The sinus cavities are hollow air spaces with openings into the nose to allow the exchange of air and mucus. They are located inside each cheekbone, behind the eyes, behind the bridge of the nose and in the forehead. Secretions from the sinus cavities normally drain into the nose.
Sinus headaches and pain occur when the sinuses are swollen and their openings into the nasal passages are obstructed, stopping normal drainage and causing pressure to build up.
Often the pain is localized over the affected sinus, perhaps causing facial pain rather than a headache. For example, if the maxillary sinus in the cheeks is obstructed, your cheeks may be tender to the touch and pain may radiate to your jaw and teeth. Obstruction in other sinuses can cause pain on the top of your head or elsewhere. Sinus pain can be dull to intense; it often begins in the morning and becomes less intense after you move from lying flat to sitting or standing in an upright position.
Similar pain can also be caused by severe nasal congestion, particularly if you have a deviated septum or a septal “spur” from a previous nasal injury. Such “headaches” or facial pain can involve one side only.