Sleep apnea, narcolepsy, and dreaming: regional cerebral hemodynamics

Neuroimaging of Narcolepsy and Primary Hypersomnias

Advances in neuroimaging open up the possibility for new powerful tools to be developed that potentially can be applied to clinical populations to improve the diagnosis of neurological disorders, including sleep disorders. At present, the diagnosis of narcolepsy and primary hypersomnias is largely limited to subjective assessments and objective measurements of behavior and sleep physiology. In this review, we focus on recent neuroimaging findings that provide insight into the neural basis of narcolepsy and the primary hypersomnias Kleine-Levin syndrome and idiopathic hypersomnia. We describe the role of neuroimaging in confirming previous genetic, neurochemical, and neurophysiological findings and highlight studies that permit a greater understanding of the symptoms of these sleep disorders. We conclude by considering some of the remaining challenges to overcome, the existing knowledge gaps, and the potential role for neuroimaging in understanding the pathogenesis and clinical features of narcolepsy and primary hypersomnias.

Cerebral Hemodynamics in Sleep Apnea and Actigraphy-Determined Sleep Duration in a Sample of the Hispanic Community Health Study/ Study of Latinos

STUDY OBJECTIVES

We sought to evaluate cerebral hemodynamics in obstructive sleep apnea (OSA) and actigraphy-defined short sleep duration using transcranial Doppler ultrasound (TCD) blood flow velocity in a subsample of Hispanics/Latinos without stroke and cardiovascular disease.

METHODS

The sample consisted of consecutive participants at the Miami site of the Hispanic Community Health Study/Study of Latinos (HCHS/SOL) with overnight home sleep testing and 7 days of wrist actigraphy in the Sueño sleep ancillary study. Ninety-five participants had sleep data and TCD determined cerebral hemodynamics. We evaluated the association between OSA (apnea-hypopnea index [AHI] ≥ 5 events/h) and short sleep duration (< 6.8 hours; sample median) with cerebral blood flow velocities (CBFV) and pulsatility index (PI) for the middle cerebral (MCA) and basilar arteries (BA).

RESULTS

Median age was 48 years (range 20-64) with 71% females. Twenty-eight percent of the sample had OSA (AHI ≥ 5 events/h) with median AHI of 10.0 (range 5.0-51.7) events/h. In unadjusted analyses, participants with OSA had lower median CBFV in the BA (30.5 cm/s [interquartile range:10.2] versus 39.4 cm/s [13.3] P < .05), but not the MCA, whereas short sleepers had higher median vascular resistance in the MCA (PI = 0.92 [0.18] versus 0.86 [0.14] P < .05) and BA (PI = 1.0 [0.17] versus 0.93 [0.24] P < .05). After full adjustment, OSA was associated with decreased CBFV (β [SE] = -5.1 [2.5] P < .05) in the BA. Short sleep was associated with increased PI (β [SE] = 0.05 [0.02] P < .05) in the MCA.

CONCLUSIONS

In this sample of Hispanic/Latinos, OSA was associated with decreased daytime blood flow velocity in the BA, whereas actigraphy-defined short sleep duration was associated with increased cerebrovascular pulsatility in the MCA.

Global brain blood-oxygen level responses to autonomic challenges in obstructive sleep apnea

Obstructive sleep apnea (OSA) is accompanied by brain injury, perhaps resulting from apnea-related hypoxia or periods of impaired cerebral perfusion. Perfusion changes can be determined indirectly by evaluation of cerebral blood volume and oxygenation alterations, which can be measured rapidly and non-invasively with the global blood oxygen level dependent (BOLD) signal, a magnetic resonance imaging procedure. We assessed acute BOLD responses in OSA subjects to pressor challenges that elicit cerebral blood flow changes, using a two-group comparative design with healthy subjects as a reference. We separately assessed female and male patterns, since OSA characteristics and brain injury differ between sexes. We studied 94 subjects, 37 with newly-diagnosed, untreated OSA (6 female (age mean ± std: 52.1±8.1 yrs; apnea/hypopnea index [AHI]: 27.7±15.6 events/hr and 31 male 54.3±8.4 yrs; AHI: 37.4±19.6 events/hr), and 20 female (age 50.5±8.1 yrs) and 37 male (age 45.6±9.2 yrs) healthy control subjects. We measured brain BOLD responses every 2 s while subjects underwent cold pressor, hand grip, and Valsalva maneuver challenges. The global BOLD signal rapidly changed after the first 2 s of each challenge, and differed in magnitude between groups to two challenges (cold pressor, hand grip), but not to the Valsalva maneuver (repeated measures ANOVA, p<0.05). OSA females showed greater differences from males in response magnitude and pattern, relative to healthy counterparts. Cold pressor BOLD signal increases (mean ± adjusted standard error) at the 8 s peak were: OSA 0.14±0.08% vs. Control 0.31±0.06%, and hand grip at 6 s were: OSA 0.08±0.03% vs. Control at 0.30±0.02%. These findings, indicative of reduced cerebral blood flow changes to autonomic challenges in OSA, complement earlier reports of altered resting blood flow and reduced cerebral artery responsiveness. Females are more affected than males, an outcome which may contribute to the sex-specific brain injury in the syndrome.

