South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Backing the horse is an additional test to assess unconscious proprioception.
Most normal horses back with a diagonal gait, whereas horses with spinal
ataxia will often pace when backing or drag the feet.
(Note that primary cerebellar diseases in horses are rare, but when
present, classically cause incoordination of the head and limbs. Weakness
is not a prominent feature. Because of the communication with the
vestibular nuclei, cerebellar diseases may sometimes present with
vestibular signs.)
Weakness
Weakness may be derived from dysfunction of either upper motor neurons
or lower motor neurons.
Weakness is normally manifest as toe dragging, stumbling, or knuckling
over. Look closely at the horse‟s hooves to see if one or more is abnormally
worn.
Lower motor neuron weakness is accompanied by muscle flaccidity and (if
chronic) with muscle atroph y. Sometimes lower motor neuron involvement
may appear similar to lameness.
Upper motor neuron weakness is manifest as a lack of control of limb
muscle movement, and may be most easily tested by pulling the tail during
walking.
Pulling the tail with the horse stationary probably tests primarily lower
motor neuron control and muscular strength of the pelvic limb.
At this stage the examiner should be able to grade the defects present. This has been done on a 5
point scale (Mayhew) where 0 is normal and 5 is recumbent. It is helpful to assess each leg in
turn, for degrees of ataxia and weakness.
DETERMINING THE SITE OF SPINAL LESIONS
Assuming that there are no brain or brainstem signs, an animal with ataxia or weakness is most
likely to have a lesion in the spinal cord. Spinal reflexes used to assess specific spinal segments in
small animals are not very useful in adult horses, even in recumbent patients. Withdrawal reflexes
(where a stimulus is applied to the distal limb and the reflex results in withdrawal of the limb
(associated with abrupt flexion) are however useful in recumbent animals with severe lesions.
Cervical lesions (C1-C5 (C6/C7))
Horses that display defects in all 4 legs are most likely to have a cervical lesion. Typically the
pelvic limbs are more severely affected than the thoracic limbs, because of the topographical
arrangement of the white matter tracts in the spinal cord. The cervicofacial reflex (segmental
afferent – VII) is tested by lightly stimulating the skin of the lateral neck and observing for a
reflex movement of the lips. There is fairly wide variation in horses. In recumbent horses,
withdrawal reflexes in both thoracic and pelvic limbs may be exaggerated due to a loss of upper
motor neuron inhibition of the reflex.
Brachial lesions (C6-T2)
Horses with a spinal lesion at the brachial outflow may display lower motor neuron signs to the
thoracic limbs (weakness, muscle atrophy) in addition to ataxia and upper motor neuron weakness
(sway, stumbling etc) in the pelvic limbs. The thoracic limb weakness may be mild and manifest
as unwillingness to take the weight on the other thoracic limb when one is picked up. Withdrawal
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