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tvpjournal.com | January/February 2016 | Today's VeTerinary PracTice obserVaTions in oPhThalmology Peer reviewed 79 alongside the ophthalmic branch of the trigeminal nerve (cn V), and ends in the periorbital muscles, third eyelid, and iris dilator muscles. dysfunction anywhere along this pathway results in a comparatively miotic pupil, which results from failure of the iris to dilate in reduced ambient light, typically accompanied by enophthalmos, ptosis, and third eyelid protrusion over the globe. LESION LOCALIZATION Neurologic Examination The neurologic examination allows the practitioner to localize the lesion to the visual, sympathetic, or parasympathetic pathways (Table 1). since each neurologic test has a sensory (afferent) and a motor (efferent) component, the examiner must determine which component is affected. Table 2 lists the tests most commonly used to evaluate the neuro-ophthalmic system. a detailed description of how to perform each test can be found elsewhere. 2 Pharmacologic Testing Pharmacologic testing can aid the practitioner with lesion localization within the sympathetic or parasympathetic system. Physostigmine (0.5%) is a cholinesterase inhibitor and, thus, requires an intact postganglionic neuron to induce miosis. • Topical administration differentiates between pregan - glionic and postganglionic parasympathetic lesions. Table 2. lesion localization for a Dog with anisocoria lesioN locaTioN MeNace Visual TracKiNg PuPillarY ligHT refleX (Direct & indirect) DarK rooM (Pupil Dilation) MeNTaTioN Iris + + Direct (−) Indirect (+) − Normal Retina − − − − Normal CN II − − − − Normal Thalamus − − + + Abnormal Visual cortex − ± + + Abnormal CN VII − + + + Normal Cerebellum +/− + + + Normal First sympathetic neuron + + + − Normal or abnormal Second sympathetic neuron + + + − Normal Third sympathetic neuron + + + − Normal First parasympathetic neuron + + − + Normal or abnormal Second parasympathetic neuron + + − + Normal FIGURE 4. The sympathetic pathway. The frst order neuron is depicted in purple, the second order neuron is depicted in blue, and the third order neuron is depicted in teal.