Neuroimaging findings in narcolepsy with cataplexy

Various brain imaging techniques have been used to study narcolepsy with cataplexy. Anatomical data with magnetic resonance imaging have characterized specific alterations in grey and white matter and their potential implications on disease severity. Functional neuroimaging studies have described changes in brain perfusion or glucose metabolism during resting wakefulness, as well as brain responses to emotional stimulation in narcoleptic patients. These different imaging modalities provide evidence for structural and functional abnormalities compatible with a deficit in the hypocretinergic system. They also indicate the involvement of other neural structures, such as the amygdala, nucleus accumbens, midbrain, thalamus, hippocampus, and fronto-temporal cortical areas. This article reviews the contribution of neuroimaging to the pathophysiology of narcolepsy with cataplexy, focusing on the most recent developments.

Transcranial magnetic stimulation and sleep disorders: pathophysiologic insights

The neural mechanisms underlying the development of the most common intrinsic sleep disorders are not completely known. Therefore, there is a great need for noninvasive tools which can be used to better understand the pathophysiology of these diseases. Transcranial magnetic stimulation (TMS) offers a method to noninvasively investigate the functional integrity of the motor cortex and its corticospinal projections in neurologic and psychiatric diseases. To date, TMS studies have revealed cortical and corticospinal dysfunction in several sleep disorders, with cortical hyperexcitability being a characteristic feature in some disorders (i.e., the restless legs syndrome) and cortical hypoexcitability being a well-established finding in others (i.e., obstructive sleep apnea syndrome narcolepsy). Several research groups also have applied TMS to evaluate the effects of pharmacologic agents, such as dopaminergic agent or wake-promoting substances. Our review will focus on the mechanisms underlying the generation of abnormal TMS measures in the different types of sleep disorders, the contribution of TMS in enhancing the understanding of their pathophysiology, and the potential diagnostic utility of TMS techniques. We also briefly discussed the possible future implications for improving therapeutic approaches.

Neuroimaging insights into the pathophysiology of sleep disorders

Neuroimaging methods can be used to investigate whether sleep disorders are associated with specific changes in brain structure or regional activity. However, it is still unclear how these new data might improve our understanding of the pathophysiology underlying adult sleep disorders. Here we review functional brain imaging findings in major intrinsic sleep disorders (i.e., idiopathic insomnia, narcolepsy, and obstructive sleep apnea) and in abnormal motor behavior during sleep (i.e., periodic limb movement disorder and REM sleep behavior disorder). The studies reviewed include neuroanatomical assessments (voxel-based morphometry, magnetic resonance spectroscopy), metabolic/functional investigations (positron emission tomography, single photon emission computed tomography, functional magnetic resonance imaging), and ligand marker measurements. Based on the current state of the research, we suggest that brain imaging is a useful approach to assess the structural and functional correlates of sleep impairments as well as better understand the cerebral consequences of various therapeutic approaches. Modem neuroimaging techniques therefore provide a valuable tool to gain insight into possible pathophysiological mechanisms of sleep disorders in adult humans.

Daily rhythm of cerebral blood flow velocity

BACKGROUND: CBFV (cerebral blood flow velocity) is lower in the morning than in the afternoon and evening. Two hypotheses have been proposed to explain the time of day changes in CBFV: 1) CBFV changes are due to sleep-associated processes or 2) time of day changes in CBFV are due to an endogenous circadian rhythm independent of sleep. The aim of this study was to examine CBFV over 30 hours of sustained wakefulness to determine whether CBFV exhibits fluctuations associated with time of day. METHODS: Eleven subjects underwent a modified constant routine protocol. CBFV from the middle cerebral artery was monitored by chronic recording of Transcranial Doppler (TCD) ultrasonography. Other variables included core body temperature (CBT), end-tidal carbon dioxide (EtCO2), blood pressure, and heart rate. Salivary dim light melatonin onset (DLMO) served as a measure of endogenous circadian phase position. RESULTS: A non-linear multiple regression, cosine fit analysis revealed that both the CBT and CBFV rhythm fit a 24 hour rhythm (R2 = 0.62 and R2 = 0.68, respectively). Circadian phase position of CBT occurred at 6:05 am while CBFV occurred at 12:02 pm, revealing a six hour, or 90 degree difference between these two rhythms (t = 4.9, df = 10, p < 0.01). Once aligned, the rhythm of CBFV closely tracked the rhythm of CBT as demonstrated by the substantial correlation between these two measures (r = 0.77, p < 0.01). CONCLUSION: In conclusion, time of day variations in CBFV have an approximately 24 hour rhythm under constant conditions, suggesting regulation by a circadian oscillator. The 90 degree-phase angle difference between the CBT and CBFV rhythms may help explain previous findings of lower CBFV values in the morning. The phase difference occurs at a time period during which cognitive performance decrements have been observed and when both cardiovascular and cerebrovascular events occur more frequently. The mechanisms underlying this phase angle difference require further exploration.

