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Epidemiology and Comorbidity
Large scale epidemiologic studies of early childhood onset neuropsychiatric disorder simply have not been undertaken thus making prevalence rates of many pediatric onset disorders relatively obscure. It is also worth noting that disorders that are relatively uncommon pose particular problems when it comes to the ascertainment of prevalence rates.
Increased awareness of ASD through laudable efforts at better education of mental health workers, pediatricians and school personnel has led to better (and earlier) identification of the core symptoms that had previously gone undiagnosed.
While there is no definitive way to know what accounts for the rise in ASD (and an increase due to, for instance, an en-vironmental insult cannot be ruled out), the current informed consensus is that this increased prevalence rate reflects better recognition and detection together with more inclusive diagnos-tic definitions.
Boys are affected with ASD more than girls in a 4 : 1 ratio. The exception is Rett’s disorder, which is found almost exclu-sively in females. Interestingly, the gender ratio in ASD increases when cases with mental retardation are excluded to about 6 : 1, and decreases to approximately 1.7 : 1 when only cases with mod-erate to severe mental retardation are considered.
The prevalence rates of ASD do not appear to be influenced by immigrant status or socioeconomic status.
With the exception of childhood disintegrative disorder (in which all affected children are mentally retarded), there is wide individual variability in intellectual functioning in ASD. Only about 20 to 25% of children with ASD have an IQ over 70, with 30 to 35% having mild to moderate mental retardation, and 40 to 45% having severe to profound mental retardation. However, with more intensive case-finding (i.e., less dependence on mental retardation for referral), the number of patients with ASD who have an IQ over 70 may be as high as 50%. Follow-up studies sug-gest that IQ levels tend to be constant from the time of diagnosis (when over 5 years old at the time of diagnosis) and are stable over time, and thus are thought to be important predictors of outcome. Particular cognitive deficits are seen in language, abstraction, se-quencing and coding operations, while visual–spatial skills are often a relative strength (Lord et al; 1997, Rutter 1983).
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