A bacterial infection within the middle ear is the most common cause of fluid in mastoid cells, according to WebMD. Mastoiditis occurs when too much fluid is found in mastoid air cells, which are honeycomb-like structures filled with air that drain into the middle ear. When the middle ear becomes infected, the inflammation may pass into the mastoid cells, causing fluid buildup in the mastoid bone.
The most common treatment for fluid in the mastoid cells is antibiotics. Most cases of mastoiditis are solved with this regimen. Minor procedures, such as draining fluid from the ear or inserting an ear tube, may resolve chronic mastoiditis. Draining the fluid involves puncturing a small hole in the eardrum to relieve pressure on the middle ear. Severe cases of mastoiditis, especially if the infection isn't treated right away, may require a mastoidectomy, a procedure that involves a surgeon removing the infected bone.
Mastoiditis is more commonly diagnosed in children, but the affliction may happen in adults. Symptoms of the disorder include pain behind the ear, swelling or redness behind the ear with a small protrusion, fever, drainage from the ear and irritability. A doctor inspects the ear with an otoscope and then cultures a sample from the infected ear to determine if mastoiditis is occurring. A head scan detects major swelling of the bone.