Isolated sleep paralysis: a web survey.

Isolated Sleep Paralysis (SP) occurs at least once in a lifetime in 40-50% of normal subjects, while as a chronic complaint it is an uncommon and scarcely known disorder. A series of messages written by subjects who experienced at least one episode of SP, containing more or less detailed descriptions of this disorder, were collected from the Sleep Web site of the University of California in Los Angeles between January 1996 and July 1998. Two hundred and sixty-four messages fulfilling the International Classification of Sleep Disorders (ICSD) (Thorpy, 1990) minimal criteria for SP were analyzed. A wide spectrum of severity was evident, with a frequency of episodes ranging from one in a lifetime to almost every night, and a variety of emotional and hallucinatory experiences associated with SP episodes were reported. Clinical similarities between the recurrent form of isolated SP and channelopathies (in particular, periodic paralyses) are discussed. An activation of limbic system structures is suggested in order to explain some of the most common subjective experiences associated with SP.