Sleep apnea in narcolepsy

Applicability of the Sustained Attention to Response Task (SART) in hypersomnolence: Experience and results from a tertiary referral center

OBJECTIVE/BACKGROUND

Evaluation of hypersomnolence disorders ideally includes an assessment of vigilance using the short Sustained Attention to Response Task (SART). We evaluated whether this task can differentiate between hypersomnolence disorders, whether it correlates with subjective and objective sleepiness, whether it is affected by the time of day, and symptoms of anxiety and depression.

PATIENTS/METHODS

We analyzed diagnostic data of 306 individuals with hypersomnolence complaints diagnosed with narcolepsy type 1 (n=100), narcolepsy type 2 (n=20), idiopathic hypersomnia (n=49), obstructive sleep apnea (n=27) and other causes or without explanatory diagnosis (n=110). We included the Multiple Sleep Latency Test (MSLT), polysomnography, Epworth Sleepiness Scale (ESS), Hospital Anxiety and Depression Scale and SART, which were administered five times during the day (outcomes: reaction time, total, commission and omission errors).

RESULTS

The SART outcomes did not differ between groups when adjusted for relevant covariates. Higher ESS scores were associated with longer reaction times and more commission errors (p<.01). The main outcome, total errors, did not differ between times of the day. Reaction times and omission errors were impacted (p<.05).

CONCLUSIONS

The SART quantifies disturbed vigilance, an important dimension of disorders of hypersomnolence. Results do not suggest that depressive symptoms influence SART outcomes. A practice session is advised. Testing time should be taken into account when interpreting results. We conclude that the SART does not differentiate between central disorders of hypersomnolence. It may be a helpful addition to the standard diagnostic workup and monitoring of these disorders.

Functional roles of orexin in obstructive sleep apnea: From clinical observation to mechanistic insights

Obstructive sleep apnea is the most common sleep-related breathing disorder. Repetitive episodes of the obstructive respiratory events lead to arousal, sleep fragmentation, and excessive daytime sleepiness. Orexin, also known as hypocretin, is one of the most important neurotransmitters responsible for sleep and arousal regulation. Deficiency of orexin has been shown to be involved in the pathogenesis of narcolepsy, which shares cardinal symptoms of sleep apnea and excessive daytime sleep with obstructive sleep apnea. However, the relationship between orexin and obstructive sleep apnea is not well defined. In this review, we summarize the current evidence, from in vitro, in vivo, and clinical data, regarding the association between orexin and obstructive sleep apnea. The effects of orexin on sleep apnea, as well as how the consequences of obstructive sleep apnea affect the orexin system function are also discussed. Additionally, the contrary findings are also included and discussed.

Pharmacologic Management of Excessive Daytime Sleepiness

Excessive daytime sleepiness (EDS) is defined as "irresistible sleepiness in a situation when an individual would be expected to be awake, and alert." EDS has been a big concern not only from a medical but also from a public health point of view. Patients with EDS have the possibility of falling asleep even when they should wake up and concentrate, for example, when they drive, play sports, or walk outside. In this article, clinical characteristics of common hypersomnia and pharmacologic treatments of each hypersomnia are described.

Narcolepsy in Children: Sleep disorders in children, A rapidly evolving field seeking consensus

Narcolepsy is a life-long sleep disorder with two distinct subtypes, narcolepsy type I and narcolepsy type II. It is now well recognized that the loss of hypocretin neurons underlies the pathogenesis of narcolepsy type I, however, the pathogenesis of narcolepsy type II is currently unknown. Both genetic and environmental factors play an important role in the pathogenesis of narcolepsy. There is increasing evidence that autoimmune processes may play a critical role in the loss of hypocretin neurons. Infections especially streptococcus and influenza have been proposed as a potential trigger for the autoimmune-mediated mechanism. Several recent studies have shown increased cases of pediatric narcolepsy following the 2009 H1N1 pandemic. The increased cases in Europe seem to be related to a specific type of H1N1 influenza vaccination (Pandemrix), while the increased cases in China are related to influenza infection. Children with narcolepsy can have an unusual presentation at disease onset including complex motor movements which may lead to delayed diagnosis. All classic narcolepsy tetrads are present in only a small proportion of children. The diagnosis of narcolepsy is confirmed by either obtaining cerebrospinal fluid hypocretin or overnight sleep study with the multiple sleep latency test (MSLT). There are limitations of using MSLT in young children such that a negative MSLT test cannot exclude narcolepsy. HLA markers have limited utility in narcolepsy, but it may be useful in young children with clinical suspicion of narcolepsy. For management, both pharmacologic and non-pharmacologic treatments are important in the management of narcolepsy. Pharmacotherapy is primarily aimed to address excessive daytime sleepiness and REM-related symptoms such as cataplexy. In addition to pharmacotherapy, routine screening of behavioral and psychosocial issues is warranted to identify patients who would benefit from bio-behavior intervention.

Narcolepsy: a model interaction between immune system, nervous system, and sleep-wake regulation

Narcolepsy is a rare chronic neurological disorder characterized by an irresistible excessive daytime sleepiness and cataplexy. The disease is considered to be the result of the selective disruption of neuronal cells in the lateral hypothalamus expressing the neuropeptide hypocretin, which controls the sleep-wake cycle. Diagnosis and management of narcolepsy represent still a substantial medical challenge due to the large heterogeneity in the clinical manifestation of the disease as well as to the lack of understanding of the underlying pathophysiological mechanisms. However, significant advances have been made in the last years, thus opening new perspective in the field. This review describes the current knowledge of clinical presentation and pathology of narcolepsy as well as the existing diagnostic criteria and therapeutic intervention for the disease management. Recent evidence on the potential immune-mediated mechanisms that may underpin the disease establishment and progression are also highlighted.

