There are certainly multiple kinds of autism, but they don't divide along functioning levels.
The most common kind of autism is polygenetic autism. This kind of autism results from the combination of hundreds of genes, each of which have only a tiny effect on autistic traits, but when added together they determine where you are on the spectrum. (Just like how height and IQ are inherited.) People with polygenetic autism can be anywhere on the spectrum, depending on the number of autism-related alleles they have as well as environmental factors. In fact, this kind of autism often results in immediate family members at different functioning levels, such as an AS kid with an LFA sibling, or an AS parent with an LFA child. (Presumably you could have an LFA parent with an AS child, but LFA people usually don't become parents.) I've even heard of twins who are both on the spectrum but at different functioning levels - although that's sometimes a mix of polygenetic autism + prematurity-related brain injury.
There are some exceptions to this pattern, though. A sizeable minority of autistic people have genetic syndromes (literally thousands of different syndromes have been linked to autism), in which one or a few high-impact genes vastly change the person's outcome. (For the height analogy, these people would be like people with genetic short stature syndromes.) Most of these conditions are associated with a particular functioning level of autism, usually LFA. For example, most people with 22q13 deletion (missing the tip of chromosome 22) are autistic and have moderate-to-severe cognitive disability (causing them to be low functioning), while 50% of people with 22q11 deletion (missing a spot near the center of chromosome 22) are autistic and most have borderline IQ or mild cognitive disability and NVLD (poor spatial skills, good verbal skills). Occasionally these syndromes are inherited, but usually they're due to a new mutation, and there's no one else in the family with autistic traits.
An even smaller proportion have non-genetic syndromes, which usually behave similarly to the genetic syndromes but are caused by an environmental factor. For example, congenital rubella syndrome usually causes autism and severe cognitive disability (along with deafblindness and medical problems). Thalidomide can cause autism if the timing of dosage is just right. (A different timing of dosage causes the more famous limb abnormalities.) Certain anticonvulsants can also cause autism (usually with moderate-to-severe cognitive disability) if taken during pregnancy. After birth, kids with cerebellar damage often act autistic, though most of them wouldn't meet the onset criteria. They can be anywhere on the spectrum.
So while most AS and LFA individuals have the same underlying cause, there are rarer forms of autism with a wide range of different causes.
12 Mar 2016, 12:27 pm
