Involuntary, jerky oscillating movement of the eyeball.
Nystagmus is a disturbance in the coordination of the muscles of the eye.
It can be caused by illness, very fast movements or the abuse of certain substances.
Oscillations can occur in a vertical, horizontal, torsional plane, or a combination of these.
The eye moves slowly in one direction, then suddenly returns to its original position with a muscular jerk.
The direction of nystagmus is defined as that of rapid movement (however, the disorder tends to cause slow movement first, before rapid movement to the original position occurs to restore the situation).