Dysregulation of the orexin/hypocretin system is not limited to narcolepsy but has far-reaching implications for neurological disorders

Neuropeptides orexin A and B (OX-A/B, also called hypocretin 1 and 2) are released selectively by a population of neurons which projects widely into the entire central nervous system but is localized in a restricted area of the tuberal region of the hypothalamus, caudal to the paraventricular nucleus. The OX system prominently targets brain structures involved in the regulation of wake-sleep state switching, and also orchestrates multiple physiological functions. The degeneration and dysregulation of the OX system promotes narcoleptic phenotypes both in humans and animals. Hence, this review begins with the already proven involvement of OX in narcolepsy, but it mainly discusses the new pre-clinical and clinical insights of the role of OX in three major neurological disorders characterized by sleep impairment which have been recently associated with OX dysfunction, such as Alzheimer's disease, stroke and Prader Willi syndrome, and have been emerged over the past 10 years to be strongly associated with the OX dysfunction and should be more considered in the future. In the light of the impairment of the OX system in these neurological disorders, it is conceivable to speculate that the integrity of the OX system is necessary for a healthy functioning body.

Effect of ambient temperature on sleep breathing phenotype in mice: the role of orexins

The loss of orexinergic neurons, which release orexins, results in narcolepsy. Orexins participate in the regulation of many physiological functions, and their role as wake-promoting molecules has been widely described. Less is known about the involvement of orexins in body temperature and respiratory regulation. The aim of this study was to investigate if orexin peptides modulate respiratory regulation as a function of ambient temperature (Ta) during different sleep stages. Respiratory phenotype of male orexin knockout (KO-ORX, N=9) and wild-type (WT, N=8) mice was studied at thermoneutrality (Ta=30°C) or during mild cold exposure (Ta=20°C) inside a whole-body plethysmography chamber. The states of wakefulness (W), non-rapid eye movement sleep (NREMS) and rapid eye movement sleep (REMS) were scored non-invasively, using a previously validated technique. In both WT and KO-ORX mice, Ta strongly and significantly affected ventilatory period and minute ventilation values during NREMS and REMS; moreover, the occurrence rate of sleep apneas in NREMS was significantly reduced at Ta=20°C compared with Ta=30°C. Overall, there were no differences in respiratory regulation during sleep between WT and KO-ORX mice, except for sigh occurrence rate, which was significantly increased at Ta=20°C compared with Ta=30°C in WT mice, but not in KO-ORX mice. These results do not support a main role for orexin peptides in the temperature-dependent modulation of respiratory regulation during sleep. However, we showed that the occurrence rate of sleep apneas critically depends on Ta, without any significant effect of orexin peptides.

Pharmacologic Management of Excessive Daytime Sleepiness

Excessive daytime sleepiness (EDS) is related to medical and social problems, including mental disorders, physical diseases, poor quality of life, and so forth. According to the International Classification of Sleep Disorders, Third Edition, diseases that result from EDS are narcolepsy type 1, narcolepsy type 2, idiopathic hypersomnia, hypersomnia due to a medical disorder, and others. EDS is usually treated using amphetamine-like central nervous system stimulants or modafinil and its R-enantiomer, armodafinil, wake-promoting compounds unrelated to amphetamines; a variety of new drugs are under development. The side effects of some stimulants are potent and careful selection and management are required.

Prevalence of Rapid Eye Movement-related Obstructive Sleep Apnea in Adult Narcolepsy

Objective The association between narcolepsy and rapid eye movement (REM)-related obstructive sleep apnea (OSA) has not been reported. This study aimed to examine the prevalence of REM-related OSA in narcolepsy patients. Methods From January 2013 to April 2018, 141 adult patients were diagnosed with narcolepsy using nocturnal polysomnography and the multiple sleep latency test. The prevalence of REM-related OSA in narcolepsy patients was retrospectively reviewed. Three criteria were used to determine REM-related OSA: Definition #1, an overall apnea-hypopnea index (AHI) ≥5 and AHI during REM (AHIREM)/AHI during non-rapid eye movement (NREM) (AHINREM) ≥2; Definition #2, an overall AHI ≥5 and AHI_REM/AHI_NREM≥2 and AHI_NREM <15; and Definition #3, an overall AHI ≥5 and AHI_REM/AHI_NREM≥2 and AHI_NREM <8 plus an REM sleep duration >10.5 minutes. Results Of the 141 narcolepsy patients, 26 were diagnosed with narcolepsy with cataplexy (NA-CA) and 115 with narcolepsy without cataplexy (NA w/o CA). Seventeen patients with NA-CA and 39 with NA w/o CA had OSA. According to Definition #1, the prevalence of REM-related OSA was 47.1% and 41.0%, respectively, in OSA patients with NA-CA and NA w/o CA; according to Definition #2, the respective prevalence was 47.1% and 38.5%, while that according to Definition #3 was 41.2% and 25.6%. No significant differences were found in the prevalence of REM-related OSA for each definition. Conclusion A high prevalence of REM-related OSA was confirmed in adult narcolepsy patients with OSA. Compared to previous reports, we noted a high frequency of REM-related OSA satisfying the relatively strict Definition #3. These results might reflect the pathophysiological characteristics of narcolepsy.

Pharmacologic Management of Excessive Daytime Sleepiness

Excessive daytime sleepiness (EDS) is related to medical and social problems, including mental disorders, physical diseases, poor quality of life, and so forth. According to the International Classification of Sleep Disorders, Third Edition, diseases that result from EDS are narcolepsy type 1, narcolepsy type 2, idiopathic hypersomnia, hypersomnia due to a medical disorder, and others. EDS is usually treated using amphetamine-like central nervous system stimulants or modafinil and its R-enantiomer, armodafinil, wake-promoting compounds unrelated to amphetamines; a variety of new drugs are under development. The side effects of some stimulants are potent and careful selection and management are required.

Orexin and Central Modulation of Cardiovascular and Respiratory Function

Orexin makes an important contribution to the regulation of cardiorespiratory function. When injected centrally under anesthesia, orexin increases blood pressure, heart rate, sympathetic nerve activity, and the amplitude and frequency of respiration. This is consistent with the location of orexin neurons in the hypothalamus and the distribution of orexin terminals at all levels of the central autonomic and respiratory network. These cardiorespiratory responses are components of arousal and are necessary to allow the expression of motivated behaviors. Thus, orexin contributes to the cardiorespiratory response to acute stressors, especially those of a psychogenic nature. Consequently, upregulation of orexin signaling, whether it is spontaneous or environmentally induced, can increase blood pressure and lead to hypertension, as is the case for the spontaneously hypertensive rat and the hypertensive BPH/2J Schlager mouse. Blockade of orexin receptors will reduce blood pressure in these animals, which could be a new pharmacological approach for the treatment of some forms of hypertension. Orexin can also magnify the respiratory reflex to hypercapnia in order to maintain respiratory homeostasis, and this may be in part why it is upregulated during obstructive sleep apnea. In this pathological condition, blockade of orexin receptors would make the apnea worse. To summarize, orexin is an important modulator of cardiorespiratory function. Acting on orexin signaling may help in the treatment of some cardiovascular and respiratory disorders.

