Several disorders commonly accompany autism. To some extent, these may be caused by a common underlying problem in brain functioning.

Mental retardation

Of the problems that can occur with autism, mental retardation is the most widespread. Seventy-five to 80 percent of people with autism are mentally retarded to some extent. Fifteen to 20 percent are considered severely retarded, with IQs below 35. (A score of 100 represents average intelligence.) But autism does not necessarily correspond with mental impairment. More than 10 percent of people with autism have an average or above average IQ. A few show exceptional intelligence.

Interpreting IQ scores is difficult, however, because most intelligence tests are not designed for people with autism. People with autism do not perceive or relate to their environment in typical ways. When tested, some areas of ability are normal or even above average, and some areas may be especially weak. For example, a child with autism may do extremely well on the parts of the test that measure visual skills but earn low scores on the language subtests.

About one-third of the children with autism develop seizures, starting either in early childhood or adolescence. Researchers are trying to learn if there is any significance to the time of onset, since the seizures often first appear when certain neurotransmitters become active.

Since seizures range from brief blackouts to full-blown body convulsions, an electroencephalogram (EEG) can help confirm their presence. Fortunately, in most cases, seizures can be controlled with medication.

Fragile X

One disorder, Fragile X syndrome, has been found in about 10 percent of people with autism, mostly males. This inherited disorder is named for a defective piece of the X-chromosome that appears pinched and fragile when seen under a microscope.

People who inherit this faulty bit of genetic code are more likely to have mental retardation and many of the same symptoms as autism along with unusual physical features that are not typical of autism.

There is also some relationship between autism and Tuberous Sclerosis, a genetic condition that causes abnormal tissue growth in the brain and problems in other organs. Although Tuberous Sclerosis is a rare disorder, occurring less than once in 10,000 births, about a fourth of those affected are also autistic.

Scientists are exploring genetic conditions such as Fragile X and Tuberous Sclerosis to see why they so often coincide with autism. Understanding exactly how these conditions disrupt normal brain development may provide insights to the biological and genetic mechanisms of autism.