Sinuses are air-filled spaces on either side of the nose, within the cheeks and forehead, and between the eyes. They can become blocked with mucus causing pressure and congestion leading to nasal blockage and headaches. If the sinuses are infected this can cause pain in the cheeks and forehead, around the eyes and sometimes in the upper teeth. Sinusitis can also affect sense of smell and breathing, and lead to increased mucus production (runny nose).