Very rare! Most of the time sinus infections are a major nuisance, painful, harm your productivity but pose little serious risk. HOWEVER dozens of times a year our hospital has to admit patients with severe infections that have gotten out of control and start to infect the eye (orbital cellulitis) or even the brain or bloodstream. These are uncommon but do happen from time to time. Longstanding low grade infections or obstructions in the sphenoid sinus can cause headaches and are the most likley to spread to serious brain or eye complications.