Pretty complicated question. I'll tell you what I know, but I'm not an expert, and you will need to get professional medical and career advice.
Some airlines, especially smaller ones and regionals, don't perform a medical exam, they just photocopy your FAA medical. It has to be a first class to get hired at most or all airlines, but it can drop to 2nd class if you are a first officer.
Others will perform a very thorough medical exam (like all day long, or even several days) and apply minimum standards that are far above FAA mins. Assume that almost all foriegn carriers will do this, and some of the US carriers.
Let's look at several Pilots:
Pilot A: Medically perfect, 20/10 vision, never been injured, no bones broken, never hospitalized, no allergies, no prescriptions, perfect body fat ratio, blood pressure, etc. This guy would hold a clean FAA medical and could pass a NASA flight physical. He might be able to get hired at Cathay Pacific or other foriegn carriers. He could get hired at any US airline.
Pilot B: Normal Guy, 20/20 vision, has some history of medical problems and/or injuries, all of which have been taken care of in the past and are not currently present. Holds a clean FAA medical, and could get hired at most US airlines, but a few might reject him for his history.
Pilot C: Just like B, but has uncorrected vision worse than 20/20, say 20/200. His vision corrects to 20/20 or better with glasses. His FAA medical will state that he must wear corrective lenses while flying. He will have the same opportunities as B, except a few US airlines might want vision that is 20/100 or better uncorrected.
Pilot D: Has some minor but active medical problems, unresolved injury, or disability. He may be able to get an FAA 1st Class medical with either a "Special Issuance" or "Statement of Demonstrated Ability". This medical will likely not be accepted by many or possibly all US airlines. If he is trying to get a job, he is probably in real trouble. If he ALREADY has an airline job, he gets to keep his job.
Pilot E: Similar to D, but his problem is bad enough that he loses his 1st. class medical, and is allowed to hold a second class medical. If he is trying to get an airline job, he's scr*wed. If he already has an airline job, he will can stay, but will be demoted to first officer and will never be able to be an airline captain unless he can regain the 1st class. He can work as a non-airline commercial pilot, but will likely have trouble getting hired by some employers.
Pilot F: Serious problem (such as diabetes, cardiovascular, etc) that causes the loss of 1st and 2nd class medical. He cannot work as a commercial pilot, and if he loses his third class cannot even fly recreationally without another pilot.
You goal is to be pilot A, B, or C. You would really prefer not be D and have to deal with the special statements on your 1st. class medical. You need to get some professional advice BEFORE you apply for a medical...that way you can have the best shot at dealing with your issue and getting a CLEAN first class medical the first time. Check out these guys, they do that kind of consulting:
http://www.aviationmedicine.com/inde...useaction=home