Cerebral haemodynamics in obstructive sleep apnoea and Cheyne-Stokes respiration

Obstructive sleep apnoeas are common among stroke patients and, as different from central apnoeas, they do not decline during stroke rehabilitation. Cerebral and cardiovascular changes display a different pattern during central and obstructive sleep apnoeas. The cerebral blood flow velocity according to transcranial Doppler increases during an obstructive apnoea and decreases after apnoea termination concomitant with changes in arterial pressure. The changes in cerebral circulation during obstructive apnoeas could be an immediate effect of rapid changes in blood pressure because cerebral autoregulation is overridden. Low cerebral blood flow, low arterial pressure and hypoxemia after apnoea termination may predispose to nocturnal cerebral ischaemia. The opposite pattern is seen during a central apnoea, with a decrease in cerebral blood flow velocity during apnoea and an increase after apnoea termination. Changes during obstructive apnoeas are probably hazardous, with adverse cardiovascular effects including stroke. This may not be the case during central apnoeas, as Cheyne-Stokes respiration with central apnoeas is a result of an underlying disorder such as heart failure and stroke and is not a disease entity in itself. It is suggested that obstructive sleep apnoea is a risk factor for stroke as it is common among stroke victims and cerebral hypoperfusion occurs after an obstructive apnoea. The treatment of sleep apnoea should also be taken into account among stroke patients. Large cohort studies, treatment studies and further studies of possible mechanisms for apnoea-induced stroke are, however, essential in order to evaluate whether obstructive sleep apnoea is an independent risk factor for stroke.

Daytime sleepiness and EEG spectral analysis in apneic patients before and after treatment with continuous positive airway pressure

BACKGROUND

Obstructive sleep apnea syndrome (OSAS) is characterized by recurrent apneas during sleep, resulting in repetitive hypoxemic episodes and interruptions of the normal sleep pattern. A previous study showed EEG slowing (ie, a higher ratio of delta + theta frequencies to alpha + beta frequencies on EEG) during rapid eye movement (REM) sleep and wakefulness in untreated OSAS patients. STUDY AND OBJECTIVES: To determine whether EEG slowing is reversible with continuous positive air pressure (CPAP) treatment and to verify whether the persistence of excessive daytime sleepiness (EDS) is correlated with residual slowing of the EEG.

PATIENTS

Ten healthy subjects (9 men and 1 woman) and 14 patients with moderate-to-severe OSAS (13 men and 1 woman) were studied before and after 6 months of treatment with CPAP.

RESULTS

Untreated OSAS patients showed EEG slowing in frontal and central cortical regions during both wakefulness and during REM sleep compared to healthy control subjects. This EEG slowing was found to be independent of time spent with arterial oxygen saturation < 90%, severity of OSAS, or mean sleep latency as determined by the multiple sleep latency test. CPAP treatment was found to correct the EEG slowing for both REM sleep and wakefulness. Daytime sleepiness also greatly improved with treatment, but some degree of somnolence remained.

CONCLUSION

CPAP treatment was found to correct the EEG slowing that was observed in untreated OSAS patients. Persistent EDS may be related to persistent obesity after CPAP treatment.

Intracerebral hemodynamics probed by near infrared spectroscopy in the transition between wakefulness and sleep

Previous imaging studies have shown that cerebral metabolism is gradually reduced at the beginning of sleep. Few studies have examined the sleep state transition periods from wakefulness to sleep and sleep to wakefulness. The current study used the Near Infrared Spectroscopy (NIRS) technique to describe the intracerebral hemodynamics at the frontal pole in the circumscribed period between wakefulness and sleep. Nine healthy young adults were studied during afternoon naps. Optical probes were placed on the forehead and EEG electrodes on the scalp. At sleep onset oxygenated hemoglobin (oxy-Hb) was reduced (P<0.01) and deoxygenated hemoglobin (deoxy-Hb) showed a near significant reduction (P<0.063). At sleep offset there were increases in oxy-Hb (P<0.005) and deoxy-Hb (P<0.05). In 18 of 26 transitions to sleep there was a coordinated fall in both NIRS parameters, we call the Switch Point, that lasted a mean of 3.6 s. In 32 of 36 transitions to wakefulness there was an analogous Switch Point that lasted a mean of 3.4 s. Before and after the Switch Point, changes were small and the relationship between oxy-Hb and deoxy-Hb was a combination of parallel and reciprocal fluctuations. A synchronized, parallel and short-lived change in oxy-Hb and deoxy-Hb is a discrete event in the transition period between wakefulness and sleep. The concentration of these light absorbing molecules is abruptly set to a new level at sleep-wake transitions and probably reflects the different perfusion demands of these states.

[Obstructive-sleep apnea syndrome: brain oxygenation measured with near-infrared spectroscopy. Preliminary results]

This study assessed cerebral oxygenation in four obstructive sleep apnea syndrome (SAOS) patients (age = 51.8 +/- 15 years, apnea-hypopnea index = 68-125 per hour), during sleep and waking time, using near infrared spectoscopy (NIRS), during a standard polysomnography. Oxyhemoglobin (HbO2, reflecting cerebral oxygenation), total hemoglobin (Hbt, reflecting cerebral blood volumes) and cerebral oxygen saturation (SaO2c = HbO2/Hbt), were compared to the data obtained in four snorers not presenting apneas (age = 51.8 +/- 6.6, apnea-hypopnea index = 2.6-6.2 per hour) examined in the same way. The main result was that HbO2 values were reduced in SAOS patients, both during sleep (at stage 2: 52.54 +/- 9.60 mumol/L versus 73.80 +/- 11.70 mumol/L) and during waking state (53.67 +/- 7.20 mumol/L versus 63.05 +/- 5.55 mumol/L). Hbt was also reduced in apneic patients as compared to snorers during waking state (72.73 +/- 13.90 mumol/L versus 96.05 +/- 6.30 mumol/L). During sleep, Hbt increased in a similar way for snorers and apneics (12.4% versus 13%), whereas HbO2 values were constant for apneics. SaO2c paralleled SaO2p in snorers and apneics, values for SaO2c being 20-30% lower than values for SaO2p. The difference in the values was probably due to the use of different monitoring techniques. Cerebral oxygenation and cerebral blood volumes were continuously low in apneic patients, and peripheral hypoxia was associated with same-range cerebral hypoxia. Cerebral hemodynamic mechanisms related to sleep, although in part efficient in apneic patients, were not able to increase cerebral oxygenation up to normal values.

