Hi,
I have a lengthy history chronic sinusitis with severe isolated sphenoid infection that I am still battling after six years. If your MRI shows complete blockage than I believe the sinus is already inflamed which requires some sort of intervention. I have had 4 operations and encountered no problems other than the usual post op fatigue and some pain. When I had my first CT scan and he saw my sphenoid was completely blocked I was booked in the next day for surgery due to the severity. Of course every person is different so I would suggest a discussion about where to go from here is needed.
Best of luck.