating trajectory of elderly patients with already-diagnosed vascular depression and cognitive impairment/dementia remains to be determined in randomized controlled trials.

Cerebrovascular reactivity and neurovascular coupling in patients with obstructive sleep apnea

AIM

Obstructive sleep apnea syndrome (OSAS) has been implicated as an independent risk factor for stroke. There are data suggesting the presence of lower cerebrovascular reactivity (CVR) as determined by transcranial Doppler (TCD) in patients with OSAS. We concurrently investigated neurovascular coupling (NVC) with visual stimulation, and CVR using breath-holding (BH) test on TCD in patients with OSAS.

MATERIALS AND METHODS

Data were collected in 49 patients with moderate to severe OSAS, and compared to 15 healthy subjects matched for age and risk factors. The CVR to hypercapnia was measured by BH test, and the NVC was performed with visual stimulation.

RESULTS

There were no significant differences in baseline characteristics of patients and controls, except for BMI, which was significantly higher in patients with OSAS (p = 0.036). OSAS patients showed significantly lower reactivity during BH in comparison to controls (36.9% ± 14.0% vs. 46.6% ± 20.1%; p = 0.019). The reactivity time was also significantly shorter in the OSAS group (8.0 ± 4.2 s) when compared to controls (10.1 ± 4.3 s; p = 0.015). The visual stimulation produced similar reactivity in patients (27.7% ± 9.4%) and controls (29.1 ± 13.9; p > 0.05).

CONCLUSIONS

Our data demonstrate a diminished vasodilator response capacity only to a strong stimulator such as hypercapnia in OSAS patients. However, the NVC, as shown by the TCD, is quite normal, suggesting that a weak or mild stimulation produces a proper reactivity among OSAS patients.

Effect of apnea duration on apnea induced variations in cerebral blood flow velocity and arterial blood pressure

Obstructive Sleep Apnea (OSA), defined by shallow breaths or complete cessation of breathing for more than 10s, is a significant contributing factor for the developments of hypertension, myocardial infarction, stroke and neuropsychological impairments. In this study, we have investigated the relation between apnea duration and apnea induced variations in cerebral blood flow velocity (CBFV) concomitant with blood pressure changes in 9 sleep apnea subjects (8 male and 1 female; Age: 46.0±11.6 years; BMI: 34.5±7.8 kg/m(2); AHI: 81.6±41). As apnea duration increased from 10s to greater than 30s, the mean percentage rise in CBFV increased from 22% to 42% for amplitude and 22% to 33% for area respectively. For blood pressure, the values increased from 14% to 26% for amplitude and 14% to 23% for area respectively. The results suggest that the apnea duration has a measurable effect on the degree of rise in both cerebral blood flow velocity and arterial blood pressure during apnea episodes (p=0.0002).

Analysis of the vertebrobasilar system in patients with obstructive sleep apnea

We conducted a prospective study to evaluate the vertebrobasilar system in adults with and without obstructive sleep apnea (OSA). Our study population was made up of 48 patients with OSA and 21 healthy volunteers who served as controls; the OSA patients were subdivided into one group with mild or moderate OSA (n = 22) and another with severe OSA (n = 26). Each participant underwent Doppler ultrasonography three times to measure the diameter of the vertebral artery, the peak systolic velocity (PSV), the resistive index (RI), and the vertebral artery flow volume; the mean of the three measurements was calculated for each patient, for the OSA and control groups, and for various subgroups. No significant differences in vessel diameter, PSV, or RI were seen among any of the subgroups. Overall, the vertebral artery flow volume was slightly, but not significantly, higher in all patients with OSA (206 ml/min) than in the control group (177 ml/min); this difference might reflect the body's daytime response to the chronic apneic events experienced during sleep. The only statistically significant difference we found was in vertebral artery flow volume between the controls and the subgroup with mild or moderate OSA (p = 0.026); no difference was seen between the controls and the patients with severe OSA (p = 0.318). Likewise, no significant difference in any of the four parameters was seen when patients were subclassified by body mass index and arterial oxygen saturation level.

