There are two reasons to take up EEG studies of sleep in mentally retarded children (MR). One is that sleep EEGs seem to be indispensable for the clinical solution of sleep disturbances in MR. The other is that the vast findings of sleep EEG studies seem to show that sleep EEGs are one of the most effective measures to examine disorders or peculiarities of brain function in MR. In this paper, we will mainly review the sleep EEGs of MR from the point of view of sleep stages, because; 1) sleep stages are fundamental to the study of sleep EEGs, 2) sleep stages which cannot be classified according to the adult classification can be found in MR. A decrease in TST, %SR, %S4 and an increase in %SW and %S1 with age is seen in MR as in normal children. But in contrast to normal children, there is a trend for decreased TST and increased %SW and %S1, for a remarkable decreased %SR and SR latency, and for a small decreased %S4, especially in 0-10 year old MR. Indetermined Sleep (IS) is found rarely in adult sleep yet, constitutes a peculiar sleep stage in MR (Petre-Quadens, 1972 et al.). IS decreases with age. Similar tendencies were seen in the Down syndrome group and the Endocrine group except for SR latency, but the PKU group showed a remarkable increase of %SW and %S1 and a decrease of %SR and %S4. It is concluded that the quality of sleep in MR is generally bad (especially in the PKU group), and that sleep in MR changes with age as in normal children.
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