Could you tell me what happens if sinusitis is left untreated?

Spread of infection: Usually a self limiting disease. In rare case it can cause infection of the eye (orbital cellulitis or abscess). It can also in rare cases extend to the brain and cause meningitis or epidural abscess. Best to see a doctor if the infection lasts more than a week or other symptoms ie eye swelling, severe eye pain, severe head pain, uncontrolled fevers, double vision or change in vision, etc.