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Autism is a developmental disability characterized by difficulties in social interaction, communication, and the presence of repetitive, stereotypical behaviors and interests. There are also frequently other symptoms, such as sensory hyper or hyposensitivites. Autism might refer to what is known as "classic" Autism, Kanner's Autism or Autistic Disorder, but it may also refer to the broader spectrum of autism and related disorders, that includes Asperger's Syndrome and PDD-NOS. When I refer to autism, I mean the whole group of autistic spectrum disorders (ASDs), because the differences are minor especially in older children and adults. In the psychiatric handbook, called DSM, the autistic spectrum disorders are categorzied as pervasive developmental disorders (PDDs).
The autistic community is extremely diverse. Some people have shown differences since early infancy, while others first developed typically and then "regressed". Some people have more apparent difficulty in one area, such as social interaction, while others may have more difficulty in another, such as language. Within categories of impairment, there is wide variation in symptom presentation. For instance, one person might be socially withdrawn, while the other frequently seeks social contact, but has a tendency to dominate conversation. Some people are called "high-functioning" and others are said to be "low-functioning". The distinction seems to be that "high-functioning" individuals have an IQ above 70, while "low-functioning" autistics have an IQ below 70. However, many assumptions made regarding the differences between those with high-funcitoning autism (HFA) and those with low-funcitoning autism (LFA), are controversial.
Like I said, there are several diagnoses that someone with autism might receive. At first, I was diagnosed with an autistic spectrum disorder, but it was not said which ASD I had. Later, my diagnosis was changed to Asperger's Syndrome - although, as I said, I don't particularly care for specific labels. Other people may be diagnosed with Autistic Disorder (also known as classic autism, core autism or Kanner's autism) or Pervasive Developmental Disorder - Not Otherwise Specified (PDD-NOS). The differences between these disorders are usually related to a child's early development. Children with Autistic Disorder have an early (before the age of three) delay in social interaction, communicaiton, and play. They usually have a speech delay and may have impairments in self-help skills, adaptive behavior or cognitive abilities. Individuals diagnosed with Asperger's Syndrome by definition did not have early global language delays and did not have an early delay in self-help skills, adaptive behavior or cognitive abilities. They always have an IQ above 70. However, all the core symptoms (except for the speech delay) are the same as those for "classic" autism. In adults and older children who have speech, the difference is insignificant. PDD-NOS, also known as atypical autism, is diagnosed when someone is on the autism spectrum, but does not meet criteria for either Autistic Disorder or Asperger's Syndrome. This may be because the person does not have enough symptoms, has atypical symptoms, and/or the symptoms did not become apparent till after the age of three. Often, a person (especially an adult) may also be diagnosed with PDD-NOS when information about their early childhood is not detailed enough to figure out whether they meet criteria for Autistic Disorder or Asperger's Syndrome.
All forms of autism are caused by neurological differences in the brain. What causes these neurological differences, is still unknown, but there is likely a strong genetic risk factor. There may also be other contributing factors, such as premature birth or certain prenatal infections (rubella, for example). Some people believe that autism is caused by heavy metal poisoning, possibly from vaccines. This theory is highly controversial and so far not supported by scientific evidence.