Body, Spinal Cord and Limb Neurological Examination - Horse

Conscious Proprioception Testing

• It is possible to test a horse’s conscious proprioception by altering the position of the limbs in space.
  • Horses should normally replace their limbs to the normal position within a few seconds.
    • There is a degress of individual variability, however, and the test can be difficult to interpret.
• Tests can also be performed by walking or trotting a horse and then suddenly stopping.
  • The animal should be observed to see how quickly it replaces its legs in a normal position.

Gait

• Gait is a combination of higher control of motor function and unconscious proprioception.
• During assessment of gait, the examiner is looking primarily for evidence of ataxia and/ or weakness.
  • Each leg should be assessed in turn.
• Defects may be graded on a five-point scale (the Mayhew scale).
  • 0 is normal and 5 is recumbent.

Ataxia

• Defects in unconscious proprioceptive pathways result in ataxia.
  • These pathways originate in muscle spindle stretch receptors and golgi tendon organs.
  • Impulses are relayed via peripheral nerves to the spinal cord.
  • Signals then ascend in pathways to the cerebellum.
    • Primary cerebellar diseases in horses are rare, but classically cause incoordination of the head and limbs.
      • Weakness is not a prominent feature.
      • The cerebellum communicates with the vestibular nuclei, and so cerebellar diseases may sometimes present with vestibular signs.
• Manifestations of ataxia:
  • Exaggerated movements - hypermetria
  • Reduced movements - stilted hypometric movements
  • Both exaggerated and reduced movements - dysmetria.
• Animals may also
  • Sway
  • Place their feet abnormally
    • Horses cross or weave their feet, with abnormal abduction or adduction.
• Signs of spinal ataxia may be exaggerated by lifting the horse's head.
  • This is beacuse it removes the horizon.
  • Similar responses can be generated by walking up and down slopes.
• Circling the horse can also reveal ataxia.
  • The horse may plant its feet and rotate around them without placing them properly.
  • Outward rotation of the pelvic limbs is also a common sign.
    • The term for this is "circumduction".
• Unconscious proprioception may also be tested by backing the horse.
  • Normal horses back with a diagonal gait.
  • Horses with spinal ataxia will often pace when backing.

Weakness

• Weakness usually manifests as toe dragging, stumbling, or knuckling over.
  • It is a good idea to look closely at the horse’s hooves to see if any are abnormally worn.
• Weakness may be caused by dysfunction of either the upper or lower motorneurons.
  • Lower motor neuron weakness.
    • Accompanied by:
      • Muscle flaccidity
      • Muscle atrophy if chronic
    • May sometimes appear similar to lameness.
  • Upper motor neuron weakness.
    • Presents as a lack of control of limb muscle movement.
    • Tested most easily tested by pulling the tail during walking.
      • Pulling the tail while stationary probably primarily tests lower motor neuron control and muscular strength of the pelvic limb.