Altered cortical excitability in patients with untreated obstructive sleep apnea syndrome

Brain structural correlates of subjective sleepiness and insomnia symptoms in shift workers

Background

Studies on the brain structures of shift workers are limited; thus, this cross-sectional study aimed to compare the brain structures and the brain structural correlates of subjective sleepiness and insomnia symptoms between shift workers and non-shift workers.

Methods

Shift workers (n = 63) and non-shift workers (n = 58) completed questionnaires assessing subjective sleepiness and insomnia symptoms. Cortical thickness, cortical surface area, and subcortical volumes were measured by magnetic resonance imaging. The brain morphometric measures were compared between the groups, and interaction analyses using the brain morphometric measures as the dependent variable were performed to test the interactions between the study group and measures of sleep disturbance (i.e., subjective sleepiness and insomnia symptoms).

Results

No differences in cortical thickness, cortical surface area, or subcortical volumes were detected between shift workers and non-shift workers. A single cluster in the left motor cortex showed a significant interaction between the study group and subjective sleepiness in the cortical surface area. The correlation between the left motor cortex surface area and the subjective sleepiness level was negative in shift workers and positive in non-shift workers. Significant interaction between the study group and insomnia symptoms was present for the left/right putamen volumes. The correlation between the left/right putamen volumes and insomnia symptom levels was positive in shift workers and negative in non-shift workers.

Conclusion

Left motor cortex surface area and bilateral putamen volumes were unique structural correlates of subjective sleepiness and insomnia symptoms in shift workers, respectively.

Effects of 3-month CPAP therapy on brain structure in obstructive sleep apnea: A diffusion tensor imaging study

White matter (WM) fiber alterations in patients with obstructive sleep apnea (OSA) is associated with cognitive impairment, which can be alleviated by continuous positive airway pressure (CPAP). In this study, we aimed to investigate the changes in WM in patients with OSA at baseline (pre-CPAP) and 3 months after CPAP adherence treatment (post-CPAP), and to provide a basis for understanding the reversible changes after WM alteration in this disease. Magnetic resonance imaging (MRI) was performed on 20 severely untreated patients with OSA and 20 good sleepers. Tract-based spatial statistics was used to evaluate the fractional anisotropy (FA), mean diffusion coefficient, axial diffusion coefficient, and radial diffusion coefficient (RD) of WM. To assess the efficacy of treatment, 20 patients with pre-CPAP OSA underwent MRI again 3 months later. A correlation analysis was conducted to evaluate the relationship between WM injury and clinical evaluation. Compared with good sleepers, patients with OSA had decreased FA and increased RD in the anterior thalamic radiation, forceps major, inferior fronto-occipital tract, inferior longitudinal tract, and superior longitudinal tract, and decreased FA in the uncinate fasciculus, corticospinal tract, and cingulate gyrus (P < 0.05). No significant change in WM in patients with post-CPAP OSA compared with those with pre-CPAP OSA. Abnormal changes in WM in untreated patients with OSA were associated with oxygen saturation, Montreal cognitive score, and the apnea hypoventilation index. WM fiber was extensively alteration in patients with severe OSA, which is associated with cognitive impairment. Meanwhile, cognitive recovery was not accompanied by reversible changes in WM microstructure after short-term CPAP therapy.

Obstructive Sleep Apnea Syndrome: A Preliminary Navigated Transcranial Magnetic Stimulation Study

PURPOSE

An increase in resting motor threshold (RMT), prolonged cortical silent period duration (CSP), and reduced short-latency afferent inhibition (SAI), confirmed with previous transcranial magnetic stimulation (TMS), suggest decreased cortical excitability in obstructive sleep apnea syndrome (OSAS). The present study included MRI of OSAS patients for navigated TMS assessment of the RMT, as an index of the threshold for corticospinal activation at rest, and SAI as an index of cholinergic neurotransmission. We hypothesize to confirm findings on SAI and RMT with adding precision in the targeting of motor cortex in OSAS.

SUBJECTS AND METHODS

After acquiring head MRIs for 17 severe right-handed OSAS and 12 healthy subjects, the motor cortex was mapped with nTMS to assess the RMT and SAI, with motor evoked potentials (MEPs) recorded from the abductor-pollicis brevis (APB) muscle. The 120%RMT intensity was used for the SAI by a paired-pulse paradigm in which the electrical stimulation to the median nerve is followed by magnetic stimulation of the motor cortex at inter-stimulus intervals (ISIs) of 18-28 ms (ISIs_18-28). The SAI control condition included a recording of MEPs without peripheral stimulation. Latency and amplitude of MEP at RMT at 120%RMT for eleven different at ISIs_18-28 were analyzed.

