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Blindness is manifested as a clinical abnormality by the animal walking into objects that it should avoid. Vision is a cerebral cortical function and is evaluated using the pupillary light reflex, the menace response, and the ability to navigate around a novel obstacle course.
The pupillary light reflex measures the integrity of the retina, optic nerves and chiasm, and oculomotor and pretectal nuclei in the midbrain, and then to a descending motor pathway that includes the oculomotor nerve, ciliary ganglion, and constrictor pupillae muscle. The technique involves pointing a focal light into one eye - the direct pupillary light reflex. Then, withdraw the light for few seconds, followed by stimulating the same eye again but this time observing the indirect, pupillary light reflex in the opposite eye. Abnormal pupillary light reflex can be found in optic nerve injury, oculomotor nerve damage, brain stem lesions, such as tumors, and medications like barbiturates.
The menace or blink response is used to test the integrity of the entire visual pathway (retina, optic nerves, optic chiasm, optic tract, lateral geniculate nucleus, and internal capsule to the visual area in the cerebrum [occipital lobe]). A threatening gesture of the hand (or even better by the index finger in a pointing manner) toward the eye elicits immediate closure of the eyelids. The finger must come close enough to the eye without touching the tactile hairs of the eyelids or creating a wind that can be felt by the animal. The menace response is a learned response that is absent in neonates. Most foals have a menace response by 9 days after birth and most calves by 5 to 7 days after birth.
The most definitive test is to make the animal walk an obstacle course and place objects in front of it so that it must step over the objects easily. A similar procedure is the only way to test for night blindness (nyctalopia). The area should be dimly lit, but the observer should be able to see the obstructions clearly. A decision that the animal is blind creates a need for examination of the visual pathways.
Central or Peripheral Blindness
Blindness may be central or peripheral. Animals with forebrain lesions are centrally blind, with depressed menace response in one or both eyes, whereas the pupillary light reflexes are usually intact. In peripheral blindness, such as hypovitaminosis A, the menace reflex is absent, and the pupillary light reflexes are also absent.
Blindness can be caused by lesions along the visual pathway, from the eye to the cerebral cortex:
The pupillary light reflex measures the integrity of the retina, optic nerves and chiasm, and oculomotor and pretectal nuclei in the midbrain, and then to a descending motor pathway that includes the oculomotor nerve, ciliary ganglion, and constrictor pupillae muscle. The technique involves pointing a focal light into one eye - the direct pupillary light reflex. Then, withdraw the light for few seconds, followed by stimulating the same eye again but this time observing the indirect, pupillary light reflex in the opposite eye. Abnormal pupillary light reflex can be found in optic nerve injury, oculomotor nerve damage, brain stem lesions, such as tumors, and medications like barbiturates.
The menace or blink response is used to test the integrity of the entire visual pathway (retina, optic nerves, optic chiasm, optic tract, lateral geniculate nucleus, and internal capsule to the visual area in the cerebrum [occipital lobe]). A threatening gesture of the hand (or even better by the index finger in a pointing manner) toward the eye elicits immediate closure of the eyelids. The finger must come close enough to the eye without touching the tactile hairs of the eyelids or creating a wind that can be felt by the animal. The menace response is a learned response that is absent in neonates. Most foals have a menace response by 9 days after birth and most calves by 5 to 7 days after birth.
The most definitive test is to make the animal walk an obstacle course and place objects in front of it so that it must step over the objects easily. A similar procedure is the only way to test for night blindness (nyctalopia). The area should be dimly lit, but the observer should be able to see the obstructions clearly. A decision that the animal is blind creates a need for examination of the visual pathways.
Central or Peripheral Blindness
Blindness may be central or peripheral. Animals with forebrain lesions are centrally blind, with depressed menace response in one or both eyes, whereas the pupillary light reflexes are usually intact. In peripheral blindness, such as hypovitaminosis A, the menace reflex is absent, and the pupillary light reflexes are also absent.
Blindness can be caused by lesions along the visual pathway, from the eye to the cerebral cortex:
- Diseases of the orbit include keratoconjunctivitis, hypopyon, cataract, panophthalmia, mixed ocular defects inherited in white Shorthorn and Jersey cattle and
and sporadic cases of blindness caused by idiopathic degeneration of the retina in cattle. - Diseases of the retina include retinal dysplasia of goats, lenticular cataracts caused by poisoning with hygromycin in pigs, and congenital ocular malformations in calves after intrauterine infection with bovine viral diarrhea virus.
- Diseases of the optic nerve and chiasma, e.g., abscess of pituitary rete mirabile, constriction of optic nerve by diet deficient in vitamin A, tumor of pituitary gland, and injury to the optic nerve, especially in horses after rearing and falling backward. There is a sudden onset of unilateral or bilateral blindness with no ophthalmologic change until 3 to 4 weeks after the injury, when the optic disc becomes paler and less vascular.
- Metabolic or ischemic lesions of the cerebral cortex as in cerebral edema, and hydrocephalus.
- Localized infectious or parasitic lesions caused by abscesses or migrating larvae.
- Functional blindness in which there is complete, often temporary, apparent blindness in the absence of any physical lesions is seen. Causes are acetonemia, pregnancy toxemia and acute carbohydrate indigestion of ruminants.
- Specific poisonings causing blindness include F. mas (male fern), Cheilanthes spp. (rock fern), and rape. Stypandra spp. cause a specific degeneration of the optic nerves. Lead poisoning in cattle can also cause blindness.