Blindness and Ocular Abnormalities
Blindness may be the result of lesions in the eye, optic nerve, optic chiasm, optic tracts, or central projections of the visual pathways. Ophthalmic examination, including fundic examination, should be part of the routine physical examination.
Animals■ TABLE 8.7
Diseases of Ruminants That May Produce Forebrain Signs
| Disease | Predominant Clinical Signs | Species Affected |
| Ammoniated feed toxicosis | Trembling, fear, uncontrolled running, crashing through objects, coma, convulsion | Cattle |
| Babesiosis (exotic) | Odontoprisis, ataxia, conscious proprioceptive deficits, coma, convulsions | Cattle |
| Border disease | Ataxia, tremors, bunny-hopping | Sheep, goat |
| Borna disease (exotic) | Head tremors, hyperesthesia, ataxia, anorexia, compulsive walking, coma, convulsions | Cattle, sheep, goat |
| Bovine spongiform | Aggression, weight loss, milk production, ataxia, recumbency, coma | Cattle |
| encephalopathy | ||
| Brain abscess, meningitis | Recumbency, opisthotonos, blindness, hyperesthesia, stiff neck, proprioceptive deficit, ataxia, head pressing, depression, coma | Cattle, sheep, goat |
| Brain tumor | Obtundation, facial paresis or paralysis, facial analgesia, head tilt, strabismus, nystagmus, loss of menace, hypermetria, ataxia | Cattle |
| Caprine arthritis-encephalitis | Obtundation, ataxia, head pressing, convulsions, coma | Goat |
| Ceroid lipofuscinosis | Blindness, ataxia, weight loss, coma, convulsions | Cattle |
| Citrullinemia | Recumbency, coma, convulsions, death by 4 days of age | Cattle |
| Coenurus cerebralis | Blindness, circling, ataxia, conscious proprioceptive deficit, head tilt, recumbency, coma, convulsions | Sheep |
| Ethylene glycol poisoning | Obtundation, somnolence, blindness, ataxia, facial analgesia, weak tongue, head pressing, opisthotonos, convulsions, odontoprisis, decerebrate rigidity | Cattle, sheep, goat |
| Globoid cell leukodystrophy | Ataxia, proprioceptive deficits, hyperreflexia, depression, coma | Sheep |
| Grass staggers | Tremor, ataxia that worsens with excitement or exercise | Cattle, sheep, goat |
| Heartwater (exotic) | Hyperesthesia, behavioral changes, muscular fasciculations, hypermetria, ataxia, head pressing | Cattle, sheep, goat |
| Hepatic encephalopathy | Hyperexcitability, aggression, rage, odontoprisis, ataxia, head pressing, coma, convulsions, semicoma, blindness, tenesmus, rectal prolapse | Cattle, sheep, goat |
| Hydrocephalus, hydranencephaly, | Blindness, ataxia, proprioceptive deficit, ventrolateral strabismus, failure to suckle, dysphonia | Cattle, sheep, goat |
| microcephaly, anencephaly | ||
| Hypocalcemia | Cattle, doe: weakness, ataxia, inappetence, bloat, proprioceptive deficit, cool extremities, weak pulse, bizarre head posture, dysuria | Cattle, goat |
| Hypoglycemia | Coma, semicoma, convulsions, blindness, hyperesthesia, cold extremities | Cattle, sheep, goat |
| Hypomagnesemia | Stiffness, hyperexcitability, recumbency, ataxia, muscle tremors | Cattle |
| Idiopathic epilepsy | Intermittent psychomotor seizures | Cattle, goat |
| Infectious bovine rhinotracheitis | Fever, bellowing, coma, convulsions, somnolence, hyperexcitability, hyperesthesia, proprioceptive deficit, recumbency | Cattle |
| Insecticide poisoning (organophosphate | Salivation, vaginal discharge, diarrhea, tremors, coma, convulsions, ataxia, recumbency | Cattle, sheep, goat |
| carbamate) | ||
| Lead poisoning | Obtundation, somnolence, blindness, ataxia, proprioceptive deficits, facial analgesia, weak tongue, anisocoria (late), head pressing, opisthotonos, odontoprisis, convulsions, decerebrate rigidity | Cattle, sheep, goat |
| Louping ill (exotic) | Fever, anorexia, obtundation, constipation, muscular tremors, head tremors, hypermetria, ataxia, proprioceptive deficits, hyperexcitability, incoordination, rabbit-hopping gait, recumbency, convulsions, coma | Sheep, cattle |
| Maedi-visna | Obtundation, ataxia, head pressing, convulsions, coma | Sheep |
| Malignant catarrhal fever | Aggression, rage, ataxia, depression, head pressing, blindness, nystagmus, bellowing, mucosal and skin erosions, lymphadenopathy, diarrhea | Cattle |