RESULTS

The study showed a significantly lower percentage deviation of MEP amplitude at ISIs_(18-28ms) from the control condition between OSAS and healthy subjects (U=44.0, p=0.01). The intensity of stimulation at RMT was significantly higher in OSAS subjects (U=55.0, p=0.04*). Correlation analysis showed that BMI significantly negatively correlated (ρ=-0.47) with MEP amplitude percentage deviation in OSAS patients.

CONCLUSION

The nTMS study results in increased RMT, and reduced cortical afferent inhibition in OSAS patients for SAI at ISIs_18-28, confirming previous findings of impaired cortical afferent inhibition in OSAS. Future nTMS studies are desirable to elucidate the role of RMT and SAI in diagnostics and treatment of OSAS, and to elucidate the usefulness of nTMS in OSAS research.

Nocturnal enuresis in obese children: a nation-wide epidemiological study from China

Childhood obesity increases the risk of obstructive sleep apnea syndrome, type 2 diabetes mellitus, cardiovascular abnormalities, and psychological and behavioral disorders. But it is unclear whether obesity is associated with childhood nocturnal enuresis (NE). This study aimed to assess the relationship between childhood obesity and NE in a nationally representative large sample in China. Subjects were enrolled from Urumqi, Chengdu, Xi'an, Hohhot, Wuhan, Canton, Shanghai, and Harbin cities in China in November and December 2005. The survey included 20,987 children aged 5-12 years and they and their caregivers completed questionnaires. Height and weight were measured by school teachers trained in healthcare. According to the WHO child growth standards, obesity was defined as a body mass index >95th percentile of peers with the same age and gender. NE was defined as bed wetting for more than twice a week for 3 consecutive months. Demographic variables were compared among different groups. The prevalence of obesity, asthma, attention-deficit/hyperactivity disorder (ADHD), depressive moods, and snoring were different between the NE and without-NE groups (P < 0.05). The raw odds ratio (OR) for NE and obesity was 1.36 (95%CI = 1.07-1.74; P = 0.013) and the adjusted OR was 1.42 (95%CI = 1.11-1.82; P = 0.005) in the multivariable analysis. When adjusting for co-occurring conditions, the results showed that asthma did not affect the risk of NE (OR = 1.42, 95%CI = 1.11-1.82; P = 0.005), but ADHD (OR = 1.41; 95%CI = 1.10-1.81; P = 0.006) and depressive moods (OR = 1.34; 95%CI = 1.07-1.76; P = 0.012) slightly weakens the association between NE in children and obesity, while snoring weakens the association between obesity and NE and the risk became non-significant (OR = 1.21; 95%CI = 0.94-1.56; P = 0.138). In conclusion, obese children were at a higher risk of incurring NE compared to non-obese children. This association was weaker in children who either snored, had ADHD, or had depressive mood.

Effects of high-altitude exposure on supraspinal fatigue and corticospinal excitability and inhibition

PURPOSE

While acute hypoxic exposure enhances exercise-induced central fatigue and can alter corticospinal excitability and inhibition, the effect of prolonged hypoxic exposure on these parameters remains to be clarified. We hypothesized that 5 days of altitude exposure would (i) normalize exercise-induced supraspinal fatigue during isolated muscle exercise to sea level (SL) values and (ii) increase corticospinal excitability and inhibition.

METHODS

Eleven male subjects performed intermittent isometric elbow flexions at 50% of maximal voluntary contraction to task failure at SL and after 1 (D1) and 5 (D5) days at 4350 m. Transcranial magnetic stimulation and peripheral electrical stimulation were used to assess supraspinal and peripheral fatigues. Pre-frontal cortex and biceps brachii oxygenation was monitored by near-infrared spectroscopy.

RESULTS

Exercise duration was not statistically different between SL (1095 ± 562 s), D1 (1132 ± 516 s), and D5 (1440 ± 689 s). No significant differences were found between the three experimental conditions in maximal voluntary activation declines at task failure (SL -16.8 ± 9.5%; D1 -25.5 ± 11.2%; D5 -21.8 ± 7.0%; p > 0.05). Exercise-induced peripheral fatigue was larger at D5 versus SL (100 Hz doublet at task failure: -58.8 ± 16.6 versus -41.8 ± 20.1%; p < 0.05). Corticospinal excitability at 50% maximal voluntary contraction was lower at D5 versus SL (brachioradialis p < 0.05, biceps brachii p = 0.055). Cortical silent periods were shorter at SL versus D1 and D5 (p < 0.05).

CONCLUSIONS

The present results show similar patterns of supraspinal fatigue development during isometric elbow flexions at SL and after 1 and 5 days at high altitude, despite larger amount of peripheral fatigue at D5, lowered corticospinal excitability and enhanced corticospinal inhibition at altitude.

