Narcolepsy

Narcolepsy is a chronic neurological disorder of rapid eye movement sleep regulation. It is characterised by excessive daytime sleepiness and, in a substantial proportion of patients, cataplexy: sudden, transient loss of muscle tone triggered by strong emotion, particularly laughter, with consciousness preserved.

It is classified into type one and type two. Type one is with cataplexy and, in the great majority, deficiency of the neuropeptide hypocretin, also termed orexin. Type two is without cataplexy, and generally with preserved hypocretin levels.

Type one is a relatively well characterised autoimmune process targeting the hypothalamic neurons that produce hypocretin.

Accurate diagnosis matters for targeted pharmacological treatment and for lifestyle and safety advice. Excessive daytime sleepiness significantly affects daily functioning and safety, particularly driving.

Pathophysiology

• Type one narcolepsy is selective, immune mediated destruction of the hypothalamic neurons producing hypocretin (orexin). Hypocretin has a central role in stabilising wakefulness and regulating the transition between sleep and wake states.
• Hypocretin deficiency produces both the excessive daytime sleepiness and cataplexy. Cataplexy is believed to reflect intrusion of rapid eye movement sleep related muscle atonia into wakefulness, triggered by strong emotional stimuli.
• A strong genetic association with the HLA-DQB1*06:02 allele supports an autoimmune mechanism. Epidemiological evidence links onset in some cases to preceding infections. It also links onset to a specific influenza vaccine used during the 2009 influenza pandemic in some regions, which further supports the immune mediated hypothesis. The complete immunopathological mechanism remains under research.
• Type two narcolepsy, without cataplexy and generally with preserved hypocretin levels, likely has a more heterogeneous pathophysiology. It overlaps clinically with type one but is less well characterised.

• Excessive daytime sleepiness is the core, defining and universally present symptom in both subtypes. It typically develops in adolescence or young adulthood. It comprises irresistible sleep attacks, even in inappropriate or stimulating circumstances, with a tendency towards refreshing, though often brief, naps.
• Cataplexy, present in type one narcolepsy, is sudden, bilateral loss of muscle tone triggered by strong emotion (classically laughter, but also anger, surprise or excitement). Severity ranges from subtle, brief facial or neck muscle weakness to complete collapse. Consciousness is characteristically preserved throughout, which distinguishes it from a seizure or syncope.
• Hypnagogic and hypnopompic hallucinations are vivid, sometimes frightening perceptual experiences at the transition into or out of sleep respectively. They are characteristic but not universal.
• Sleep paralysis is a transient inability to move or speak at the sleep wake transition, with preserved consciousness and awareness of the surroundings. Together with the hallucinations, it reflects abnormal intrusion of rapid eye movement sleep into wakefulness.
• Disrupted nocturnal sleep, with frequent waking despite significant daytime sleepiness, is a further, sometimes underappreciated feature.
• Impact on daily functioning is common and warrants sensitive clinical attention. It includes safety (particularly driving, given the risk of sudden sleep onset) and psychosocial wellbeing. Before diagnosis, patients risk social embarrassment and misattribution of symptoms to laziness or a psychiatric condition.

• Overnight polysomnography followed by a multiple sleep latency test is the standard, essential investigation. The test is a standardised series of daytime nap opportunities. It assesses the speed of sleep onset and, critically, whether rapid eye movement sleep occurs abnormally early within the brief naps (sleep onset rapid eye movement periods). A short mean sleep latency with two or more sleep onset rapid eye movement periods supports the diagnosis.
• Cerebrospinal fluid hypocretin measurement, where available, directly and specifically confirms type one narcolepsy when significantly reduced. It is particularly valuable in diagnostic uncertainty or atypical presentation. It is not universally available, and is not required where the clinical presentation and multiple sleep latency test are already sufficiently characteristic.
• HLA-DQB1*06:02 testing may support the diagnosis, given its strong association with type one narcolepsy. The allele is also present in a meaningful proportion of the general population without narcolepsy, so its presence is supportive, not diagnostic. Its absence makes type one narcolepsy less likely but does not entirely exclude it.
• Exclude alternative causes of excessive daytime sleepiness before confirming narcolepsy: obstructive sleep apnoea, inadequate sleep duration or poor sleep hygiene, depression, and certain medications.
• Obstructive sleep apnoea may coexist with narcolepsy. It contributes independently to daytime sleepiness, so assess for it specifically and treat it where present.

• Modafinil or armodafinil are first line pharmacological treatment for the excessive daytime sleepiness of narcolepsy. These wake promoting agents are better tolerated than traditional stimulants, and are typically given once daily in the morning.
• Sodium oxybate is given as a divided nocturnal dose, at bedtime and again several hours later because of its short half life. It is highly effective for excessive daytime sleepiness and cataplexy simultaneously, and improves the disrupted nocturnal sleep.
• Sodium oxybate needs careful patient selection and monitoring. It is structurally related to gamma-hydroxybutyrate and has misuse potential, so most jurisdictions require controlled prescribing and dispensing arrangements.
• Traditional stimulants (methylphenidate or amphetamine based preparations) are an alternative for excessive daytime sleepiness, particularly where modafinil or armodafinil give inadequate benefit. Their tolerability and safety profile is generally less favourable than that of the more selective wake promoting agents.
• Cataplexy not adequately controlled by sodium oxybate may be targeted with venlafaxine or another antidepressant with anticataplectic effect. These agents suppress rapid eye movement sleep, the process from which cataplexy is believed to arise. This is an important option for a sometimes highly distressing and functionally impairing symptom.
• Structured, scheduled daytime napping, where feasible within the patient's routine, gives meaningful adjunctive benefit alongside medication, because naps are characteristically refreshing, if brief.
• Educate patients about the condition. Give practical safety counselling on driving, following local licensing authority guidance, given the risk of sudden sleep onset, and on other hazardous activities.
• Provide psychosocial support for the impact on education, employment and social functioning. This is particularly important because diagnostic delay is frequent and substantial, leaving a period of unexplained, often significantly impairing symptoms before diagnosis and treatment.
• Management is delivered through specialist sleep medicine services.