Idiopathic hypersomnia (ORPHA:33208)
A rare neurologic disease characterized by an excessive daytime sleepiness with long and unrefreshing naps, and/or prolonged and undisturbed nocturnal sleep, impaired daytime alertness, and/or sleep inertia (ie, great difficulty in waking up after sleep) and where other causes have been excluded.
Epidemiology
The prevalence estimates were initially based on the proportion of patients with idiopathic hypersomnia (IH) over those with narcolepsy in the sleep medicine population. The consensus currently estimates prevalence of IH in the range of 0.002%-0.010% in the general population. A higher frequency in women is reported.
Clinical description
Symptoms frequently start during adolescence or young adulthood, but may occur at any age. Idiopathic hypersomnia with long sleep time is characterized by a prolonged (more than 10 hours) nocturnal sleep of good quality, and / or excessive daytime sleepiness with prolonged unrefreshing sleep episodes, and sometimes difficulty in awakening in the morning or after naps, with sleep inertia or drunkenness. Idiopathic hypersomnia without long sleep time is characterized by isolated excessive daytime sleepiness, with irresistible and more or less refreshing diurnal naps. Nocturnal sleep is normal or slightly prolonged but lasts less than 10 hours, and quality of awakening is often normal. Idiopathic hypersomnia is never associated with cataplexy (which would lead to a suspicion of narcolepsy type 1).
Etiology
Etiology is still unknown. There is no association with any particular HLA (human leukocyte antigen) marker or with a decrease in the orexin/hypocretin levels, as in narcolepsy type 1.
Diagnostic methods
Diagnosis is complex and must exclude other causes of sleepiness and document the excess of nighttime and daytime sleep. Definitive diagnosis is based on polysomnography completed with multiple sleep latency tests (MSLT), performed without any medication with an effect on sleep. These tests reveal a sleep of good quality and reveal a mean sleep latency of less than 8 min with a maximum of one episode of paradoxical sleep. In case of idiopathic hypersomnia with long sleep time, a 24h (or 32h) continuous sleep recording shows a nocturnal sleep episode of over 10 hours with a daytime nap of more than an hour, i.e. over 11 hours/ 24h (or over 19 hours/32h).
Differential diagnosis
Clinical examination (based on a clinical interview, a sleep diary or on actimetry) eliminates chronic insufficient sleep syndrome. Sleep recordings exclude narcolepsy, circadian rhythm disorders or fragmented night sleep due to motor or respiratory events. A psychological examination excludes hypersomnia of psychiatric origin (especially depressive symptoms). Finally, neuro-radiological tests, which are rarely performed, exclude cerebral lesions.
Genetic counseling
IH appears to be familial; however, there is no information on inheritance or penetrance.
Management and treatment
To date, no drug has authorisation for the treatment of patients affected with idiopathic hypersomnia worldwide; recommendations are based on expert opinion only. Stimulant drugs (i.e. modafinil, methylphenidate, pitolisant) have a good benefit risk ratio except for the post-awakening confusion (sleep inertia). Amphetamines are third-line therapies. Sodium oxybate could be effective on sleepiness and inertia (ongoing clinical trials).
Prognosis
The disease has a negative social and professional impact. Spontaneous evolution of the disease is variable and may remain stable, resolve or evolve to narcolepsy type 2.

                            Last update: 
                            November 2020


                            - Expert reviewer(s): 
                            Dr Lucie BARATEAU  - Pr Yves DAUVILLIERS