The neurobiology of narcolepsy-cataplexy syndrome

The pathophysiology of narcolepsy is closely related to the abnormalities of REM sleep that are the electrophysiologic signature of the syndrome. Evidence from studies of canine narcolepsy and postmortem human narcoleptic brain tissue provide strong evidence that cholinergic and monoaminergic systems involved in REM sleep regulation are abnormal in narcolepsy but the primary neurochemical abnormality has not yet been determined. There is now conclusive evidence that a genetic basis is required for all or almost all cases of narcolepsy. In the vast majority of narcoleptics, a gene closely linked to the HLA-DR/DQ region appears to confer narcoleptic susceptibility, but the penetrance of the gene is low and additional environmental and perhaps genetic factors are required to express the disease. In a minority of narcoleptics, there may be a second autosomal dominant gene not linked to HLA-DR2 that facilitates the occurrence of narcolepsy. This gene may be related to the mu-immunoglobulin heavy-chain switch-like segment that has been implicated in canine narcolepsy. There appear to be at least two narcoleptic phenotypes associated with the narcoleptic susceptibility gene or genes: narcolepsy-cataplexy syndrome and monosymptomatic narcolepsy, or narcolepsy with REM sleep abnormalities but without cataplexy. Idiopathic hypersomnia without cataplexy or REM sleep abnormalities may represent a third phenotype, although most cases of idiopathic hypersomnia are probably unrelated to the HLA-D linked gene. The link between the genetic basis of narcolepsy and its neurochemical abnormalities is still entirely unknown. Although the hypothesis that a transient immune-mediated reaction leads to a permanent alteration of monoaminergic function is appealling, there is no direct evidence to support this hypothesis. Several important questions concerning the neurobiology of narcolepsy remain to be answered. What is the specific gene in the HLA-D region that is linked to human narcolepsy and what are the products or functions of the gene that predispose to narcolepsy? Does the human mu-switch region contain genetic material homologous to the 85-kb band linked to canarc-1 that predisposes to narcolepsy? What are the environmental factors required for expression of the disease in susceptible individuals and do they incite immunologic processes? Which of the neurochemical abnormalities are primary, which are secondary or compensatory, and how do they relate to the predisposing genetic and environmental elements? Additional familial, genetic, and neurochemical studies over the next decade should lead to more complete understanding of the neurobiology of narcolepsy and ultimately to better treatments for this chronic disabling disease.

Hypersomnolence and narcolepsy; a pragmatic diagnostic neurophysiological approach

Out of a group of 250 consecutive patients who were examined for various disorders of sleep and waking at Ghent University Hospital within a period of 24 months, 30 patients with hypersomnolence associated with a suspected underlying neurological etiology were selected. The population consisted of 15 males and 15 females with mean age of 36 years (range: 16-60 years). Twenty-one patients had had hypersomnolence for more than 2 years. All patients underwent a single night polysomnography (PSG) and a 4-nap multiple sleep latency test (MSLT). PSG was normal in 23 patients. Sleep onset REM period (SOREMP) was defined as the occurrence of REM sleep within 15 min. after initiation of sleep. PSG demonstrated SOREMP's in only 1 patient and showed evidence of obstructive sleep apnea in 4 patients. Two patients had a low sleep efficiency. MSLT demonstrated hypersomnolence in 17 patients of whom 6 showed SOREMP. Significant hypersomnolence was defined as a mean sleep latency < or = 5 min. 4 patients fulfilled the classical clinical and polygraphic criteria (> or = 2 SOREMP) of narcolepsy. In 8 patients the tentative diagnosis of idiopathic CNS hypersomnolence was made. 13 patients did not sleep during MSLT. These results emphasize the relative importance of MSLT. Our limited 4-nap MSLT protocol proved useful in distinguishing narcolepsy from idiopathic CNS hypersomnolence.

Practice parameter for the use of stimulant medications in the treatment of children, adolescents, and adults

This practice parameter describes treatment with stimulant medication. It uses an evidence-based medicine approach derived from a detailed literature review and expert consultation. Stimulant medications in clinical use include methylphenidate, dextroamphetamine, mixed-salts amphetamine, and pemoline. It carries FDA indications for treatment of attention-deficit/hyperactivity disorder and narcolepsy.

A randomized, double blind, placebo-controlled multicenter trial comparing the effects of three doses of orally administered sodium oxybate with placebo for the treatment of narcolepsy

STUDY OBJECTIVES

To evaluate and compare the efficacy and safety of three doses of sodium oxybate and placebo for the treatment of narcolepsy symptoms.

