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Neuro Exam of The Horse - Pathology (view source)
Revision as of 10:56, 18 August 2008
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==Introduction==
* Investigation of neurological disease combines:
** Case history
** Neurological examination
** Neuroanatomical knowledge
** Experience
===Aims of Neurological Examination===
* There are two main aims of neurological examination:
*# To aid determination of the presence or absence of neurological disease.
*# To localise the lesion(s) when neurological disease is suspected.
===Indications for Neurological Examination===
* Neurological testing may be indicated by:
*# '''History'''
*#* For example, the owner reports a seizure-like episode.
*# '''Clinical examination'''
*#* For example, an unexplained area of muscle atrophy or patchy sweating.
*# '''Information from other tests'''
*#* Particularly negative, confusing or corroborating evidence from these.
*#* For example:
*#** A horse evaluated for pelvic limb lameness that may in fact have a neurological problem.
*#** A horse with altered mentation or depression that has already been found to have abnormal hepatic function.
===Approaches to Neurological Examination===
* A methodical approach is necessary, whatever the background, to ensure no aspect of examination is omitted.
* Two different approaches may be taken:
*# '''Systems based'''
*#* The different neurological modalities of the systems are examined in turn.
*# '''Anatomic'''
*#* Tests are performed in turn working from head to tail.
* A combination of the two approaches tends to be used in practice.
==Neuroanatomy==
* Certain basic facts must be understood before a decent neurological examination may be performed.
===White and Grey Matter===
* '''White matter''' consists of myelinated axons of nerve cells.
* '''Grey matter''' is made up of neuronal cell bodies, containing the nucleus.
===Afferent and Efferent===
* '''Afferent''' pathways relay sensory information from receptors towards the spinal cord or brain.
** For example:
*** Touch
*** Muscle stretch
*** Vision
*** Balance
* '''Efferent''' pathways relay motor or effector information from the brain or spinal cord to muscles or organs.
===Motor Neurons and Motor Reflexes===
* '''Lower motor neurons''' are the last link in the efferent pathway.
** Directly innervate skeletal muscles.
** Cell bodies are located in the ventral horn of the grey matter of the spinal cord.
** Their axons run in peripheral nerves and synapse at the neuromuscular junction.
* '''Upper motor neurons''' communicate with lower motor neurons.
** They:
*** Relay information to lower motor neurons
*** Control the output of lower motor neurons.
** Upper motor neurons are found in the brain and spinal cord.
* The term '''reflex''' describes a certain sort of nervous pathway.
** The sequence of events in a reflex is as follows:
**# Signals are derived from receptors.
**#* For example, tendon stretch.
**# Signals are conveyed directly in sensory (afferent) fibres.
**# Afferent signals reach the central nervous system.
**# Effector signals are generated in the CNS.
**#* This generally occues via an intermediate neuron, known as the internuncial neuron.
**# Effector signals are relayed in the lower motor neurons to the muscles.
** A reflex does NOT require:
*** Conscious perception of the stimulus
*** Ascending or descending upper motor neuron pathways
*** Ascending or descending proprioceptive pathways
==Signalment and History==
===Signalment===
* The term "signalment" describes the animal's age, sex and breed.
* The horse's signalemt may be relevant to the neurological examination.
** For example:
*** A 1 year old thoroughbred colt with ataxia is more likely to have cervical compression than a space occupying lesion.
*** An Arabian foal suffering seizures may have idiopathic epilepsy.
===History===
* History is a very important component of the neurological examination.
** An owner's description of the animal's behaviour or abnormality may both
*** Help direct the neurological examonation.
*** Suggest possible differential diagnoses.
* For example, a recumbent horse with a history of stumbling or knuckling that has recently fallen:
** May have underlying cervical vertebral malformation or stenosis, OR
** It is possible that the onset of pelvic limb weakness may have followed a spate of respiratory disease or an abortions, if the animal has [[Herpesviridae|EHV1]] myelitis.
==Demeanour/ Behaviour==
* The horse should be examined from a distance to assess behaviour and demeanour.
* Diseases associated with altered mentation or behaviour tend to involve the forebrain.
* Levels of consciousness are determined partly by the cerebrum and partly by the reticular activating system in the brainstem.
===What to Look For===
* Assymetrical changes.
** These include:
*** Circling
*** Head turning
** Horses usually circle or turn towards the affected side.
* Excessive yawning.
* Head pressing.
