Therefore, it does not test vision, and a normal PLR may be found in a cortically blind animal. Unlike the menace response, the pupillary light reflex (PLR) is a subcortical reflex. The pupillary light reflex and dazzle reflex. Some animals, especially cats, will follow the red light emitted by a laser pointer. Avoid throwing heavier objects that cause significant air movement or noise, because the animal may respond to these stimuli. A normal, alert animal that may not respond readily to a menace gesture will follow the cotton ball. Vision can be assessed in young puppies and kittens that may not yet have learned the menace response, and occasionally in stoic older animals, by throwing cotton balls in the air in front of the animal. A normal animal will extend its legs toward the surface before its paws touch the table ( Figure 1). For this test, lift the animal toward the table, allowing it to see the approaching surface. The visual placing response is useful when results of the obstacle course and menace response are equivocal. Be consistent in the obstacle course you construct, and make sure it can be navigated by normal animals. Vision also can be assessed using an obstacle course, navigated by the patient in both light and dim environments and with one eye patched. The menace response is absent in animals younger than about 10 to 12 weeks and may also be affected by the patient’s mental state.Īdditional visual tests. ![]() Therefore, in the absence of a menace response always test the blinking reflex by touching of the skin at both the lateral and medial canthi. False-negative responses may be caused by facial nerve paralysis. False-positive responses can also result from touching the patient’s eyelashes or hair, or causing air movement near the patient’s eyes. To avoid false-positive responses from the visual, contralateral eye (in cases of unilateral blindness), evaluate the menace response in one eye while the other eye is covered. Rather, it is a cortical response that requires the entire peripheral and central visual pathways, as well as the visual cortex and the facial nucleus of cranial nerve VII, to be intact. It is important to note that the menace response involves cerebral cortical integration and interpretation, and thus is not a reflex. This involves making a sudden threatening gesture that is supposed to elicit a blink response. Assessing the visual system in the blind animal Finally, perform a comprehensive ophthalmic examination, and evaluate the patient’s visual system. Furthermore, a complete physical examination and a thorough neurologic assessment are part of the workup of every case of blindness. Therefore, a complete (non-ophthalmic) history of the patient is required. Blindness may be caused by numerous systemic or neurologic diseases that frequently are accompanied by additional clinical signs. Next, ask about the animal’s overall health and other signs of illness. However, these changes are usually too subtle to be detected by owners. Other rod functions (such as detection of moving objects or objects in the peripheral visual field) are also affected before cone functions (color vision, detection of objects in the central visual field). Be careful of how you phrase your questions and do not "lead" the owners (that is, ask whether there is a difference between the pet’s daytime and nighttime vision rather than “is your pet seeing worse at night?”). One of the first behavioral signs of inherited, degenerative diseases of the outer retina (commonly known as progressive retinal atrophy) is loss of night vision as rods are affected before cones. Inquire whether the blindness was associated with preferential loss of night vision. The change in environment causes the animal to bump into objects, misleading the owners into believing that the blindness is acute. We can assume that even though an animal has been blind for some time, the gradual onset of the disease enabled it to learn to navigate at home. Questioning will likely reveal that the blindness was noticed when the animal’s environment was changed (e.g. Owners often report sudden loss of vision in the pet, even though ophthalmic examination reveals changes associated with chronic disease. Ask whether the blindness came on abruptly or gradually. HistoryĪs with any patient, begin by taking a history. Therefore, all patients that present with blindness-even those that do not regain vision-should undergo a comprehensive examination to reach an ophthalmic and systemic diagnosis, as the latter may have profound and long-term implications for the animal’s health. However, veterinarians and pet owners should bear in mind that acute blindness can be, and often is, a manifestation of systemic disease. ![]() Fortunately, in many cases vision can be restored. Acute blindness in a pet is distressing for both the animal and the owner.
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