Impaired orexin receptor expression in the Kölliker-Fuse nucleus in sudden infant death syndrome: possible involvement of this nucleus in arousal pathophysiology

OBJECTIVES

As well known, the sudden infant death syndrome (SIDS) is characterized by the sudden death of a seemingly healthy infant during sleep, frequently resulted from a deficit in arousal phase. Awakening from sleep requires a fully developed and functioning neuronal respiratory network to modulate the ventilation as needed. The pontine Kölliker-Fuse nucleus (KFN) plays a pivotal role in breathing control, thanks to its interconnections with the widespread serotonin and noradrenaline neurons in the brainstem. Numerous studies to date have focused on the implication of orexin, a neuropeptide synthesized by neurons of the lateral hypothalamus, with major projections to the brainstem raphé nuclei and locus coeruleus, in arousal, a neurobiological process closely linked to breathing modifications. The aim of our research has been to demonstrate that also the KFN is a fundamental component of the orexin system, actively involved in arousal.

METHODS

We have evaluated the expression and distribution of the orexin receptors (orexin-1 and orexin-2 receptors) particularly in the rostral pons, where the KFN is located, of 25 SIDS cases and 18 controls.

RESULTS

An intense orexin-1 innervation around the KF neurons has been detected in almost all the controls and only in 20% of SIDS cases.

DISCUSSION

On the basis of these results, we believe that: (1) the KFN plays a leading role not only in providing a regular breathing rhythm but also in the coordination of the sleep-to-wake transition; (2) a defective orexin expression in the KFN could prevent arousal, thus assuming a crucial importance in causing SIDS.

Histamine Transmission Modulates the Phenotype of Murine Narcolepsy Caused by Orexin Neuron Deficiency

Narcolepsy type 1 is associated with loss of orexin neurons, sleep-wake derangements, cataplexy, and a wide spectrum of alterations in other physiological functions, including energy balance, cardiovascular, and respiratory control. It is unclear which narcolepsy signs are directly related to the lack of orexin neurons or are instead modulated by dysfunction of other neurotransmitter systems physiologically controlled by orexin neurons, such as the histamine system. To address this question, we tested whether some of narcolepsy signs would be detected in mice lacking histamine signaling (HDC-KO). Moreover, we studied double-mutant mice lacking both histamine signaling and orexin neurons (DM) to evaluate whether the absence of histamine signaling would modulate narcolepsy symptoms produced by orexin deficiency. Mice were instrumented with electrodes for recording the electroencephalogram and electromyogram and a telemetric arterial pressure transducer. Sleep attacks fragmenting wakefulness, cataplexy, excess rapid-eye-movement sleep (R) during the activity period, and enhanced increase of arterial pressure during R, which are hallmarks of narcolepsy in mice, did not occur in HDC-KO, whereas they were observed in DM mice. Thus, these narcolepsy signs are neither caused nor abrogated by the absence of histamine. Conversely, the lack of histamine produced obesity in HDC-KO and to a greater extent also in DM. Moreover, the regularity of breath duration during R was significantly increased in either HDC-KO or DM relative to that in congenic wild-type mice. Defects of histamine transmission may thus modulate the metabolic and respiratory phenotype of murine narcolepsy.

Effects of opioid, hypnotic and sedating medications on sleep-disordered breathing in adults with obstructive sleep apnoea

BACKGROUND

Obstructive sleep apnoea (OSA) is a common sleep disorder characterised by partial or complete upper airway occlusion during sleep, leading to intermittent cessation (apnoea) or reduction (hypopnoea) of airflow and dips in arterial oxygen saturation during sleep. Many patients with recognised and unrecognised OSA receive hypnotics, sedatives and opiates/opioids to treat conditions including pain, anxiety and difficulty sleeping. Concerns have been expressed that administration of these drugs to people with co-existing OSA may worsen OSA.

OBJECTIVES

To investigate whether administration of sedative and hypnotic drugs exacerbates the severity of OSA (as measured by the apnoea-hypopnoea index (AHI) or the 4% oxygen desaturation index (ODI)) in people with known OSA.

SEARCH METHODS

We searched the Cochrane Airways Group Specialised Register (CAGR) of trials. The search was current as of March 2015.

SELECTION CRITERIA

Randomised, placebo-controlled trials including adult participants with confirmed OSA, where participants were randomly assigned to use opiates or opioids, sedatives, hypnotics or placebo. We included participants already using continuous positive airway pressure (CPAP) or a mandibular advancement device.

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures as recommended by The Cochrane Collaboration.