Obstructive sleep apnea and other sleep-related syndromes

Obstructive sleep apnea syndrome (OSAS) is a common disorder characterized by repetitive episodes of breathing cessation due to complete or partial collapse of the upper airway therefore affecting ventilation. It is quite common, with a prevalence of about 2-4%, has a strong genetic component, and creates a proinflammatory state with elevated TNFα and other cytokines. If untreated, OSA can lead to significant neurological problems that include stroke, cognitive decline, depression, headaches, peripheral neuropathy, and nonarteritic ischemic optic neuropathy (NAION). Treatment reverses some of these neurological problems. Treatment includes continuous positive airway pressure and its variants, oral appliances, weight loss, upper airway surgery, and rarely maxillofacial procedures. Other sleep breathing disorders such as hypoventilation, central sleep apnea, complex sleep apnea, and Cheyne-Stokes respiration are less common and are sometimes associated with neuromuscular disorders causing diaphragmatic paralysis, but can also be seen in opiate exposure and severe obesity.

Serial monitoring of CO2 reactivity following sport concussion using hypocapnia and hypercapnia

PRIMARY OBJECTIVE

This study examined the effects of mild traumatic brain injury (mTBI) on cerebrovascular reactivity (CVR).

RESEARCH DESIGN

A repeated measures design was used to examine serial changes in CVR.

METHODS AND PROCEDURES

Twenty subjects who recently suffered a mTBI were subjected to a respiratory challenge consisting of repeated 20 s breath-holds (BH) and hyperventilations (HV). Testing occurred on days 2 (D2), 4 (D4) and 8 (D8) post-injury as well as a baseline (BASE) assessment (after return-to-play). Transcranial Doppler was used to assess mean cerebral blood velocity (vMCA) and expired gas analysis provided end-tidal carbon dioxide (PETCO2) levels.

RESULTS

There was no significant difference in resting vMCA across all testing days for mTBI. No significant differences in PETCO2 were found throughout the testing protocol. A significant effect (p < 0.001) of testing day on vMCA was found during BH and HV challenges for mTBI. Post-hoc analysis revealed significant differences (p < 0.05) in vMCA between D2 and the other testing days.

CONCLUSIONS

These data suggest that, following mTBI: (1) CVR is not impaired at rest; (2) CVR is impaired in response to respiratory stress; and (3) the impairment may be resolved as early as 4 days post-injury.

A modeling study of idiopathic intracranial hypertension: etiology and diagnosis

OBJECTIVE

To investigate the relationship between idiopathic intracranial hypertension (IIH) and transverse sinus stenosis through experiments performed on a validated mathematical model.

METHODS

A mathematical model of intracranial pressure (ICP) dynamics has been extended to accommodate venous sinus compression through the introduction of a Starling-like resistor between the sagittal and transverse sinuses.

RESULTS

In the absence of this type of resistor, the sinuses are rigid, and the model has only a unique, stable steady state with normal pressures. With resistance a function of the external pressure on the sinus, a second stable steady state may exist. This state is characterized by elevated ICP concurrent with a compressed transverse sinus. Simulations predict that a temporary perturbation that causes a transient elevation of ICP can induce a permanent transition from the normal to the higher steady state. Comparisons to clinical data from IIH patients provide supporting evidence for the validity of the model's predictions. Simulations suggest a possible clinical diagnostic technique to determine if an individual has a compressible transverse sinus and is at risk for developing IIH.

CONCLUSIONS

Results of the model experiments suggest that the primary cause of IIH may be a compressible, as opposed to rigid, transverse sinus, and that the observed stenosis is a necessary characteristic of the elevated pressure state.

Worsening of central sleep apnea at high altitude--a role for cerebrovascular function

Although periodic breathing during sleep at high altitude occurs almost universally, the likely mechanisms and independent effects of altitude and acclimatization have not been clearly reported. Data from 2005 demonstrated a significant relationship between decline in cerebral blood flow (CBF) at sleep onset and subsequent severity of central sleep apnea that night. We suspected that CBF would decline during partial acclimatization. We hypothesized therefore that reductions in CBF and its reactivity would worsen periodic breathing during sleep following partial acclimatization. Repeated measures of awake ventilatory and CBF responsiveness, arterial blood gases during wakefulness. and overnight polysomnography at sea level, upon arrival (days 2-4), and following partial acclimatization (days 12-15) to 5,050 m were made on 12 subjects. The apnea-hypopnea index (AHI) increased from to 77 ± 49 on days 2-4 to 116 ± 21 on days 12-15 (P = 0.01). The AHI upon initial arrival was associated with marked elevations in CBF (+28%, 68 ± 11 to 87 ± 17 cm/s; P < 0.05) and its reactivity to changes in PaCO2 [>90%, 2.0 ± 0.6 to 3.8 ± 1.5 cm·s(-1)·mmHg(-1) hypercapnia and 1.9 ± 0.4 to 4.1 ± 0.9 cm·s(-1)·mmHg(-1) for hypocapnia (P < 0.05)]. Over 10 days, the increases resolved and AHI worsened. During sleep at high altitude large oscillations in mean CBF velocity (CBFv) occurred, which were 35% higher initially (peak CBFv = 96 cm/s vs. peak CBFv = 71 cm/s) than at days 12-15. Our novel findings suggest that elevations in CBF and its reactivity to CO(2) upon initial ascent to high altitude may provide a protective effect on the development of periodic breathing during sleep (likely via moderating changes in central Pco2).

