According to the International Classification of Sleep Disorders (ICSD 2005), there are 12 core categories of parasomnias, mainly classified according to the sleep state of origin. The last three categories consist of parasomnias: unspecified, due to drug or substance, and due to medical conditions. The essential feature of this last diagnosis is that a parasomnia emerges as a manifestation of an underlying neurological or medical condition. Some authors categorize the parasomnias as “primary” (disorders of the sleep state per se) and “secondary” (organ-system disorders that appear during sleep). The secondary sleep parasomnias can be further classified by the organ system involved: Nevertheless, these secondary phenomena represent different events and symptoms arising from specific organ systems and occurring preferentially during the sleep period rather than sleep-related manifestations of an underlying medical or neurological disorder. The REM sleep behavior disorder (RBD) is the parasomnia most commonly associated with an underlying neurological condition (the so-called “symptomatic RBD”) and will be analyzed in detail in Chapter 14. Another parasomnia often associated with different medical conditions such as delirium tremens, Morvan's fibrillary chorea, fatal familial insomnia and sporadic Creutzfeldt-Jakob disease, is called “agrypnia excitata”.
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