What is Idiopathic Hypersomnia (IH)? The main symptom of IH is pervasive daytime sleepiness despite adequate, or more typically, extraordinary sleep amounts (e.g., > 10 hours per night).  Additional symptoms and complaints commonly include unrefreshing or non-restorative sleep, and sleep inertia and sleep drunkenness (difficulty awakening from sleep, accompanied by feelings of grogginess and disorientation upon awakening).  Symptoms usual start in the mid-to-late teens or early twenties, although it can begin at a later age.   Symptom intensity often varies between weeks, months, or years, sometimes worsen just prior to menses in women, and can spontaneously remit in 10-15% of patients.  Sleep is usually described as “deep” and arousal from sleep is usually difficult, often requiring multiple alarm clocks and morning rituals to ensure that patients arise for school or work.  In contrast to the short and generally refreshing daytime naps observed in genuine narcolepsy, those in IHS patients can be very long – on the scale of hours – and are unrefreshing.  Some patients exhibit hypersensitivity to sedating medications such as anesthetics, sleeping pills, or alcohol.

What causes Idiopathic Hypersomnia? IH is a disorder of the nervous system in which there often appears to be over production of a small molecule that acts like a sleeping pill (e.g., a sedative-hypnotic drug such as Versed®), or anesthetic (e.g., propofol).  Although the exact composition of this small molecule is yet to be determined, much is known about how it interacts with γ-aminobutyric acid (GABA), a principal player in the brain mechanisms that promote sleep.  In the presence of this substance, the inhibitory and sleep promoting actions of GABA are enhanced at the receptors at which it acts.

Diagnosing IH The symptoms of IH can be mimicked by other disorders, so it is important for a sleep physician to evaluate for other contributing causes including depression or other mood disorders, insufficient sleep time, and disturbance of circadian rhythms (e.g., advanced sleep phase syndrome and delayed sleep phase syndrome). In clinical experience, hypothyroidism and iron deficiency with or without anemia can also cause excessive sleep and daytime sleepiness and should be assessed for and corrected as appropriate.  IHS is then diagnosed by a night-time sleep test (polysomnogram) followed by multiple sleep latency testing (MSLT) the day after.  During the MSLT, the patient is given an opportunity to fall asleep on 4 or 5 occasions at 2-hour intervals.  The polysomnogram rules out certain causes of secondary hypersomnia, including that caused by obstructive sleep apnea.  Current criteria for the diagnosis of IH by MSLT requires falling asleep in an average of less than 8 minutes in all the nap opportunities, without frequent dream sleep (which distinguishes it from narcolepsy).  Unfortunately, several groups of researchers have shown that patients who otherwise seem to have IH have a normal MSLT, raising concerns that this test may not be accurate to make the diagnosis in all patients.  If the results do not paint a clear picture (e.g., there is a compelling history of excessive sleep, sleepiness, or slowed reaction times, but a normal or ambiguous MSLT result), diagnosis can be clarified with a lumbar puncture to obtain cerebrospinal fluid (CSF) in order to measure hypocretin (to test for narcolepsy with cataplexy), and determine to what extent a patient’s CSF might enhance GABA receptor function in human cells grown in a petri dish in the laboratory (thought to underlie many cases of idiopathic hypersomnia and narcolepsy without cataplexy).