MAIN RESULTS

Fourteen studies examining the effects of 10 drugs and including a total of 293 participants contributed to this review. Trials were small, with only two trials, which used sodium oxybate, recruiting more than 40 participants, and all but three trials were of only one to three nights in duration. Most participants had mild to moderate OSA with a mean AHI of 11 to 25 events/h, and only two trials recruited patients with severe OSA. Two trials investigating the effects of ramelteon, a treatment option for insomnia, recruited adults over 60 years of age with OSA and concomitant insomnia.The drugs studied in this review included remifentanil (infusion) 0.75 mcg/kg/h, eszopiclone 3 mg, zolpidem 10 and 20 mg, brotizolam 0.25 mg, flurazepam 30 mg, nitrazepam 10 mg to 15 mg, temazepam 10 mg, triazolam 0.25 mg, ramelteon 8 mg and 16 mg and sodium oxybate 4.5 g and 9 g. We were unable to pool most of the data, with the exception of data for eszopiclone and ramelteon.None of the drugs in this review produced a significant increase in AHI or ODI. Two trials have shown a beneficial effect on OSA. One study showed that a single administration of eszopiclone 3 mg significantly decreased AHI compared with placebo (24 ± 4 vs 31 ± 5; P value < 0.05), and a second study of sodium oxybate 4.5 g showed a significant decrease in AHI compared with placebo (mean difference (MD) -7.41, 95% confidence interval (CI) -14.17 to -0.65; N = 48).Only four trials reported outcome data on ODI. No significant increase, in comparison with placebo, was shown with eszopiclone (21 (22 to 37) vs 28.0 (15 to 36); P value = NS), zolpidem (0.81 ± 0.29 vs 1.46 ± 0.53; P value = NS), flurazepam (18.6 ± 19 vs 19.6 ± 15.9; P value = NS) and temazepam (6.53 ± 9.4 vs 6.56 ± 8.3; P value = 0.98).A significant decrease in minimum nocturnal peripheral capillary oxygen saturation (SpO2) was observed with zolpidem 20 mg (76.8 vs 85.2; P value = 0.002), flurazepam 30 mg (81.7 vs 85.2; P value = 0.002), remifentanil infusion (MD -7.00, 95% CI -11.95 to -2.05) and triazolam 0.25 mg in both rapid eye movement (REM) and non-REM (NREM) sleep (MD -14.00, 95% CI -21.84 to -6.16; MD -10.20, 95% CI -16.08 to -4.32, respectively.One study investigated the effect of an opiate (remifentanil) on patients with moderate OSA. Remifentanil infusion did not significantly change AHI (MD 10.00, 95% CI -9.83 to 29.83); however it did significantly decrease the number of obstructive apnoeas (MD -9.00, 95% CI -17.40 to -0.60) and significantly increased the number of central apnoeas (MD 16.00, 95% CI -2.21 to 34.21). Similarly, although without significant effect on obstructive apnoeas, central apnoeas were increased in the sodium oxybate 9 g treatment group (MD 7.3 (18); P value = 0.005) in a cross-over trial.Drugs studied in this review were generally well tolerated, apart from adverse events reported in 19 study participants prescribed remifentanil (n = 1), eszopiclone (n = 6), sodium oxybate (n = 9) or ramelteon (n = 3).

AUTHORS' CONCLUSIONS

The findings of this review show that currently no evidence suggests that the pharmacological compounds assessed have a deleterious effect on the severity of OSA as measured by change in AHI or ODI. Significant clinical and statistical decreases in minimum overnight SpO2 were observed with remifentanil, zolpidem 20 mg and triazolam 0.25 mg. Eszopiclone 3 mg and sodium oxybate 4.5 g showed a beneficial effect on the severity of OSA with a reduction in AHI and may merit further assessment as a potential therapeutic option for a subgroup of patients with OSA. Only one trial assessed the effect of an opioid (remifentanil); some studies included CPAP treatment, whilst in a significant number of participants, previous treatment with CPAP was not stated and thus a residual treatment effect of CPAP could not be excluded. Most studies were small and of short duration, with indiscernible methodological quality.Caution is therefore required when such agents are prescribed for patients with OSA, especially outside the severity of the OSA cohorts and the corresponding dose of compounds given in the particular studies. Larger, longer trials involving patients across a broader spectrum of OSA severity are needed to clarify these results.

Respiration and autonomic regulation and orexin

Orexin, a small neuropeptide released from neurons in the hypothalamus with widespread projections throughout the central nervous system, has broad biological roles including the modulation of breathing and autonomic function. That orexin activity is fundamentally dependent on sleep-wake state, and circadian cycle requires consideration of orexin function in physiological control systems in respect to these two state-related activity patterns. Both transgenic mouse studies and focal orexin receptor antagonism support a role for orexins in respiratory chemosensitivity to CO₂ predominantly in wakefulness, with further observations limiting this role to the dark period. In addition, orexin neurons participate in the regulation of sympathetic activity, including effects on blood pressure and thermoregulation. Orexin is also essential in physiological responses to stress. Orexin-mediated processes may operate at two levels: (1) in sleep-wake and circadian states and (2) in stress, for example, the defense or "fight-or-flight" response and panic-anxiety syndrome.

Orexin neurons and emotional stress

Stress increases cardiac function, ventilation, and body temperature and induces analgesia. These changes, which result in an increase in metabolic rate, oxygen supply, and the conduction velocity of nerve impulses, prepare the body for a fight-or-flight response. A part of the hypothalamus called the defense area has long been known to play a key role in these responses, but the precise mechanisms are largely unknown. Our recent findings suggest that orexin (hypocretin) neurons act as a master switch of the fight-or-flight response. In addition, our results, as well as those from other researchers, suggest that orexin neurons do not modulate specific behaviors such as the fight-or-flight responses but rather integrate the autonomic functions and behaviors in a broad sense or in a vigilance state-dependent manner. The orexin system seems to be a pivotal link between the subconscious and the conscious brain functions.

Respiratory regulation in narcolepsy

INTRODUCTION

Narcolepsy is a debilitating sleep disorder characterized by excessive daytime sleepiness, cataplexy and intrusive REM sleep. Deficits in endogenous orexins are a major pathogenic component of the disease. This disorder is also associated with the gene marker, HLADQB1*0602. An increased prevalence of sleep apnea in narcolepsy suggested interactions among ventilatory chemosensitivity, narcolepsy-cataplexy, and sleep apnea.

RESULTS

Evidence from animal studies using orexin knockout mice and focal microdialysis of an orexin receptor antagonist demostrated that orexins are also contributed to respiratory regulation in a vigilance state-dependent manner, as animals with orexins dysregulation have attenuated hypercapnic ventilatory responses predominately in wakefulness, which is consistent with the notion that the activity of orexinergic neurons is higher during wake than sleep periods. Human model of hypocretin deficiency is patients with narcolepsy-cataplexy. In contrast to findings suggested by animal studies, we found significant decrease in hypoxic responsiveness but not in hypercapnic responsiveness in narcoleptics, and further analysis indicated that decreased ventilatory responses to hypoxia in human narcolepsy-cataplexy is in relation to HLA-DQB1*0602 status, not hypocretin deficiency.

CONCLUSION

Unlike in mouse, hypocretin-1 is not a major factor contributing to chemoresponsiveness in human. Species differences may exist.

Restless legs syndrome is frequent in narcolepsy with cataplexy patients

STUDY OBJECTIVES

To investigate the occurrence of restless legs syndrome (RLS) in narcolepsy with cataplexy (NC).

