Localization of Neurologic Diseases by Major Clinical Signs

Abnormal Mentation and Behavior and Seizures

Decreased mental alertness (dullness, obtundation, stupor, coma) is the most common change of mental status in animals with neurologic disease, although increased responsiveness to external stimuli (anxiety, mania, aggression) sometimes occurs.

LETHA GY. Animals respond to normal stimuli but are duller than normal.

STUPOR. Animals appear to be asleep and will respond only to vigorous and painful stimuli. Responses are blunted even to these stimuli. Semicoma is a severe form of stupor wherein patients are too obtunded to remain standing.

COMA. Animals are unconscious and will not respond even to the most painful stimuli. Animals in coma are recumbent.

DEMENTIA. Animals with dementia exhibit abnormal reactions and responses to their environments, including to inanimate objects, humans, and other animals within those environments. Signs are extremely variable, from head press­ing and yawning to compulsive walking, bellowing, rearing, self-mutilation, and aggression.

SEIZURES, COLLAPSE. Episodic abnormalities of behavior or consciousness are usually the result of seizure activity, narcolepsy or cataplexy, or syncopal attacks caused by cardiovascular or respiratory dysfunction. Intermittent toxicities or fluctuating metabolic abnormalities such as occasionally occur with hepatic encephalopathy may also cause episodic changes in mentation and behavior. Animals with a history of episodic collapse should undergo a thorough physical examination to determine whether disease of the cardiovascular system (e.g., cardiac arrhythmias, intermittent hemorrhage) or respiratory system (e.g., laryngeal paralysis) is present.

Diffuse forebrain disease often results from metabolic, toxic, or infectious diseases. Increased intracranial pressure, the conse­quence of early acquired hydrocephalus, mass lesions within the cranial vault, inflammatory diseases, or cerebral edema, tends to produce signs of diffuse forebrain dysfunction, which can range from mild to severe. Mild to moderate forebrain dysfunction usually results in an animal with decreased mental awareness or, more rarely, excitement and overreaction. Diffuse disease does not result in circling, and gait on a level surface may appear normal, or almost so. Gait may be abnormal, however, when the animal is challenged to ascend or descend slopes, step over objects on the ground, step onto and off curbs, circle, or back up. Postural and proprioceptive reflexes and reactions similarly may be abnormal. When an animal is walking at normal speed on a level surface, local reflexes in the spinal cord and regulatory information from the red and reticular nuclei in the brainstem control simple gait patterns. Movements that require visual input or complex limb and body integration of movements are initiated in motor centers of the cerebral cortex and regulated by the cerebellum. The combination of normal gait on a level surface with obvious proprioceptive and postural deficits should immediately alert the examiner to the likelihood of forebrain disease.

Vision and the menace response may be defective (see Blindness and Ocular Abnormalities, later). Pupillary light reflexes and oculocephalic reflexes are usually normal in animals with forebrain disease. Responses to noxious tactile stimuli, especially around the head, are reduced.

The hypothalamus regulates primitive functions such as eating, drinking, cardiovascular function, and sexual behavior.

Seizures are the physical manifestations of spontaneous paroxysmal electrical activity arising in the forebrain. When seizure activity is limited to a small area of the cerebral cortex, the seizure is focal, resulting in localized abnormal motor activity such as muscular twitching in the face or in one limb or episodes of lateralizing abnormal behavior. More commonly the seizure is generalized or starts focally and becomes general­ized to the entire cerebral cortex. Generalized seizures cause loss of consciousness, collapse, and generalized tonic-clonic motor activity. The presence of seizures indicates forebrain dysfunction, but the initiating cause may lie elsewhere in the brain; the origin may even be extracranial. Epilepsy is a term that means repeated seizures of any cause, although it is often used to indicate seizures of unknown cause. The nature of the seizure, whether focal or generalized, is not a reliable indicator of the underlying cause. Congenital, familial, or idiopathic epilepsy syndromes such as benign epilepsy of Arabian foals are usually characterized by generalized seizures. Partial or focal seizures more commonly indicate an acquired cause. Animals with seizures should undergo a complete physical examination, together with diagnostic testing for suspected toxins and underlying metabolic diseases, as well as a thorough neurologic examination to localize any interictal neurologic signs. Further diagnostics such as CSF analysis and MRI are performed as indicated after this initial work-up.

When forebrain disease is unilateral or asymmetric, asymmetry of clinical signs becomes apparent. Circling often occurs, ranging from a tendency to drift toward one side to obvious and com­pulsive circling around the inside of an enclosure. It is usually possible to stop the circling, although the animal may be reluctant to turn in the opposite direction. Proprioceptive and postural reaction deficits may be present in the limbs on the side of the body opposite to the lesion (contralateral) and vary in severity with the severity of the underlying neurologic disease. The head and neck may be turned to one side (usually toward the side of the lesion), but the head tilt characteristic of vestibular disease is not seen. The absence of signs such as head tilt, nystagmus, and strabismus, together with the presence of contralateral proprioceptive and postural reaction deficits, distinguishes forebrain lesions from those affecting the vestibular system.

Specific diseases associated with the forebrain of ruminants and horses are given in Tables 8.7 and 8.8, respectively.