South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Note that the sympathetic supply to the skin of most of the neck caudal to
C2 is via segmental cervical vertebral nerves that each carry post ganglionic
sympathetic fibres. This explains why a caudal cervical lesion affecting the
sympathetic trunk, results in sweating of the face down to the level of C2,
but not C2-C8 (the supply to each cervical vertebral nerve is not carried in
the sympathetic trunk, but comes from fibres that follow the vertebral artery
after leaving the stellate cranial thoracic ganglion).
The face and the area of skin at the base of the ear down to about the atlas
(C1) is affected with sweating with post (cranial cervical) ganglionic lesions
(for instance in guttural pouch disease).
Vision: The easiest way to determine blindness in horses is to create an
obstacle course. Each eye can be covered separately to enable assessment of
each eye in turn.
Ophthalmological examination: should be performed in any animal where
impaired visual pathways, reflexes or responses are discovered.
Eye position
Eye position is controlled primarily by the action of the extraocular eye
muscles (innervated by the oculomotor nerve (III), the trochlear nerve (IV)
and the abducens nerve (VI). Dysfunction results in strabismus. Because it is
necessary for the eyes to move in relation to the position of the head and
neck, pathways mediate reciprocal movement of both eyes in response to
head and neck movement. As a result, neck and vestibular problems can
sometimes result in a perceived strabismus.
The elongated pupil of the horse makes assessment of strabismus relatively
easy. In a normal horse, elevation of the head results in the eye being moved
ventrally (usually fixed on a point in space). Likewise, when the head and
neck is moved laterally, the normal response is for the eyes to move
rhythmically in response to the motion (doll‟s eye nystagmus) (akin to our
fixing on a point out of a moving train‟s window). This normal nystagmus is
characterised by the fast phase being in the direction of movement, or in the
train example, in the direction of movement of the train).
True strabismus is relatively rare in horses, however oculomotor nerve
dysfunction may result in lateral deviation of the eyeball (and often a
mydriasis (dilated pupil) due to the interruption to the parasympathetic
supply (see discussion above). Because of the interactions of the vestibular
system with eye position, apparent strabismus may be seen in horses with
vestibular disease, but here eye movements to and away from the apparent
direction of strabismus are still possible.
Sensation to the face
Sensory nerve fibres from the skin of the face, ears, the nasal mucosa and
the cornea are carried in the sensory branches of the trigeminal nerve (V) –
in its mandibular, maxillary and ophthalmic divisions. These modalities can
be tested by stimulating within the nostrils, the ears, and between the
hemimandibles, and observing for an avoidance response or reflex
movement. The corneal reflex (where pressure is exerted on the corneal
surface through the eyelid tests the ability of the eye to retract through the
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