It is really a symptom and not a disease. It can be caused by many things including organ failure or damage (usually caused by poor hydration or oversupplementaion) heart problems, circulatory problems, reaction to medication. Sometimes it reverses itself when the problem is fixed sometimes not. It is often fatal because it is usually a symptom of a serious problem.
I just bought a female panther and she came in with a huge swelling on her chest/gular region. The seller was kind enough to offer me a full refund if it does not go away by next week and told me to keep her any way.
It does not look to be improving at all so it looks like I will be nursemaid for a while. I am bummed because I wanted her for breeding but I don't want to get rid of her either because she is very sweet. It is sad because I will probably get attached to a pet I won't have around for long. I have not even been able to give her a name because if I get too attached it will be even harder if she does not make it. At least she is eating well so keep your fingers crossed for her. 
Sonia