Neurologic Examination
Evaluation of Cranial Nerve Function
Overview and Goal(S)
Evaluation of cranial nerve (CN) function can provide information on:
Evaluation of the CNs and cervicofacial reflexes:
• Head and neck posture are influenced by the cerebellar and vestibular systems.
• Horses with cerebellar disease often show fine resting tremors of the head that become exaggerated by intentional movements.
• The level of consciousness can be evaluated as alert, depressed, stuporous, semicomatose, or comatose.
• The response of an animal to its environment is controlled by the cerebrum and brainstem. The normal horse should appear alert and responsive to external stimuli.
• Lesions of the cerebral hemispheres can result in comatose or semicomatose animals, whereas severe systemic illnesses can produce depression.
• Horses with cerebral disease may demonstrate inappropriate behavior, head pressing, or compulsive walking and circling.
• Recumbent horses with spinal cord disease often appear bright and alert, at least during the early stages of the problem.
• Horses with vestibular disease often have a head tilt in which the poll is deviated toward the side of the lesion. There will be weakness on this side as well, with increased tone on the opposite side that often results in circling toward the side of the lesion.
• CN examination can help localize a problem in horses with neurologic disease along the brainstem.
CN II: The Optic Nerve
• Courses from the eye to the optic chiasm, where a significant number of the fibers cross over to the contralateral side (about 80%).
• From the optic chiasm, the fibers travel to the pretectal nucleus in the midbrain and from there to the Edinger-Westphal nucleus in the brainstem, which provides input to the oculomotor nerve (CN III) and ciliary ganglia.
• Evaluation of visual perception:
Requires input from the retina along the optic nerve to the optic chiasm, where it crosses over and continues along the optic tract to the lateral geniculate nucleus in the midbrain.
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