DESIGN

A case-control study assessing the frequency of comorbidity of RLS and NC in three European sleep disorder centers.

PATIENTS

Three sleep research centers recruited 184 NC patients and 235 age-matched controls.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

NC patients and controls underwent a face-to-face interview investigating demographics, medical and drug history, sleep habits, and sleep disorders, in particular RLS based on the 4 international criteria and on a frequency > or =2 times/week, with a detailed description of RLS symptoms when present. RLS was significantly more prevalent among NC patients (14.7%) than in controls (3.0%). The age at onset of RLS in NC patients fits with the age at onset in idiopathic RLS, and RLS appeared more than 10 years after NC onset. Unlike idiopathic RLS, RLS in NC subjects was not more prevalent in women and was less familial (15.4% of cases). Lastly, NC patients with RLS showed a moderate disease severity and an almost daily occurrence of symptoms, which were also diurnal in 35% of cases. Older age, higher blood ferritin levels, and sleep paralysis seem to have a predictive value for RLS in NC. The higher ferritin levels indicate that different pathophysiological mechanisms underlie secondary RLS associated with NC.

CONCLUSIONS

This study highlights the association between RLS and NC. The nature of this association is still investigational, but it does indicate that RLS must be addressed in the evaluation and management of nocturnal sleep impairment in NC patients.

Emerging treatments for narcolepsy and its related disorders

IMPORTANCE OF THE FIELD

Narcolepsy is a chronic sleep disorder, characterized by excessive daytime sleepiness (EDS), cataplexy, hypnagogic hallucinations, sleep paralysis and nocturnal sleep disruption. Non-pharmacological treatments (i.e., behavioral modification) are often helpful for the clinical management of narcoleptic patients. As these symptoms are often disabling, most patients need life-long treatments. Over 90% of diagnosed narcoleptic patients are currently prescribed medications to control their symptoms; however, available treatments are merely symptomatic.

AREAS COVERED IN THIS REVIEW

This review presents a description of the clinical symptoms of narcolepsy, followed by a discussion of the state-of-the-art knowledge regarding the disorder and related emerging treatments. In preparing this review, an extensive literature search was conducted using Pubmed. Only selected references from 1970 to 2008 are cited.

WHAT THE READER WILL GAIN

This review focuses on emerging treatments for human narcolepsy, and the reader will gain significant knowledge of current and future treatment for this and related disorders. Traditionally, amphetamine-like stimulants (i.e., dopaminergic release enhancers) have been used for clinical management to improve EDS, and tricyclic antidepressants have been used as anticataplectics. However, treatments have recently evolved which utilize better tolerated compounds, such as modafinil (for EDS) and adrenergic/serotonergic selective reuptake inhibitors (as anticataplectics). In addition, night time administration of a short-acting sedative, gamma-hydroxybutyrate, has been used for the treatment for EDS and cataplexy. As a large majority of human narcolepsy is hypocretin peptide deficient, hypocretin replacement therapy may also be a new therapeutic option; yet, this option is still unavailable. In addition to the hypocretin-based therapy, a series of new treatments are currently being tested in animal and/or humans models. These potential options include novel stimulant and anticataplectic drugs as well as immunotherapy, based on current knowledge of the pathophysiology of narcolepsy with cataplexy.

TAKE HOME MESSAGE

We expect that more pathophysiology-based treatments, capable of curing and/or preventing narcolepsy and related diseases, will be available in near future. As cases of EDS, associated with other neurological conditions (i.e., symptomatic narcolepsy or narcolepsy due to medical conditions), are often linked with hypocretin deficiency, these novel therapeutic options may also be applied to treatment of these disabling conditions.

Hypothalamic modulation of breathing

Hypothalamus has long been known to be involved in the regulation of breathing. For example, many neurons are activated by hypoxia and hypercapnia and stimulation to the hypothalamus increases respiration. However, precise characters of these neurons have not well understood until recently presumably because hypothalamus is a heterogeneous structure intermingly containing many kind of neurotransmitters. The situation has dramatically changed by a discovery of hypothalamic neuropeptide orexin in 1998 and subsequent development of orexin-knockout mice in 1999. Here I summarize our recent discovery of the possible contribution of orexin to the vigilance-state-dependent adjustment of central respiratory regulation. Orexin-deficient mice show an attenuated hypercapnic ventilatory response during the awake but not during the sleep period, whereas basal ventilation remained normal, irrespective of the vigilance state. Orexin supplementation remedied the defect, and the administration of an orexin receptor antagonist to wild-type mice mimicked the abnormality. Hypercapnic stimulation activated orexinergic neurons in the wild-type mice. Orexin-deficient mice also showed frequent sleep apneas and loss of repetitive intermittent hypoxia-induced ventilatory and phrenic long-term facilitation. Hence, it is possible that the orexin system is one of the essential modulators required for coordinating the circuits controlling respiration and behavior.

A safety trial of sodium oxybate in patients with obstructive sleep apnea: Acute effects on sleep-disordered breathing

OBJECTIVE

Sodium oxybate (SXB) is an approved drug for the treatment of excessive daytime sleepiness (EDS) and cataplexy in narcolepsy. Obstructive sleep apnea syndrome (OSAS) is a condition that frequently co-occurs with narcolepsy. Given the known central nervous system (CNS) depressant effects of SXB, this study aimed to examine its effects on sleep-disordered breathing (SDB) and sleep architecture in patients with OSAS.

METHODS

Sixty patients with a history of mild to moderate OSAS (apnea-hypopnea index [AHI]>or=10 and <or=40, mean oxygen saturation [SaO(2)] >or=75%) received one of four treatments of the following: (1) 9g SXB, (2) 9g SXB/modafinil 200mg, (3) zolpidem 10mg, and (4) placebo (PBO) in a randomized, crossover design on four consecutive nights followed by overnight polysomnography.

RESULTS

Forty-two patients (70%) completed the study. The mean change from baseline in AHI and mean SaO(2) was not significantly different among groups following treatment. Central apneas in patients treated with SXB increased, and clinically significant oxygen desaturations were seen in three patients with SXB treatment. The most common treatment related adverse events were headache and nausea.