** This is often a sign of severe obtundation, which may be caused by:
*** Diffuse cerebral disease.
*** Metabolic problems, such as hepatic encephalopathy.
==Cranial Nerve Examination==
* The cranial nerves are numbered 1-12 from the most rostral to the most caudal.
* A systematic examination of the cranial nerves can aid accurate identification of the site of a lesion.
===Cranial Nerve I===
* Cranial nerve I is the olfactory nerve.
** Abnormalities are rarely detected in this nerve.
===Eyes===
====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.
*# 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.
=====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.
* 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.
* 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'''.
* 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.
* 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.
===Sensation to the Face===
* Nerve fibres giving sensation to the face are carried in cranial nerve V - the trigeminal nerve.
** The trigeminal nerve provides sensory innervation to:
*** The skin of the face
*** The ears
*** The nasal mucosa
*** The cornea
** There are three divisions of the trigeminal nerve
*** The mandibular branch
*** The maxillary branch
*** The opthalmic branch
* Facial sensation is tested by observation for an avoidance response or reflex movement following stimulation.
** All the divisions of the trigeminal nerve should be tested. This is achieved by:
*** Stimulation within the nostrils
*** Stimulation within the ears
*** Stimulation between the mandibles
*** Testing the corneal reflex.
***# Pressure is exerted on the corneal surface through the eyelid.
***# The opthalmic branch of the trigeminal nerve provides sensory (afferent) input.
***# Efferent signals are sent via the abducens nerve (CN VI)
***# The retractor oculi muscle retracts the eye
====Masticatory muscles====
* The mandibular branch of the trigeminal nerve carries motor fibres.
** It is the only division of the trigeminal nerve that does so.
* The muscles of mastication are innervated by these fibres.
** The masseter and temporalis muscles are particulaly influenced by the mandibular brach of the trigeminal nerve.
*** These muscles of mastication are also the easiest to assess.
*** These muscles should be closely examined for signs of atrophy.
**** '''Atrophy of the temporalis and masseter muscle indicates damage to the mandibular branch of CN V.'''
====Facial Expression====
* The muscles of facial expression are innervated by branches of cranial nerve VII - the facial nerve.
* The facial nerve also carries the parasympathetic supply to the lacrimal glands.
** Horses with facial nerve dysfunction are therefore prone to corneal ulceration, due to :
*** An inability to blink
*** Poor or absent tear production
=====Facial Nerve Dysfunction=====
* Facial nerve dysfunction is common in the horse.
* It is readily identified by one or more of the following:
** A lip droop
** Muzzle deviation
*** Deviation is towards the normal side.
** An ear droop
* Early or mild dysfunction may be reflected by:
** Slight changes to nostril size
** Nostril flare
** Reduced ear movements in response to audible stimuli
* The branch of CN VII supplying the muzzle and nostrils crosses the vertical mandibular ramus and the surface of the masseter muscle.
** A lesion to that side of the face, for example a kick, may cause signs confined to the nose.
** Alternatively, a more central lesion will give both ear and nostril signs.
=====The Palpebral Reflex=====
* The palpebral reflex examines the function of both CN V (afferent) and CN VII (efferent).
* The reflex is elicited by lightly touching the eyelids and watching for reflex closure.
===The Vestibular System===
====The Vestibulocochlear Nerve====
* Cranial nerve VIII is the vestibulocochlear nerve.
* The vestibulocochlear nerve carries
** Auditory, or cochlear, signals.
** Balance, or vestibular, signals.
* Signals from CN VIII are relayed to the vestibular nuclei in the brainstem.
** The nuclei in turn relay information to
*** The eyes
*** The body and limbs
*** Higher centres.
* Many efferent signals are controlled in part by cerebellar input.
====Unilateral Vestibular Disease====
* Horses with unilateral vestibular lesions often have a head tilt towards the side of the lesion.
* 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.
** In peripheral diease, the horse may be ataxic but weakness is not normally seen.
=====Acute=====
* In the acute stages of vestibular disease, spontaneous nystagmus may be present.
** The appearance of nystagmus is different depending on the type of vestibular disease present.
*** Central vestibular disease
**** Nystagmus often is variable; i.e. rotary, horizontal and vertical.
*** Peripheral vestibular disease
**** The fast phase of the nystagmus is away from the side of the lesion.
=====Chronic=====
* Nystagmus may resolve in more chronic lesions.
** However, it may return with changes in head position.
*** I.e. positional nystagmus.
* Visual accomodation improves ataxic movements.
** However, these may return dramatically on blindfolding.
====Bilateral Vestibular Disease====
* An observable head tilt may not be present.
