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| | ==[[Cranial Nerve Examination - Horse]]== | | ==[[Cranial Nerve Examination - Horse]]== |
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| − | * The cranial nerves are numbered 1-12 from the most rostral to the most caudal.
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| − | * A systematic examination of the cranial nerves can aid accurate identification of the site of a lesion.
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| − | ===Cranial Nerve I===
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| − | * Cranial nerve I is the olfactory nerve.
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| − | ** Abnormalities are rarely detected in this nerve.
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| − | ===[[Neurological Eye Examination - Horse]]===
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| − | ====Menace Response====
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| − | * The reflex is assessed by observing the horse blink in response to a visual "threat".
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| − | ** The menace reflex is a learned response.
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| − | * When testing the menace reflex, it should be ensured that the hand does not create air movements.
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| − | ** These may be sensed, for example by the vibrissae, rather than seen.
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| − | * A positive menace reflex confirms normal function of:
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| − | *# The particular optic nerve (CN II)
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| − | *# The optic chiasm
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| − | *#* Nearly all optic nerve fibres cross at the chiasm in the horse.
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| − | *# Pathways through the thalamus to the occipital visual cortex on the opposite side.
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| − | *# Afferent pathways to the facial nerve (VII) nucleus in the brainstem on the original side.
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| − | *#* It is assumed that the afferent pathway from the visual cortex passes through the cerebellum.
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| − | *#** Horses with cerebellar disease may lack or have diminished menace responses.
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| − | *# The facial nerve on the original side (efferent pathway).
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| − | * '''The menace response therefore assesses both visual pathways and the facial nerve.'''
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| − | ====Pupillary Responses====
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| − | * Pupil diameter is controlled by:
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| − | ** Parasympathetic function for constriction.
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| − | *** Occulomotor nerve (CN III)
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| − | ** Sympathetic function for dilation.
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| − | *** For example, in fear or excitement.
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| − | =====The pupillary light reflex (PLR)=====
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| − | * The PLR is a true reflex; the pathways remain in the thalamus and brainstem, and the stimulus need not be perceived.
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| − | * Shining a light into the eye should result in:
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| − | *# A reflex constriction of the pupil in the same eye.
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| − | *#* The direct response
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| − | *# A partial constriction of the other eye.
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| − | *#* The consensual response
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| − | *#* This is difficult to see in large animals because of the shape of the head.
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| − | * The PLR examines:
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| − | ** Optic nerve function
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| − | ** The parasympathetic fibres within the occulomotor nerve.
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| − | =====Horner’s syndrome=====
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| − | * Horner's syndrome is a clinical syndrome caused by damage to the sympathetic nervous system.
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| − | * Signs include:
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| − | ** '''Ptosis'''
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| − | *** Drooping of the upper eyelid
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| − | ** '''Miosis'''
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| − | *** Constriction of the pupil
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| − | ** '''Enophthalmus'''
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| − | *** Sinking of the eyeball into the orbital cavity
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| − | ** '''Protrusion of the third eyelid'''
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| − | ** In horses, Horner's syndrome is often seen in combination with '''regional sweating'''.
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| − | *** Unlike in other animals, sweating in horses is largely dependent on regional increases in blood flow.
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| − | *** Parasympathetic dilation of peripheral blood vessels predominates when sympathetic pathways are interrupted.
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| − | **** This causes regional sweating.
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| − | * The sympathetic supply reaches the eye via the spinal cord; Horner’s syndrome can therefore be caused by spinal cord disease.
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| − | ** 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.
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| − | ** Second order preganglionic neurones exit the spinal cord via spinal nerves.
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| − | *** Preganglionic fibres destined for the head leave the spinal cord at spinal nerves T1-T3.
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| − | ** Fibres pass through the thorax, travelling via the cranial stellate ganglion (where they do not synapse), and the vagosympathetic trunk up the neck.
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| − | ** Preganglionic fibres then synapse in the cranial cervical ganglion.
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| − | ** From here, 3rd order postganglionic neurons pass to:
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| − | *** The eye, via branches of the internal carotid artery.
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| − | *** The skin of the top of the head.
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| − | * The sympathetic supply to the skin the neck caudal to C2 is via segmental cervical vertebral nerves.
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| − | ** Cervical vertebral nerves each carry postganglionic sympathetic fibres.
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| − | ** These fibres follow the vertebral artery after leving the stellate ganglion.
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| − | ** A caudal cervical lesion may therefore affect the sypathetic trunk, causing sweating to C2 but not C2-C8.
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| − | *** I.e. C2-C8 has alternative sympathetic supply, and so is not affected by a lesion of this sort.
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| − | * 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.
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| − | ** For example, lesions in guttural pouch disease.
