Page 1073 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1048 CHAPTER 10

VetBooks.ir 10.4 10.5

Fig. 10.4 Unilateral
masseter muscle atrophy
in a horse with equine
protozoal myelitis.

Fig. 10.5 Left-sided
facial nerve injury.
Note how the muzzle
is deviated to the
unaffected side (right).

Facial nerve (CN VII) which can be picked up by assessing response to a
This primarily motor nerve innervates the muscles of handclap. Unilateral deafness, however, can be hard
facial expression. Facial paralysis results in drooping of to detect.
the eyelid and ear to the side of the lesion, and retrac- The vestibular system comprises the sensory
tion of the muzzle away from the side of the lesion structures in the inner ear, the vestibular branch
(Fig. 10.5). Interestingly, chronic unilateral paralysis of CN VIII and the central components in the
may eventually result in muzzle deviation to the affected cerebellum and medulla. The vestibular system
side due to muscle contracture of the denervated mus- is responsible for maintaining proper orientation
cle. Clinically, cases of facial paralysis may present as of the eyes, head, neck and limbs with respect to
corneal ulceration due to inability to close the eyelid, gravity and motion. A loss of function results in
inspiratory stertor at exercise due to nostril collapse and a loss of ipsilateral antigravitational tone result-
difficulty in prehension of food due to lip paralysis. ing in staggering, circling, falling over, a head tilt
The facial nerve tract runs through the petrous (Fig. 10.6) and spontaneous nystagmus. Visual
temporal bone close to the middle ear, and so lesions cues will usually compensate over time, which
of the inner or middle ear can result in both ves- may reduce the severity of the vestibular signs,
tibular and facial paralysis. Facial grimacing can be a and blindfolding (with care) will usually exacer-
sign of pain in a conscious horse. Grimacing is some- bate the signs (Fig. 10.7).
times subconscious, as a result of damage to inhibi- Peripheral lesions in the inner ear or CN VIII
tory upper motor neuron centres controlling facial usually result in nystagmus that is only horizontal,
movement. Peracut encephalitides may cause facial and the fast phase is usually away from the side of the
grimacing for this reason. lesion and away from the direction of the head tilt.
Central lesions tend to result in an inconsistent and
Vestibulocochlear nerve (CN VIII) variable nystagmus, and these horses usually have
The cochlear portion of this nerve is responsible for other adjacent pathology resulting in ataxia, weak-
hearing. Bilateral middle ear disease causes deafness, ness and depression.