The sinuses are spaces in the skull that are lined with a mucous membrane. The mucous that is produced has to drain out and air has to circulate. Most sinusitis is caused by blockage of drainage areas. These are very narrow channels that can be blocked by structural factors such as a septal deviation, enlargement of the turbinate bones, abnormally small drainage passages, polyps or tumors. They can also be blocked by inflammation due to allergies or infection. Decongestants along with antibiotics are used to treat symptoms caused by sinusitis.