The mastoid is a part of the side (temporal bone) of the skull. It can be felt as a bony bump just behind and slightly above the level of the earlobe. The mastoid has been described as resembling a "honeycomb" of tiny partitioned-off airspaces. The mastoid is connected with the middle ear, so that when there is a collection of fluid in the middle ear, there is usually also a slight collection of fluid within the airspaces of the mastoid.

Mastoiditis can range from a simple case of some fluid escaping into the mastoid air cells during a middle ear infection, to a more complex infection which penetrates through to the lining of the mastoid bone, to a very severe and destructive infection of the mastoid bone itself.

Mastoiditis is caused by the same types of bacteria which cause middle ear infections (Streptococcus pneumoniae and Haemophilus influenzae), as well as by a variety of other bacteria (Staphylococcus aureus, Pseuodomonas aeruginosa, Klebsiella, Escherichia coli, Proteus, Prevotella, Fusobacterium, Porphyromonas, and Bacteroides). Mastoiditis may occur due to the progression of an untreated, or undertreated, middle ear infection.

Symptoms of mastoiditis may at first be the same as symptoms of an early middle ear infection. With progression, however, the swollen mastoid may push the outer ear slightly forward and away from the head. The area behind the ear will appear red and swollen, and will be very sore. There may be drainage of pus from the infected ear. In some cases, the skin over the mastoid may develop an opening through which pus drains. Fever is common.

Mastoiditis is usually suspected when a severe middle ear infection is accompanied by redness, swelling, and pain in the mastoid area. A computed tomography scan (CT scan) will show inflammation and fluid within the airspaces of the mastoid, as well as the erosion of the little walls of bone that should separate the air spaces. If there is any fluid draining from the ear or mastoid, this can be collected and processed in a laboratory to allow identification of the causative organism. If there is no fluid available, a tiny needle can be used to obtain a sample of the fluid which has accumulated behind the eardrum.

Identification of the causative organism guides the practitioner's choice of antibiotic. Depending on the severity of the infection, the antibiotic can be given initially through a needle in the vein (intravenously or IV), and then (as the patient improves) by mouth.

In the case of a very severe infection of the mastoid bone itself, with a collection of pus (abscess), an operation to remove the mastoid part of the temporal bone is often necessary (mastoidectomy).

With early identification of mastoiditis, the prognosis is very good. When symptoms are not caught early enough, however, a number of complications can occur. These include an infection of the tissues covering the brain and spinal cord (meningitis), a pocket of infection within the brain (abscess), or an abscess within the muscles of the neck. All of these complications have potentially more serious prognoses.

Prevention of mastoiditis involves careful and complete treatment of any middle ear infections.