CONCLUSION

These results suggest that nighttime administration of 9g SXB in patients with mild to moderate OSAS does not negatively impact SDB, as measured by mean change from baseline in AHI and SaO(2), but might increase central apneas and cause oxygen desaturation in some individuals and should be used with caution.

Obstructive sleep apnea in narcolepsy

STUDY OBJECTIVES

Narcolepsy and obstructive sleep apnea syndrome (OSAS) are two conditions associated with excessive daytime sleepiness (EDS). They may coexist in the same patient but the frequency of this association and its clinical significance is unknown. The presence of obstructive sleep apnea (OSA) in a narcoleptic patient may interfere with the diagnosis of narcolepsy. The aim of the study was to determine the prevalence of OSA in narcolepsy.

DESIGN AND SETTING

University hospital sleep clinic series of narcoleptic patients diagnosed with nocturnal polysomnography and multiple sleep latency test. Patients were systematically interviewed evaluating narcoleptic and OSAS features and their response to continuous positive airway pressure (CPAP) treatment when applied.

PATIENTS

One hundred and thirty-three patients with narcolepsy.

RESULTS

Thirty-three patients (24.8%) had an apnea-hypopnea index greater than 10 with a mean index of 28.5+/-15.7. Ten of them were initially diagnosed only with OSAS and the diagnosis of narcolepsy was delayed 6.1+/-7.8years until being evaluated in our center for residual EDS after CPAP therapy. In the remaining 23 patients, narcolepsy and OSA were diagnosed simultaneously. Cataplexy occurred with similar frequency in both groups. EDS did not improve in 11 of the 14 patients who were treated with CPAP. The presence of OSA was associated with male gender, older age and higher body mass index.

CONCLUSIONS

OSA occurs frequently in narcolepsy and may delay the diagnosis of narcolepsy by several years and interfere with its proper management. In patients with OSA, cataplexy should be actively looked for to exclude the presence of narcolepsy. Treatment with CPAP does not usually improve EDS in narcoleptics with OSA.

Treatment of narcolepsy

Narcolepsy is a neurological disorder characterized, in its classical form, by excessive daytime sleepiness (EDS) with irresistible episodes of sleep, cataplexy, disrupted nocturnal sleep, hypnagogic/hypnopompic hallucinations and sleep paralysis. It is often under-diagnosed, however, if it is suitably diagnosed, symptoms can be well treated by means of targeted drugs, such as modafinil to treat EDS, sodium oxybate for cataplexy, as well as EDS and disrupted nocturnal sleep, and tricyclic and newer antidepressants for cataplexy. Hallucinations and sleep paralysis can be treated with the same drugs used for cataplexy. Amphetamines and amphetamine-like stimulants are used less nowadays. Behavioral measures are also important and useful. The discovery of hypocretin deficiency in narcoleptic patients opens new perspectives for the development of newer therapeutic approaches for both EDS and cataplexy. Therapy for narcolepsy is chronic, hence symptomatic. However, the correct use of available drugs enables patients to gain a better quality of life, keeping the symptoms under control, which, mainly from a social point of view, are heavily disabling.

Orexinergic modulation of breathing across vigilance states

Basal respiration and respiratory reflex regulations are considerably different during the awake and sleep states. Tidal volume and respiratory frequency diminish during sleep, and hypoxic and hypercapnic ventilatory responses also decline during sleep. Reduced metabolic demand during sleep cannot completely explain these phenomena because PaCO2 increases during sleep. In this review, I will summarize our recent discovery of the possible contribution of orexin, a hypothalamic neuropeptide, to the vigilance state-dependent adjustment of central respiratory regulation. Orexin-deficient mice show an attenuated hypercapnic ventilatory response during the awake but not during the sleep period, whereas basal ventilation remained normal, irrespective of the vigilance state. Orexin supplementation remedied the defect, and the administration of an orexin receptor antagonist to wild-type mice mimicked the abnormality. Orexin-deficient mice also showed frequent sleep apneas and loss of repetitive intermittent hypoxia-induced ventilatory long-term facilitation. Hence, it is possible that the orexin system is one of the essential modulators required for coordinating the circuits controlling respiration and behavior.

The dual role of the orexin/hypocretin system in modulating wakefulness and respiratory drive

PURPOSE OF REVIEW

Today, numerous studies show that orexin peptides act as regulators of many functions including the control of sleep-wake states, breathing, and central chemosensitivity. However, little is known on neuronal mechanisms by which orexin regulates breathing in a state-dependent manner. This review summarizes recent data on the control of neuronal circuits by orexin, with a special emphasis on breathing, central chemosensitivity, and obstructive sleep apneas.

RECENT FINDINGS

Activity of hypothalamic orexinergic neurons is subjected to maturation and is mandatory to maintain long bouts of wakefulness in adults. At wake onset, this activity progressively builds up as a result of synaptic interactions and reinforces the awake state. Orexin deficiency attenuates the hypercapnic reflex only during wakefulness and is correlated with an increase in sleep apneas. Intrinsic sensitivity to CO2/pH of orexin neurons may impact on brainstem chemosensitive neurons, and this effect likely involves TWIK (tandem of P domains in a weak inwardly rectifying K+ channel)-related acid sensitive K+ (TASK)-like potassium currents.

SUMMARY

Orexin signaling is directly involved in the control of upper airway patency in particular during wakefulness, whereas decreasing activity of orexinergic neurons may contribute to upper airway collapse during sleep causing obstructive sleep apnea. Future research should focus on the role of orexin in upper airway control, which may lead to new clinical strategies for treating breathing disorders associated with sleep.

