Today, with the DSM-5, there is just one "autism spectrum disorder" -- and everyone is lumped under that single diagnosis. But that doesn't mean we've stopped using the older or informal terms! Welcome to the complex world of many autisms.
The "autism spectrum" describes a set of developmental delays and disorders which affects social and communication skills and, to a greater or lesser degree, motor and language skills. It is such a broad diagnosis that it can include people with high IQ's and mental retardation - and people with autism can be chatty or silent, affectionate or cold, methodical or disorganized. .
Until May, 2013, official diagnoses within the autism spectrum included autistic disorder, pervasive developmental disorder not otherwise specified (PDD-NOS), Asperger syndrome, Childhood Disintegrative Disorder and Rett Syndrome. Today, there is just one Autism Spectrum Disorder, with three levels of severity -- but many therapists, clinicians, parents and organizations continue to use terms like PDD-NOS and Asperger syndrome.
Asperger syndrome describes individuals at the highest-functioning end of the autism spectrum. The term -- and the diagnosis -- was removed from the diagnostic manual in 2013, but virtually everyone in the autism community continues to use it because of its usefulness in describing a very specific group of people. People with Asperger syndrome generally develop spoken language in the same way as typically developing children, but have a tough time with social communication. These difficulties that become more obvious as they get older and social expectations rise.; Because people with Asperger syndrome are often very intelligent - but "quirky" - the disorder is sometimes nicknamed "geek syndrome" or "little professor syndrome."
Like "mild" autism, high functioning autism (sometimes shortened to HFA) is a made-up term that's become more and more commonly used. HFA is a tricky term, because it can be hard to distinguish a person with HFA from a person with Asperger syndrome. The official distinction is that people with HFA had or have speech delays, while people with Asperger Syndrome have normal speech development. But there may also be very real differences in terms of social awareness, personality characteristics, and other traits. The jury is still debating the fine distinctions.
"Pervasive Developmental Disorder Not Otherwise Specified" is a mouthful of words that are often applied to people on the autism spectrum. It describes individuals who don't fully fit the criteria for other specific diagnoses, but are nevertheless autistic. Because there is no easy way to define the symptoms of PDD-NOS, which may range from very mild to very severe, the diagnostic category no longer exists, though a new diagnosis, Social Communication Disorder, may become a similar "catchall" category. The term PDD-NOS is rarely used outside of practioners' offices: most parents, therapists and teachers prefer to use more descriptive (though less official) terms to describe their children, students and patients with "most but not all symptoms of autism."
Severe autism is not an official diagnosis; instead, it is a descriptive term along with profound autism, low functioning autism, and classic autism. People with autistic disorder are often non-verbal and intellectually disabled, and may have very challenging behaviors.
Rett syndrome is a genetic disorder that affects only girls. It is the only one of the former autism spectrum disorders that can be diagnosed medically (so far) -- and as of May, 2013 it is no longer included in the Autism Spectrum. Girls with Rett syndrome develop severe symptoms including the hallmark social communication challenges of autism. In addition, Rett syndrome can profoundly impair girls' ability to use their hands usefully.
If Asperger syndrome is considered "mild" autism, then the broad autism phenotype includes those people with the merest touch of autism. Is this really autism? Or just a personality type? As with many issues related to autism, it depends on who you ask.