With my Crohn's disease I really never needed any long term periods off work but I often needed a day or two here or there due to symptoms so I applied for intermittent FMLA which worked best for me. My family doctor filled out the FMLA paperwork.
My immediate supervisors understood my plight and were okay w/it. I had an assigned substitute so as long as she was available on short notice (God bless her she almost always was!) it didn't create a scheduling problem for my office or supervisors.
There were those higher up the food chain who were NOT happy about the frequent days off irregularly thru the year simply because they wanted reliability despite there wasn't a scheduling problem. The excessive sick leave bothered them no end.
Whatever, intermittent FMLA worked for me for 3 years. I sincerely hope you are approved and that it works for you! Good luck and God bless.