Positron emission tomography studies of sleep and sleep disorders

Using positron emission tomography (PET) it is possible to perform an in vivo study of cerebral physiological and biochemical processes in man. Employing this technique in sleep studies, decreased cerebral metabolic rates for glucose during slow wave sleep compared with those seen during wakefulness were first demonstrated, whereas similar rates of cerebral glucose metabolism were observed during paradoxical sleep and wakefulness. More recently, regional modifications of cerebral blood flow during sleep have also been demonstrated. During slow wave sleep, cerebral blood flow is decreased particularly in the prefrontal cortex. Rapid eye movement sleep is characterized by activation of the pons, thalami, amygdaloid complexes and a number of cortical areas (e.g. the anterior cingulate cortex). Although data remain incomplete, a variety of sleep disorders, including narcolepsy, fatal familial insomnia and continuous spike-and-wave discharges during slow sleep have been investigated. These results are briefly reviewed.

Sleep apnea syndrome and cerebral hemodynamics

The dynamics of cerebral blood flow velocity (CBFV) during sleep were investigated in the right middle cerebral artery of 10 patients with sleep apnea syndrome (SAS) (mean age, 37 years) and 10 healthy control subjects (mean age, 32 years) throughout the entire sleep period. A computer-assisted pulsed (2 MHz) transcranial Doppler ultrasonography system was modified for continuous long-term and on-line recording of cerebral hemodynamics. Concurrently, simultaneous polysomnography, continuous BP recordings, and measurement of the end-expiratory carbon dioxide were undertaken. CBFV showed comparable nocturnal profiles in both groups with decreases during non-rapid eye movement (NREM) sleep and increases during rapid eye movement (REM) sleep, indicating that the general pattern of brain perfusion during normal sleep is maintained in SAS. Sleep stage changes were not regularly accompanied by corresponding changes in CBFV. This reflected a quantitative uncoupling between cerebral electrical activity and cerebral perfusion during sleep and indicated a dissociation in the activity of central regulatory mechanisms. Sleep stage-related analysis showed slightly reduced CBFV in patients with SAS compared with healthy control subjects during wakefulness and the first NREM sleep period, suggesting depressed brain activity in the patient group. The higher CBFV values observed in patients with SAS compared with control subjects during REM sleep and sleep stage 2, both preceding and following REM sleep, underline the influence of dynamically changing sleep patterns on cerebral perfusion in these patients. Reproducible rapid decreases in CBFV were related to EEG arousals. Since apneas are terminated by arousals, these results showed that direct neuronal influences on brain perfusion during apnea are evident.

Cerebral perfusion during sleep-disordered breathing

Snoring, a leading symptom of the sleep apnoea syndrome (SAS), has been reported to be one of the risk factors for sleep-related cerebral strokes. Episodes of apnoea are accompanied by hypoxaemia as well as hypercapnia. As CO2 constitute a major regulatory factor controlling cerebral blood flow, it is likely that changes in cerebral perfusion are to be found in patients with SAS, which may be related to nocturnal stroke. A computer-assisted pulsed (2 mHz) Doppler ultrasonography system has been modified for continuous long-term and on-line recording of cerebral haemodynamics together with simultaneous polysomnography, continuous blood pressure recordings, and measurement of the end-expiratory CO2. The dynamics of cerebral blood flow velocity (CBFV) during sleep were measured in the right middle cerebral artery in 10 SAS patients. CBFV showed a characteristic nocturnal pattern with decreases during non-rapid eye movement (NREM) sleep and increases during REM sleep. Changes in sleep stage patterns as well as awakenings from NREM sleep were not regularly accompanied by corresponding changes in CBFV. Dramatic increases in CBFV could be observed during apnoeic episodes, with maximum increases during REM sleep. CO2 reactivity and changes in CBFV related to apnoea duration were markedly increased during sleep compared with the waking state in SAS patients. The dynamic feature of CBFV in relation to sleep patterns reflects quantitative uncoupling between cerebral electrical activity and cerebral perfusion during sleep in SAS patients as has been previously reported for normal subjects (Hajak et al. 1994). It supports a dissociation in the activity of central regulatory mechanisms during human sleep which might cause abnormal cerebral perfusion under certain circumstances. The increased CO2 reactivity during sleep in SAS suggests a 'hypersensitivity' of intracranial vasoactive receptors and/or disturbances in the central autonomic control of cerebrovascular functions. It may be concluded that, under certain conditions, the interaction of decreased cerebral perfusion in SAS patients with sleep-related cerebral perfusion patterns and haemodynamic changes during apnoeic episodes might lead to a critical reduction in cerebral perfusion.