DESIGN

A multicenter, double blind, placebo-controlled trial.

SETTING

N/A.

PARTICIPANTS

Study subjects were 136 narcolepsy patients with 3 to 249 (median 21) cataplexy attacks weekly.

INTERVENTIONS

Prior to baseline measures, subjects discontinued anticataplectic medications. Stable doses of stimulants were permitted. Subjects were randomized in blinded fashion to receive 3, 6, or 9 g doses of sodium oxybate or placebo taken in equally divided doses upon retiring to bed and 2.5-4 hours later for 4 weeks.

MEASUREMENTS AND RESULTS

Disease symptoms and adverse events were recorded in daily diaries. The primary measure of efficacy was the change from baseline in weekly cataplexy attacks. Secondary measures included daytime sleepiness using the Epworth Sleepiness Scale (ESS), inadvertent daytime naps/sleep attacks and nighttime awakenings. Investigators assessed changes in disease severity using Clinical Global Impression of Change (CGI-c). Compared to placebo, weekly cataplexy attacks were decreased by sodium oxybate at the 6 g dose (p=0.0529) and significantly at the 9 g dose (p=0.0008). The ESS was reduced at all doses, becoming significant at the 9 g dose (p=0.0001). The CGI-c demonstrated a dose-related improvement, significant at the 9 g dose (p=0.0002). The frequency of inadvertent naps/sleep attacks and the nighttime awakenings showed similar dose-response trends, becoming significant at the 9 g dose (p=0.0122 and p=0.0035, respectively). Sodium oxybate was generally well-tolerated at all three doses. Nausea, headache, dizziness and enuresis were the most commonly reported adverse events.

CONCLUSIONS

Sodium oxybate significantly improved symptoms in patients with narcolepsy and was well tolerated.

Gamma-Hydroxybutyrate (orphan medical)

Orphan Medical is developing gamma-hydroxybutyrate (Xyrem) for the potential treatment of narcolepsy [183352]. In October 2000, an NDA was filed with the FDA [384422], [405504] and Xyrem received an FDA approvable letter in July 2001. Orphan Medical stated that it believed it could meet the requirements in the letter, including a trial in respiratory-compromised patients, by the end of 2001 [414461]. The FDA also requested follow-up safety data from patients in previous Xyrem trials. At that time, the drug was not expected to be launched until mid-2002 [415301], [416305]. In October 1999, the US House of Representatives passed the HR 2130 bill, allowing the medical use of gamma-hydroxybutyrate, which is classified as a Schedule I controlled substance in the US [343562]; the Senate approved this legislation in November 1999 [348206]. In February 2000, a congressional bill supporting the continued development of medically formulated gamma-hydroxybutyrate was passed, making medically formulated gamma-hydroxybutyrate products Schedule III substances [354108], [356597]. GHB occurs naturally in many human tissues. It has previously been used in the treatment of narcolepsy and is not patentable for that indication.

[Hypocretin (orexin) deficiency in narcolepsy-cataplexy]

A mutation in the HCRT locus was proved in 18-yrs old male suffering from narcolepsy-cataplexy. He has demonstrated cataplectic attacks (brief spells of head dropping provoked by laughter) as well as imperative sleep in spells of several minutes up to one hour since the age of six months. He has suffered from severe bulimia since five years; later hypnagogic hallucinations, sleep paralysis and unquiet nocturnal sleep accompanied by periodic limb movements appeared. Symptoms are partially controlled with methylphenidate and either imipramine, clomipramine or fluoxetine. Periodic leg movements poorly responded to L-DOPA and clonazepam treatment. He is HLA-DQB1*0602 negative. Repeated MSLT (over 16 years followed-up period) showed extremely short latency with predominant SOREMPs and also nocturnal PSG recordings revealed fragmented sleep with SOREMPs. This case report demonstrates that hypocretin (orexin) mutations in human can produce the full narcolepsy phenotype and validates data recently reported in dog and mouse models suggesting a role for hypocretin (orexin) in the pathophysiology of narcolepsy and the regulation of REM sleep.

Systemic administration of hypocretin-1 reduces cataplexy and normalizes sleep and waking durations in narcoleptic dogs

Recent work has implicated the hypocretin (orexin) system in the genesis of narcolepsy. In the current study we demonstrate that systemically administered hypocretin-1 (Hcrt-1) produces an increase in activity level, longer waking periods, a decrease in REM sleep without change in nonREM sleep, reduced sleep fragmentation and a dose dependent reduction in cataplexy in canine narcoleptics. Repeated administration of single daily doses of Hcrt-1 led to consolidation of waking and sleep periods and to a complete loss of cataplexy for periods of three or more days after treatment in animals that were never asymptomatic under control conditions. Systemic administration of Hcrt-1 may be an effective treatment for narcolepsy.