Echocardiographic Findings in Healthy Elderly People with Unrecognized Sleep Disordered Breathing

OBJECTIVE

Sleep disordered breathing (SDB) is associated with cardiovascular disease such as hypertension and left ventricular hypertrophy in middle-aged patients; however, this association is not well described in the elderly. The aim of this study was to evaluate the impact of unrecognized SDB on cardiac function and remodeling in a population-based sample of healthy elderly without cardiac disease.

METHODOLOGY

A total of 405 healthy elderly (age ≥ 65 years) were examined by echocardiography and respiratory polygraphy. According to the apnea-hypopnea index (AHI), subjects were stratified in four categories: snorers (AHI < 5), mild (AHI: 5-15), moderate (AHI: 15-30), and severe (AHI > 30) cases.

RESULTS

Comparative analysis between snorers and SDB cases revealed that left atrial (LA) diameter and surface increased according to SDB severity (p < 0.05) without differences in LA mass index. In subjects with an AHI > 30, an increase was found for LV end-diastolic and end-systolic dimension (p < 0.001), as well as for LV mass (p < 0.03) and LV index (p < 0.05). The current study showed a weak but significant correlation between altered LA and LV measurements versus AHI and hypoxemia indices (p < 0.001). In the regression analysis, AHI and hypoxemia had a minimal effect, body mass index and male gender being the most significant predictors.

CONCLUSIONS

In a population of healthy elderly with SDB, slight changes in left atrial and ventricular measurements occur in severe cases (AHI > 30). Irrespective of the lack of a strong association between SDB and cardiac dysfunction, the presence of slight cardiac pathology in severe SDB cases might be considered.

CLINICAL TRIAL REGISTRATION

NCT 00759304 and NCT 00766584.

Prevalence and determinants of subjective sleepiness in healthy elderly with unrecognized obstructive sleep apnea

OBJECTIVE

Obstructive sleep apnea (OSA) is associated with behavioral consequences such as excessive daytime sleepiness (EDS). The aim of this study was to establish the presence of sleepiness in elderly with unrecognized OSA and the factors explaining its occurrence.

METHODOLOGY

A total of 825 healthy elderly (aged ≥65 years) undergoing clinical, respiratory polygraphy, and heart-rate variability analysis were studied. According to the apnea-hypopnea index (AHI), the subjects were stratified in four categories: no-OSA (AHI <5), mild (5 > AHI <15), moderate (15 ≥ AHI <30), and severe OSA (AHI ≥30). Participants with EDS were defined on the basis of the Epworth Sleepiness Scale (ESS) score ≥10.

RESULTS

Overall, the mean ESS score was 5.7 ± 5.6, 14.9% showing EDS with an increasing trend according to OSA severity (p <0.001). Sleepy subjects were more frequently men, having a greater body mass index (BMI), alcohol intake, depression score, and indices of OSA severity (23% of severe OSA cases being sleepy). No differences were present between sleepy and non-sleepy subjects in regard to other clinical, metabolic, and autonomic nervous system measurements. Regression analyses adjusted for age, gender, BMI, hypertension, depression score, autonomic sleep fragmentation, and AHI and/or oxygen desaturation index revealed that gender (p <0.0001), depression score (p <0.0001), and BMI (p = 0.01) were the only significant factors affecting the presence of sleepiness in this population.

CONCLUSIONS

In healthy elderly with OSA, the prevalence of EDS was low, and it affected only severe cases. Despite a weak association between the ESS score and the AHI index, male gender and depression score were the most significant predictors for the occurrence of sleepiness.

Distinctive patterns of cortical excitability to transcranial magnetic stimulation in obstructive sleep apnea syndrome, restless legs syndrome, insomnia, and sleep deprivation

Altered responses to transcranial magnetic stimulation (TMS) in obstructive sleep apnea syndrome (OSAS), restless legs syndrome (RLS), insomnia, and sleep-deprived healthy subjects have been reported. We have reviewed the relevant literature in order to identify eventual distinctive electrocortical profiles based on single and paired-pulse TMS, sensorimotor modulation, plasticity-related and repetitive TMS measures. Although obtained from heterogeneous studies, the detected changes might be the result of the different pathophysiological substrates underlying OSAS, RLS, insomnia and sleep deprivation rather than reflect the general effect of non-specific sleep loss and instability. OSAS tends to exhibit an increased motor cortex inhibition, which is reduced in RLS; intracortical excitability seems to be in favor of an "activating" profile in chronic insomnia and in sleep-deprived healthy individuals. Abnormal plasticity-related TMS phenomena have been demonstrated in OSAS and RLS. This review provides a perspective of TMS techniques by further understanding the role of neurotransmission pathways and plastic remodeling of neuronal networks involved in common sleep disorders. TMS might be considered a valuable tool in the assessment of sleep disorders, the evaluation of the effect of therapy and the design of non-pharmacological approaches.

