DS1 (3yo) mostly likely has Aspergers - I've known since he was a few months old & people used to comment on what a "good" baby he was for not getting upset when left with unfamiliar people. We've been talking to his nursery about the next steps wrt a diagnosis & it's got me thinking about his behaviours.
He doesn't seem to have "classic" meltdowns, he has standard toddler tantrums but even those aren't particularly extreme or common. I guess part of that is because we don't do a lot of things - if he's not at nursery (2 days a week), toddler group or church, we're generally pottering around the house & he gets to choose what to do. I know there are certain things he dislikes (anything involving getting dirty usually) so I rarely suggest it. Any time something happens that he doesn't like he just doesn't seem to react, at all. It's like he goes blank & wanders off. It happened the other day when one of his friends shouted at him - no reaction, wandered away - but later on I asked him casually "what happened at toddlers?" & he refused to answer initially but eventually told me that his friend shouted & he loved his friend & why didn't his friend want to play with him?! It had obviously upset him but he didn't show it. He was almost completely emotionless, yet confused. It's like he doesn't know how to express fear/hurt/anger - sometimes he'll get frustrated & cross, like any other child, but even when he's fallen over & has clearly hurt himself he'll only be upset for about 30 seconds before he seems "fine" again.
The only extreme emotion we seem to get from him is hyperness. He can go from seeming perfectly normal to completely manic in the blink of an eye. It happens most commonly at bedtime, either when he's had a sleep in the afternoon or when he's had a busy day with no alone time. It does happen at other times too, often exhibited as sudden bolting while out, accompanied by hysterical laughter & octopus-like squirming when we do finally catch him. We've yet to find anything that will break his behaviour when he's like that, other than when his flailing results in him hurting himself or we put him in his room & shock, horror shut the door that he's perfectly capable of opening! 
At other times he runs around the room, flings things, flails & repeats phrases he's heard but in a more aggressive way - usually "that is unacceptable behaviour" type comments that we use to calmly explain when he or his brother aren't behaving but with an added "I'm going to hit and smash with my sword" - no idea where he gets that from, we're pretty zero-tolerance when it comes to violence, mostly because he's very suggestible & will come out with these things when he's in these moods.
We tried to explain this manic state to the SENCO at his nursery & she explained how they support children who throw but clearly didn't understand the scale of the issue. It's not just "throwing" - that's a perfectly normal toddler phase AFAIC I have early years education training - it's the manic can't-stop-until-he-hurts-himself state that concerns me! She was talking about how they "get down to their level to calmly explain what is & isn't ok to throw" then apparently they turn it into a game & get them throwing in an appropriate way... She's clearly never tried that with DS1 when he's like that - "get down to his level" & the flailing will knock your glasses across the room, "calmly explain" - good luck getting him to stand still & listen, "what is & isn't ok" will just be met with "I'm going to throw it & smash it" etc. being growled at her!
Sorry, it's a bit of a brain-dump, just trying to get my head around it so I might stand some chance of explaining it to them! I might just have to try & film it sometime so they can see for themselves. She did ask what happens if we leave him to it but I chickened out of telling her that he'll eventually run out of steam after an hour or 2 & stay still long enough to start humping the floor & will do that for anther couple of hours until he falls asleep. It was 1am before we had peace the other day.
He's always laughing but it's completely out-of-control hysterical laughter & is clearly an emotional release for him so I guess it's reasonable to call it a meltdown, of sorts. Does anyone else have experience of this sort of behaviour? I still haven't figured out how best to deal with it - if I leave him I'm terrified he's going to leap off his bed & injure himself but staying just winds us both up! Most of the rest of the time he's a bright, inquisitive child but it's like a switch gets flipped in his brain & it's genuinely quite terrifying sometimes! I feel completely useless - I've got a level 5 in EYE, been a nanny, looked after various other children but never experienced anything quite like DS1's manic-meltdowns. I'm utterly baffled - if anyone's found anything that works I'd really like to know! 