[Unwanted napping]

Three patients, one woman aged 43, and two men aged 32 and 51, suffered from excessive daytime sleepiness with different causes. The woman experienced nocturnal motor attacks of epileptic origin, responding to carbamazepine. The diagnosis was based on polysomnographic recordings combined with video monitoring. Narcolepsy was diagnosed in the 32-year-old man. He also suffered from cataplexy. The diagnosis was supported by a multiple sleep latency test and HLA-DR2 positivity. He was treated with clomipramine. In the 43-year-old man an obstructive sleep apnoea syndrome was diagnosed by polysomnographic recording. He was treated successfully with continuous positive airway pressure. These patients show that excessive daytime sleepiness is sometimes difficult to recognise and can be a potentially incapacitating symptom of a treatable underlying disorder. Diagnosis is made by careful history taking and through the use of different types of somnographic recordings.

[Narcolepsy in children]

INTRODUCTION

Narcolepsy is a neurological disorder characterized by excessive somnolence during the daytime, with recurrent, irresistible episodes of sleepiness. The complete forms are associated with cataplexy, hypnagogic hallucinations and sleep paralysis. The incidence reported in adults is 4 10/10,000. A considerable proportion of adults consider their disorder to have started before the age of 15 years. It is essential to have neurophysiological confirmation of the short period of the onset of REM sleep for diagnosis.

CLINICAL CASE

An 11 year old boy with diurnal hypersomnolence, behavior disorder and weight gain. He was evaluated in the Sleep Disorder Unit by polysomnography and the multiple latency test, which confirmed the suspicion of narcolepsy.

CONCLUSIONS

Narcolepsy is a disorder which starts during childhood and usually goes unnoticed or is erroneously diagnosed and treated. At the present time there are valid criteria for the identification and diagnosis of children with this disorder. Treatment of these patients should be orientated towards adaptation of the environment to the child and prevention of the psychosocial problems which may be caused by this disorder, in view of the poor response to the drugs used.

New insights into the pathogenesis and treatment of narcolepsy

Narcolepsy is a complex neurologic disorder, which has significant negative impacts on the lives of those who have it. Although the disorder is treatable, traditional methods do not alleviate symptoms completely and often produce unwanted side effects. Fortunately, recent advances in the understanding of narcolepsy offer the promise of improved treatments in the foreseeable future.

[Narcolepsy in a prepubertal boy]

We reported a 10-year-old boy with narcolepsy, the onset of which was at the age of 8 years and 6 months. The initial symptom was excessive daytime sleepiness, followed by cataplexy and disrupted nocturnal sleep. There was neither hallucination nor sleep paralysis. A daytime polysomnogram showed a sleep-onset rapid eye movement period (SOREMP), and human leukocyte antigen (HLA) analysis revealed HLA-DR2/DQB1 * 0602. Treatment with methylphenidate and clomipramine was effective; methylphenidate (30 mg/day) improved his wakefulness and alertness throughout the day, and clomipramine (20 mg/day) reduced the number of cataplexic episodes. Because of their abnormal behavior, prepubertal narcoleptic children may often be misdiagnosed as having epilepsy or an attention deficit hyperactivity disorder. Therefore, they need early diagnosis and treatment. Assistance should be provided to protect them from being labeled as lazy by their parents and school teachers.

A commentary on the neurobiology of the hypocretin/orexin system

Hypocretins/orexins are rapidly emerging as functionally important neurotransmitters. Two related neuropeptides (Hcrt-1/OXA, Hcrt-2/OXB) encoded by the same precursor gene and two G-protein coupled receptors (Hcrtr1/OXR1, Hcrtr2/OXR2) are currently known. Hypocretin-containing cells are discretely localized within the perifornical hypothalamus but have widespread projections, with generally excitatory postsynaptic effects. Dense excitatory projections to all monoaminergic cell groups have been reported. A major emerging function for this system is likely to be the regulation of sleep. Alterations in hypocretin neurotransmission causes the sleep disorder narcolepsy in mice, dogs and humans. Effects on appetite, neuroendocrine and energy metabolism regulation are also suggested by other studies. Hypocretins are uniquely positioned to link sleep, appetite and neuroendocrine control, three behaviors of major importance in psychiatry. The potential role of this system in regulating the sleep cycle, modulating wakefulness at selected circadian times and in mediating the deleterious effects of sleep deprivation is discussed.

