Conscious Proprioception and Postural Reactions
The integrity of conscious proprioceptive pathways may be tested by means of the postural reactions. Normal animals stand at rest with the limbs in line with the abaxial boundaries of the trunk.
Normal alert animals do not permit the limbs to be placed outside of the body axis or across the midline. After the examiner places the limbs in an abnormal position, the neurologically intact animal returns to a normal stance within a few seconds. Animals with conscious proprioceptive deficits allow the limb to remain in the abnormal position for longer than the usual period. This can vary from animals in which replacement of the limb into a normal position is slightly slowed to animals that do not try to replace the limb at all. The examiner should cross one of the animal's limbs over the opposite limb or abduct one limb; the normal response is for the limb to be placed back into the resting position. Normal animals often strongly resist attempts to place the limbs in abnormal positions. It should be noted that normal animals are extremely variable in their responses to this maneuver, so undue reliance should not be placed on its results, particularly if there are no other signs of ataxia. Animals with proprioceptive deficits may spontaneously place the limbs in abnormal positions: excessively adducted, abducted, or even crossed. Abnormalities of proprioception alone are poorly localizing signs, although a couple of generalities may be stated. Unilateral lesions rostral to the medulla oblongata produce mild to moderate proprioceptive and postural deficits in the contralateral limbs. Unilateral lesions in the medulla oblongata or spinal cord produce more severe proprioceptive and postural deficits in the ipsilateral limbs. Lesions of the cerebellum rarely result in postural deficits.Additional postural reactions such as hopping, hemiwalking, and wheelbarrowing can be tested in small ruminants, calves, and some foals.
Hopping is tested in the thoracic limbs by lifting the pelvic limbs a few inches off the ground by means of a hand and arm placed around the abdomen, flexing one thoracic limb slightly, and moving the animal away from the side of the flexed thoracic limb so that it must hop laterally on the thoracic limb still in contact with the ground. It is easiest if the examiner stands in one place and turns clockwise when testing the animal's right thoracic limb and counterclockwise when testing the left thoracic limb. Hopping in the pelvic limbs can be tested similarly, supporting both thoracic limbs off the ground with an arm around the chest. Hemiwalking is done by supporting both limbs on one side of the body in a slightly flexed position and pushing the animal toward the opposite side so that it must walk laterally on the two limbs still in contact with the ground. Both hopping and hemiwalking should be done with care not to push the patient over. Adult horses can be made to hop on one thoracic limb by picking up the opposite limb and firmly pushing the head and shoulders laterally. Hopping and hemiwalking involve the same ascending and descending motor tracts involved in gait on a level surface but also require input from the forebrain. These maneuvers are abnormal on the ipsilateral side in animals with lesions in the skeletal muscles, peripheral nerves, spinal cord, and medulla oblongata and on the contralateral side in animals with lesions in the midbrain or forebrain. Animals with forebrain lesions have normal gait on a level surface but subtle deficits in hemiwalking and hopping.
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