What causes it?

Mastoiditis is usually due to a bacterial infection and is almost always associated with otitis media. As the mucous lining of the mastoid cells is so closely linked to that of the middle ear it probably does occur to some extent in most cases of acute otitis media but in most cases it resolves within a few days either spontaneously or after treatment without becoming fully evident. In infants the immaturity of both the immune system and the structure of the mastoid process probably facilitate the development of infection within the mastoid bone.

It has been found that in around a third of cases there is a history of preceding acute otitis media and in 30% there is a history of recurrent otitis media. The infection may penetrate the thin walls of the air cells and cause infection in the bone with destruction of the structure of the mastoid bone.