When a horse moves its head around, their brain and inner ear control eye movement. This is known as normal or physiologic nystagmus. When this system is damaged, inappropriate or jerking movements of the eye are known as pathologic nystagmus. These abnormal movements can be seen when the horse's head is still.

Abnormal nystagmus is rare in horses and is most likely associated with brain damage, recovery from anesthesia, or middle ear dysfunction. Nystagmus is most often seen in horses that are suffering from severe brain injury, as can occur after a horse rears over backwards and lands on their poll. In most cases, horses with obvious pathologic nystagmus are lying on the ground or are in danger of falling to the ground.