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| − | ====Vision====
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| − | * The easiest way to determine blindness in horses is to create an obstacle course.
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| − | ** Cover the eyes separately to assess each in turn.
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| − | * Ophthalmological examination should be performed if any any of the followinf are found to be imparied:
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| − | ** Visual pathways
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| − | ** Reflexes
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| − | ** Responses
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| − | ====Eye position====
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| − | * Eye position is controlled by the actions of the extraocular eye muscles.
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| − | ** These muscles are innervated by:
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| − | *** The oculomotor nerve (CN III)
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| − | *** The trochlear nerve (CN IV)
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| − | *** The abducens nerve (CN VI).
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| − | ** '''Dysfunction of these nerves results in strabismus'''.
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| − | * The eyes must move in relation to the position of the head and neck.
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| − | ** Pathways exist that mediate the movement of the eyes in response to head and neck movement.
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| − | *** Vestibular and neck problems can therefore result in a perceived strabismus.
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| − | * Normally, elevation of the head results in ventral movement of the eye.
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| − | ** The eye is usually fixed on a point in space.
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| − | * Lateral head and neck movement results in rhythmic eye movement in response to motion - "doll’s eye vestibular nystagmus".
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| − | ** This is similar to a human fixing its eyes on a point out of a window of a moving train.
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| − | ** This form of nystagmus is normal.
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| − | *** It is characterised by the fast phase being in the direction of movement.
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| − | * Strabismus is relatively easy to asses in the horse due to the elongated shape of the pupil.
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| − | ** True strabismus is relatively rare in horses.
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| − | ** Occulomotor nerve dysfunction may result in lateral deviation of the eyeball.
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| − | *** Parasympathetic supply is often also interrupted, giving mydriasis.
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| − | ** Apparent strabismus may be seen in horses with vestibular disease, since the vestibular system interacts with eye positioning.
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| − | *** However, in this scenario eye movements to and away from the apparent direction of strabismus are still possible.
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| − | [[Category:Neurological Examination - Horse|C]]
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| − | ===[[Face Sensation]]===
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| − | * Nerve fibres giving sensation to the face are carried in cranial nerve V - the trigeminal nerve.
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| − | ** The trigeminal nerve provides sensory innervation to:
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| − | *** The skin of the face
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| − | *** The ears
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| − | *** The nasal mucosa
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| − | *** The cornea
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| − | ** There are three divisions of the trigeminal nerve
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| − | *** The mandibular branch
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| − | *** The maxillary branch
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| − | *** The opthalmic branch
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| − | * Facial sensation is tested by observation for an avoidance response or reflex movement following stimulation.
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| − | ** All the divisions of the trigeminal nerve should be tested. This is achieved by:
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| − | *** Stimulation within the nostrils
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| − | *** Stimulation within the ears
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| − | *** Stimulation between the mandibles
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| − | *** Testing the corneal reflex.
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| − | ***# Pressure is exerted on the corneal surface through the eyelid.
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| − | ***# The opthalmic branch of the trigeminal nerve provides sensory (afferent) input.
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| − | ***# Efferent signals are sent via the abducens nerve (CN VI)
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| − | ***# The retractor oculi muscle retracts the eye
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| − | ====Masticatory muscles====
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| − | * The mandibular branch of the trigeminal nerve carries motor fibres.
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| − | ** It is the only division of the trigeminal nerve that does so.
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| − | * The muscles of mastication are innervated by these fibres.
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| − | ** The masseter and temporalis muscles are particulaly influenced by the mandibular brach of the trigeminal nerve.
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| − | *** These muscles of mastication are also the easiest to assess.
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| − | *** These muscles should be closely examined for signs of atrophy.
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| − | **** '''Atrophy of the temporalis and masseter muscle indicates damage to the mandibular branch of CN V.'''
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| − | ====Facial Expression====
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| − | * The muscles of facial expression are innervated by branches of cranial nerve VII - the facial nerve.
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| − | * The facial nerve also carries the parasympathetic supply to the lacrimal glands.
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| − | ** Horses with facial nerve dysfunction are therefore prone to corneal ulceration, due to :
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| − | *** An inability to blink
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| − | *** Poor or absent tear production
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| − | =====Facial Nerve Dysfunction=====
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| − | * Facial nerve dysfunction is common in the horse.
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| − | * It is readily identified by one or more of the following:
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| − | ** A lip droop
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| − | ** Muzzle deviation
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| − | *** Deviation is towards the normal side.
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| − | ** An ear droop
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| − | * Early or mild dysfunction may be reflected by:
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| − | ** Slight changes to nostril size
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| − | ** Nostril flare
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| − | ** Reduced ear movements in response to audible stimuli
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| − | * The branch of CN VII supplying the muzzle and nostrils crosses the vertical mandibular ramus and the surface of the masseter muscle.