Sleep apnea in patients with hemispheric stroke

Sleep pattern and breathing in humans are altered following cerebrovascular accidents involving the brainstem. Sleep apnea is a well-established complication of stroke involving the brainstem. On the other hand, the effect of cerebral stroke on sleep and breathing has not been well defined. The diffuse cerebral symptoms such as cognitive deficits, depression or fatigue, after hemispheric stroke mimic those present in patients with sleep apnea. To define the breathing pattern in patients with stroke involving cerebral hemispheres without brainstem lesion and without the prior history of sleep-disordered breathing, we studied 10 patients within 1 year of their stroke. The data collected during polysomnography from the stroke patients were compared with a group of subjects matched for age, body mass index, presence of hypertension, and smoking history without stroke. Patients with stroke had an abnormal sleep architecture with significantly lower slow wave sleep and rapid eye movement (REM) sleep when compared with controls. Sleep was fragmented because of the presence of increased respiratory disturbances. Stroke patients had a respiratory disturbance index of 52 +/- 10 events per hour when compared with 3 +/- 1 in controls (p < .05). Majorities of respiratory events were obstructive apneas and were associated with arterial oxygen desaturations and arousals. The pathogenic mechanism of sleep-disordered breathing in patients with hemispheric stroke seems to be related to the physiological effect of sleep on already compromised upper airway muscle control. Patients with stroke and diffuse cerebral symptoms should be investigated for the possibility of sleep-disordered breathing.

Regional metabolic dependency in obstructive sleep apnea

Abnormalities of oxygen use occur in obstructive sleep apnea, as do impaired cerebral perfusion and alterations of cerebral function. In this case study, the authors quantitated the local cerebral glucose metabolic rate in two patients with obstructive sleep apnea (one with and one without oxygen supply dependency) and assessed cerebral glucose use by increasing oxygen delivery through passive leg elevation. Obstructive sleep apnea was confirmed by visual analysis of nocturnal pulse oximetry traces in two patients and its severity assessed from the respiratory disturbance index and minimum oxygen saturation. Awake local cerebral glucose metabolic rate (microM/min/100 g) was determined by positron-emission tomography using [18F]-2-Fluoro-2-Deoxy-D-Glucose at baseline and on the following day during passive leg elevation. Conditions otherwise were unchanged. The patient with global oxygen supply dependency exhibited a significant increase in the local cerebral glucose metabolic rate. In contrast, the patient without global supply dependency had no change in the local cerebral glucose metabolic rate. These case studies demonstrate the first evidence of improvement in regional metabolic consumption in response to increased oxygen delivery and in the presence of global oxygen supply dependency.

Methylphenidate decreases regional cerebral blood flow in normal human subjects

To assess the effects of methylphenidate (MP) on cerebral blood flow (CBF), 5 healthy males were studied using 15O-water and positron emission tomography before and after MP (0.5 mg/kg iv). MP significantly decreased whole brain CBF at 5-10 minutes (25 +/- 11%) and at 30 minutes (20 +/- 10%) after its administration. Decrements in CBF were homogeneous throughout the brain (regional decrements 23-30%) and probably reflect the vasoactive properties of MP. The vasoactive properties of MP should be considered when prescribing this drug chronically and/or when giving it to subjects with cerebrovascular compromise.

ST-segment depression during sleep in obstructive sleep apnea

It was hypothesized that obstructive sleep apnea may precipitate myocardial ischemia, reflected by ST-segment depression, in some patients during sleep. Overnight sleep studies and simultaneous 3-channel Holter monitoring were performed on 23 consecutive patients with obstructive sleep apnea without a history of coronary artery disease. Each patient was randomly assigned to nasal continuous positive airway pressure for the first half of the night. An episode of significant ST depression was defined as > 1 mm from baseline for > 1 minute. The total duration (minutes) of ST depression was indexed to the total sleep time (minutes per hour of sleep). Seven patients (30%) had ST depression during sleep. In all 7 patients the duration of ST depression decreased during nasal continuous positive airway pressure (30 +/- 18 vs 11 +/- 13 minutes per hour of sleep) in association with a reduction in the apnea-hypopnea index (65 +/- 35 vs 7 +/- 6/hour), arousal index (49 +/- 14 vs 6 +/- 4/hour) and the duration that oxygen saturation was < 90% (44 +/- 27 vs 12 +/- 23% total sleep time). When patients were not on nasal continuous positive airway pressure, the apnea-hypopnea and arousal indexes were higher during periods of ST depression than when ST segments were isoelectric, whereas oxygen saturation was not different. These 7 patients underwent exercise testing, which was positive for inducible myocardial ischemia in 1 patient. It is concluded that ST depression is relatively common in patients with obstructive apnea during sleep and that the duration of ST depression is significantly reduced by nasal continuous positive airway pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Persistent neuropsychological deficits and vigilance impairment in sleep apnea syndrome after treatment with continuous positive airways pressure (CPAP)

The obstructive sleep apnea syndrome is characterized by nocturnal sleep disturbance, excessive daytime sleepiness and neuropsychological deficits in the areas of memory, attention, and executive tasks. In the present study, these clinical manifestations were assessed in apneic patients before and 6 months after treatment with nasally applied continuous positive airway pressure (CPAP). CPAP treatment was found to restore normal respiration during sleep and to normalize sleep organization. Daytime vigilance greatly improved with treatment but some degree of somnolence as compared to normal controls persisted. Similarly, most neuropsychological deficits normalized with treatment. The exception was for planning abilities and manual dexterity, two neuropsychological deficits that have been found to be highly correlated with the severity of nocturnal hypoxemia. These results raise the possibility that anoxic brain damage is a pathogenic factor in severe obstructive sleep apnea syndrome.

