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tvpjournal.com | January/February 2016 | Today's VeTerinary PracTice obserVaTions in oPhThalmology Peer reviewed 81 • Pilocarpine can cause mild uveitis, resulting in blepharospasm, redness, and aqueous flare for up to 24 hours after administration. Dilute phenylephrine (1%) can be used to test the sympathetic system. • When applied topically to both eyes, it should cause rapid (typically 20 minutes or less) dilation in an eye with a postganglionic sympathetic neuron dysfunction. • administration in the normal eye has no effect. DIFFERENTIAL DIAGNOSIS lesion localization permits the clinician to assemble an appropriately ranked list of potential causes (Table 3). common differential diagnoses are discussed below using the damniTV schema. Iris Atrophy iris atrophy—thinning of the iris stroma, especially at the pupillary margin where the iris sphincter muscle is located—is a common fnding in older dogs. This condition may result in anisocoria and/ or P lr abnormalities if dysfunction of the sphincter muscle is notably asymmetric (Figure 5 ). diagnosis is made by careful examination of the pupillary margin. • iris atrophy appears as scalloping along the edge of the pupil and/or thinning of the tissue, which allows light reflected from the tapetum to pass through the translucent, atrophied areas. • in very early iris atrophy, loss of the iris stroma may expose the posterior pigmented epithelium of the iris, resulting in the darker areas in the pupillary margin, which are actually iris atrophy. a direct Plr will be slow or absent, while the consensual (indirect) refex in the contralateral eye should be normal. Neoplasia neoplasia may occur anywhere throughout the central or peripheral nervous system. Intracranial Neoplasia The most common intracranial neoplasm in dogs and cats is meningioma. anisocoria may result from disruption of either the afferent or efferent pathways of pupil innervation. meningioma routinely occurs in the cn ii, cerebrum, brainstem, and spinal cord. glioma, lymphoma, peripheral nerve sheath tumors, and cranial thoracic masses may cause anisocoria but are less common. Iris & Ciliary Body Neoplasia neoplasia of the iris and ciliary body can also cause anisocoria, through prostaglandin-mediated uveitis (miosis), secondary glaucoma (mydriasis), synechia formation (miosis, mydriasis, or dyscoria), or mass infltration of the iris with subsequent physical obstruction of the pupil. Idiopathic Sympathetic Dysfunction approximately 50% of dogs with sympathetic dysfunction (commonly termed horner's syndrome) are diagnosed with idiopathic dysfunction. 3 The lesion may be located in the second or third order sympathetic neuron based on pharmacologic testing. 4-6 no treatment is indicated for these animals and many spontaneously recover. 6 Dysautonomia dysautonomia is an idiopathic disease that affects both branches of the autonomic system. risk factors for development of dysautonomia include young to middle age, medium to large breed dogs, and living in rural housing. 7 many affected animals are visual, but demonstrate mydriasis with absent Plr, along with other systemic signs of autonomic dysfunction. dysautonomia is not treatable; therefore, humane euthanasia is usually recommended due to poor quality of life. Infectious/Infammatory Otitis Media otitis media accounts for 1% to 4% of the cases of sympathetic dysfunction in dogs and cats. 3,8 While management of otitis is important, miosis often persists. FIGURE 5. A 9-year-old, castrated male dachshund exhibiting anisocoria secondary to iris atrophy of the right eye.