Large Animal Neurology. Joe Mayhew

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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 changes (including seizures), blindness, and lethargy. Although often sluggish in movement, little or no alteration in gait occurs, at least in the subacute to chronic stages. Central motor pathways to and sensory inputs from cranial nerves can be disrupted, and such syndromes are discussed under the individual cranial nerve disorders. Particularly with metabolic diseases such as hyperammonemia, and with those focal and diffuse disorders accompanied by inflammatory lesions or by perilesional edema, there can be dramatic fluctuations in the severity of behavioral signs displayed over periods ranging from minutes to weeks. This can even mean that there are no overt signs demonstrated at a point in time, and within a few hours there may be prominent behavioral changes and even an uncontrollable patient.

Nondomesticated and unhandled domestic large animals, particularly adult bulls, boars and horses, can become exceedingly defensive when incapacitated by all sorts of disorders and behave quite violently and aggressively. Additionally, Equidae routinely respond to visceral pain in surprisingly violent ways (Figure 5.5) such that colic always needs to be considered differentially when dealing with aberrant behavior in these species.

Figure 5.3 Self‐inflicted lesions caused by biting are quite unusual and can be spectacular with horses suffering from the self‐mutilation syndrome. In the Arabian stallion shown here, such signs were due to rabies virus infection, as was a proclivity to assault learned professors wearing white coats. Possibly associated with aggressive anti‐inflammatory therapy, these signs progressed slowly over many days, with the patient surprisingly surviving for well over a week after the onset of abnormal behavior. Self‐inflicted biting trauma can also be seen in horses with other encephalomyelitides and with metabolic encephalopathies.

      A period of calm or of induced sedation may be taken as an opportunity to search for localizing signs of brain disease that are often overshadowed by any accompanying wildly aberrant behavior. These will include subtle signs such as asymmetric menace responses, anisocoria, asymmetric nasal sensation, head tilt, head turn, facial hypotonia, and drifting to one side walking undirected with blindfold applied.

      Faced with an animal showing aggressive or violent behavior, for safety reasons the clinician must consider sedating the patient. Most times diazepam will not be at hand but expedient IM or IV administration of moderate to high doses of a readily available α‐2 agonist drug combined with a synthetic opioid drug is perfectly satisfactory in most circumstances. Obviously, with a patient from a rabies‐endemic area, this diagnosis must be considered of paramount importance. Some diseases that result in such a fulminant syndrome of wildly abnormal behavior can have a positive outcome, and therefore euthanasia must be given careful consideration while sedation takes effect. Examples of those diseases with a more favorable prognosis include thiamine‐responsive polioencephalomalacia, hypoadrenocorticism,¹ neonatal hypoxic and ischemic encephalopathy, salt poisoning, hypomagnesemia, hypocalcemia, hypoglycemia, hepatic, exogenous and intestinal ammonia intoxication, ketosis, metaldehyde toxicosis, macrocyclic lactone overdose, and immediate post‐head trauma delirium.

Figure 5.4 This milking Friesian cow likely was suffering from ketosis with episodes of bizarre behavior characterized by vigorously attacking animate and especially inanimate objects such as the metal bars of her pen as shown; she recovered fully with treatment. Such extremes of abnormal behavior in cattle can also be seen with many morbid and functional encephalopathies including rabies, hepatoencephalopathy, ammonia intoxication, and lead poisoning.

Asymmetric forebrain lesions frequently cause an animal to hold its head and neck turned to one side, usually toward the same side as the lesion. This can be difficult to distinguish from a vestibular head tilt where there is rotation of the poll around the muzzle. In a more prominent form, a head turn due to vestibular or cerebral disease may involve bending of the whole neck and head toward the flank. The presence or absence of a vestibular head tilt is best evaluated after the nose and neck have been held back into a midline position. In some animals with forebrain lesions, the eyes are also deviated and the whole body spins in circles in the direction of the head turn. Such episodes may be precipitated by any stimulus to either side of the animal, when such a response can be regarded as an adversive movement. This sign of cerebral disease is mostly seen with prominently asymmetric lesions, such as lateral ventricle cholinisteric granuloma, forebrain abscess, parasitic thromboembolism or verminous migrations, and with head trauma.

Figure 5.5 Painful processes, perhaps especially abdominal pain, frequently cause unexpected СКАЧАТЬ