Nystagmus is an involuntary movement of the eyes associated with vestibular or visual stimuli. Nystagmus is not always abnormal. Normal vestibular nystagmus occurs when the head moves in any direction. When the head moves in the horizontal plane to the right, the eyes will stay focused on an object in their visual field, and as the head moves out of that field, the eye will suddenly jerk in the direction of that movement. This is referred to as physiologic or vestibulo-ocular nystagmus. This type of nystagmus will also occur in a blind dog.4 Congenital nystagmus in dogs is usually associated with ocular abnormalities and visual defects.
Abnormal nystagmus is characterized by the presence of continual jerk nystagmus with the head motionless in any position. This is a spontaneous or resting nystagmus.4 The movement is either smooth and equal in both directions (pendular nystagmus), or alternates between a slow drift and a quick, jerk-like movement (jerk nystagmus)). Pendular nystagmus is usually seen in congenitally blind eyes. Horizontal nystagmus is a side-to-side movement most often seen in cats and dogs with vestibular disease. The "fast" component always "kicks away" from the side of the brain where there is a lesion; then this is followed by a slower return phase.5 Rotatory nystagmus involves more violent movement of the ball and usually indicates a more severe brain damage.
Animals with congenital eye abnormalities often show abnormal nystagmus. Pendular nystagmus nystagmus is observed in Collies with known congenital and hereditary eye disorders.3 Dogs affected by microphthalmia, persistent pupillary membrane, cataracts, or conditions with a very early onset of blindness, often show abnormal eye movements of wandering nature termed as "searching nystagmus."6 The Biard breed is affected by an autosomal recessively inherited retinal disease accompanied by nystagmus.7
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- Retinal degenerations: mechanisms and experimental therapy. Matthew M. LaVail, Joe G. Hollyfield, Robert Eugene Anderson