South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Eyes
Menace response - Care should be taken when testing the menace response
to ensure that the hand does not create air movements that may be sensed
(rather than seen). Since nearly all optic nerve (II) fibres cross at the optic
chiasm in the horse, a positive menace response confirms normal function of
that particular optic nerve, the chiasm, pathways through the thalamus to the
occipital visual cortex (on the opposite side). The response is assessed by
observing the horse blink, which confirms that appropriate afferent signals
have reached the facial nerve (VII) nucleus and nerve in the brainstem on
the same side. The response therefore assesses both visual pathways and the
facial nerve. Note that it is assumed that the afferent pathway from the
visual cortex passes through the cerebellum, hence horses with cerebellar
disease may lack or have diminished menace responses.
Pupillary reflexes. The pupillary light reflex (PLR) is a true reflex (there is
no necessity for perception of the stimulus) as the pathways remain in the
thalamus and brainstem. A light shined into the eye should normally result
in a reflex constriction of the pupil in the same eye (the direct reflex) and a
partial constriction of the other eye (consensual reflex). Note that this test,
examines both optic nerve (II) function and also the parasympathetic fibres
within the occulomotor nerve (III) that control pupillary constriction. It is
difficult, but possible, to obtain consensual pupillary light reflexes in large
animals for a single observer because of the shape of the head.
The pupil diameter is controlled by both parasympathetic (occulomotor
nerve III) function (see PLR above) for constriction and sympathetic
function for dilation (fear / excitement). Horner’s syndrome (ptosis –
drooping of the upper eyelid, miosis - constriction of the pupil, and
enophthalmus with protrusion of the 3rd eyelid) is seen in horses often in
combination with sweating, since in horses (unlike other animals) sweating
is largely dependent on regional increases in blood flow. Interruption to
sympathetic pathways means that parasympathetic (dilation) of peripheral
blood vessels predominates, causing regional sweating.
Because the sympathetic supply reaches the eye via the spinal cord, Horner‟s
syndrome can be caused by spinal cord disease. First order preganglionic
fibres that originate i n the hypothalamus pass via the brainstem and cervical
spinal cord to the ventral grey matter of the thoraco-lumbar spinal cord.
From here 2nd order preganglionic neurones exit via spinal nerves and
synapse in a ganglion before post-ganglionic fibres emanate.
2nd order preganglionic fibres destined for the head leave the spinal cord at
T1-T3 spinal cord segments and pass through the thorax close to the cranial
apex of the lung and the thoracic inlet. They travel via the cranial thoracic
(stellate) ganglion (they do not synapse here), and thence via the (vago)
sympathetic trunk back up the neck to the cranial cervical ganglion (close to
the wall of the guttural pouch) where they synapse. From here, 3rd order
(post-ganglionic) neurons pass to the eye via branches of the internal carotid
artery and to the skin of the head.
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