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| − | ** A lesion to that side of the face, for example a kick, may cause signs confined to the nose.
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| − | ** Alternatively, a more central lesion will give both ear and nostril signs.
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| − | =====The Palpebral Reflex=====
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| − | * The palpebral reflex examines the function of both CN V (afferent) and CN VII (efferent).
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| − | * The reflex is elicited by lightly touching the eyelids and watching for reflex closure.
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| − | [[Category:Neurological Examination - Horse|D]]
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| − | ===[[Vestibular System Examination]]===
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| − | ====The Vestibulocochlear Nerve====
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| − | * Cranial nerve VIII is the vestibulocochlear nerve.
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| − | * The vestibulocochlear nerve carries
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| − | ** Auditory, or cochlear, signals.
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| − | ** Balance, or vestibular, signals.
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| − | * Signals from CN VIII are relayed to the vestibular nuclei in the brainstem.
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| − | ** The nuclei in turn relay information to
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| − | *** The eyes
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| − | *** The body and limbs
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| − | *** Higher centres.
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| − | * Many efferent signals are controlled in part by cerebellar input.
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| − | ====Unilateral Vestibular Disease====
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| − | * Horses with unilateral vestibular lesions often have a head tilt towards the side of the lesion.
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| − | * If nystagmus is absent, determining whether a horse with a head tilt is also weak is helpful in deciding whether vestibular disease is central or peripheral.
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| − | ** In peripheral diease, the horse may be ataxic but weakness is not normally seen.
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| − | =====Acute=====
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| − | * In the acute stages of vestibular disease, spontaneous nystagmus may be present.
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| − | ** The appearance of nystagmus is different depending on the type of vestibular disease present.
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| − | *** Central vestibular disease
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| − | **** Nystagmus often is variable; i.e. rotary, horizontal and vertical.
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| − | *** Peripheral vestibular disease
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| − | **** The fast phase of the nystagmus is away from the side of the lesion.
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| − | =====Chronic=====
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| − | * Nystagmus may resolve in more chronic lesions.
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| − | ** However, it may return with changes in head position.
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| − | *** I.e. positional nystagmus.
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| − | * Visual accomodation improves ataxic movements.
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| − | ** However, these may return dramatically on blindfolding.
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| − | ====Bilateral Vestibular Disease====
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| − | * An observable head tilt may not be present.
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| − | * Movements are likely to be markedly ataxic.
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| − | ** This is probably due in part to involvement of ascending proprioceptive and descending motor pathways that run through the brainstem.
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| − | [[Category:Neurological Examination - Horse|E]]
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| − | ===[[Pharyngeal and Laryngeal Function]]===
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| − | * The pharynx and larynx are innervated by:
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| − | ** CN IX - the glossopharyngeal nerve
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| − | ** CN X - the vagus nerve
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| − | ** CN XI - the accessory nerve
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| − | * Pharyngeal and laryngeal function is best studied by:
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| − | *# Observing the horse eat and swallow
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| − | *# Endoscopy
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| − | ====Laryngeal Dysfunction====
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| − | * Left recurrent laryngeal hemiplegia is the most common dysfunction in horses.
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| − | ** The left recurrent laryngeal nerve is a branch of the vagus nerve.
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| − | ** Horses are known as "roarers".
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| − | ====Pharyngeal Dysfunction====
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| − | * Pharyngeal paralysis in horses is commonly associated with peripheral disease.
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| − | ** Especially guttural pouch disease.
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| − | *** Endoscopy of the guttural pouches should be therefore be performed in horses that are dysphagic.
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| − | [[Category:Neurological Examination - Horse|F]]
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| − | ===[[Tongue Movement]] ===
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| − | * The hypoglossal nerve, CN XII provides motor innervation to the [[Tongue - Anatomy & Physiology|tongue]].
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| − | * Normally, a horse resists the [[Tongue - Anatomy & Physiology|tongue]] being retracted from the mouth.
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| − | ** The horse will quickly return the tongue after it is pulled out to the side.
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| − | * Horses with hypoglossal weakness appear differently.
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| − | ** The [[Tongue - Anatomy & Physiology|tongue]] may protrude from the mouth.
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| − | ** Chronic unilateral hypoglossal lesions may result in unilateral tongue atrophy.
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| − | ** Bilateral hypoglossal lesions may result in difficulty prehending food.
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| − | * Horses with cerebral dysfunction may have signs corresponding to tongue weakness.
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| − | ** However, [[Tongue - Anatomy & Physiology|tongue]] atrophy is not present in these animals.
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| − | [[Category:Neurological Examination - Horse|G]]
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