Cerebral blood flow, sympathetic nerve activity and stroke risk in obstructive sleep apnoea. Is there a direct link?

Obstructive sleep apnoea (OSA) is significantly associated with the risk of stroke, and this association is independent of other risk factors, including hypertension, atrial fibrillation and diabetes mellitus. Therefore, additional pathogenic mechanisms may exist, which contribute to the increased risk of stroke. OSA is characterized by prolonged sympathetic overactivity; however the role of the sympathetic nervous system in regulating cerebral circulation remains a matter of controversy. Converging data indicate that brain perfusion is significantly distorted in OSA, with reported decreases in cerebral blood flow as well as intermittent surges in blood pressure and cerebral blood flow velocity. Based on recent research, there is accumulating evidence that sympathetic nerve activity is an important element in brain protection against excessive increases in perfusion pressure during blood pressure surges and flow during rapid eye movement sleep. The aim of this article was to review: (i) the current physiological knowledge related to the role of the sympathetic system in the regulation of cerebral blood flow, (ii) how the influence of the sympathetic system on cerebral vessels is affected by apnoea (increased PaCO(2)) and (iii) the potential significance of the pathological sympathetic system/PaCO(2) interplay in OSA. Sympathetic system seems to be at least partially involved in pathogenesis of distorted haemodynamics and stroke in OSA patients. However, there are still several open questions that need to be addressed before the effective therapeutic strategies can be implemented.

The effect of continuous positive airway pressure on total cerebral blood flow in healthy awake volunteers

PURPOSE

Continuous positive airway pressure (CPAP) is the gold standard treatment for obstructive sleep apnea. However, the physiologic impact of CPAP on cerebral blood flow (CBF) is not well established. Ultrasound can be used to estimate CBF, but there is no widespread accepted protocol. We studied the physiologic influence of CPAP on CBF using a method integrating arterial diameter and flow velocity (FV) measurements obtained for each vessel supplying blood to the brain.

METHODS

FV and lumen diameter of the left and right internal carotid, vertebral, and middle cerebral arteries were measured using duplex Doppler ultrasound with and without CPAP at 15 cm H(2)O, applied in a random order. Transcutaneous carbon dioxide (PtcCO(2)), heart rate (HR), blood pressure (BP), and oxygen saturation were monitored. Results were compared with a theoretical prediction of CBF change based on the effect of partial pressure of carbon dioxide on CBF.

RESULTS

Data were obtained from 23 healthy volunteers (mean ± SD; 12 male, age 25.1 ± 2.6 years, body mass index 21.8 ± 2.0 kg/m(2)). The mean experimental and theoretical CBF decrease under CPAP was 12.5 % (p < 0.001) and 11.9 % (p < 0.001), respectively. The difference between experimental and theoretical CBF reduction was not statistically significant (3.84 ± 79 ml/min, p = 0.40). There was a significant reduction in PtcCO(2) with CPAP (p = <0.001) and a significant increase in mean BP (p = 0.0017). No significant change was observed in SaO(2) (p = 0.21) and HR (p = 0.62).

CONCLUSION

Duplex Doppler ultrasound measurements of arterial diameter and FV allow for a noninvasive bedside estimation of CBF. CPAP at 15 cm H(2)O significantly decreased CBF in healthy awake volunteers. This effect appeared to be mediated predominately through the hypocapnic vasoconstriction coinciding with PCO(2) level reduction. The results suggest that CPAP should be used cautiously in patients with unstable cerebral hemodynamics.

