There is lots of data that shows sinus cultures done at the same time as nasal cultures in kids with sinusitis generally grow different organisms. I choose antibiotics on the basis of actual reports of bacteria found in infected sinuses in large studies rather than nasal cultures in individual patients, although I do culture the sinuses of children that come to surgery. No child should undergo sinus surgery who has not had an aggressive course of medical treatment. Sinus infections can be caused by enlarged adenoids in children. Adenoids, like other lymphoid tissue, get bigger when they get infected.
Size matters when a breathing passage is concerned.
Big adenoids mean less room for air passage through the nose. Very large adenoids can block the back of the nose completely. This blockage interferes with the normal circulation of sinus and nasal secretions. Secretions cannot clear in a natural way. Sometimes a sinus infection was the cause for adenoids to enlarge enough to block the back of the nose in the first place.