root canal on upper of lower teeth can cause sinus infections. The best method is get to the MD request a NASAL SMEAR forget about the ENT they will charge you a bunch of money I've seen tons of specialist and the ENT really charge the most and they do the least...my experience anyway You want the ENT tests if you want to know the level of inflammation and you suspect polyups I prefer Thermagrapy no radiation better results and cheaper .. MY dentist is the one that told me I had a very bad sinus infection when I went to the dentist for routine visit they did the typcial dental x-ray showed major sinus infection that x-ray cost like 30.00 at the dentist and the ENT with great insurance was 265.00. ..anyhow my nasal smear found I had Ecoli YIKES and fungus infection. Apparently caused by serious lower teeth root canal infections... I did not even know I had this until I decided to have both root canal extracted because I read they can make you super sick. The key to sinus infection is first identify what is in you sinuses then begin a attack plan of total saturation of the nasal tissues with a nasal irrigation tool and the antagonist of choice, Seriously this will work if done right and, breathing in steam with powerful oils helps and be consistenT. Take high doses of vitamin C for four months to be sure. Work closely with ND because they have other effective natural ideas that you MD will never have in a million years! However if you want the hammer treatment go see the MD