Unilateral Vestibular Loss

Unilateral vestibular nerve loss simply means that one of the two vestibular nerves (nerves that run from each ear to the brainstem) is not functioning properly.  There are many causes for this, and when this occurs suddenly, it is quite symptomatic, causing vertigo (an illusory sense of movement or "spinning") and often vomiting.  The end result is that for at least several days following such a sudden injury, a very typical pattern of nystagmus is observed (see movie below).  The eye slowly drifts to the side of the vestibular nerve damage, then quickly moves away from the side of vestibular nerve damage.  This is called 'jerk' nystagmus, and in this case 'left-beating' nystagmus, suggesting disfunction of the right vestibular nerve.  This type of nystagmus follows 'Alexander's Law', in that the amplitude of nystagmus increases when the patient looks to the left and decreases when the patient looks to the right.  This nystagmus will typically decrease and resolve over a period of weeks or months.  The most common cause for this is vestibular neuritis (labyrinthitis if hearing loss also occurs) and most patients will recover from this completely with vestibular rehabilitation therapy.  Other causes include Meniere's syndrome, head truma and a benign tumor known as a vestibular schwannoma.

The video above is of a patient who has suffered vestibular neuritis of the right vestibular nerve.  Note the 'jerk' 'left-beating' nystagmus present which increases in the gaze direction of the nystagmus (left) and decreases in gaze to the right, following 'Alexander's Law'.  Below is a tracing from caloric testing, revealing a weakness of the right vestibular nerve of 38%.

The videonystagmogram (VNG) tracing above displays a 'left-beating' nystagmus and reveals another characteristic of a peripheral (from the inner ear to the brainstem) vestibular nerve disfunction, that of 'fixation suppression'.  This means that the amount of nystagmus is reduced when the patient fixates on something, in this case, a light inside the goggles that are worn.  The VNG tracing below displays 'left-beating' nystagmus, which is continuous throughout the entire recording time.

Below is a Rotary Chair tracing revealing typical findings of an acute unilateral vestibular loss, including reduced VOR gain, VOR asymmetry and phase lead.  Once central compensation occurs, the VOR gain may improve and the VOR asymmetry resolved, but the phase lead often remains.

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