Forebrain Diseases of the Horse: Relevant Examination Techniques and Illustrative Video Segments

1. Introduction

The forebrain comprises the diencephalon (thalamus and hypothalamus) and the cerebral hemispheres. Both intrinsic and learned behaviors are forebrain-dependent, as are central perception of vision and touch. Forebrain injury thus may cause demented behavior, central blindness, and reduced response to touch. Seizures usually arise in the forebrain. Careful evaluation of mentation, menace responses, and vision and reaction to touching the nasal septum, in conjunction with other parts of the neurologic examination, should provide accurate localization of forebrain lesions. Recognition of signs of forebrain dysfunction is an important step in the processes of diagnosis and treatment.

2. Anatomy and Nomenclature

The forebrain or prosencephalon includes the telencephalon (cerebrum) and diencephalon (thalamus and hypothalamus).¹  

3. Functions (Location)

Level of Consciousness (Cerebrum, Brainstem, Especially Front Half)

One of the critical functions of the reticular formation of the brainstem is activation of the cerebral cortex for the awake state.^2-4 This component of the formation, known as the ascending reticular activating system, is an ill-defined meshwork of cells concentrated in the midbrain and rostral brainstem that receives afferent input from all parts of the CNS and projects excitatory stimuli cortically.² Focal to extensive lesions in the midbrain, or, to a lesser degree, anywhere else in the brainstem may reduce the level of consciousness, whereas cerebral injury must be diffuse to cause noticeable obtundation. The ascending reticular activating system also is involved in the initiation and maintenance of sleep. Abnormalities of production or action of hypothalamic arousal peptides (hypocretins/orexins)⁵ or imbalances of brainstem neurotransmitters may result in narcolepsy/cataplexy sleep disorders.

Behavior (Limbic System, Temporal Lobes)

Normal behavior requires integration of signals from the entire CNS but principally involves the forebrain. Most important in controlling intrinsic behavior is the limbic system— a connected series of structures in the cerebrum and diencephalon. A minor component is also found in the midbrain. Included are the amygdala, hippocampus, fornix, cingulate gyrus, and septal area. A closely associated region, which is important in primate behavior, is the temporal lobe of the cerebrum. It is thought that behavior based on conditioning and experience (i.e., learning) is controlled by the temporal lobes. Structural, metabolic, or psychological disturbances affecting these areas may result in behavioral abnormalities (i.e., dementia). Dementia can be defined as changes in normal habits, personality, attitude, reaction to the environment, or loss of learned skills. Some of the signs that may be seen include disorientation in a familiar environment, failure to recognize a handler or object, loss of the ability to be led, frequent yawning, head-pressing, irritability, unprovoked kicking or biting, compulsive walking or circling, and dramatic changes in eating or drinking habits.

Almost any disturbance of the forebrain potentially can cause dementia. Encephalitis, head trauma, space-occupying lesion, malformation, infarct, and metabolic disorders all are likely to cause changes in behavior. It is likely that structural or metabolic forebrain disease is the cause of dementia if other neurologic abnormalities are found by neurologic examination or imaging studies. In the absence of such supportive findings, abnormal behavior such as self-mutilation⁶ may have a psychological basis.

Seizures

Seizures are sudden, transient attacks of abnormal motor and/or behavioral activity attributable to paroxysmal depolarization of part to all of the brain. Depolarization occurs either simultaneously throughout the brain or originates from a hyperirritable focus in the forebrain. Seizures originating from a focus probably will initially have asymmetric clinical signs, and there may be additional signs of forebrain disease between seizures that are revealed by neurologic examination. Seizures frequently originate in the frontal (motor) cortex and involve muscle fasciculations and tremors around the head or abnormal movements of the jaws and tongue (“chewing gum fits”). Convulsions characteristic of neonatal encephalopathy often are of this type. In their most severe (generalized) form, seizures manifest as sudden recumbency, with a brief phase of extensor tonus, followed by clonic (“galloping”) movements of the legs, loss of consciousness, and a variety of signs of autonomic discharge (e.g., sweating, urination, defecation, pupillary dilation). Mild motor seizures are often accompanied by behavioral signs such as obtundation, compulsive walking, hyperresponsiveness to stimuli, or other signs of dementia. Seizure foci in the forebrain may occur at sites of previous or current trauma or inflammation. Syndromes of more than multiple seizure episodes without interictal evidence of brain disease often are described as epilepsy.

Perception of Pain/Touch (Parietal Cortex, Cranial Nerve V)

Pain/touch sensation is transmitted from the body to the brain in multisynaptic spinothalamic tracts. Signals initiated by stimulation of pain receptors on one side of the body pass through spinothalamic tracts on both sides of the spinal cord. Axons in these tracts course rostrally to terminate in the thalamus. From there, cell bodies project axons to the sensory (somesthetic) cortex for conscious perception of pain or other sensory modalities. It is thought that the somesthetic cortex is located principally in the parietal lobe of the cerebrum. The pathways for pain perception in the head pass through the maxillary, ophthalmic, and mandibular branches of the trigeminal nerve. The central component of this pathway is predominantly contralateral. Unilateral lesions of the sensory parts of the forebrain (thalamus, internal capsule, sensory cortex) thus cause contralateral facial hypalgesia. Because the central components of vision and facial pain/touch perception are close anatomically, it is common to find unilateral facial hypalgesia and blindness (on the same side) in the same horse.

Smell (Olfactory Bulbs, Cranial Nerve I)

Olfactory nerves pass through the cribriform plate and into the olfactory bulbs of the cerebrum. The sense of smell is relayed through the thalamus to centers in the unconscious (limbic system) and conscious cerebrum.

Vision (Thalamus, Occipital Cortex, Cranial Nerve II)

In horses, 80% to 90% of optic nerve fibers from one eye cross at the optic chiasm and 80% of fibers in the optic tracts synapse at the lateral geniculate nucleus in the thalamus. The remainder course to the mid-brain to function in the pupillary light reflex. Neurons in the lateral geniculate nucleus project via the internal capsule to the visual cortex in the occipital lobe of the cerebrum. This area is caudal in the cerebrum, immediately rostral to the tentorium cerebelli and caudal to the parietal cortex. The pathway from the eye to the contralateral visual cortex via the optic nerve, optic tract, lateral geniculate nucleus, and internal capsule must be intact for normal vision. Lesions in the pathway caudal to the optic chiasm result in blindness predominantly in the opposite eye and lesions rostral to the chiasm affect vision in the ipsilateral eye. Damage to the cortex should not affect the pupillary reflex pathway. Visual perception is evaluated by obstacle tests (with and without blindfolding of one eye) and by the menace response. The menace response requires the central visual pathway just described plus normal facial nerve function. Integrity of the cerebellar cortex is also needed, although it is not known if the pathway that mediates this response actually passes through the cerebellum. It is important to note that the menace response does not develop in foals until they are 1 to 2 weeks old. [...]