Assessment of intracranial hemodynamics in sleep apnea syndrome

BACKGROUND AND PURPOSE

Sleep apnea syndrome may lead to changes in cerebral hemodynamics due to altered alveolar ventilation. We investigated the dynamics of CO2- and blood pressure-regulated alterations of cerebral blood flow velocities during apneic episodes and evaluated CO2 reactivity during different sleep stages.

METHODS

A computer-assisted pulsed Doppler system (2 MHz) was used for continuous overnight recordings of middle cerebral artery flow patterns together with simultaneous polysomnography, continuous blood pressure recordings, and measurements of end-expiratory CO2 in six patients with sleep apnea syndrome.

RESULTS

Increases in mean flow velocity of 19-219% and in blood pressure of 12.5-83.1% could be observed during the apneic episodes, with maximum increases during rapid eye movement (REM) sleep. CO2 reactivity was in the normal range (4.4 +/- 1.2%) in the waking state and was markedly increased during sleep stages 1 and 2 (p less than 0.005 compared with awake). The greatest increase was found during REM sleep, with a rise of up to three times the waking value (p less than 0.0001 compared with sleep stage 2).

CONCLUSIONS

The changes of mean flow velocity could be interpreted as reactive adaptation processes because of CO2 and blood pressure increases corresponding to apnea. The increased CO2 reactivity during sleep may indicate a "hypersensitivity" of intracranial vascular CO2 or pH receptors and a disturbance of central catecholaminergic and cholinergic systems. The pronounced velocity changes during apneic episodes and the concomitant alterations of vessel wall tension might lead to microangiopathies and macroangiopathies due to chronic strain on the brain vessels.

Obstructive sleep apnea syndrome: pathogenesis of neuropsychological deficits

Neuropsychological deficits have been documented in patients with obstructive sleep apnea syndrome (OSAS). Both nocturnal hypoxemia and impairement of daytime vigilance have been suggested as the pathogenesis of these deficits, yet it remains difficult to find good correlations between cognitive deficits and either of these physiological parameters. In the present study, 10 normal controls were compared to 10 moderately and 10 severely apneic patients, all recorded in a sleep laboratory for two consecutive nights, with a vigilance and neuropsychological assessment made during the intervening day. Relative to the controls, moderate and severe OSAS showed differences in many cognitive functions, although the severely affected showed the greater differences. Moreover, severe apneics were also worse than moderate apneics on tests that were found to be normal in the latter group. This suggests a discontinuity in the appearance of neuropsychological deficits as OSAS progresses. Further analyses revealed that reductions in general intellectual measures, as well as in executive and psychomotor tasks were all attributable to the severity of hypoxemia, while other attention and memory deficits were related to vigiance impairment. Therefore, both vigilance impairment and nocturnal hypoxemia may differentially contribute to the cognitive dysfunctions found in OSAS.

Nocturnal hypoxemia as a determinant of vigilance impairment in sleep apnea syndrome

In sleep apnea syndrome (SAS), vigilance impairment is typically associated with highly disrupted sleep, but recently, nocturnal hypoxemia has also been identified as a second pathogenetic factor in patients with a high degree of desaturation. However, although sleep disruption has been demonstrated to play a role in both the propensity to fall asleep and the capacity to stay awake, the role of nocturnal hypoxemia has been implicated only in the latter. In the present study, both sleep disruption and nocturnal hypoxemia were assessed in 20 moderately to severely apneic patients. During the day, vigilance was assessed both by the multiple sleep latency test (MSLT), as a measure of the propensity to fall asleep, and by the four-choice reaction time test (FCRTT), as a measure of the capacity to stay awake in a performance task. Severity of nocturnal hypoxemia was found to predict performance on the MSLT, as well as on the FCRTT, but sleep disruption was found to predict performance only on the FCRTT. These results suggest that in moderately to severely affected SAS patients, nocturnal hypoxemia may play a primary role in the pathogenesis of vigilance impairment.

Cerebral blood flow and metabolism in sleep

A review is presented of the electrical activity of the brain and its global and regional blood flow and metabolism in the different stages of sleep and in wakefulness in animals and humans. During slow-wave sleep (SWS), the blood flow and metabolism of the brain decrease slightly below the level of wakefulness. During rapid eye movement the activity of the brain increases above that of SWS and sometimes above that of wakefulness. Some studies suggest that both at sleep onset and at arousal the brain stem-cerebellar complex (BSC) may be activated before the cortex and the right hemisphere before the left. Variation of hemispheric dominance seems to be a phenomenon of both wakefulness and sleep.

Auditory event-related potentials and brain dysfunction in sleep apnea

Auditory event-related potentials (ERPs) were recorded from 14 subjects with obstructive sleep apnea (OSA) before and after treatment with nasal continuous positive airway pressure (nCPAP). After 2 nights of treatment, there was dramatic improvement in the sleep patterns of the OSA patients, improvements in measures of apnea severity and oxygenation, and decrease in daytime sleepiness. The results of neuropsychological tests of a broad range of cognitive functions failed to confirm the patients' subjective reports of improvement in psychological functioning after treatment. The latencies of the N2 and P3 components were significantly prolonged prior to treatment, and there was a trend towards smaller N2 and P3 amplitude in the apneic subjects. The latency of P3 (but not N2) changed with treatment, decreasing almost to normative values. The results suggest that ERPs may be useful in documenting neural dysfunction in patients with OSA, in evaluating treatment efficacy, and possibly in determining the causes of the daytime symptoms of OSA.