Ventilatory long-term facilitation in mice can be observed during both sleep and wake periods and depends on orexin

Respiratory long-term facilitation (LTF) is a long-lasting (>1 h) augmentation of respiratory motor output that occurs even after cessation of hypoxic stimuli, is serotonin-dependent, and is thought to prevent sleep-disordered breathing such as sleep apnea. Raphe nuclei, which modulate several physiological functions through serotonin, receive dense projections from orexin-containing neurons in the hypothalamus. We examined possible contributions of orexin to ventilatory LTF by measuring respiration in freely moving prepro-orexin knockout mice (ORX-KO) and wild-type (WT) littermates before, during, and after exposure to intermittent hypoxia (IH; 5 x 5 min at 10% O2), sustained hypoxia (SH; 25 min at 10% O2), or sham stimulation. Respiratory data during quiet wakefulness (QW), slow wave sleep (SWS), and rapid-eye-movement sleep were separately calculated. Baseline ventilation before hypoxic stimulation and acute responses during stimulation did not differ between the ORX-KO and WT mice, although ventilation depended on vigilance state. Whereas the WT showed augmented minute ventilation (by 20.0 +/- 4.5% during QW and 26.5 +/- 5.3% during SWS; n = 8) for 2 h following IH, ORX-KO showed no significant increase (by -3.1 +/- 4.6% during QW and 0.3 +/- 5.2% during SWS; n = 8). Both genotypes showed no LTF after SH or sham stimulation. Sleep apnea indexes did not change following IH, even when LTF appeared in the WT mice. We conclude that LTF occurs during both sleep and wake periods, that orexin is necessary for eliciting LTF, and that LTF cannot prevent sleep apnea, at least in mice.

Clinical and neurobiological aspects of narcolepsy

Narcolepsy is characterized by excessive daytime sleepiness (EDS), cataplexy and/or other dissociated manifestations of rapid eye movement (REM) sleep (hypnagogic hallucinations and sleep paralysis). Narcolepsy is currently treated with amphetamine-like central nervous system (CNS) stimulants (for EDS) and antidepressants (for cataplexy). Some other classes of compounds such as modafinil (a non-amphetamine wake-promoting compound for EDS) and gamma-hydroxybutyrate (GHB, a short-acting sedative for EDS/fragmented nighttime sleep and cataplexy) given at night are also employed. The major pathophysiology of human narcolepsy has been recently elucidated based on the discovery of narcolepsy genes in animals. Using forward (i.e., positional cloning in canine narcolepsy) and reverse (i.e., mouse gene knockout) genetics, the genes involved in the pathogenesis of narcolepsy (hypocretin/orexin ligand and its receptor) in animals have been identified. Hypocretins/orexins are novel hypothalamic neuropeptides also involved in various hypothalamic functions such as energy homeostasis and neuroendocrine functions. Mutations in hypocretin-related genes are rare in humans, but hypocretin-ligand deficiency is found in many narcolepsy-cataplexy cases. In this review, the clinical, pathophysiological and pharmacological aspects of narcolepsy are discussed.

Persistent sleepiness in CPAP treated obstructive sleep apnea patients: evaluation and treatment

Nasal continuous positive airway pressure (CPAP) is an effective treatment for most patients with obstructive sleep apnea syndrome (OSAS), improving sleepiness, cognitive function and mood. A number of patients, however, complain about persistent sleepiness after CPAP. In these cases another clinical history should be carried out to confirm the diagnosis of OSAS, to check CPAP compliance and to exclude associated conditions such as poor sleep hygiene, depression, narcolepsy or idiopathic hypersomnia. If necessary, a full polysomnography (PSG) followed by a multiple sleep latency test or even a full PSG with CPAP titration should be performed. Experimental data in animals suggest that long-term intermittent hypoxia related to the apneic events could deteriorate the brain structures that regulate alertness. This impairment, if present in humans, could be another reason for residual sleepiness after CPAP. Modafinil has been shown to reduce subjective sleepiness after CPAP in OSAS patients. Further studies are warranted to clarify the way in which CPAP modifies sleepiness.

Vigilance state-dependent attenuation of hypercapnic chemoreflex and exaggerated sleep apnea in orexin knockout mice

Exogenous administration of orexin can promote wakefulness and respiration. Here we examined whether intrinsic orexin participates in the control of breathing in a vigilance state-dependent manner. Ventilation was recorded together with electroencephalography and electromyography for 6 h during the daytime in prepro-orexin knockout mice (ORX-KO) and wild-type (WT) littermates. Respiratory parameters were separately determined during quiet wakefulness (QW), slow-wave sleep (SWS), or rapid eye movement (REM) sleep. Basal ventilation was normal in ORX-KO, irrespective of vigilance states. The hypercapnic ventilatory response during QW in ORX-KO (0.19 +/- 0.01 ml.min(-1).g(-1).%CO(2)(-1)) was significantly smaller than that in WT mice (0.38 +/- 0.04 ml.min(-1).g(-1).%CO(2)(-1)), whereas the responses during SWS and REM in ORX-KO were comparable to those in WT mice. Hypoxic responses during wake and sleep periods were not different between the genotypes. Spontaneous but not postsigh sleep apneas were more frequent in ORX-KO than in WT littermates during both SWS and REM sleep. Our findings suggest that orexin plays a crucial role both in CO(2) sensitivity during wakefulness and in preserving ventilation stability during sleep.

Respiratory and cardiovascular actions of orexin-A in mice

Ample evidence has been reported to show a probable contribution of orexin in the central cardiovascular regulation. Although cardiovascular and respiratory centers in the brain are located close to each other and are interconnected, the possible participation of orexin in respiratory regulation has not been fully documented. Here we examined the effects of intracerebroventricular administration of orexin-A on respiratory and cardiovascular parameters in urethane-anesthetized mice. Respiratory frequency and tidal volume were recorded simultaneously with blood pressure and heart rate. Orexin-A (0.003-3 nmol in 2 microL) or vehicle was administered into the lateral ventricle or cisterna magna. Lateral ventricular administration induced a rise in respiratory frequency (by 11% at the highest dose), tidal volume (76%), blood pressure (13%) and heart rate (6%) in a dose-dependent manner. With intracisternal administration, however, respiratory frequency did not change while a similar increase in tidal volume (75%) was observed. A relatively larger cardiovascular response was elicited with intracisternal administration (blood pressure 26%, heart rate 9%). On the other hand, with either administration route, orexin-A did not affect reflex increases in respiratory frequency and tidal volume in response to hypoxia and hypercapnia. These results show possible participation of orexin-A not only in the cardiovascular regulation but also in the respiratory control system. Moreover, orexin can affect the cardiorespiratory control system at multiple sites in different ways. Orexin-A seems not to be involved in respiratory reflex regulation in mice at least under anesthetized condition.