Direct comparison of cortical excitability to transcranial magnetic stimulation in obstructive sleep apnea syndrome and restless legs syndrome

OBJECTIVE

Changes to transcranial magnetic stimulation (TMS) have been reported in obstructive sleep apnea syndrome (OSAS) and restless legs syndrome (RLS), although no direct comparison study is available. The aim of this new investigation is to assess and compare cortical excitability of OSAS and RLS patients using the same methodology and under the same experimental conditions.

METHODS

Fourteen patients with OSAS and 12 with RLS were compared to 14 age-matched controls. All patients were untreated and had a severe degree of disease. Resting motor threshold (rMT), cortical silent period (CSP) and motor evoked potentials MEPs, as well as intracortical inhibition (ICI) and facilitation at interstimulus interval (ISI) of 3 and 10 ms, respectively, were explored from the right first dorsal interosseous muscle, during wakefulness.

RESULTS

rMT was higher in OSAS than in RLS and controls. CSP was shorter in RLS only when compared to apneic patients, whereas it was similar between OSAS and controls. OSAS subjects exhibited slightly prolonged central motor conductivity, whereas MEP amplitude was smaller in both patient groups. The ICI ratio at ISI of 3 ms was decreased in RLS patients only.

CONCLUSIONS

Distinct changes of responses at TMS were found, probably connected with the different neurophysiological substrates underlying OSAS and RLS and could not be interpreted as a mere reflection of the effects of sleep architecture alteration. TMS can be considered an additional tool for the understanding of clinical and pathophysiological aspects of sleep disorders, and possibly for the evaluation of the effect of therapy.

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.

Narcolepsy-cataplexy: deficient prepulse inhibition of blink reflex suggests pedunculopontine involvement

Hypocretin (orexin) deficiency plays a major role in the pathophysiology of narcolepsy-cataplexy. In animal models, hypocretinergic projections to the pedunculopontine nucleus are directly involved in muscle tone regulation mediating muscle atonia - a hallmark of cataplexy. We hypothesized that pedunculopontine nucleus function, tested with prepulse inhibition of the blink reflex, is altered in human narcolepsy-cataplexy. Twenty patients with narcolepsy-cataplexy and 20 healthy controls underwent a neurophysiological study of pedunculopontine nucleus function. Blink reflex, prepulse inhibition of the blink reflex and blink reflex excitability recovery were measured. Blink reflex characteristics (R1 latency and amplitude, and R2 and R2c latency and area under the curve) did not differ between patients and controls (P > 0.05). Prepulse stimulation significantly increased R2 and R2c latencies and reduced R2 and R2c areas in patients and controls. However, the R2 and R2c area suppression was significantly less in patients than in controls (to 69.8 ± 14.4 and 74.9 ± 12.6%, respectively, versus 34.5 ± 28.6 and 43.3 ± 29.5%, respectively; each P < 0.001). Blink reflex excitability recovery, as measured by paired-pulse stimulation, which is not mediated via the pedunculopontine nucleus, did not differ between patients and controls (P > 0.05). Our data showed that prepulse inhibition is reduced in narcolepsy-cataplexy, whereas unconditioned blink reflex and its excitability recovery are normal. Because the pedunculopontine nucleus is important for prepulse inhibition, these results suggest its functional involvement in narcolepsy-cataplexy.

Using actigraphy and transcranial magnetic stimulation to assess the relationship between sleep and visuomotor skill learning

PURPOSE

Sleep following training can enhance motor skill memory consolidation while chronic sleep disruption can have the converse effect. The aim of this investigation was to explore the relationship between sleep measured by wrist actigraphy, motor skill consolidation and primary motor cortex excitability in young, healthy individuals.

METHODS

Training was a visuospatial finger-tracking task. Dependent measures included tracking skill performance, cortical excitability, measures of sleep, and level of arousal. Assessments occurred pre-training, post-training and at 12 h and 24 h retention. An activity monitor was worn on the wrist during the nights preceding and following training.

RESULTS

Results indicate that sleep during the night following training was predictive of 1) offline skill consolidation following training (R² = 0.34) and 2) cortical excitability at 24 h follow-up (R² = 0.35) with less time spent awake associated with better skill performance and lower cortical excitability at 24 h follow-up. No difference in measures of sleep was observed between nights of sleep (p > 0.05). Sleep the night before training did not influence skill performance, skill acquisition during training, nor measures of cortical excitability at pre-training assessment.

CONCLUSIONS

These findings suggest a relationship between motor skill development, cortical excitability and sleep following training. These results invite further investigation into the utility of actigraphy as a low-cost, easy-to-administer alternative to polysomnography for short and long-term evaluation of the relationship between sleep, cortical excitability and motor skill learning in healthy and patient populations.