* Movements are likely to be markedly ataxic.
** This is probably due in part to involvement of ascending proprioceptive and descending motor pathways that run through the brainstem.
===Pharyngeal and Laryngeal Function===
* The pharynx and larynx are innervated by:
** CN IX - the glossopharyngeal nerve
** CN X - the vagus nerve
** CN XI - the accessory nerve
* Pharyngeal and laryngeal function is best studied by:
*# Observing the horse eat and swallow
*# Endoscopy
====Laryngeal Dysfunction====
* Left recurrent laryngeal hemiplegia is the most common dysfunction in horses.
** The left recurrent laryngeal nerve is a branch of the vagus nerve.
** Horses are known as "roarers".
====Pharyngeal Dysfunction====
* Pharyngeal paralysis in horses is commonly associated with peripheral disease.
** Especially guttural pouch disease.
*** Endoscopy of the guttural pouches should be therefore be performed in horses that are dysphagic.
===Movement of the Tongue===
* The hypoglossal nerve, CN XII provides motor innervation to the [[Oral Cavity - Tongue - Anatomy & Physiology|tongue]].
* Normally, a horse resists the [[Oral Cavity - Tongue - Anatomy & Physiology|tongue]] being retracted from the mouth.
** The horse will quickly return the tongue after it is pulled out to the side.
* Horses with hypoglossal weakness appear differently.
** The [[Oral Cavity - Tongue - Anatomy & Physiology|tongue]] may protrude from the mouth.
** Chronic unilateral hypoglossal lesions may result in unilateral tongue atrophy.
** Bilateral hypoglossal lesions may result in difficulty prehending food.
* Horses with cerebral dysfunction may have signs corresponding to tongue weakness.
** However, [[Oral Cavity - Tongue - Anatomy & Physiology|tongue]] atrophy is not present in these animals.
==The Body, Spinal Cord and Limbs==
===Testing Conscious Proprioception===
* It is possible to test a horse’s conscious proprioception by altering the position of the limbs in space.
** Horses should normally replace their limbs to the normal position within a few seconds.
*** There is a degress of individual variability, however, and the test can be difficult to interpret.
* Tests can also be performed by walking or trotting a horse and then suddenly stopping.
** The animal should be observed to see how quickly it replaces its legs in a normal position.
===Gait===
* Gait is a combination of higher control of motor function and unconscious proprioception.
* During assessment of gait, the examiner is looking primarily for evidence of '''ataxia''' and/ or '''weakness'''.
** Each leg should be assessed in turn.
* Defects may be graded on a five-point scale (the Mayhew scale).
** 0 is normal and 5 is recumbent.
====Ataxia====
* Defects in unconscious proprioceptive pathways result in ataxia.
** These pathways originate in muscle spindle stretch receptors and golgi tendon organs.
** Impulses are relayed via peripheral nerves to the spinal cord.
** Signals then ascend in pathways to the cerebellum.
*** Primary cerebellar diseases in horses are rare, but classically cause incoordination of the head and limbs.
**** Weakness is not a prominent feature.
**** The cerebellum communicates with the vestibular nuclei, and so cerebellar diseases may sometimes present with vestibular signs.
* Manifestations of ataxia:
** Exaggerated movements - hypermetria
** Reduced movements - stilted hypometric movements
** Both exaggerated and reduced movements - dysmetria.
* Animals may also
** Sway
** Place their feet abnormally
*** Horses cross or weave their feet, with abnormal abduction or adduction.
* Signs of spinal ataxia may be exaggerated by lifting the horse's head.
** This is beacuse it removes the horizon.
** Similar responses can be generated by walking up and down slopes.
* Circling the horse can also reveal ataxia.
** The horse may plant its feet and rotate around them without placing them properly.
** Outward rotation of the pelvic limbs is also a common sign.
*** The term for this is "circumduction".
* Unconscious proprioception may also be tested by backing the horse.
** Normal horses back with a diagonal gait.
** Horses with spinal ataxia will often pace when backing.
====Weakness====
* Weakness usually manifests as toe dragging, stumbling, or knuckling over.
** It is a good idea to look closely at the horse’s hooves to see if any are abnormally worn.
* Weakness may be caused by dysfunction of either the upper or lower motorneurons.
** Lower motor neuron weakness.
*** Accompanied by:
**** Muscle flaccidity
**** Muscle atrophy if chronic
*** May sometimes appear similar to lameness.
** Upper motor neuron weakness.
*** Presents as a lack of control of limb muscle movement.
*** Tested most easily tested by pulling the tail during walking.
**** Pulling the tail while stationary probably primarily tests lower motor neuron control and muscular strength of the pelvic limb.