Compound narcolepsy: sleep pattern and involution

A review of studies of sleep in three-month-old infants, narcoleptics, and normal adults indicates that the sleep pattern in compound narcolepsy is in many ways involuted. Similarities in sleep onsets, REM-specific movement, REM dissociation, ambiguous sleep, nocturnal arousals, diurnal sleep-wake cycles, and relatively limited quiet-awake time are discussed as part of a global inability to inhibit state changes common to both infants and compound narcoleptics. The analogy with infant sleep patterns and results of studies of brain function in narcoleptics suggest that forebrain inhibitory processes are more important in narcoleptic symptomology than is brainstem dysfunction. Puberty and old age are critical periods for the development or exacerbation of the involuted sleep pattern. Closer study of the early development of narcoleptics and of lability of state changes in narcolepsy may aid in diagnosis and prognosis for susceptible individuals.

Compound narcolepsy: development of biochemical imbalance

In a review of existing literature, compound narcolepsy is shown to involve not only problems of sleep regulation, but also autonomic, hormonal, emotional, and possibly motor and cognitive dysfunctions, strongly implicating the hypothalamus, limbic system, and possibly the striatum and cortex in the disorder. Neurochemical studies and the pattern of narcoleptic symptoms support the idea of a dynamic imbalance between dopamine and acetylcholine in the etiology of the disorder. What is known about the natural history of compound narcolepsy suggests a developmental course beginning with fluctuations in dopamine release, followed by supersensitivity of dopamine autoreceptors, and later followed by a pattern of intrinsic oscillations and reciprocal "overshoots" in release of dopamine and acetylcholine to account for the typical sequence of appearance of narcoleptic symptoms.

Cerebral blood flow in normal and abnormal sleep and dreaming

Measurements of regional or local cerebral blood flow (CBF) by the xenon-133 inhalation method and stable xenon computerized tomography CBF (CTCBF) method were made during relaxed wakefulness and different stages of REM and non-REM sleep in normal age-matched volunteers, narcoleptics, and sleep apneics. In the awake state, CBF values were reduced in both narcoleptics and sleep apneics in the brainstem and cerebellar regions. During sleep onset, whether REM or stage I-II, CBF values were paradoxically increased in narcoleptics but decreased severely in sleep apneics, while in normal volunteers they became diffusely but more moderately decreased. In REM sleep and dreaming CBF values greatly increased, particularly in right temporo-parietal regions in subjects experiencing both visual and auditory dreaming.

Narcolepsy following cerebral hypoxic ischemia

A 51-year-old man with non-HLA-DR2 histocompatibility developed classic signs and symptoms of the narcoleptic tetrad soon after recovering from an episode of cardiopulmonary insufficiency, which occurred during induction of surgical anesthesia. Symptoms included excessive daytime sleepiness, hypnagogic hallucinations, sleep paralysis, and cataplexy. The diagnosis was confirmed by repeated polysomnographic examinations in the sleep laboratory. Cerebral hemodynamic changes during the onset of sleep showed remarkable increases of cerebral blood flow during the onset of rapid-eye-movement sleep similar to those reported previously in patients with narcolepsy. Magnetic resonance imaging showed focal regions of abnormal spin-echo signals in the ventral pons.

Cognitive asymmetries after waking from REM and NONREM sleep: effects of delayed testing and handedness

The present paper describes two experiments designed to investigate cognitive asymmetry after awakening from REM and NONREM sleep as a function of time after the awakening and handedness. Two groups of 9 right-handed male subjects each participated in the first experiment. Subjects were awakened from REM and NONREM sleep and after 35 min (Group 1), or 75 min (Group 2), tested on a 6-test battery designed to measure cognitive asymmetry. In contrast to our previous findings in subjects tested within 1-2 min after the awakening who showed a significant sleep-stage dependent shift in cognitive asymmetry, the present results did not show significant differences in performance between the two awakening conditions for any of the groups. Two groups of 12 male and 12 female left-handed subjects participated in the second experiment. All subjects were tested within 1-2 min after the awakenings from REM and NONREM sleep on the same 6-test battery as in experiment 1. Also in contrast to our previous findings in right handers, left-handers did not show any consistent shifts in cognitive asymmetry after the awakenings.

Cognitive asymmetries after wakings from REM and NONREM sleep in right-handed females

Eleven right-handed females were awakened from REM and NONREM sleep and tested on three tests designed to measure functions attributed to the left hemisphere and on three tests designed to measure right hemisphere function. A significant shift in cognitive profile was found in the direction of right hemisphere dominance after waking from REM sleep, and left hemisphere dominance after waking from NONREM sleep. Comparison of the cognitive shifts in females to that observed in males revealed a significant interaction of gender and awakening condition. Females showed a larger increase in right hemisphere performance following awakening from REM sleep relative to NONREM sleep, and a smaller increase in left hemisphere performance following awakening from NONREM sleep relative to REM sleep. The reverse trend was found for males.