A Moderate Increase of Physiological CO(2) in a Critical Range during Stable NREM Sleep Episode: A Potential Gateway to REM Sleep

Sleep is characterized as rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. Studies suggest that wake-related neurons in the basal forebrain, posterior hypothalamus and brainstem, and NREM sleep-related neurons in the anterior-hypothalamic area inhibit each other, thus alternating sleep-wakefulness. Similarly, pontine REM-ON and REM-OFF neurons reciprocally inhibit each other for REM sleep modulation. It has been proposed that inhibition of locus coeruleus (LC) REM-OFF neurons is pre-requisite for REM sleep genesis, but it remains ambiguous how REM-OFF neurons are hyperpolarized at REM sleep onset. The frequency of breathing pattern remains high during wake, slows down during NREM sleep but further escalates during REM sleep. As a result, brain CO(2) level increases during NREM sleep, which may alter REM sleep manifestation. It has been reported that hypocapnia decreases REM sleep while hypercapnia increases REM sleep periods. The groups of brainstem chemosensory neurons, including those present in LC, sense the alteration in CO(2) level and respond accordingly. For example, one group of LC neurons depolarize while other hyperpolarize during hypercapnia. In another group, hypercapnia initially depolarizes but later hyperpolarizes LC neurons. Besides chemosensory functions, LC REM-OFF neurons are an integral part of REM sleep executive machinery. We reason that increased CO(2) level during a stable NREM sleep period may hyperpolarize LC neurons including REM-OFF, which may help initiate REM sleep. We propose that REM sleep might act as a sentinel to help maintain normal CO(2) level for unperturbed sleep.

Effect of transnasal insufflation on sleep disordered breathing in acute stroke: a preliminary study

BACKGROUND AND PURPOSE

Sleep disordered breathing (SDB) is frequent in acute stroke patients and is associated with early neurologic worsening and poor outcome. Although continuous positive airway pressure (CPAP) effectively treats SDB, compliance is low. The objective of the present study was to assess the tolerance and the efficacy of a continuous high-flow-rate air administered through an open nasal cannula (transnasal insufflation, TNI), a less-intrusive method, to treat SDB in acute stroke patients.

METHODS

Ten patients (age, 56.8 ± 10.7 years), with SDB ranging from moderate to severe (apnea-hypopnea index, AHI, >15/h of sleep) and on a standard sleep study at a mean of 4.8 ± 3.7 days after ischemic stroke (range, 1-15 days), were selected. The night after, they underwent a second sleep study while receiving TNI (18 L/min).

RESULTS

TNI was well tolerated by all patients. For the entire group, TNI decreased the AHI from 40.4 ± 25.7 to 30.8 ± 25.7/h (p = 0.001) and the oxygen desaturation index >3% from 40.7 ± 28.4 to 31 ± 22.5/h (p = 0.02). All participants except one showed a decrease in AHI. The percentage of slow-wave sleep significantly increased with TNI from 16.7 ± 8.2% to 22.3 ± 7.4% (p = 0.01). There was also a trend toward a reduction in markers of sleep disruption (number of awakenings, arousal index).

CONCLUSIONS

TNI improves SDB indices, and possibly sleep parameters, in stroke patients. Although these changes are modest, our findings suggest that TNI is a viable treatment alternative to CPAP in patients with SDB in the acute phase of ischemic stroke.

Snoring and insomnia are not associated with subclinical atherosclerosis in the Northern Manhattan Study

BACKGROUND

Sleep-disordered breathing is a risk factor for stroke, but its association with subclinical atherosclerosis remains controversial. Snoring and insomnia are frequently comorbid with sleep-disordered breathing and may contribute to stroke. Data on the relationship between snoring and insomnia with atherosclerotic disease are sparse. We investigated the relationship between markers of subclinical atherosclerosis, insomnia, snoring, and carotid intima-media thickness, in the Northern Manhattan Study.

METHODS

A group of 1605 participants (mean age 65 +/- 8 years; 40% men; 61% Hispanic, 19% black, 20% white) who had carotid intima-media thickness measurements performed was assessed for self-reported sleep habits. Habitual snoring was defined as self-reported snoring greater than four times per week. Presence of insomnia was based on three items extracted from the Hamilton Rating Scale for Depression. Carotid intima-media thickness was expressed as a mean composite measure of intima-media thickness in the carotid bifurcation, common, and internal carotid artery. Multivariate linear regression models were used to identify associations between snoring, insomnia, and carotid intima-media thickness.

RESULTS

Habitual snoring was present in 29% of the subjects and insomnia in 26%. There was a higher prevalence of self-reported snoring (84%) and insomnia (66%) among Hispanics than non-Hispanics. The mean total carotid intima-media thickness was 0.95 +/- 0.09 mm; among those with self-reported snoring was 0.94 +/- 0.09 mm; and among those with insomnia was 0.95 +/- 0.08 mm. After controlling for age, gender, race-ethnicity, body mass index and cardiovascular risk factors, snoring (P=0.986) and insomnia (P=0.829) were not significantly associated with increased carotid intima-media thickness.

