After you apply to MSC it will email you a preliminary medical survey. Based on your answers you will receive a Medical Summary Form noted for your specific health issue(s) to be completed by your health care provider.
If your provider indicates the issue is controlled and considers you to be fit for duty (FFD) at sea MSC will evaluate the info on the Form and either find you FFD (or not) and instruct you if NFFD.
Good luck.