South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Pharyngeal and laryngeal function
Cranial nerves (IX, X and XI): the glossopharyngeal, vagus and accessory
nerves innervate these areas. Dysfunction in horses is most commonly
appreciated in roarers – horses with left recurrent laryngeal hemiplegia (a
branch of the vagus). Traditionally, this nerve was evaluated with the „slap‟
test; however pharyngeal and laryngeal function is best studied by (1)
observing the horse eat and swallow, and by (2) endoscopy.
Pharyngeal paralysis in horses may commonly be associated with peripheral
(especially guttural pouch) disease: as such, endoscopy of the guttural
pouches should be performed in horses that are dysphagic.
(Note: the spinal division of the accessory-spinal nerve innervates the
trapezius muscle and cranial portions of the strap cervical strap muscles;
dysfunction is rare. The nerve is accessible for nerve biopsies in the throat
latch region).
Movement of the tongue
The motor innervation of the tongue is derived from the hypoglossal nerve
(XII). A normal horse will resist the tongue being retracted from the mouth,
and will quickly return it after it is pulled out to the side. In contrast, horses
with hypoglossal weakness may have the tongue protruding from the mouth;
chronic hypoglossal lesions may result in unilateral tongue atrophy, whereas
bilateral hypoglossal lesions may result in difficulty prehending food.
Note that horses with cerebral dysfunction may have signs compatible with
tongue weakness: in these cases though, tongue atrophy is not present.
BODY, SPINAL CORD AND LIMBS
3. Testing conscious proprioception
By altering the position of the limbs in space it is possible to test a horse‟s conscious
proprioception. Normally horses should replace their limbs to a normal position
within a few seconds. There is however considerable variability and the test can be
difficult to interpret. These tests can also be performed by walking or trotting and then
suddenly stopping a horse to see how quickly it replaces its legs in a normal position.
4. Gait – higher control of motor function and unconscious proprioception
During assessment of gait, the examiner is looking primarily for evidence of ataxia
and/ or weakness.
Ataxia
Ataxia reflects defects in unconscious proprioceptive pathways that
originate in muscle spindle (stretch) receptors and golgi tendon organs,
relayed via peripheral nerves to the spinal cord. They then ascend in
pathways to the cerebellum. Ataxia is manifest as exaggerated movements
(hypermetria) or reduced (stilted hypometric) movements or both
(dysmetria). Sometimes animals will sway or place their feet abnormally,
either crossing or weaving the feet with abnormal abduction or adduction.
By lifting the horse‟s head it is sometimes possible to exaggerate signs of
spinal ataxia. 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 during circling (“circumduction”) is also a common sign.
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