The overall prevalence of the autistic state has been calculated at 4.5 to 20 per 10,000. Although there was said in the past to be no familial tendency, this is almost certainly incorrect; we have seen the disease in both identical twins and in brothers, and small familial subgroups are known to exist. Autistic traits, without the full syndrome, are being found with increasing frequency in sibs and other family members, suggesting a polygenic inheritance. DeMyer found that 4 of 11 monozygotic twins were concordant for autism and that siblings have a 50 times greater risk of developing the disorder than normal children. Bailey and associates and also LeCouteur and associates have reported a concordance rate in monozygotic twins of 71 percent for the autistic spectrum disorder (as defined below) and 92 percent for an even broader phenotype of disordered social communication and stereotypic or obsessive behaviors. DeLong has found an increased incidence of bipolar disease in the families of one group of autistic children and superior mathematical aptitudes in other family members.
The elucidation of microdeletions and microduplications within chromosome 16p by Weiss and colleagues, working with the Autism Consortium was the first hint of a genetic locus for susceptibility to autism. Despite a high degree of penetrance in individuals with these changes, the importance of these findings is as a biologic direction for research as it explains no more than 1 percent of cases. Furthermore, certain polymorphisms, particularly in the SHANK3 gene may explain a small number of cases. Shank proteins serve as scaffolds on the post-synaptic membranes of glutamatergic synapses, and are highly expressed in the striatum. This gene has variants that appear disproportionately often in schizophrenia as well, leading to speculation on common biologic processes.
The autistic child is ostensibly normal at birth and may continue to be normal in achieving early behavioral sequences until 18 to 24 months of age. Then an alarming regression occurs, sometimes fairly abruptly. In some instances, the abnormality appears even before the first birthday and the child is identified as different in some way by the mother; or, if there had been a previously autistic child, she recognizes the early behavioral characteristics of the disorder. The level of activity is reduced or increased. There may be less crying and an apparent indifference to surroundings. Toys are ignored or held tenaciously. Spinning toys or running water may hold a strange fascination for the child. Cuddling may be resisted. Motor developments, on the other hand, proceed normally and may even be precocious. Later there may be an unusual sensitivity to all modes of sensory stimulation. Occasionally the onset appears to have a relationship to an injury or an upsetting experience.