South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
action of the retractor oculi muscle (abducens nerve (VI) in response to
sensory input from the ophthalmic branch of the trigeminal nerve.
Masticatory muscles
The mandibular branch of the trigeminal nerve is the only division of the
trigeminal nerve that carries motor fibres. It innervates the muscles of
mastication, in particular (and most easily assessed), the masseter and
temporalis muscles (in addition to the caudal half of the digastricus). Look
closely for muscle atrophy of these structures.
Facial expression
The so-called muscles of facial expression are innervated by branches of the
facial nerve (VII). In horses, facial nerve dysfunction is common, and
readily identified by a lip droop and/or muzzle deviation or an ear droop.
(Muzzle deviation is towards the normal side). Early or mild dysfunction
may be reflected by slight changes to nostril size or flare and to reduced ear
movements in response to audible stimuli. Because the facial nerve branch
that supplies the muzzle and nostrils crosses the vertical ramus of the
mandible and the surface of the masseter muscle, a lesion (for example a
kick) to the side of the face may result in signs confined to the nose; in
contrast, a more central lesion tends to result in both ear and nostril signs.
The parasympathetic supply to the lacrimal glands is carried in the facial
nerve. As such, horses with facial nerve dysfunction are prone to corneal
ulceration through (i) inability to blink and (ii) poor or absent production of
tears.
The palpebral reflex (V and VII) is readily elicited by ligh tly touching the
lids around the eyes and examining for reflex closure.
Vestibular system
The vestibulocochlear nerve (VIII) carries both auditory (cochlear) signals
and balance (vestibular) signals. The vestibular nuclei in the brainstem then
relay information to the eyes (see above), to the body and limbs and to
higher centres. Many efferent signals are controlled in part by cerebellar
input.
Horses with unilateral vestibular (VIII) lesions often have a head tilt towards
the side of the lesion and in the acute stages may have spontaneous
nystagmus. With central vestibular disease the nystagmus often is variable
(rotary, horizontal and vertical). With peripheral vestibular disease the fast
phase of the nystagmus is away from the side of the lesion.
With more chronic lesions, the nystagmus may have resolved (although it
may return with a change in the position of the head (positional nystagmus).
In addition, ataxic movements often improve with visual accommodation;
however they can return (often dramatically) upon blindfolding.
Horses with bilateral vestibular disease may not have an observable head
tilt, but movements are likely to be markedly ataxic (in part probably due to
involvement of ascending proprioceptive and descending motor pathways
that run through the brainstem.
In the absence of nystagmus, determining whether a horse with a head tilt is
in addition, weak, is helpful in deciding whether vestibular disease is central
or peripheral. In the latter scenario, the horse may be ataxic, but weakness is
not normally a feature.
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