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5 Disorders of behavior
The location of lesions resulting in disorders of behavior essentially involves the forebrain, and such lesions can be focal, multifocal, and diffuse. Because normal behavior is extremely variable among species, breeds, individuals, and especially stages of reproductive cycles, if after a neurologic examination the only finding is a history of or evidence of a subtle change in expected behavior, the examiner must be cautious while assuming that a morbid lesion in the forebrain accounts for the signs.
Frantic behavior of a bull during the mating season, jerky collapsing in a white pig with sunburn in full sun, bizarre antics of a mare in diestrus, and violent kicking in a colt having a new bandage applied over the hock all attest to unusual behavior syndromes that can easily be mistaken for morbid brain disorders.
Subtle alterations in behavior resulting from organic brain disorders include the following: continual yawning—that can be prominent in hepatoencephalopathy; a tendency to drift to one side when walking—that is often present with asymmetric lesions; and a lack of recognition of familiar animals (Figure 5.1), people, and objects—that is often the earliest expression of cerebral disease in foals. More prominent signs are seizures (Chapter 6), compulsive walking and circling (Figure 5.2), pressing the head on objects, running around frantically, biting at animate (including self) (Figure 5.3) and inanimate (Figure 5.4) objects, leaving food in the mouth, and adopting bizarre postures of the head, neck, trunk and limbs. Such syndromes are usually referable to lesions in the frontal lobes, temporal lobes, internal capsule, limbic system, thalamus or basal nuclei, or due to diffuse brain disease.
Figure 5.1 A newborn Thoroughbred foal that is not distracted by the presence of people, does not attend to the dam, and postures with its head flagging alongside its flank is behaving very abnormally and is likely suffering from forebrain disease. With such a syndrome, this foal did have an undiagnosed, aseptic meningoencephalitis from which it survived.
Figure 5.2 A patient that is variably obtunded and spontaneously turns and walks toward one side will have an asymmetric lesion in the forebrain, usually worse on the side toward which it turns—right in this case. This lamb has a chronic suppurative and granulomatous ependymitis and ventriculitis on the right side. Additional signs of forebrain involvement were poor vision and poor menace response in the left eye (with normal pupils) and decreased sensation perceived from the left nasal septum compared with the right. Typical of many lesions associated with perilesional edema, and especially those involving forebrain, the signs would wax and wane in severity so that at times the lamb would walk, trot, and run quite well in both directions. The only consistent abnormality with gait and posture was a delay in protracting the left limbs, especially the left forelimb as shown here, while turning right and while attempting to hop on each left limb in turn. This likely represents abnormal conscious proprioceptive processing from the left side of the body and limbs.
Usually, localized and diffuse lesions affecting only the forebrain result in combinations of behavioral