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Today's VeTerinary PracTice | January/February 2016 | tvpjournal.com obserVaTions in oPhThalmology Peer reviewed 78 The parasympathetic pathway is best assessed using the pupillary light refex (Plr): • When a bright light enters the eye, a proportion of crossed cn ii fibers enter the pretectal nucleus in the midbrain to synapse with neurons which, in turn, synapse with efferent parasympathetic fibers in Psn cn iii. • These parasympathetic fibers transmit this information to the eye, resulting in pupillary constriction. • a relay between the paired Psn cn iii in the midbrain results in indirect (or consensual) Plr. • clinically, this can be observed when a bright light is shone in one eye and the opposite eye also constricts. The degree of constriction is lesser in the opposite eye. Sympathetic Function: Pupil Dilation The opposing system is the sympathetic system, which is responsible for pupillary dilation. The sympathetic pathway (Figure 4) is a 3-neuron pathway that takes a longer course to the eye compared with the parasympathetic system. • sympathetic function originates in the hypothalamus of the brain and courses as the first order neuron through the brainstem and cervical spinal cord to thoracic spinal cord segments T1 to T3. • The second order neuron exits the spinal cord between T1 and T3, courses cranially through the thoracic cavity, out the thoracic inlet, and along the jugular groove to the cranial cervical ganglion. • The third order neuron exits the cranial cervical ganglion, runs through the middle ear, then FIGURE 3. The parasympathetic pathway. The frst order neuron is depicted in orange and the second order neuron is depicted in yellow. In a patient with left eye miosis, left thoracic limb paresis, and absent to decreased left thoracic limb refexes and postural reactions, the practitioner would localize the lesion to a site that would affect all these structures simultaneously. In this patient, loss of left brachial plexus function and sympathetic innervation to the left eye could be explained by a single lesion at the brachial plexus, which is located between spinal cord segments C6 and T2. I Table 1. anisocoria of Neurologic origin: Key Neurologic Tests TesT NorMal reacTioN lesio N localiZaTioN (if abnormal response) Visual Pathway Menace response Blinking of eyelid • Cerebellum • Cerebrum • CN II • CN VII • Eyelid • Orbit (exoph- thalmos) • Retina* • Thalamus Visual tracking (cotton ball test) Following a soundless, odorless object with eyes/head • Brainstem • Cerebrum • CN II • Retina Pupillary light refex Pupil constriction with direct bright light • CN II • CN III • Iris • Midbrain • Retina sympathetic Pathway Dark room Dilation of pupil • CN II • Iris • Retina • Sympathetic pathway Parasympathetic Pathway Pupillary light refex Ipsilateral and contralateral pupil constriction with direct bright light • Brainstem • CN II • CN III • Iris • Optic chiasm • Retina Swinging light test Ipsilateral and contralateral (indirect) pupil constriction with bright light as it is quickly moved between eyes • CN II • Retina * An unobstructed or minimally obstructed optical pathway is required for menace response. For example, lenticular pa- thology may obstruct the patient's visual feld suffciently to reduce the response to menace. Maze test or visual tracking may be useful in this situation.