==Determining the Site of Spinal Lesions==
* If there are no brain or brainstem signs, an animal with ataxia or weakness is most likely to have a lesion in the spinal cord.
* The spinal reflexes used in small animals to assess specific segments are not very useful in adult horses.
** However, withdrawal reflexes are useful in recumbent horses with severe lesions.
*** A stimulus is applied to the distal limb and the reflex results in limb withdrawal.
===Cervical Lesions===
* Cervical lesions occur from C1 to C5
** C6 and C7 may also be included.
* Horses with defects in all four legs are most likely to have a cervical lesion.
* The pelvic limbs tend to be more severely affected than the thoracic limbs.
** This is due to the topographical arrangement of the white matter tracts in the spinal cord.
====Effects on Reflexes====
* Cervicofacial reflex
** The cervicofacial reflex is tested by lightly stimulating the skin of the lateral neck.
*** In an intact reflex movement of the lips should be observed.
*** This reflex varies widely between individual horses.
*** This is not intact in cervical lesions.
* Withdrawal reflexes
** Only of use in recumbent horses.
** In cervical lesions, reflexes in both thoracic and pelvic limbs may be exaggerated.
*** This is due to a loss of upper motor neuron inhibition.
===Brachial Lesions===
* Brachial lesions occur from C6 to T2.
* Signs of a spinal lesion at this level differ in the thoracic and pelvic limbs.
** Thoracic limbs
*** Lower motor neuron signs
**** Weakness
***** May be mild
***** May manifest unwillingness to take the weight on the other thoracic limb when one is picked up.
**** Muscle atrophy
*** Withdrawal reflex in recumbent animals may be reduced.
** Pelvic limbs
*** Ataxia
*** Upper motor neuon weakness
**** For example, swaying and stumbling.
*** Withdrawal reflex in recumbent animals may be exaggerated.
===Thoraco-lumbar Lesions===
* Thoraco-lumbar lesions occur in the segments T3 to L3 of the spinal cord.
* In an animal that has pelvic limb ataxia and weakness but no thoracic limb involvement, the lesion will be between T2 and L3.
** I.e. caudal to the brachial plexus.
** The animal should be closely examined for symmetry over the gluteals and pelvic limbs.
*** If there is symmetry, the lesion is likely to be at this level.
*** If there is no symmetry, the lesion is likely to be more caudal, and involve the pelvic outflow.
* Asymmetry may be detected in the cutaneous trunic reflex.
** This is tested by lightly stimulating the skin of the lateral thorax.
* The withdrawal reflex in the pelvic limb may be exaggerated.
* Flexibility of the thoracolumbar spine may be assessed.
** The skin of the longissimus musculature is firmly stimulated, and observed for:
*** A lordotic movement over the thorax and cranial lumbar region, and
*** A kyphotic movement over the caudal lumbar region and gluteals.
===Lumbosacral Lesions===
* Lumbosacral lesions occur between the segments L4 and S3.
* Horses with such lesions may:
** Exhibit atrophy of one or more muscles of the pelvic region, and so
** Be easy to pull over when standing, and
** Be unwilling or unable to take weight on the affected side when the other limb is lifted.
* More caudal lesions may:
** Affect the anal and perineal reflexes.
*** These are tested by lightly stimulating the skin surrounding the anus and observing for a reflex contraction of the anal sphincter.
** Cause paresis of the tail.
* The withdrawal reflex may be reduced or absent in the pelvic limb.
===Sacral Lesions===
* Damage to the savral spinal cord can affect bladder function and control of defaecation.
====Bladder Incontinence====
* The bladder is reflexely controlled, via higher order neurones in the caudal brainstem and cerebrum, by the:
** Pelvic nerve
*** Parasympathetic fibres leave the spinal cord in the sacral segments and combine to form the pelvic nerve.
**** These fibres innervate the smooth detrusor muscle of the bladder wall.
** Pudendal nerve
*** The pudendal nerve innervates the striated muscle of the bladder neck.
* During micturition, the bladder is evacuated as:
** The striated muscle is relaxed.
** Stimulation of the pelvic nerve causes the detrusor smooth muscle to contract.
====Faecal Incompetence====
* Lesions in the sacral region may also be manifest as faecal incompetence.
** In horses, problems are usually caused by defects in the lower motor neurons in the pelvic nerve.
*** Faeces is retained.