CONCLUSION

Snoring and insomnia were not significantly associated with subclinical atherosclerosis in this population-based community cohort.

Choroidal blood-flow responses to hyperoxia and hypercapnia in men with obstructive sleep apnea

STUDY OBJECTIVES

Obstructive sleep apnea (OSA) impacts on macrovasculature and autonomic function and may therefore interfere with ocular microvascular regulation. We hypothesized that choroidal vascular reactivity to hyperoxia and hypercapnia was altered in patients with OSA compared with matched control subjects and would improve after treatment with continuous positive airway pressure (CPAP).

METHODS

Sixteen healthy men were matched 1:1 for body mass index, sex, and age with 16 men with newly diagnosed OSA without comorbidities. Subjects underwent sleep studies, 24-hour blood pressure monitoring, arterial stiffness measurements, and cardiac and carotid echography. Overall, patients were middle-aged, lean, and otherwise healthy except for having OSA with a limited amount of desaturation, with, at most, subclinical lesions of the cardiovascular system, stage 1 hypertension, or both. Choroidal laser Doppler flowmetry provides a unique opportunity to assess microvascular function by measuring velocity, (ChBVel), volume (ChBVol), and relative subfoveal choroidal blood flow (ChBF). Vascular choroidal reactivity was studied during hyperoxia and hypercapnia (8% CO2) challenges before and after treatment with nasal CPAP.

RESULTS

Patients with OSA and control subjects exhibited similar choroidal reactivity during hyperoxia (stability of choroidal blood flow) and hypercapnia (significant increases in ChBVel of 13.5% and in ChBF of 16%). Choroidal vasoreactivity to CO2 was positively associated with arterial stiffness in patients with OSA. Gas choroidal vasoreactivity was unchanged after 6 to 9 months of CPAP treatment.

CONCLUSION

This study showed unimpaired choroidal vascular reactivity in otherwise healthy men with OSA. This suggests that patients with OSA, without comorbidities, have long-term adaptive mechanisms active in ocular microcirculation.

Obstructive sleep apnea-hypopnea and incident stroke: the sleep heart health study

RATIONALE

Although obstructive sleep apnea is associated with physiological perturbations that increase risk of hypertension and are proatherogenic, it is uncertain whether sleep apnea is associated with increased stroke risk in the general population.

OBJECTIVES

To quantify the incidence of ischemic stroke with sleep apnea in a community-based sample of men and women across a wide range of sleep apnea.

METHODS

Baseline polysomnography was performed between 1995 and 1998 in a longitudinal cohort study. The primary exposure was the obstructive apnea-hypopnea index (OAHI) and outcome was incident ischemic stroke.

MEASUREMENTS AND MAIN RESULTS

A total of 5,422 participants without a history of stroke at the baseline examination and untreated for sleep apnea were followed for a median of 8.7 years. One hundred ninety-three ischemic strokes were observed. In covariate-adjusted Cox proportional hazard models, a significant positive association between ischemic stroke and OAHI was observed in men (P value for linear trend: P = 0.016). Men in the highest OAHI quartile (>19) had an adjusted hazard ratio of 2.86 (95% confidence interval, 1.1-7.4). In the mild to moderate range (OAHI, 5-25), each one-unit increase in OAHI in men was estimated to increase stroke risk by 6% (95% confidence interval, 2-10%). In women, stroke was not significantly associated with OAHI quartiles, but increased risk was observed at an OAHI greater than 25.

CONCLUSIONS

The strong adjusted association between ischemic stroke and OAHI in community-dwelling men with mild to moderate sleep apnea suggests that this is an appropriate target for future stroke prevention trials.

