cortisone injection helpful?

[DCS627]

I had a positive emg, my dr. gave me another cortisone injection in my carpal tunnel. He said if this helps the tingling go away and the tingling comes back, that means the surgery will be a positive result. He said if it doesn't help, the surgery will be useless. Does this make sense to anyone?
How long will it take before the injection starts to work, if at all, and I'd like to hear if it has helped anyone permanently. My dr. won't do surgery unless he is sure the surgery could work as he doesn't want to make me worse. This is a w/c case. Thanks for everyone's advice.

[NancyH]

They often use that injection as a diagnostic tool, if it works it is carpal tunnel, if not it could mean something mechanical going on. The injection should help within a day as well!