Today's Veterinary Practice

JAN-FEB 2016

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tvpjournal.com | January/February 2016 | Today's VeTerinary PracTice obserVaTions in oPhThalmology Peer reviewed 77 anisocoria is defned as pupil asymmetry, and may be seen with ocular or neurologic dysfunction (Figure 1). 1 When anisocoria is caused by neurologic disease, unequal pupil size may result from malfunction of the sympathetic, parasympathetic, or visual systems. When evaluating patients with asymmetric pupils, the practitioner needs to: 1. determine whether one or both pupils are abnormal in size 2. localize the lesion responsible for anisocoria. Miosis refers to smaller than normal pupil size, while mydriasis refers to larger than normal pupil size. NEUROANATOMY Visual Pathway The visual pathway (Figure 2 ) is composed of the retina, optic nerve (also known as cranial nerve ii), lateral geniculate nuclei (lgn) in the thalamus, and occipital cortex in the cerebrum. When light enters the eye, it activates the retinal photoreceptors. This information travels along cranial nerve (cn) ii via the optic chiasm to the optic tracts and then the lgn. optic radiations relay the visual information from the lgn to the occipital cortex. in dogs, about 75% of optic nerve fbers cross to the opposite cerebral cortex at the optic chiasm. 1 Parasympathetic Function: Pupil Constriction The parasympathetic pathway to the eye (Figure 3, page 78) is a short, 2-neuron pathway that originates in the midbrain. The paired parasympathetic nuclei of cranial nerve iii (Psn cn iii), along with the somatic nerves from the oculomotor nerve (cn iii), send fbers—called frst order neurons, or preganglionic fbers—to the eye. after synapsing in the ciliary ganglion, the short, postganglionic fbers course to the iris sphincter muscle and cause pupil constriction. The Practitioner's guide to Neurologic causes of caNiNe aNisocoria Heidi Barnes Heller, DVM, Diplomate ACVIM (Neurology), and Ellison Bentley, DVM, Diplomate ACVO University of Wisconsin–Madison FIGURE 1. Representation of a dog with anisocoria. Cranial Nerves Relevant to Anisocoria CN II Optic nerve CN III Oculomotor nerve PSN CN III Parasympa- thetic nucleus of cranial nerve III (previously Edinger- Westphal nucleus) CN V Trigeminal nerve CN VII Facial nerve FIGURE 2. Visual pathway from the ventral aspect of the brain. The blue and orange lines represent visual felds from each eye. White arrow—optic chiasm. Black arrows—lateral geniculate nuclei in the thalamus.

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