CN IV, the trochlear nerve, innervates the dorsal oblique muscle that passes over a trochlea to rotate the globe medially. CN III also innervates the levator palpebrae superioris muscle (to elevate the upper eyelid) and most of the extraocular muscles. Parasympathetic fibers of CN III, the oculomotor nerve, comprise the efferent arm of the PLR. Cranial nerve II (CN II), or the optic nerve, is the afferent arm of the PLR and vision and transmits sensory information to the brain. Most of the cranial nerves are involved in vision, PLRs, eye position, and movement. Role of the Cranial Nerves in the Neuro-ophthalmic Examination Recording all examination findings is essential ( Web Figure 80-1) to compile the information that must be reviewed before making a decision regarding the cause of the neuro-ophthalmic defect or anisocoria. Behavioral changes often accompany visual deficits because the pathways for both are intimately associated in the brain, particularly the forebrain.Ī complete ophthalmic examination (see Chapter 242), including measurement of intraocular pressure, examination of the cranial nerves, and a full physical examination, always should be done. Alternatively, owners may try home remedies, which can include medications used for a previous condition, that may alter the pupil size. For example, if atropine is used as a mydriatic instead of a more appropriate short-acting agent such as tropicamide, the resulting mydriasis could last up to 2 weeks. The owner should be asked about any trauma or absence of the pet from its environment, as well as prior treatment by either the owner or a previous veterinarian. This is termed physiologic dynamic anisocoria and does not indicate a pathologic condition.įor all patients in which a neuro-ophthalmic defect, anisocoria, or abnormal PLR is diagnosed, a thorough medical history that includes past medical conditions, current medications, current medical conditions, behavioral changes, and physical signs should be elicited. Therefore it is normal for the pupil receiving direct stimulation to be smaller than the fellow (nonstimulated) pupil. Although pupils of both eyes react to light when it is directed at only one eye, the iris sphincter muscle of the directly stimulated eye receives more efferent impulses than the nonstimulated eye. At the optic chiasm, nerve fibers cross to the contralateral side, whereas at the pretectal nucleus, nerve fibers cross back to the original (ipsilateral) stimulated side. Some degree of dynamic anisocoria is normal because of incomplete decussation of afferent fibers at two locations: the optic chiasm and the pretectal nucleus. In a patient with dynamic anisocoria, the difference in pupil size depends on the stimulation of one pupil. Static anisocoria is unequal pupils when both eyes are receiving equal illumination. There are two types of anisocoria: static and dynamic. Once the cause is known, a prognosis can be given. Identification of lesions enables a short list of potential causes to be assessed. This chapter provides a logical guide to address frequent owner concerns: What is wrong? What caused it? How is it treated? Will it get better? These goals are best achieved by first examining the eye with the normal pupil, assessing the PLRs, and then evaluating the eye with the abnormal pupil. The clinician’s role is to determine which pupil is abnormal and whether the underlying cause is ophthalmic or neurologic. This abnormality also may be associated with abnormalities of the pupillary light reflex (PLR). This chapter summarizes key aspects of this examination and interpretation of the findings.Īnisocoria (unequal size of the pupils) is abnormal and is a common neuro-ophthalmic problem. The ultimate goal is to provide the information necessary to proceed with diagnosis, prognosis, and treatment of the pet. The neuro-ophthalmic examination is a tool that localizes a defect to the nervous system or the eye.
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