Increased dopaminergic transmission mediates the wake-promoting effects of CNS stimulants

Amphetamine-like stimulants are commonly used to treat sleepiness in narcolepsy. These compounds have little effect on rapid eye movement (REM) sleep-related symptoms such as cataplexy, and antidepressants (monoamine uptake inhibitors) are usually required to treat these symptoms. Although amphetamine-like stimulants and antidepressants enhance monoaminergic transmission, these compounds are non-selective for each monoamine, and the exact mechanisms mediating how these compounds induce wakefulness and modulate REM sleep are not known. In order to evaluate the relative importance of dopaminergic and noradrenergic transmission in the mediation of these effects, five dopamine (DA) uptake inhibitors (mazindol, GBR-12909, bupropion, nomifensine and amineptine), two norepinephrine (NE) uptake inhibitors (nisoxetine and desipramine), d-amphetamine, and modafinil, a non-amphetamine stimulant, were tested in control and narcoleptic canines. All stimulants and dopaminergic uptake inhibitors were found to dose-dependently increase wakefulness in control and narcoleptic animals. The in vivo potencies of DA uptake inhibitors and modafinil on wake significantly correlated with their in vitro affinities to the DA and not the NE transporter. DA uptake inhibitors also moderately reduced REM sleep, but this effect was most likely secondary to slow wave sleep (SWS) suppression, since selective DA uptake inhibitors reduced both REM sleep and SWS proportionally. In contrast, selective NE uptake inhibitors had little effect on wakefulness, but potently reduced REM sleep. These results suggest that presynaptic activation of DA transmission is critical for the pharmacological control of wakefulness, while that of the NE system is critical for REM sleep regulation. Our results also suggest that presynaptic activation of DA transmission is a key pharmacological property mediating the wake-promoting effects of currently available CNS stimulants.

Comparison in symptoms between aged and younger patients with narcolepsy

We investigated the age-related changes in symptoms in narcolepsy. Fifty patients, 65-year-old and over (aged group), were recruited from the National Narcolepsy Registry. Thirty-four patients, younger than 65 (younger group), were selected by random sampling. Although there was no difference in the age of disease onset between the two groups, the age of diagnosis was significantly earlier for the younger group. Methylphenidate was used significantly more in the aged group, and modafinil in the younger group. The aged group had lower total scores on the Ullanlinna Narcolepsy Scale, because the scores for cataplexy were significantly less for the aged group. There was no significant difference in excessive daytime sleepiness between the two groups.

Pharmacokinetics of imipramine in narcoleptic horses

OBJECTIVE

To validate use of high-performance liquid chromatography (HPLC) in determining imipramine concentrations in equine serum and to determine pharmacokinetics of imipramine in narcoleptic horses.

ANIMALS

5 horses with adult-onset narcolepsy.

PROCEDURE

Blood samples were collected before (time 0) and 3, 5, 10, 15, 20, 30, and 45 minutes and 1, 2, 3, 4, 6, 8, 12, and 24 hours after IV administration of imipramine hydrochloride (2 or 4 mg/kg of body weight). Serum was analyzed, using HPLC, to determine imipramine concentration. The serum concentration-versus-time curve for each horse was analyzed separately to estimate pharmacokinetic values.

RESULTS

Adverse effects (muscle fasciculations, tachycardia, hyperresponsiveness to sound, and hemolysis) were detected in most horses when serum imipramine concentrations were high, and these effects were most severe in horses receiving 4 mg of imipramine/kg. Residual adverse effects were not apparent. Value (mean +/- SD) for area under the curve was 3.9 +/- 0.7 h X microg/ml, whereas volume of distribution was 584 +/- 161.7 ml/kg, total body clearance was 522 +/- 102 ml/kg/h, and mean residence time was 1.8 +/- 0.6 hours. One horse had signs of narcolepsy 6 and 12 hours after imipramine administration; corrresponding serum imipramine concentrations were less than the therapeutic range.

CONCLUSIONS AND CLINICAL RELEVANCE

Potentially serious adverse effects may be seen in horses administered doses of imipramine that exceed a dosage of 2 mg/kg. Total body clearance of imipramine in horses is slower than that in humans; thus, the interval between subsequent doses should be longer in horses.

