Somniloquy, Other Episodic Sleep Disorders and the Question of Hereditary Predisposition
As previously stated, clinical observation indicates that certain major sleep disorders -- somnambulism, nocturnal enuresis, and night terrors -- are often concurrent. Epilepsy, various EEG abnormalities, CNS infection and trauma, familial tendencies, and psychopathology are commonly part of the background of each of these conditions. (See Williams, Karacan, & Hursch, 1974, for a discussion and extensive bibliography pertaining to this issue.) Sleep-walking, enuresis, and night terrors tend to occur predominantly (but by no means exclusively) in association with the first third of the night's sleep, and because they are concommitant with arousal processes from slow-wave sleep, Broughton ( 1968) has grouped them together as "Disorders of Arousal."
Despite the difficulties in arriving at valid figures for the incidence of these "classical" sleep disorders in the general population, the consensus suggested by study of the available literature and clinical experience is that sleep-talking occurs more frequently than any other single disorder of this group and very likely even exceeds the sum of their total combined frequencies, especially in adults. (See Williams & Karacan, 1975.) But sleep-talking is also frequently associated with each of these syndromes, as we have seen (Chapter 3). If somniloquy was rarely or never observed in their absence, it would be reasonable to uniformly classify sleep-speech as a component of a larger diathesis that becomes manifest in varying modes and degrees -- sometimes minimal (as mere nonspecific sleep-vocalization) and at other times fully developed (somnambulism, nocturnal enuresis, and night terrors and sleep- speech combined). Such a view would be strengthened by evidence of a hereditary predisposition for this entire group of disorders.