As with most body systems, the vestibular system is bilateral. We have two eyes, two ears, and thus, a two sided sense of motion. A vestibular problem can persist for days, or evolve into ongoing problems when the head moves, which may persist and even become permanent.
Like a teeter totter when one side is suddenly empty, the vestibular system is thrown out of balance when signals into the brain stem are unequal from different sides of the head. When one side of the vestibular system is disrupted, for any reason, a person is likely to experience intense vertigo, nausea and dysequilibrium. This imbalance results in a condition called nystagmus, with accompaning nausea and vertigo.
The most important function performed by the Vestibular System isn't balance, but allowing the eyes to focus when the head is moving. The retina of the eye, is closely analogous to the film in a camera. As anyone who uses a camera soon learns, if the camera moves while the shutter of the camera is open, the image will be blurry. The basic optic rules which govern a camera lens and film, also apply to the lens and the retina of the eye.
Our eyes are not fixed on top of a tripod, but mounted in a contantly moving object, the head. Thus, without some compensatory mechanism, we could not focus when our head was moving. The compensatory mechanism is called the Vestibular Ocular Reflex ("VOR"), which is in response to the input of the Vestibular System, as to head movements. In essense, the VOR, responds with a counter motion of the eye, whenever the head moves.
When the Vestibular System isn't working properly, Nystagmus occurs. Nystagmus is a rhythmic, repetitive, oscillatory eye movement. It may occur as a result of vestibular problems, as a result of visual problems, or as a result of certain brain stem or cerebellar abnormalities. Vestibular Nystagmus is characterized by a slow migration of the eyes in the wrong direction, followed by a sudden jerking back response. Nystagmus is easy to detect by a trained observer, and the ability to measure it and to identify its cause, through the ENG test, provide us one of the clearest footprints of pathology after head injury and associated brain damage.
