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Image by Florian Schneider

Ocular Position

Clinical Exam

Abnormalities of ocular position can be fixed/static related to dysfunction of cranial nerves or extraocular muscle disease. These disorders can also cause abnormal eyeball movement (see section on Nystagmus). A positional strabismus can be seen in association with vestibular disease, which does not cause eyeball deviation when the head is in a normal position. Evaluation is simply performed by visual examination of the position of each eyeball in its orbit with the head at rest in a level position.

Anatomic Pathway

The oculomotor and trochlear nerves can be seen to arise from the brainstem within the midbrain, However, CN IV is the only cranial nerve to completely cross over. The abducens nerve originates near the vestibular nucleus in the medulla of the brainstem.

Overview of Anatomy

Eye movements are controlled by the oculomotor (III), trochlear (IV) and abducent nerves (VI). Damage to one of these nerves results in deviation of the eye (strabismus), because the muscles innervated by the functioning nerves pull the eye away from the denervated muscles. Thus with loss of CN III a ventrolateral strabismus occurs. CN IV loss results in an outward rotation of the top of the eye. IN an animal with a round pupil like the dog, this can be detected by the position of the vessels in the fundus. Loss of CN VI function results in medial strabismus and absent globe retraction.

Anatomic Sections

Cross sections of the brainstem are illustrated here depicting the origin of the cranial nerves responsible for ocular position and movement.


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