I was thinking Angelman Syndrome. She seems happy all the time and laughs at nothing.
People with Angelman Syndrome have particular facial features and Abbie doesn't have any of those. Yes, she does share
some of the behavioral traits but having her lack the common physical characteristics, I'd say she most likely does not have it. I'm not a medical professional (of course).
Common physical characteristics include:
- Prominent mandible (jaw or jawbone)
- Wide mouth with widely spaced teeth
- Flat back of the head
- Microcephaly (abnormally small skull)
- Usually has light colored hair and eyes compared to other family members
- Frequently protruding tongue
(Info found through google search)