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Rett's Disorder is included as a Pervasive Developmental Disorder because there is some potential confusion with autism - particularly in the preschool years (Tsai, 1992). Otherwise the course and onset of this condition is very distinctive. In people with Rett's Disorder (first reported by Rett in 1966), very early development is normal. Head growth then decelerates, usually in the first months of life, and a loss of purposeful hand movements occurs. Motor involvement is quite striking and profound mental retardation is typical. Characteristic hand-washing stereotypies develop.  Mental retardation inevitably develops in Rett’s Disorder. While the DSM-IV does not list male sex in the exclusionary criteria, the existing literature on Rett’s syndrome documents the condition primarily in girls. The DSM-IV field trial sample included only girls and a recent, very well executed epidemiological investigation documented a prevalence of 3.8 per 10,000 girls; boys were not included. Since the discovery of the MECP2 gene, responsible for Rett’s, variants of the syndrome have been reported in males who have mutations of MECP2, with some overlap in the symptomatology observed in girls (Amir, Van de Veyver, Wan, Tran, Franke, & Zoghbi, 1999; Schwartzman, Zatz, Vasquez, Gomes, Koiffman, Fridman & Otto, 1999; Schanen, Kurczynski, Brunelle, Woodcock, Dure, & Percy 1998).


Pervasive Developmental Disorder, Not Otherwise Specified (PDD-NOS) is a 'sub threshold' condition in which some - but not all - features of autism or another explicitly identified Pervasive Developmental Disorder are identified. PDD-NOS is often incorrectly referred to as simply "PDD." The term PDD refers to the class of conditions to which autism belongs. PDD is NOT itself a diagnosis, while PDD-NOS IS a diagnosis. The term Pervasive Developmental Disorder - Not Otherwise Specified (PDD-NOS; also referred to as "atypical personality development," "atypical PDD," or "atypical autism") is included in DSM-IV to encompass cases where there is marked impairment of social interaction, communication, and/or stereotyped behavior patterns or interest, but when full features for autism or another explicitly defined PDD are not met. It should be emphasized that this ''sub threshold'' category is thus defined implicitly, that is, no specific guidelines for diagnosis are provided. While deficits in peer relations and unusual sensitivities are typically noted, social skills are less impaired than in classical autism. The lack of definition(s) for this relatively heterogeneous group of children presents problems for research on this condition. The limited available evidence suggests that children with PDD-NOS probably come to professional attention rather later than is the case with autistic children, and that intellectual deficits are less common.

Information for this section is primarily from the Yale Developmental Disabilities Clinic.

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