My nine year old son is autistic and has similar behaviors. I believe it to be sensory prossessing disorder which commonly occurs with autism. It's like having the "heebee jeebee's" or "ants in the pants". She is responding to some inner stimuli. My suggestion would be to try some deep pressure massage, maybe a nightly bath with a lotion massage the arms and legs. How is your daughters diet? I know that when my son gets caffiene or candy that can happen. It can also happen after too much intense stimuli, like video games, watching videos, or playing with toy that has a lot of lights, bells and whistles. Autistic children crave this stimulation, but they can also get overstimulated, and thats when you see the inappropriate laughing, pacing, hand wringing, and echolalia, and other vocalizations etc. When they are going through one of these "states" its hard to get there attention. Try using a "visual" cue. Pecs Cards. With pictures for "too loud", "nice voice" and "too low". You could put them on one piece of paper and make it small enough to fit in your purse so if you are at a restaurant you can pull it out and show it to her. Also, don't forget to show her the "nice voice" picture when she's calmed down and using her nice voice to reinforce the good behavior. If you aren't sure what PECS cards are, there are many references to them on the internet. If your daughter has difficulty processing verbal language and/or the written word, then visual pictures will help better. I really hope this helps you and your daughter. I understand how difficult it is raising an autistic child. Best wishes to you both.
My 2 years old son too has autism and SPD and also has these laughing fits but I have a hunch that there may be something more neurological going on. I read online about gelastic and dacrystic seizures and from what I understand these can be really scary for kids even though they appear to be having a good laugh. Try this link http://www.epilepsyfoundation.org/aboutepilepsy/seizures/Gelastic-and-Dacrystic-Seizures.cfm and see what you think about them. I'm having my son go in for an EEG at the end of the month to test this for these seizures. I hope this is not the case and that he's just really happy but I can't help worry that he might be frightened and I've been laughing "with" him all these times ...
If you get back on this site please update what happened. my 20 month old is concerning me with this too and not sure what I should do.
I really want to know an update on this issue after all these years. I am facing a similar situation with my 10 year old daughter these days, and talking to her, or explaining her, doesn't work. There is nothing that can stop her once she starts to laugh and its becoming stronger and unctrollable every day.