Bells Palsy is caused by an inflammation within a small bony tube called the fallopian canal. The canal is an extremely narrow area. An inflammation within it is likely to exert pressure on the nerve, compressing it. Likewise, if the nerve itself becomes inflamed within this small canal, it can encounter pressure, with the same result of compression. The nerve has not yet exited the skull and divided into its several branches, resulting in impairment of all functions controlled by the 7th nerve. If only part of the face is affected, the condition is not Bell's palsy. If, for example, the mouth area is weak but the forehead moves, Bells palsy is ruled out. Trauma induced by tumor, surgery, etc. can occur at a location where the nerve has already divided into its main branches. This type of trauma may spare one or more branches and allow some muscles to remain functional.