Kleine-Levin syndrome: a disease in its own right?

This Practice Point commentary describes the findings of a cross-sectional study by Arnulf and co-workers on Kleine-Levin syndrome (KLS), a rare disorder of unknown pathophysiology and etiology that is characterized by periodic hypersomnolence and associated symptoms such as hyperphagia and hypersexuality. Arnulf et al. report data collected through a comprehensive questionnaire that help to elucidate the KLS phenotype. In addition, they report some evidence to suggest that an underlying biological abnormality could lead to this disorder. We discuss how the current lack of understanding regarding how the brain constructs the mind makes the categorization of clinical syndromes with unknown pathophysiological foundations, such as KLS, particularly challenging in psychiatry and neurology. We further consider the validity of the claim by Arnulf et al. that KLS represents a genuine disease entity.