Incoordination, Hypermetria, Hypometria, Dysmetria, and Intention Tremor
Clinical signs that occur in animals with cerebellar disorders include hypermetria, hypometria, intention tremor, and truncal ataxia (excessive body sway during movement along a straight path).
Conscious proprioceptive fibers do not pass through the cerebellum. Consequently, postural placement of the limbs is normal. Animals with cerebellar disease move the limbs with imprecise and variable rate, range, and force. There is a slight delay in lifting the limb from the ground. At the peak of protraction the limbs are lifted too high and too far anteriorly. The legs then hit the ground with excessive force. During the exaggerated protraction, thoracic limbs may flex excessively, termed hypermetria, or may appear stiff because of reduced flexion, termed hypometria or spasticity. The animal may violently thrust the outside pelvic limb backward and laterally when turned. The thoracic limbs and the pelvic limbs occasionally collide during the turn (interference). At rest the animal stands with the legs abducted, in a basewide stance. This is not a conscious proprioceptive deficit, however, because the animal consciously returns the limbs to the basewide posture if the leg position is manually corrected. There is intention tremor, most marked in the head. When the animal attempts to reposition the head, it overshoots the intended position, corrects, and then overshoots again. The sequence of overcompensation and overcorrection results in a coarse oscillation or bobbing of the head. The head tremor is most conspicuous when the animal is alert, especially when eating. Intention tremor disappears when the animal is recumbent and the musculature is relaxed. In animals with cerebellar disorders, the extensor muscles of the limbs may be hypertonic, and spinal reflexes occasionally are exaggerated. Lesions of the rostral cerebellum can result in opisthotonos. Animals with cerebellar cortical disease may lack a menace response but retain their vision and can negotiate around obstacles. Animals with pure cerebellar dysfunction remain bright, alert, and responsive to external stimuli. Animals with severe lesions of the cerebellum may be recumbent and unable to rise and show opisthotonos. Cerebellar disease is often bilaterally symmetric, but lateralized lesions cause signs on the ipsilateral side of the body. Diseases that cause spasticity or tremors in livestock are listed in Table 8.9.Involvement of the vestibular components of the cerebellum (caudal cerebellar peduncle, flocculonodular lobe, and fastigial nucleus) results in signs of paradoxical vestibular syndrome, described earlier.
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