It depends on a lot of things and on a case by case basic. It will depend mostly on the type of the disorder, the severity, and the current or potential financial burden it can cause to the Canadian government. For example, if it is something like a down's syndrome and you don't have to pay too much for the health costs (there is a maximum amount, you can find that somewhere else), then there shouldn't be too much problem. But, if it is something like cancer and you have to spend substantially, then there is a higher chance of getting rejected.
There are other threads in this forum that talk a lot about medical related issues. You can do a search and read them. I think a forum member called 'computergeek' is the go-to guy for this kind of stuff.