Sinusitis. Illustration of a cutaway view of a woman's nasal cavities and facial bones, showing sinusitis on her left side, an inflammation of the mucous membranes that line the paranasal sinuses. Shown here are the frontal sinus, the ethmoid sinus and the maxillary sinus. These are air spaces in the facial bones around the eyes and nose. An infection of the membranes in these spaces causes an increase in mucus production, which can block the sinuses, causing internal pressure. It is this pressure that leads to pain and discomfort. Blockage of the sinuses often occurs when suffering from a cold or influenza. Treatment is with antibiotics and decongestants.