Difference between revisions of "Neurological Eye Examination - Horse"
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(Created page with "====Menace Response==== * The reflex is assessed by observing the horse blink in response to a visual "threat". ** The menace reflex is a learned response. * When testing the me...") |
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* The easiest way to determine blindness in horses is to create an obstacle course. | * The easiest way to determine blindness in horses is to create an obstacle course. | ||
** Cover the eyes separately to assess each in turn. | ** Cover the eyes separately to assess each in turn. | ||
− | * Ophthalmological examination should be performed if any any of the | + | * Ophthalmological examination should be performed if any any of the followinf are found to be imparied: |
** Visual pathways | ** Visual pathways | ||
** Reflexes | ** Reflexes | ||
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− | [[Category: | + | [[Category:Neurological Examination - Horse]] |
Revision as of 16:02, 9 March 2011
Menace Response
- The reflex is assessed by observing the horse blink in response to a visual "threat".
- The menace reflex is a learned response.
- When testing the menace reflex, it should be ensured that the hand does not create air movements.
- These may be sensed, for example by the vibrissae, rather than seen.
- A positive menace reflex confirms normal function of:
- The particular optic nerve (CN II)
- The optic chiasm
- Nearly all optic nerve fibres cross at the chiasm in the horse.
- Pathways through the thalamus to the occipital visual cortex on the opposite side.
- Afferent pathways to the facial nerve (VII) nucleus in the brainstem on the original side.
- It is assumed that the afferent pathway from the visual cortex passes through the cerebellum.
- Horses with cerebellar disease may lack or have diminished menace responses.
- It is assumed that the afferent pathway from the visual cortex passes through the cerebellum.
- The facial nerve on the original side (efferent pathway).
- The menace response therefore assesses both visual pathways and the facial nerve.
Pupillary Responses
- Pupil diameter is controlled by:
- Parasympathetic function for constriction.
- Occulomotor nerve (CN III)
- Sympathetic function for dilation.
- For example, in fear or excitement.
- Parasympathetic function for constriction.
The pupillary light reflex (PLR)
- The PLR is a true reflex; the pathways remain in the thalamus and brainstem, and the stimulus need not be perceived.
- Shining a light into the eye should result in:
- A reflex constriction of the pupil in the same eye.
- The direct response
- A partial constriction of the other eye.
- The consensual response
- This is difficult to see in large animals because of the shape of the head.
- A reflex constriction of the pupil in the same eye.
- The PLR examines:
- Optic nerve function
- The parasympathetic fibres within the occulomotor nerve.
Horner’s syndrome
- Horner's syndrome is a clinical syndrome caused by damage to the sympathetic nervous system.
- Signs include:
- Ptosis
- Drooping of the upper eyelid
- Miosis
- Constriction of the pupil
- Enophthalmus
- Sinking of the eyeball into the orbital cavity
- Protrusion of the third eyelid
- In horses, Horner's syndrome is often seen in combination with regional sweating.
- Unlike in other animals, sweating in horses is largely dependent on regional increases in blood flow.
- Parasympathetic dilation of peripheral blood vessels predominates when sympathetic pathways are interrupted.
- This causes regional sweating.
- Ptosis
- The sympathetic supply reaches the eye via the spinal cord; Horner’s syndrome can therefore be caused by spinal cord disease.
- First order preganglionic fibres originate in the hypothalamus, and pass via the brainstem and cervical spinal cord to the ventral grey matter of the thoraco-lumbar spinal cord.
- Second order preganglionic neurones exit the spinal cord via spinal nerves.
- Preganglionic fibres destined for the head leave the spinal cord at spinal nerves T1-T3.
- Fibres pass through the thorax, travelling via the cranial stellate ganglion (where they do not synapse), and the vagosympathetic trunk up the neck.
- Preganglionic fibres then synapse in the cranial cervical ganglion.
- From here, 3rd order postganglionic neurons pass to:
- The eye, via branches of the internal carotid artery.
- The skin of the top of the head.
- The sympathetic supply to the skin the neck caudal to C2 is via segmental cervical vertebral nerves.
- Cervical vertebral nerves each carry postganglionic sympathetic fibres.
- These fibres follow the vertebral artery after leving the stellate ganglion.
- A caudal cervical lesion may therefore affect the sypathetic trunk, causing sweating to C2 but not C2-C8.
- I.e. C2-C8 has alternative sympathetic supply, and so is not affected by a lesion of this sort.
- Lesions occuring post- cranial cervical ganglion result in sweating of the face and the area of skin at the base of the ear down to about C1.
- For example, lesions in guttural pouch disease.
Vision
- The easiest way to determine blindness in horses is to create an obstacle course.
- Cover the eyes separately to assess each in turn.
- Ophthalmological examination should be performed if any any of the followinf are found to be imparied:
- Visual pathways
- Reflexes
- Responses
Eye position
- Eye position is controlled by the actions of the extraocular eye muscles.
- These muscles are innervated by:
- The oculomotor nerve (CN III)
- The trochlear nerve (CN IV)
- The abducens nerve (CN VI).
- Dysfunction of these nerves results in strabismus.
- These muscles are innervated by:
- The eyes must move in relation to the position of the head and neck.
- Pathways exist that mediate the movement of the eyes in response to head and neck movement.
- Vestibular and neck problems can therefore result in a perceived strabismus.
- Pathways exist that mediate the movement of the eyes in response to head and neck movement.
- Normally, elevation of the head results in ventral movement of the eye.
- The eye is usually fixed on a point in space.
- Lateral head and neck movement results in rhythmic eye movement in response to motion - "doll’s eye vestibular nystagmus".
- This is similar to a human fixing its eyes on a point out of a window of a moving train.
- This form of nystagmus is normal.
- It is characterised by the fast phase being in the direction of movement.
- Strabismus is relatively easy to asses in the horse due to the elongated shape of the pupil.
- True strabismus is relatively rare in horses.
- Occulomotor nerve dysfunction may result in lateral deviation of the eyeball.
- Parasympathetic supply is often also interrupted, giving mydriasis.
- Apparent strabismus may be seen in horses with vestibular disease, since the vestibular system interacts with eye positioning.
- However, in this scenario eye movements to and away from the apparent direction of strabismus are still possible.