Health-related quality of life in narcolepsy

Narcolepsy is a chronic sleep disorder characterised by symptoms of excessive daytime sleepiness and cataplexy. The aim of this study was to describe the health-related quality of life of people with narcolepsy residing in the UK. The study comprised a postal survey of 500 members of the UK narcolepsy patient association, which included amongst other questions the UK Short Form 36 (SF-36), the Beck Depression Inventory (BDI), and the Ullanlinna Narcolepsy Scale (UNS). A total of 305 questionnaires were included in the final analysis. The results showed that the subjects had significantly lower median scores on all eight domains of the SF-36 than normative data, and scored particularly poorly for the domains of role physical, energy/vitality, and social functioning. The BDI indicated that 56.9% of subjects had some degree of depression. In addition, many individuals described limitations on their education, home, work and social life caused by their symptoms. There was little difference between the groups receiving different types of medication. This study is the largest of its type in the UK, although the limitations of using a sample from a patient association have been recognised. The results are consistent with studies of narcolepsy in other countries in demonstrating the extensive impact of this disorder on health-related quality of life.

Dopaminergic role in stimulant-induced wakefulness

The role of dopamine in sleep regulation and in mediating the effects of wake-promoting therapeutics is controversial. In this study, polygraphic recordings and caudate microdialysate dopamine measurements in narcoleptic dogs revealed that the wake-promoting antinarcoleptic compounds modafinil and amphetamine increase extracellular dopamine in a hypocretin receptor 2-independent manner. In mice, deletion of the dopamine transporter (DAT) gene reduced non-rapid eye movement sleep time and increased wakefulness consolidation independently from locomotor effects. DAT knock-out mice were also unresponsive to the normally robust wake-promoting action of modafinil, methamphetamine, and the selective DAT blocker GBR12909 but were hypersensitive to the wake-promoting effects of caffeine. Thus, dopamine transporters play an important role in sleep regulation and are necessary for the specific wake-promoting action of amphetamines and modafinil.

Methylphenidate and narcolepsy: new indication. When modafinil fails

(1) Narcolepsy (daytime bouts of drowsiness) is sometimes associated with cataplexy, and can be incapacitating. The best-assessed treatment is modafinil, which has no demonstrated efficacy on cataplexy. (2) After many years of off-license use in the treatment of narcolepsy, methylphenidate is now licensed for this indication in France. (3) According to the results of our literature search, which includes a clinical expert report from Novartis Pharma, the file on methylphenidate in narcolepsy is mainly based on "clinical experience": only three case series, totalling fewer than 200 patients, have been published. (4) These non comparative studies suggest an effect of high dose methylphenidate on daytime drowsiness and cataplexy. (5) The adverse effects of methylphenidate are those of amphetamine psychostimulants, i.e. mainly neuropsychological disorders, cardiovascular effects, loss of appetite and a limited risk of excessive use by some patients.

[Treatment of the neurological sleep disorders restless legs syndrome and narcolepsy]

Pharmacological therapies are presented for two typical neurological sleep-wake disorders, restless legs syndrome (RLS) and narcolepsy. The individual discomfort caused by RLS and the accompanying problems with initiating and maintaining sleep often require a therapy with L-dopa and dopamine agonists. Positive treatment effects on sensory and motor symptoms have been shown in open trials and controlled studies. Development of time shift and/or augmentation of symptoms is a problem of L-dopa therapy. Further efficient drugs are opioids and benzodiazepines. The therapy of narcolepsy depends on its severity and the pattern of the symptoms. Excessive daytime sleepiness and sleep attacks are the most impairing symptoms, which are difficult to treat. These symptoms require an optimal combination of CNS stimulants with regular napping. Cataplexy and other REM sleep associated symptoms are effectively treated with REM sleep suppressing antidepressants.

Update on nonapnea sleep disorders

Much of the attention in the field of sleep disorders focuses on the obstructive sleep apnea syndrome. However, for the pulmonary physician interested in a wider breadth of sleep knowledge, important and interesting developments have been witnessed in the area of nonapneic sleep disorders. Exciting new breakthroughs have illuminated the pathogenesis of narcolepsy and are expected to lead to new treatment options for narcoleptic patients. For the surprisingly common but poorly understood restless legs syndrome (RLS), an expanding armamentarium of medications aids the knowledgeable physician in caring for his or her patients. The physiology of circadian rhythms is better understood due to basic scientific advances. Finally, new drugs used to treat insomnia offer more flexible treatment options and an old and previously vilified drug may allow sleep specialists to better understand the mechanisms of sleep.