What Is Hypersomnia?
The word “hypersomnia” is derived from “hyper” (too much) and “somnia” (sleep), i.e., it refers to the condition of sleeping excessive amounts. However, in current medical terminology, both “hypersomnia” and the related word “hypersomnolence” are used more broadly to indicate long sleep durations, excessive daytime sleepiness, or both. In some classification systems (such as the ICSD-3, which the HF follows), “hypersomnia” is reserved to refer to specific disease entities, such as idiopathic hypersomnia and the related sleep disorders, while “hypersomnolence” refers to the symptoms of long sleep and/or excessive daytime sleepiness regardless of cause.
Hypersomnolence is a complex trait. Formal study of hypersomnolence is in its infancy compared to many other medical symptoms and disorders, yet the socioeconomic burden of hypersomnolence is enormous. Based on current knowledge, different hypersomnia disorders can be classified based on clinical symptoms, diagnostic testing, and, in some cases, underlying biological cause.
One commonly used classification system for hypersomnia disorders is the International Classification of Sleep Disorders (ICSD). The 3rd edition of the ICSD defines the broad category of “central disorders of hypersomnolence,” (CDH) meant to encompass disorders of excessive daytime sleepiness related to the central nervous system, i.e., the brain. These disorders share in common the predominant symptom of daytime sleepiness. Furthermore, to meet criteria for one of these diagnoses, patients cannot have sleepiness caused solely by problems with night sleep (such as sleep apnea, a sleep-related breathing disorder) or with the circadian rhythm.
The ICSD-3 lists 8 different central disorders of hypersomnolence, including: narcolepsy type 1 (with cataplexy), narcolepsy type 2, idiopathic hypersomnia, Kleine-Levin syndrome, hypersomnia due to a medical condition, hypersomnia due to a medication or substance, hypersomnia associated with a psychiatric condition, and insufficient sleep syndrome.
Classifying Hypersomnia Disorders as Primary or Secondary
Although the ICSD-3 does not explicitly divide these eight disorders into two categories, these disorders can be divided into those that are “secondary”, i.e., caused by or intimately related to a different condition, and those that are “primary”, i.e., occurring on their own, not caused by another condition.
The secondary forms of central disorders of hypersomnolence include: 1) hypersomnia due to a medical condition (e.g., caused by a head injury, a neurodegenerative disease such as Parkinson’s disease, or a neuromuscular disorder such as myotonic dystrophy); 2) hypersomnia due to a medication or substance (e.g., sleepiness caused by a prescription or non-prescription medication or drug); 3) insufficient sleep syndrome (i.e., not sleeping enough hours per night on a regular basis – which for adults should be 7-9 hours, with individual variation in duration of sleep needed within that range); and 4) hypersomnia associated with a psychiatric disorder. Unlike the first three of these syndromes, in which sleepiness is believed to be the direct cause of the other disorder, medication, or short habitual sleep times, hypersomnolence occurring in patients with psychiatric disorders is not necessarily caused by the psychiatric disorder. For example, a patient who is diagnosed with hypersomnia associated with depression must have both hypersomnia and depression, and the two must be related, but it is not presently known whether the depression causes the hypersomnia, the hypersomnia causes the depression, or the two are related in some other way.
Classifying Primary Hypersomnias by Symptoms and Sleep Test Results
For the primary hypersomnia disorders, classification is based mostly on symptoms and sleep testing results. Excessive sleepiness is present in all four disorders. However, in Kleine-Levin syndrome, sleepiness is episodic, occurring for days to weeks, separated by months without excessive sleepiness. Patients typically sleep very long durations when they are in a symptomatic spell, and during those times they also have alterations in their thinking, behaviors, and/or personality. In the remaining three hypersomnia syndromes, excessive daytime sleepiness is chronic, without long periods free from sleepiness.
Patients with narcolepsy type 1 generally have cataplexy, which differentiates it from narcolepsy type 2 and idiopathic hypersomnia. Cataplexy is a sudden episode of weakness (not sleep) triggered by a strong emotion such as humor or anger. It very rarely occurs in any disorders other than narcolepsy type 1, so if it occurs it is very useful for diagnosis.
Otherwise, the central disorders of hypersomnolence are separated by a daytime nap test called the Multiple Sleep Latency Test (MSLT). In all three disorders (idiopathic hypersomnia and both types of narcolepsy), patients fall asleep in less than 8 minutes on average. In both forms of narcolepsy, REM sleep (aka dreaming sleep) occurs in at least two naps; in idiopathic hypersomnia, REM sleep doesn’t occur or occurs in only one nap.
Idiopathic hypersomnia patients frequently, but not always, sleep for very long amounts of time. Patients with idiopathic hypersomnia can be diagnosed with sleep testing that documents at least 11 hours of sleep per 24-hour period. Some patients with narcolepsy, especially type 2, may also sleep long durations, but this is not considered part of the diagnostic criteria.
Other clinical features, e.g., episodes of paralysis upon awakening, may be more likely in particular disorders but can occur in any of these three disorders and are therefore not part of the core diagnostic criteria.
Many symptoms experienced by people with CDH overlap between IH, NT2, and NT1, but may be more or less common depending on the disease. This chart summarizes the typical pattern of symptoms by diagnosis, based on review of the medical literature and clinical experience. However, because many of these symptoms occur in more than one disorder, most are not part of the official diagnostic criteria for each disorder, and not every patient with a particular diagnosis will have this pattern of symptoms.
Despite classification rules, some people end up getting diagnosed with both NT2 and IH during the course of their disease. The clinical features of these two disorders are very similar, particularly for people who have IH but don’t sleep very long durations. In fact, in a study known as a “cluster analysis”, where a computer algorithm attempts to determine diagnosis based on symptoms, people with NT2 and people with IH without long sleep time were sorted by the computer into the same cluster. People with NT1 made up their own cluster, as did people with IH with long sleep times. That is, the computer could reliably tell the difference between NT1 and IH with long sleep time, but could not distinguish those with NT2 from those with IH without long sleep time. Furthermore, the only feature that distinguishes IH and NT2 for certain, per current diagnostic criteria, is REM sleep on the MSLT. However, multiple studies have shown that the MSLT results can change when the MSLT is repeated (especially for people with disorders other than NT1), so the same person might initially be diagnosed with IH and then NT2, or vice versa, if two MSLTs are performed.
Classifying Primary Hypersomnias by Underlying Cause
As the above discussion shows, different primary hypersomnia syndromes sometimes have overlapping clinical features because they were defined before the underlying causes were known. In practice, it can be helpful to cluster hypersomnia syndromes by underlying cause when it is known.
Cerebrospinal fluid analysis (collected via lumbar puncture, aka “spinal tap”) is most helpful in this regard. In particular, if levels of hypocretin are low, it confirms the diagnosis of narcolepsy type 1, because this type of narcolepsy is known to be caused by an attack, likely auto-immune, on the hypocretin-producing nerve cells in a part of the brain known as the hypothalamus. In patients with narcolepsy type 2 and idiopathic hypersomnia, hypocretin levels are normal.