It's happened to me all my life, too. I remember in school I wouldn't be able to grasp the pen/pencil to write after I had laughed really hard...it still happens to me to this day (I'm 40).
I have the same problem, Its always happened ever since i was a kid but i always thought it was normal and never thought anything of it till i pointed it out one day to one my boyfriend and he said that he never had that problem. It kinda scares me a little bit and i would like to know why my hands lose strength whenever i laugh real hard.
I would mention it to your neurologist, especially if your hands become so weak you have trouble picking up a cup. Lots of strange things have very logical explanations. It doesn't sound as if it had anything to do with your surgery, though, if the weakness happened when laughing even as a child (unless the surgery was done in your childhood, too). Could it possibly be you lose strength when you just become very emotional (laughing hard would qualify I would think)? I know I became weaker a couple times (especially with my speech) when I was upset/stressed/angry. You should do a search typing in the keywords schwanomma (sp?), laughing, muscle weakness, etc. and see what you come up with, if anything.
look i know this sounds like im at it, but please be assured im not. Breif history... Had a schwannoma tumour removed four years ago from C4, because it had been there so long ( approx 12 years maybe more!!) I have permanent damage so lose strenght in hands,legs etc. Problem I have is when I really laugh I have no fine motor movements in fingers for at least half an hour afterwards. Like i cant lift a cup or use a pen. On thinking back this used to happen as a small child in school. I dont have any appointment with surgeon now as my last MRI was clear. apart from confirming the kink in spinal cord, is it worth mentioning it to him in a call? Thanks for any input folks Anne x