Lesions of the efferent pathway will not affect vision and include a disease or condition affecting the mobility of the iris/pupil to constrict or a lesion along the parasympathetic pathway. The pupil will be mydriatic because there is no afferent signal telling the pupil to constrict. Lesions of the afferent pathway (retina, optic nerve, chiasm) will also affect vision. Anisocoria due to an abnormal mydriatic pupil can be due to a lesion in the afferent or efferent PLR pathway. Therefore, a lesion along the parasympathetic pathway will result in a dilated (mydriatic) pupil and a lesion along the sympathetic pathway will result in a constricted (miotic) pupil. Mark Neer, Oklahoma State University, Center for Veterinary Health Sciences).Īlthough there is thought to be some crossover, in general the parasympathetic pathway is responsible for pupillary constriction and the sympathetic pathway is responsible for pupillary dilation. Diagram illustrating the visual pathway, pupillary light reflex pathway, and the sympathetic nerve pathway (with permission for use from Dr. Because the efferent arm of the PLR breaks away from the visual pathway fibers long before the end of the visual pathway (occipital cortex), the PLR pathway is independent of vision.įIGURE 47-1. Meanwhile, the afferent arm of the visual pathway continues beyond the lateral geniculate body to the optic radiation and occipital (visual) cortex. The parasympathetic fibers that run with the oculomotor nerve and the iris constrictor muscle make up the efferent arm of the PLR. Both pathways are the same until just before the lateral geniculate body of the thalamus where sensory fibers for the PLR leave the optic tracts to activate the parasympathetic centers of the oculomotor nerve. The afferent arm of the PLR and visual pathway is composed of the retina, optic nerve, chiasm, and a portion of the optic tract. To work through a case of anisocoria requires an understanding of the PLR pathway, the visual pathway, and sympathetic innervation to the eye (see Fig. Careful examination can narrow the list of possible etiologies for anisocoria and help the clinician diagnose potentially blinding or even life-threatening disease. Ophthalmic and neurologic etiologies overlap for the optic nerve. Anisocoria can occur with primary ophthalmic disease defined as anterior to and including the retina or it can occur with primary neurologic disease defined as posterior to and including the optic nerve. It is often the first abnormality noted by an owner or detected on a routine physical examination. The remaining ophthalmic examination is unremarkable.Īnisocoria is defined as unequal pupil size. In the right eye there is a negative direct PLR but positive consensual reflex to the left eye in the left eye there is a positive direct PLR but a negative consensual reflex to the right eye. Ophthalmic examination reveals anisocoria with the right pupil larger than the left. Menace reflex is positive in both eyes and direct and consensual pupillary light reflexes (PLRs) are positive however, movement of the left pupil is less due to the initially small pupil size.Ī 10-year-old male/neutered miniature poodle presents for annual vaccinations and wellness examination. Ophthalmic examination of both eyes is normal with the exception of anisocoria (left pupil smaller than the right), smaller palpebral fissure, and partially prolapsed nictitans in the left eye. You might have a serious eye, brain, or neck injury that requires emergency treatment.A 9-year-old female/spayed, domestic shorthair cat presents with the left eye appearing smaller to the owner for 1 week. If you experienced a head injury before your pupils changed in size, contact 911 or go to the hospital immediately. For example, be sure to mention if you’ve recently experienced:ĭepending on your symptoms and medical history, your doctor might order one or more tests to help diagnose the underlying cause of your anisocoria. You should also discuss any other symptoms you’ve been experiencing. If you don’t already have a provider, our Healthline FindCare tool can help you connect to physicians in your area.ĭuring your appointment, your doctor will examine your eyes and have your vital signs taken. If you notice a difference in size between your pupils, contact your doctor right away. How will your doctor diagnose the cause of anisocoria?
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