[Kleine-Levin syndrome: state of the art]

Kleine-Levin syndrome is a rare neurological disorder (1-2 cases per million inhabitants) primarily affecting young subjects. It is characterized by relapsing-remitting episodes of hypersomnia in association with cognitive and behavioral disturbances. Case-reports, small series, meta-analysis and a recent large, prospective trio study are consistent with a homogeneous, genuine disease entity.

Patients are mostly male (68-78%) and adolescents (81%), with mean onset at 15 years (range 4-82 years). The first episode is triggered by an infection in 72% of patients. Patients experience an average of 7-19 episodes of 10-13 days each, relapsing every 3.5 months. Episodes recur more quickly in patients with childhood onset. The median disease course is 8-14 years, with longer course in men, in patients with hypersexuality, and when onset is after age 20. During episodes, all patients have hypersomnia (with sleep lasting 15-21 heures per day), cognitive impairment (apathy, confusion, slowness, amnesia) and a specific feeling of derealization (dreamy state, altered perception). Less frequently, patients experience hyperphagia (66%), hypersexuality (53%, principally men) and depressed mood (53%, predominantly women). Patients are remarkably similar to controls between episodes regarding sleep, vigilance, mood, and eating attitude, but have increased body mass index. Structural brain imaging, evaluation of the cerebrospinal fluid and serological inflammatory markers are unremarkable. EEG slowing is notable in 70% of cases during episodes, without epileptic activity. Sleep structure varies from harmonious hypersomnia to hypo-arousal with low sleep efficiency. The brain scintigraphy may show hypoperfusion, mostly focused on the thalamic, hypothalamic and fronto-temporal areas, especially when contrasted to images obtained between episodes. Newly identified factors include increased birth and developmental problems, Jewish heritage, genetics (5% multiplex families, suggesting autosomal recessive transmission). The association of KLS with HLA-DQ2, found in a small series, is not replicated in a larger independent sample. There is no increased family history for neuropsychiatric disorders. Some stimulants (amantadine, but more rarely modafinil or amphetamins) and mood stabilizers (lithium, valproate, but not carbamazepine) have marginal efficacy. In the 10% KLS cases secondary to various genetic, inflammatory, vascular or paraneoplasic conditions, patients are older, have more frequent and longer episodes, but their clinical symptoms, disease course and treatment response are similar to primary cases.

The most promising findings are the familial clustering and a potential Jewish founder effect, supporting a role for genetic susceptibility factors.

KLS is a puzzling and disabling disease. Until its cause will be identified, disease management should be primarily supportive and educational.