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. In acute mastoiditis, infection may spread outside of the mastoid bone and cause serious health complications.
Mastoiditis typically affects children, but adults can also be affected.