Page 1071 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1046 CHAPTER 10

VetBooks.ir Optic nerve (CN II) eye in turn and a consistent blink (and often the
head jerking away from the hand) is the normal
The optic nerve is part of the visual pathway and
both the nerve and vision can be assessed in a variety
occur if the hand movement is too vigorous and
of ways. The horse’s ability to navigate safely in its response (Figs. 10.2, 10.3). False positives can
usual environment, or a constructed obstacle course, creates an air current picked up by the cornea, or if
is the simplest way of assessing vision; however, some the hand touches the whiskers or eyelashes. False
confusion can occur in owners of horses with vestib- negatives can occur if the horse is depressed, or
ular disease, who may mistakenly think their horse stoic, or if the test is repeated too many times in
is blind because it stumbles into objects. quick succession.
The visual pathway can also be assessed by the A negative menace response has several causes
menace response. A hand is flicked towards each that are summarised in Table 10.2.

10.2 10.3

Fig. 10.2 Two-week-old foal with dementia Fig. 10.3 Same foal as in 10.2 with no menace response
and normal menace response in the left eye. in the right eye. The foal was also noted to circle
(Photo courtesy FT Bain) continually to the left. The foal was diagnosed with a
left-sided cortical lesion. (Photo courtesy FT Bain)

Table 10.2 Causes of a negative menace response

CAUSE OF NEGATIVE
MENACE RESPONSE ADDITIONAL DIAGNOSTICS INTERPRETATION OF FURTHER TESTS
True visual deficit • Obstacle course Unilateral central blindness is usually in the contralateral eye
• Ophthalmological examination to the lesion because of the almost complete cross-over at
the optic chiasm in the horse
Central blindness is usually bilateral, due to cerebral swelling
Eye and optic nerve disease will cause ipsilateral blindness
and abnormalities of the pupillary light response
Lesions of the optic tracts and lateral geniculate nucleus
within the brain will cause contralateral blindness.
Facial nerve paralysis • Rest of cranial nerve examination Usually there is additional evidence of facial paralysis, such
(inability to blink) • Will usually have head jerk even if blink as facial drooping on the same size and muzzle pulled to the
is absent contralateral side
Cerebellar disease • Normal vision but no menace response Unknown mechanism possibly involving the upper motor
• Other signs of cerebellar disease neuron to the eye
(ataxia, intention tremor)