Episodic hypersomnia, hyperphagia, and hypersexual behavior has been referred to as the classical symptom triad but is presented by only half of KLS patients. In the majority of patients, cognitive dysfunction, hallucinations, and déjà vu-like experiences accompany hypersomnia [4].

In detail, the following symptoms have been related to the disease [5]:

• Sleep disorders- Besides hypersomnia (100%), intense dreaming (59%), sleep paralysis (14%) and post-episode insomnia (72%) have been described in affected individuals.
• Eating disorders comprise hyperphagia (66%) and excessive drinking (16%), but a loss of appetite (34%) may also be claimed.
• An excess sexual drive (59%) is much more common than decreased sexuality (6%) and is more often observed in male patients [6].
• Cognitive impairment may manifest in form of impaired speech (94%), concentration problems (91%), temporal and spatial disorientation (87 and 43%, respectively).
• Visual, hearing, olfactory and gustatory impairment as well as an altered perception of tactile stimuli and pain have been reported (in total, 100%).
• An altered mental state may lead to irritability (65%), anxiety (45%) and depression (53%). The latter mainly affects females [6].
• Constitutive symptoms described in KLS patients include fever (68%), hot flashes (24%) and sweating (46%), migraine-like headaches (48%), photophobia (59%) and nausea (18%).

During early stages of the disease, KLS episodes may be experienced several times a year, but frequency declines over time. Similarly, the severity of symptoms gradually decreases with disease progression. A single episode typically lasts about ten days [3], but may also extend over more than a month [7]. Between episodes, affected individuals may present unaltered sleep, eating and sexual patterns, but a trend towards fatigue and higher incidence rates of anxiety and depression has been registered [8].

Patients may or may not be able to recall a KLS episode.

According to The International Classification of Sleep Disorders [9], KLS falls into the categories "hypersomnias of central origin" and "recurrent hypersomnia." Diagnostic criteria are defined as follows:

• A minimum of two episodes of hypersomnia each lasting more than two days and less than five weeks.
• A month to one and a half years lie between single episodes.
• Episodes are further characterized by at least one of the following symptoms: eating disorder, disinhibited behavior, cognitive impairment, altered perception.
• Between episodes, consciousness, cognitive function, mood, and behavior remain unaltered.
• Symptoms cannot be explained by other pathologies, e.g., epilepsy, narcolepsy, bipolar disorder, or medication.

Thus, thorough anamnesis, general and neurological examinations are the mainstays of KLS diagnosis. Additional measures may be carried out to rule out psychiatric disorders.

In this context, polysomnography [10], brain imaging, laboratory analyses of blood or cerebrospinal fluid, and neuropsychological tests may be performed [4].

Kleine-Levin syndrome (KLS) is a rare disease with as-of-yet unknown etiology. Affected individuals suffer from episodic hypersomnia, cognitive impairment, and an altered mental state. It has been speculated that these symptoms indicate hypothalamic lesions, although scientific evidence regarding this hypothesis is scarce. Disease onset has been related to encephalitis [1], though, single episodes may be triggered by infection, head trauma, alcohol or drug abuse. Familial KLS has also been described [2].

To date, symptomatic treatment is not available. Spontaneous remission is likely, though, particularly in patients who experience first episodes of hypersomnia during adolescence. However, years may pass until resolution occurs [3].

1. Fenzi F, Simonati A, Crosato F, Ghersini L, Rizzuto N. Clinical features of Kleine-Levin syndrome with localized encephalitis. Neuropediatrics. 1993;24(5):292-295.
2. Nguyen QT, Groos E, Leclair-Visonneau L, et al. Familial Kleine-Levin Syndrome: A Specific Entity? Sleep. 2016;39(8):1535-1542.
3. Arnulf I, Zeitzer JM, File J, Farber N, Mignot E. Kleine-Levin syndrome: a systematic review of 186 cases in the literature. Brain. 2005;128(Pt 12):2763-2776.
4. Miglis MG, Guilleminault C. Kleine-Levin syndrome: a review. Nat Sci Sleep. 2014;6:19-26.
5. Arnulf I, Lin L, Gadoth N, et al. Kleine-Levin syndrome: a systematic study of 108 patients. Ann Neurol. 2008;63(4):482-493.
6. Arnulf I, Rico TJ, Mignot E. Diagnosis, disease course, and management of patients with Kleine-Levin syndrome. Lancet Neurol. 2012;11(10):918-928.
7. Miglis MG, Guilleminault C. Kleine-Levin Syndrome. Curr Neurol Neurosci Rep. 2016;16(6):60.
8. Millichap JG. Differential Diagnosis of Kleine-Levin Syndrome. Pediatr Neurol Briefs. 2015;29(3):24.
9. American Academy of Sleep Medicine. International classification of sleep disorders: diagnostic and coding manual. 3rd ed. Westchester, USA: American Academy of Sleep Medicine;2014.
10. Huang YS, Lin YH, Guilleminault C. Polysomnography in Kleine-Levin syndrome. Neurology. 2008;70(10):795-801.