Pituitary secretions and wake-sleep cycle

Neurohumoral correlations of sleep are considered from three aspects: 1. Metabolism and cerebral blood circulation (CBF, EEG, endocranial pressure, cerebral temperature); 2. Neuromediators and neuropeptides (5-HT, hypnogen neuropeptides); 3. The influence of the sleeping-waking cycle on adeno-hypophyseal secretion rhythms (GH, PRL, LH, TSH). Variations of these parameters can play an important role in the onset of night crises of migraine and cluster headache.

Sleep related breathing disorders in older men: a search for underlying mechanisms

The incidence of sleep-related breathing disorders (SRBDs) associated with hemoglobin desaturation was determined by nocturnal polygraphic evaluations in 26 healthy men, aged 55-70 years. Sixteen subjects (62%) had abnormal rates of at least 12 episodes per hour of sleep: 8 had occlusive, and 8 had central apnea or hypopnea. During waking ten of 16 SRBD subjects and only one subject without SRBDs exhibited either an elevated nasopharyngeal airway resistance (n = 4) or a reduced ventilatory response to hypercapnia (n = 4) and/or hypoxia (n = 3). However, these abnormalities were not related to the type or severity of SRBDs, and 6 subjects with SRBDs demonstrated no respiratory defect. We conclude that SRBDs have a very high incidence in older males and are not usually secondary to pulmonary cardiac, neurological, or behavioral disorders. Additionally, we hypothesize that abnormalities in ventilatory control or upper airway resistance contribute to SRBDs, but depression of brain stem reticular formation activity during sleep plays a primary role in these disorders. Factors related to both aging and SRBDs are reviewed. These include reduced chemoreceptor responses, altered steroid hormone metabolism, and use and metabolism of hypnotic drugs and alcohol.

Primary sleep apnoea syndrome

Polygraphic study in 18 men with the sleep apnoea syndrome showed central, upper airway obstructive, and mixed apnoeas. Fifty per cent of the total apnoea time was central, 33% was obstructive, and 17% was mixed. Apnoeic episodes were accompanied by oxygen desaturation, relative bradycardia and hypotonia of orofacial muscles innervated by ponto-medullary neurons. During regular breathing these muscles revealed tonic and phasic inspiratory EMG activities. The data suggest that the primary sleep apnoea syndrome results from a dysfunction of the central control of breathing.

Mapping local blood flow of human brain by CT scanning during stable xenon inhalation

Non-invasive methods are described for estimating local cerebral blood flows (LCBF) and partition coefficients (L lambda) during inhalation of 35% stable xenon gas (Xes) in oxygen during CT scanning. After denitrogenation by 100% oxygen breathing, 35% Xes is breathed for 7-8 minutes to minimize subanesthetic effects. Mean changes in brain Hounsfield units extrapolated to 15 minutes were 7.7 units for white matter and 5.3 units for gray matter. They were measured from volumes 80 cubic mm (10 mm2 area x 8 mm), or larger with an EMI 1010 scanner at 1 minute intervals. These data were used for computing LCBFs and L lambdas. Irradiation measured at the center of brain slices was 1 rad per minute. To calculate L lambdas about 6 exposures are necessary, thereafter, each 1 minute scan provides LCBF measurements for 2 adjacent 8 mm slices. Reproducibility for LCBF was r = 0.85 (P less 0.001). Mean L lambdas were 0.86 +/- 0.08 for gray and 1.34 +/- 0.10 for white matter. Normative mean flows (mls/100 g brain/min) were: basal ganglia = 79.6 +/- 9.3, cortex = 82.3 +/- 8.5, white matter = 29.2 +/- 5.9, midbrain tegmentum = 94.3 +/- 14.8, cerebellar cortex = 80.1 +/- 10.9, dorsal pons = 89.3 +/- 4.7, brachium pontis = 35.0 +/- 4.2. Subject finger exercises produced increases of LCBF in contralateral pre-central and post-central gyri. Eye closure decreased flow values limited to the visual system. Gray matter flow values diffusely decreased in non-REM sleep but increased above normal in REM sleep. Cerebral infarction and hemorrhage resulted in zones of zero flow with borders having reduced lambdas and low flows attributed to edema.

An etiology of narcolepsy-cataplexy and a proposed cataplexy neuromechanism

Narcolepsy-cataplexy is an idiopathic sleep disorder that reflects a complex neuropathology. Surveys and physiological investigations indicate that genetic and stress factors are involved in its onset and that stress is associated with symptomatic fluctuations and exacerbations of its clinical course. This paper summarizes the literature regarding the evolution, characteristics and treatment of the disorder. A comprehensive etiology is advanced, integrating neurophysiological and psychological factors specific to narcolepsy-cataplexy with recent advances in blood pressure regulation. Moreover, a testable neuromechanism of cataplexy is proposed, based on longitudinal effects of chronic drowsiness, the strong hypnogenic effect obtained by carotid sinus stimulation, an experimental animal model of narcolepsy-cataplexy, the adaptive characteristics of baroreceptors and, finally, the interconnections between CNS sleep and blood pressure regulators of the brain stem. Through better understanding of the causes and mechanisms of narcolepsy-cataplexy, more effective treatments and preventive measures can be developed, high risk populations identified, and, perhaps, a cure found. Suggestions for future physiological and epidemiological research are made.