Fatigue in the executive cortical network demonstrated in narcoleptics using functional magnetic resonance imaging—a preliminary study

BACKGROUND AND PURPOSE

To demonstrate dynamic changes in cerebral functional activation during a working memory task in a state of severe excessive daytime sleepiness.

PATIENTS AND METHODS

Omitting the usual morning dose of stimulants in three narcoleptics induced sleepiness. Functional magnetic resonance imaging (fMRI) was used to map cerebral activation during the performance of a 2-back verbal working memory task. Repeated 9.5 min scans were performed, until the subjects felt they could not continue. This was the functional imaging equivalent of the maintenance of wakefulness test.

RESULTS

Bilateral and widespread activation in known nodes of the executive network were seen during the first scan in all subjects, including the lateral prefrontal, posterior parietal and anterior cingulate cortex. There was a reduction in cerebral activation, especially but not exclusively in the prefrontal cortex, associated with slowing of performance from the first to the last tolerated scan. On stimulants, subjective alertness, activation and objective performance were readily maintained.

CONCLUSION

This preliminary study suggests that fatigue in the executive cortical network may be demonstrated by a progressive reduction in regional cerebral activation across scans, which may be prevented by stimulant use. Averaging multiple scan runs, a typical practice in fMRI, could blur important dynamic components of activation.

Narcolepsy and Disorders of Excessive Somnolence

Recent studies provide valid criteria that help differentiate idiopathic narcolepsy from other disorders of excessive daytime somnolence [3]. Research to date suggests that idiopathic narcolepsy might properly be considered a disorder of excessive sleepiness with dysfunctional REM-sleep mechanisms, clinically evidenced as cataplexy and electrophysiologically recognized as SOREMPs. Given these criteria, a diagnosis can generally be made using a combination of history, PSG, and MSLT. Traditionally, the medical treatment of idiopathic narcolepsy has centered on a two-drug regimen (stimulants for sleepiness and TCAs for cataplexy and auxiliary symptoms). Some newer medications are proving efficacious for sleepiness with minimal adverse effects, whereas others may provide a single-drug regimen that simultaneously addresses sleepiness and cataplexy [18]. New research has allowed some experts to hypothesize that idiopathic narcolepsy may be the result of a genetic predisposition to autoimmune disease [176]. It is possible that aberrant genetic coding of elements in the hypocretin/orexin systems allows a sensitivity to inducible and possibly virally mediated changes, which leave cells in the lateral hypothalamus susceptible to autoimmune attack [96]. As such, genetic screening of high-risk individuals might eventually rationalize the prophylactic use of immunosuppressants in some instances. In the future, for atypical cases(poorly responsive to therapy), genetic, CSF, and brain imaging studies, and possibly even neuronal transplantation may prove beneficial in the assessment and treatment of idiopathic narcolepsy.

Narcolepsy in the older adult: epidemiology, diagnosis and management

Narcolepsy is a disorder of impaired expression of wakefulness and rapid-eye-movement (REM) sleep. This manifests as excessive daytime sleepiness and expression of individual physiological correlates of REM sleep that include cataplexy and sleep paralysis (REM sleep atonia intruding into wakefulness), impaired maintenance of REM sleep atonia (e.g. REM sleep behaviour disorder [RBD]), and dream imagery intruding into wakefulness (e.g. hypnagogic and hypnopompic hallucinations). Excessive sleepiness typically begins in the second or third decade followed by expression of auxiliary symptoms. Only cataplexy exhibits a high specificity for diagnosis of narcolepsy. While the natural history is poorly defined, narcolepsy appears to be lifelong but not progressive. Mild disease severity, misdiagnoses or long delays in cataplexy expression often cause long intervals between symptom onset, presentation and diagnosis. Only 15-30% of narcoleptic individuals are ever diagnosed or treated, and nearly half first present for diagnosis after the age of 40 years. Attention to periodic leg movements (PLM), sleep apnoea and RBD is particularly important in the management of the older narcoleptic patient, in whom these conditions are more likely to occur. Diagnosis requires nocturnal polysomnography (NPSG) followed by multiple sleep latency testing (MSLT). The NPSG of a narcoleptic patient may be totally normal, or demonstrate the patient has a short nocturnal REM sleep latency, exhibits unexplained arousals or PLM. The MSLT diagnostic criteria for narcolepsy include short sleep latencies (<8 minutes) and at least two naps with sleep-onset REM sleep. Treatment includes counselling as to the chronic nature of narcolepsy, the potential for developing further symptoms reflective of REM sleep dyscontrol, and the hazards associated with driving and operating machinery. Elderly narcoleptic patients, despite age-related decrements in sleep quality, are generally less sleepy and less likely to evidence REM sleep dyscontrol. Nonpharmacological management also includes maintenance of a strict wake-sleep schedule, good sleep hygiene, the benefits of afternoon naps and a programme of regular exercise. Thereafter, treatment is highly individualised, depending on the severity of daytime sleepiness, cataplexy and sleep disruption. Wake-promoting agents include the traditional psychostimulants. More recently, treatment with the 'activating' antidepressants and the novel wake-promoting agent modafinil has been advocated. Cataplexy is especially responsive to antidepressants which enhance synaptic levels of noradrenaline (norepinephrine) and/or serotonin. Obstructive sleep apnoea and PLMs are more common in narcolepsy and should be suspected when previously well controlled older narcolepsy patients exhibit a worsening of symptoms. The discovery that narcolepsy/cataplexy results from the absence of neuroexcitatory properties of the hypothalamic hypocretin-peptidergic system will significantly advance understanding and treatment of the symptom complex in the future.

Migraine and idiopathic narcolepsy--a case-control study

In order to evaluate a possible association between migraine and idiopathic narcolepsy, we performed a multicentre case-control study on the comorbidity of narcolepsy and different headaches. In total, 96 patients with idiopathic narcolepsy were enrolled. The migraine frequency in the patients and in the control group was 21.9% and 19.8%, respectively (P = 0.722). The migraine features did not differ significantly between both groups. However, headache fulfilling the criteria for tension-type headache was significantly more often reported by narcolepsy patients than by the control group (60.3% vs. 40.7%, P= 0.006). We conclude that there is no association between migraine and narcolepsy but that patients with narcolepsy show more unspecific headache, probably due to sleep disturbances.