| Narcolepsy | Sleep state, recumbency, loss of consciousness, loss of motor activity, rapid eye movement | Cattle |
■ TABLE 8.7
Diseases of Ruminants That May Produce Forebrain Signs—cont'd
| Disease | Predominant Clinical Signs | Species Affected |
| Nervous coccidiosis | Diarrhea, recumbency, obtundation, somnolence, blindness, ataxia, compulsive walking, head pressing | Cattle |
| Nervous ketosis | Aggressiveness, tremors, ataxia, paresthesia, recumbency, hyperesthesia, bellowing | Cattle |
| Nitrofurazone poisoning | Obtundation, proprioceptive deficit, recumbency, convulsions, coma | Cattle, sheep, goat |
| Nitrofurazone toxicosis | Hyperirritability, compulsive running, muscular tremors, blindness, convulsions | Cattle |
| Organochlorine poisoning | Tremors, hyperesthesia, recumbency, coma, convulsions | Cattle, sheep, goat |
| Plant poisonings | Convulsions, blindness, ataxia, compulsive walking, head pressing, odontoprisis, hyperexcitability, salivation, proprioceptive deficit, sudden death, vomiting, fetal malformations | Cattle, sheep, goat |
| Polioencephalomalacia | Obtundation, somnolence, blindness, ataxia, proprioceptive deficits, facial analgesia, weak tongue, anisocoria (late), dorsomedial strabismus, head pressing, opisthotonos, convulsions, odontoprisis, decerebrate rigidity | Cattle, sheep, goat |
| Propylene glycol poisoning | Depression, bloat, ataxia, recumbency | Cattle, sheep, goat |
| Propylene glycol toxicosis | Ataxia, obtundation, bloat, characteristic garlic-like odor | Cattle, sheep, goat |
| Pseudorabies | Obtundation, ataxia, hyperesthesia, paresthesia, self-mutilation, aggressiveness, fear, head pressing, compulsive walking, hypersexuality, salivation, coma, convulsions, recumbency, conscious proprioceptive deficit | Cattle, sheep, goat |
| Rabies | Obtundation, excitement, aggressiveness, hyperesthesia, analgesia, anesthesia, proprioceptive deficits, recumbency, compulsive walking, head pressing, tenesmus, hypersexuality, salivation | Cattle, sheep, goat |
| Salt poisoning | Obtundation, somnolence, blindness, ataxia, proprioceptive deficits, opisthotonos, facial analgesia, weak tongue, convulsions, anisocoria (late), head tilt, head pressing, decerebrate rigidity | Cattle, sheep, goat |
| Sarcocystis | Fever, weight loss, tremors, weakness, diarrhea, loss of hair on the tail switch, abortions | Cattle |
| Sarcocystis species infection | Seizures, blindness, opisthotonos, nystagmus, ataxia, muscular weakness, tremors, hyperexcitability, hypersalivation, recumbency | Cattle |
| Scrapie | Chewing, licking, wool break, depression, weight loss, ataxia, reduced menace, hypertonicity, hyperreflexia, proprioceptive deficit, recumbency, | Sheep, goat |
| Sporadic bovine encephalomyelitis | coma Blindness, circling, ataxia, proprioceptive deficits, pleural friction rubs, pericardial friction rubs, abdominal tenderness | Cattle |
| Theileriosis (central nervous system form, exotic) | Depression, hypersensitivity, ataxia, circling, paralysis, convulsions | Cattle |
| Trauma, hematoma, brain edema | Obtundation, somnolence, blindness, ataxia, proprioceptive deficits, opisthotonos, facial analgesia, weak tongue, convulsions, anisocoria (late), head tilt, head pressing, blood from ears or nose, decerebrate rigidity | Cattle, sheep, goat |
| Trypanosomiasis (exotic) | Ataxia, somnolence, circling, head pressing | Cattle |
| Urea poisoning | Muscle tremor, bloat, salivation, incoordination, struggling, ataxia, proprioceptive deficit, recumbency, bellowing, coma, convulsion | Cattle |
| Vitamin A deficiency | Obtundation, somnolence, blindness with fixed pupils, ataxia, facial analgesia, weak tongue, head pressing, opisthotonos, convulsions, odontoprisis, decerebrate rigidity | Cattle, sheep, goat |
presented with the complaint of blindness should receive a more detailed ophthalmic examination to determine whether primary ocular disease is the cause of the problem (see Chapter 39). Sophisticated diagnostics such as electroretinography (ERG) may be indicated in some animals.
