Page 1072 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1047

VetBooks.ir Oculomotor nerve (CN III) dysfunction results in deviation of the globe that
persists in all head positions and is called stra-
The diameter of the pupil is controlled by two mus-
cle groups: the constrictor muscles innervated by the
nerves to each other in the horse’s skull, they are
parasympathetic fibres of the oculomotor nerve, and bismus. Given the close proximity of these three
the dilator muscles that are innervated by sympa- often damaged simultaneously. Their function is
thetic fibres from the cranial cervical ganglion. The assessed by moving the horse’s head up and down
innervation originates in the brainstem and changes and from side to side. Normal vestibular nys-
pupil diameter in response to light (oculomotor), and tagmus should be present – with a slow phase of
to fear or excitement (sympathetic). eyeball rotation away from the direction of head
The parasympathetic component of this nerve is movement, and then a rapid ‘catch-up’ in the
tested by the pupillary light response (PLR). First, direction of the head movement. This characteris-
check there are no physical obstructions to inhibit tic movement pattern requires an intact vestibular
iris movement, such as synechiae, and then shine a system, intact CNs III, IV and VI, and functional
bright light into each eye in turn, while observing connection between them. Hence, lesions of the
changes in the pupil size. An immediate direct (ipsi- vestibular system may result in abnormal eye posi-
lateral) and consensual (contralateral) constriction of tion (strabismus that varies with head position) or
the pupil is normal. If the constriction response is abnormal or spontaneous movement (nystagmus).
weak, it is worth repeating in a darkened room in Periorbital space-occupying lesions often cause
case the ambient light is too bright. mechanical globe deviations.
The afferent pathway of the PLR is the optic nerve,
which projects to the contralateral Edinger Westphal Trigeminal nerve (CN V)
nucleus. Efferent projections from this nucleus medi- The trigeminal nerve is responsible for facial sensa-
ate the direct and consensual responses. Lesions of tion and the muscles of mastication (via the man-
the central visual pathway (thalamus and cerebral dibular branch). Sensory function can be assessed
cortex) do not affect the PLR. Therefore, a widely by lightly stimulating the skin at the ears, medial
dilated (mydriatic) pupil that fails to respond to direct and lateral canthi of the eyes (palpebral response),
light, in an eye with normal vision, suggests an ocu- nostrils and lips and watching for a correspond-
lomotor nerve abnormality. If the contralateral eye ing reflex in that part of the head. Together, these
has normal oculomotor function it will have a direct are referred to as the facial reflexes. A normal
and consensual PLR. A retrobulbar lesion affecting reflex requires an intact brainstem and trigeminal
CN II and CN III will be unresponsive to light in and facial nerves (CN VII), but does not require
either eye and will not have a menace response. conscious perception of the skin stimulation.
The oculomotor nerves can be damaged by oedema, Unilateral loss of facial sensation could be due to
haemorrhage and space-occupying lesions in the fore- a lesion affecting the peripheral portion of the tri-
brain that put pressure ventrally on the brainstem. geminal nerve, the nuclei in the petrosal bone or
Forebrain swelling can lead to herniation beneath spinal tracts in the medulla and brainstem. If the
the tentorium cerebella, and crushing of the Edinger latter are damaged the lesion is likely to be fatal
Westphal nucleus in the midbrain and underlying ocu- because the lesion will usually affect the adjacent
lomotor nerves. When managing a horse with head cardiovascular and respiratory centres.
trauma or one in a semi-comatose or comatose state, The motor component of CN V innervates
uni- or bilateral pupillary dilation is a grave prognostic the muscles of mastication (except the digastricus
sign as it suggests substantial brainstem compression. muscle). A lesion of the motor component of the
trigeminal results in a dropped jaw with a pro-
Oculomotor (CN III), trochlear truding tongue that can be retracted when stimu-
(CN IV) and abducens (CN VI) lated (bilateral) or a weak jaw, difficulty chewing
These nerves are responsible for the normal and muscle atrophy in advanced cases (unilateral)
position and movement of the eye, and their (Fig. 10.4).