Infection of the small chambers containing air in the mastoid bones, which can be felt as bumps behind the ears. The bone contains small cavities lined with mucous membrane and connected with the middle ear. In cases of acute otitis media (inflammation of the middle ear) the mucous membrane of these cells is always painful; this is noticeable as pressure pain behind the ear. If the connection between mastoid and middle ear is closed, pus accumulates in the cavities and the bone also becomes inflamed. This condition is known as acute mastoiditis, and it should be suspected if the ear is not dry four weeks after acute otitis, or if swelling occurs behind the ear, associated with severe pressure pain and excessive protrusion of the ear. Other symptoms resemble those of acute inflammation of the middle ear: earache and fever. Undiscovered mastoiditis can sustain otitis, and cause glue ear. In the early stages the inflammation can sometimes still be treated with antibiotics. Later it is usually necessary to operate: an incision is made behind the ear, and the diseased cells and pus removed.