When no ocular disease can be found to account for blindness, a lesion in the nervous system is likely to be responsible. Observing the animal’s ability to negotiate its environment, particularly in unfamiliar surroundings, and testing the menace responses are the primary methods of determining visual function. Further testing can be performed by setting up a maze of objects for the animal to negotiate, by using different light levels and assessing vision in bright versus dim light, and by blindfolding each eye in turn when unilateral deficits are suspected. About 90% of optic nerve fibers (i.e., axons of retinal ganglion cells) cross to the opposite side of the brain in the optic chiasm of ungulates; thus central representation of vision in these species is predominantly contralateral. Fibers that remain uncrossed originate from the temporal extreme of the retina. Lesions in the visual apparatus distal to the optic chiasm (i.e., lesions of the globe, the retina, or the optic nerve) produce ipsilateral visual deficits. Lesions proximal or central to the optic chiasm produce lesions in the opposite visual field.The following discussion refers to severe or complete lesions because these are most easily understood and described. Partial lesions will produce similar but milder signs (e.g., reduced visual acuity rather than complete blindness). Absent or reduced menace response can be caused by lesions of the facial nerve (cranial nerve VII), the cerebellum, or the forebrain. Animals with facial nerve lesions can see, but palpebral reflexes cannot be elicited. Animals with cerebellar disease can see and have normal palpebral reflexes. Forebrain disease causes defective menace responses with intact pupillary light and palpebral reflexes. Localization of lesions causing blindness is summarized in Table 8.6.
■ TABLE 8.8
Diseases of the Horse That Produce Forebrain Disease
Disease
Predominant Clinical Signs
Brain abscess, meningitis
Brain tumor
Hepatoencephalopathy, gastrointestinal hyperammonemia
Hydrocephalus
Idiopathic epilepsy
Leukoencephalomalacia
Rabies
Trauma, hematoma
Verminous encephalitis
Viral encephalomyelitis (nonrabies)
Eastern equine encephalomyelitis
Western equine encephalomyelitis Venezuelan equine encephalomyelitis Borna disease
Head pressing, blindness, conscious proprioceptive deficit, ataxia, circling, depression, convulsions, hyperexcitability, stiff neck, rigid legs, fever, compulsive walking Depression, facial paresis or paralysis, facial analgesia, head tilt, strabismus, nystagmus, loss of menace, hypermetria, ataxia
Aggression, rage, hyperexcitability, odontoprisis, ataxia, head pressing, convulsions, obtundation, coma, semicoma, blindness, fear, red urine (hemolysis), icterus
Coma, semicoma, blindness, somnolence, head pressing, dysphonia, ataxia, weak tongue Intermittent psychomotor seizures, normal interictal periods
Recumbency, ataxia, aggression, obtundation, coma, semicoma, head pressing, circling, compulsive walking, mydriasis, tenesmus, fear, continual chewing
Recumbency, ataxia, aggression, depression, coma, semicoma, head pressing, circling, compulsive walking, mydriasis, tenesmus, fear, continual chewing
Head pressing, blindness, ataxia, circling, depression, convulsions, hyperexcitability, stiff neck, rigid legs, fever, compulsive walking, blood from ear or nose
Head tilt, hyperexcitability, odontoprisis, ataxia, head pressing, circling, coma, semicoma, blindness, anisocoria, convulsion, tongue dystonia
Head pressing, blindness, ataxia, circling, depression, coma, convulsions, recumbency, hyperexcitability, stiff neck, rigid legs, fever, compulsive walking
Pupillary size and movement of the globes are affected by cranial nerves II, III, IV and VI and the sympathetic innervation of the eye.
Clinical signs of diseases affecting these nerves are described earlier.
More on the topic Blindness and Ocular Abnormalities:
- Blindness and Ocular Abnormalities
- Abnormalities of Cranial Nerve Function
- Signs of Ocular Disease*
- Ophthalmic History and Examination
- Terminology and Description of Clinical Signs of Neurologic Disease (See Table 8.1)
- Immune-Mediated Ocular Diseases
- Localization of Neurologic Diseases by Major Clinical Signs
- Diagnosis of Neurologic Disease by Presenting Signs
- MANIFESTATIONS OF DISEASE
- Amaurosis