Autism spectrum disorders occur in approximately 1 in 150 children according to a 2007 study conducted by the Centers for Disease Control and Prevention’s Autism and Developmental Disabilities Monitoring (ADDM) Network. There are certain behavioral, psychiatric and medical conditions that are seen alongside a pervasive developmental disorder diagnosis. These diagnoses are called co-morbidities and are simply any two conditions that co-exist in a single patient.
One set of co-morbid conditions that are experiencing an upsurge in research is the existence of an autism spectrum disorder in an individual who also has Down syndrome (Trisomy 21). According to the National Down Syndrome Society (NDSS), Down syndrome occurs in approximately one in 733 births.
Dual Diagnosis Rates
Elisabeth Dyken’s “Psychiatric and Behavioral Disorders in Persons with Down Syndrome” study [Mental Retardation and Developmental Disabilities Research Reviews Volume 13: 2007] confirms that initial co-morbidity rates of Down Syndrome and an ASD were approximately one to two percent of the Down Syndrome population. However, further research shows that the rates are actually closer to seven percent of the Down syndrome population also meeting the criteria for an autism spectrum diagnosis.
Delayed Diagnosis
Part of the reason for the drastic discrepancy between studies is that the original studies were done only of children who actually held a dual diagnosis instead of individuals with Down syndrome who also exhibited symptoms of autism. Part of the problem is that children with Down syndrome are diagnosed with an ASD later than their peers without Down syndrome.
In her study, Dyken states "later ages of diagnoses likely stem from diagnostic overshadowing as well as the prevalent stereotype and view of children with Down syndrome as being friendly, social, and charming; a far cry from autism.“ Dyken goes on to describe that “syndrome overshadowing” may play a part in the delayed diagnosis – symptoms of one disorder or condition overshadow that of a co-existing condition.
Symptoms
A 2007 study by John C. Carter, et al, “Autism-spectrum Disorders in Down Syndrome: Further Delineation and Distinction from Other Behavioral Abnormalities” was published in the American Journal of Medical Genetics Part B (Neuropsychiatric Genetics); the study looked at symptoms of individuals with a dual diagnosis of Down syndrome and an autism spectrum disorder. Based on results from the Stanford-Binet IV IQ test that was given during the Carter study, cognitive functioning among individuals with a dual diagnosis was significantly lower than individuals with a standalone Down syndrome diagnosis.
In addition to the differences in the IQ scores, children with Down syndrome and an ASD were more socially withdrawn, had a higher level of anxiety and were plagued with more severe stereotypical behaviors.
Parents of children with Down syndrome who are concerned that their child may also be on the autism spectrum can consult the National Down Syndrome Society (NDSS) for guidance. The NDSS maintains a resource list for the dual diagnosis of autism and Down syndrome. The NDSS can be contacted at 1-800-221-4602.
