Mastoiditis is a bacterial infection of the mastoid bone. The mastoid bone, which sits behind the ear, consists of air spaces that help drain the middle ear. When the mastoid cells become infected or inflamed, often as a result of an unresolved middle ear infection (otitis media), mastoiditis can develop.astoiditis symptoms may include: 1  Fever, irritability, and lethargy. 2  Swelling of the ear lobe. 3  Redness and tenderness behind the ear. 4  Drainage from the ear. 5  Bulging and drooping of the ear (in acute mastoiditis).