Pathophysiology of sleep apnea

Sleep-induced apnea and disordered breathing refers to intermittent, cyclical cessations or reductions of airflow, with or without obstructions of the upper airway (OSA). In the presence of an anatomically compromised, collapsible airway, the sleep-induced loss of compensatory tonic input to the upper airway dilator muscle motor neurons leads to collapse of the pharyngeal airway. In turn, the ability of the sleeping subject to compensate for this airway obstruction will determine the degree of cycling of these events. Several of the classic neurotransmitters and a growing list of neuromodulators have now been identified that contribute to neurochemical regulation of pharyngeal motor neuron activity and airway patency. Limited progress has been made in developing pharmacotherapies with acceptable specificity for the treatment of sleep-induced airway obstruction. We review three types of major long-term sequelae to severe OSA that have been assessed in humans through use of continuous positive airway pressure (CPAP) treatment and in animal models via long-term intermittent hypoxemia (IH): 1) cardiovascular. The evidence is strongest to support daytime systemic hypertension as a consequence of severe OSA, with less conclusive effects on pulmonary hypertension, stroke, coronary artery disease, and cardiac arrhythmias. The underlying mechanisms mediating hypertension include enhanced chemoreceptor sensitivity causing excessive daytime sympathetic vasoconstrictor activity, combined with overproduction of superoxide ion and inflammatory effects on resistance vessels. 2) Insulin sensitivity and homeostasis of glucose regulation are negatively impacted by both intermittent hypoxemia and sleep disruption, but whether these influences of OSA are sufficient, independent of obesity, to contribute significantly to the "metabolic syndrome" remains unsettled. 3) Neurocognitive effects include daytime sleepiness and impaired memory and concentration. These effects reflect hypoxic-induced "neural injury." We discuss future research into understanding the pathophysiology of sleep apnea as a basis for uncovering newer forms of treatment of both the ventilatory disorder and its multiple sequelae.

Obstructive sleep apnea and stroke

Obstructive sleep apnea (OSA) and stroke are frequent, multifactorial entities that share risk factors, and for which case-control and cross-sectional studies have shown a strong association. Stroke of respiratory centers can lead to apnea. Snoring preceding stroke, documentation of apneas immediately prior to transient ischemic attacks, the results of autonomic studies, and the circadian pattern of stroke, suggest that untreated OSA can contribute to stroke. Although cohort studies indicate that OSA is a stroke risk factor, controversy surrounds the cost-effectiveness of the screening for and treatment of OSA once stroke has occurred.

Cerebrovascular Diseases and Sleep-Disordered Breathing

Sleep-disordered breathing (SDB) is more probably the cause rather than the consequence of stroke because: apneas are essentially obstructive rather than central, the frequency of SDB is not different between transient ischemic attack and cerebral infarction; and previous excessive daytime sleepiness is significantly more frequent among stroke patients with SDB than those without. The presence of SDB in stroke patients could lead to a poor outcome. Pathophysiological relationships between strokes and SDB are multiple. Experimental and clinical studies have shown that both short- and long-term factors may play a role in increasing the susceptibility to stroke in patients with obstructive sleep apnea syndrome. The former include changes in cerebral hemodynamics, hematologic alterations, and cardiocirculatory dysfunctions that typically and repeatedly occur during apnea episodes and also may persist during wakefulness. Regarding long-term factors, some changes in the anatomical characteristics of carotid arteries wall have been recognized in SDB patients. This finding seems to suggest that the link between SDB and cerebrovascular disease might be explained, at least in part, by an increase in the progression of the atherosclerosis process involving cerebral vessels. There are several practical implications from the demonstrated significant role of sleep apnea in increasing the predisposition to developing stroke. Specific investigation is fundamental in the presence of a clinical suspect of SDB, especially in patients with history of transient ischemic attacks and stroke. Specific treatment of SDB may reduce the possibility of further cerebrovascular disturbances.

Obstructive sleep apnea and cardiovascular disease

Over the past few decades, sleep apnea has emerged as an important potential etiologic factor in a broad range of cardiac and vascular diseases. These disease conditions include hypertension, coronary artery disease, myocardial infarction, heart failure, and stroke. Recognition of the role of sleep apnea in clinical cardiology is also increasing in Japan. Although sleep apnea has been strongly linked to obesity in Western populations, in Japanese and other Asian populations there is evidence to indicate that sleep apnea may be prevalent even at lower levels of obesity. In this review we address the epidemiology of sleep apnea. Since sleep apnea includes the combined stresses of hypoxemia, apnea, and disrupted sleep, we also review briefly the potential disease mechanisms that may be activated as a consequence of sleep apnea. We further examine the role of sleep apnea in the pathophysiology and management of specific cardiovascular conditions. Overall, while the evidence of sleep apnea as a causal mechanism in cardiovascular disease is strong and increasing, definitive evidence of the etiologic role of sleep apnea has yet to be obtained. The evidence is most clear in patients with hypertension. Also remaining to be established is whether the treatment of sleep apnea prevents cardiac and vascular events. With regard to this question, although the available data strongly suggest that continuous positive airway pressure treatment is beneficial, randomized control trials are needed in order to confirm this.