Vestibular nystagmus, the most common type of nystagmus, is caused by dysfunction of the vestibular part of the inner ear, the nerve, the vestibular nucleus within the brainstem, or parts of the cerebellum that transmit signals to the vestibular nucleus.
Vestibular nystagmus is a "linear slow phase" nystagmus. The eye travels at constant speed in one direction, and then there is a resetting saccade in the opposite direction. The nystagmus is generally identified by the "beat direction". As right is generally up and left down, the nystagmus above would be left-beating.
Vestibular nystagmus is vectored in "canal planes". The semicircular canals are oriented into 3, nearly perpendicular planes. Each canal on one side of the head is "paired" -- roughly in the same plane, as another on the opposite side.
The combined lateral canal plane is then the "lrh". There are also two oblique pairs -- the "larp" -- left anterior-right posterior, and the "ralp" -- right anterior, left posterior. The vector of excitation from each canal is combined to provide the brain with a 3-dimensional vector reflecting head velocity.
Damage (lesions) or excitation (usually BPPV) of the canals causes nystagmus in the plane of the affected canal. This relationship is variously called "Flouren's laws", or "Ewald's 1st law", after the physiologists who first observed this in the 1800's.
|Waveform||Jerk with linear slow phase|
|Vector of common lesion||Canal plane or combination of canal planes|
|Lateral canal||Inhibition beats mainly opposite, excitation mainly same.|
|Posterior canal||Excitation, such as in BPPV, beats upward and ipsitorsion (to the down ear)|
|Superior canal||Excitation, such as in AC BPPV, beats downward and ipsitorsional. As in AC BPPV the excited canal is up, practically the torsion should be contra to the down ear.|
|Effect of light||reduces nystagmus other than torsional component.|
|Effect of gaze||Increases when looking in direction of fast phase. This is called Alexander's law.|
A more subtle point concerning the vector observed by the examiner, is that it "depends" on where the eye is looking. What actually moves in canal planes is not the center of the pupil, but the eyeball as a whole. When the eye is "straight ahead", the vectors above apply. When the eye is away from straight ahead, the apparent vector of the center of the eye changes.
Consider the effect of exciting the right posterior canal. If a person looks in the plane of the right posterior canal (about 45 degrees to the left), then the center of the pupil moves vertically. If the person looks perpendicular to the plane, (about 45 degrees to the right), then the movement apparent to the outside observer is pure torsion.
Of course, this then means that the direction of visual vertigo percieved by the person, with eyes open, depends on where they are looking. This is mainly relevant in vertical canal plane disorders (e.g. BPPV).