Labyrinthitis is an inflammation of the inner ear. The labyrinth is a structure of fluid-filled sacs and tubes just inside the skull. It contains two important organs: the cochlea, which is necessary for hearing, and the balance organs (vestibular system), which tell people which way is up and down, even when their eyes are shut. Either one or both of these organs can be affected. The entire labyrinth is less than a half inch (1.25 cm) across, so infection can easily spread throughout.
The inner ear also contains a bundle of nerves leading from these organs to the brain. Inflammation of these nerves is called vestibular neuronitis (or vestibular neuritis). It's sometimes difficult to find out whether it is the labyrinth or the nerves that are inflamed.