Untreated sinus infections can lead to bronchitis, asthma, worsening of asthma, chronic cough, ear infections and throat infections. Continued infection also leads to swelling of the nasal membranes, nasal obstruction, and mouth breathing. The nasal congestion can cause snoring and lead to sleep apnea as well.
If acute sinusitis is not treated, it can lead to a low grade chronic infection. Chronic sinusitis is more likely to result in the development of nasal polyps. The presence of nasal polyps makes treatment of the sinusitis more difficulty and the polyps will ultimately cause complete nasal blockage.
In severe cases, sinus infections will spread into the blood stream, or spread to adjacent structures such as the brain and eyes. Such spread of infection could lead to blindness or double vision and can be life threatening.