Idiopathic hypersomnia (IH) is a neurological disorder which is characterized primarily by excessive sleep and excessive daytime sleepiness (EDS). Idiopathic hypersomnia was first described by Bedrich Roth in 1956, and it can be divided into two forms: polysymptomatic and monosymptomatic. The condition typically becomes evident in early adulthood and most patients diagnosed with IH will have had the disorder for many years prior to their diagnosis. As of August 2021, an FDA-approved medication exists for IH called Xywav, which is an oral solution of calcium, magnesium, potassium, and sodium oxybates; in addition to several off-label treatments (primarily FDA-approved narcolepsy medications). Idiopathic hypersomnia may also be referred to as IH, IHS, or primary hypersomnia, and belongs to a group of sleep disorders known as central hypersomnias, central disorders of hypersomnolence, or hypersomnia of brain origin. Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) defines idiopathic hypersomnia as EDS without narcolepsy or the associated features of other sleep disorders. It occurs in the absence of medical problems or sleep disruptions, such as sleep apnea, that can cause secondary hypersomnia. It is relatively common for mood disorders (depression or bipolar disorder) to occur at the same time as IH, with a small increase in anxiety disorder diagnoses.
Signs and symptoms Individuals with IH share common symptoms including excessive daytime sleepiness, sleep inertia, brain fog, and long sleep periods.
Excessive daytime sleepiness, characterized by persistent sleepiness throughout the day and often a general lack of energy, even during the day after apparently adequate or even prolonged nighttime sleep. People with EDS nap repeatedly throughout the day and have strong urges to sleep while driving, working, eating, or conversing with others. Sleep inertia (also known as sleep drunkenness), characterized by having extreme difficulty waking up and feeling an uncontrollable desire to go back to sleep. Clouding of consciousness characterized by inattention, thought process abnormalities, comprehension abnormalities, and language abnormalities. These symptoms may affect performance on perception, memory, learning, executive functions, language, constructive abilities, voluntary motor control, attention, and mental speed. Affected individuals can complain of forgetfulness, confusion, or inability to think clearly. Excessive sleep (9 hours or more over a full 24-hour period), without feeling refreshed after waking. Daytime naps can be up to several hours and are also unrefreshing. Some studies have shown increased frequencies of palpitations, digestive problems, difficulty with body temperature regulation, and other symptoms in patients with IH. Anxiety and depression are frequent, and are likely to be a response to chronic illness. A case series in 2010 found that peripheral vascular symptoms, such as cold hands and feet (e.g., Raynaud syndrome), were more common in people with IH than in controls. Other autonomic dysfunctional symptoms, such as fainting episodes, dizziness upon arising, possibly migrainous headaches, food cravings, and impotence may also be correlated with IH. Researchers have found that people reporting IH symptoms report high levels of autonomic dysfunction on par with other conditions of autonomic failure (i.e., MSA and diabetes). As of 2019 there is no evidence that such symptoms are related to IH.
Causes Unlike narcolepsy with cataplexy, which has a known cause (autoimmune destruction of hypocretin-producing neurons), the cause of IH is largely unknown. As of 2012, researchers have identified a few abnormalities associated with IH, which with further study may help to clarify the etiology. Destruction of noradrenergic neurons has produced hypersomnia in experimental animal studies, and injury to adrenergic neurons has also been shown to lead to hypersomnia. IH has also been associated with a malfunction of the norepinephrine system and decreased cerebrospinal fluid (CSF) histamine levels. Researchers have recently found an abnormal hypersensitivity to GABA (the major brain chemical responsible for sedation) in a subset of patients with central hypersomnia i.e.: IH, narcolepsy without cataplexy and long sleepers. They have identified a small (500 to 3000 daltons) naturally occurring bioactive substance (most likely a peptide as it is trypsin-sensitive) in the CSF of affected patients. Although this substance requires further identification of its chemical structure, it is currently referred to as a "somnogen" because it has been shown to cause hyper-reactivity of GABAA receptors, which leads to increased sedation or somnolence. In essence, it is as though these patients are chronically sedated with a benzodiazepine (medication which acts through the GABA system) such as midazolam or alprazolam, even though they do not take these medications.
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