Welcome to Healing Well.
Sorry I don't have any specific info about any differences for a person with Down syndrome, but I can comment on the nissen in general.
I no longer responded to meds and the nissen was the best thing I ever did for my GERD. Please don't think that because I have had 3 of them that it is flawed. My case is that 1 in a million. I would have it redone tomorrow if mine was to be damaged today as it has given me back a normal life as far as dealing with GERD goes. As DOGG said, the nissen is the gold standard surgery for this and though very promising the others are still relatively new compared to it. What this means is that there are less surgeons who have done 100s to 1000s of these surgeries. This is important as the more a surgeon does the better they are at it.
Please feel free to ask any questions you have we love to help. Also if you wish my email is on my profile and I am happy to respond to any emails.
Nissen 6/06 and 5/09
#3 on 8/24/11