Neuro Exam of The Horse - Pathology
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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.
- History
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.
- Systems based
- 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
- For example:
- 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.
- They:
- 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.
- Signals are derived from receptors.
- A reflex does NOT require:
- Conscious perception of the stimulus
- Ascending or descending upper motor neuron pathways
- Ascending or descending proprioceptive pathways
- The sequence of events in a reflex is as follows:
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.
- For example:
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.
- An owner's description of the animal's behaviour or abnormality may both
- 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 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.
- These include:
- 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.
- This is often a sign of severe obtundation, which may be caused by:
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.
- 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.
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
- The trigeminal nerve provides sensory innervation to:
- 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
- All the divisions of the trigeminal nerve should be tested. This is achieved by:
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.
- The masseter and temporalis muscles are particulaly influenced by the mandibular brach of the trigeminal nerve.
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
- Horses with facial nerve dysfunction are therefore prone to corneal ulceration, due to :
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.
- The nuclei in turn relay information to
- 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.
- Central vestibular disease
- The appearance of nystagmus is different depending on the type of vestibular disease present.
Chronic
- Nystagmus may resolve in more chronic lesions.
- However, it may return with changes in head position.
- I.e. positional nystagmus.
- However, it may return with changes in head position.
- 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.
- Especially guttural pouch disease.
Movement of the Tongue
- The hypoglossal nerve, CN XII provides motor innervation to the tongue.
- Normally, a horse resists the 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 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, 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.
- Horses should normally replace their limbs to the normal position within a few seconds.
- 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.
- Primary cerebellar diseases in horses are rare, but classically cause incoordination of the head and limbs.
- 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.
- Accompanied by:
- 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.
- Lower motor neuron weakness.
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.
- However, withdrawal reflexes are useful in recumbent horses with severe lesions.
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.
- The cervicofacial reflex is tested by lightly stimulating the skin of the lateral neck.
- 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
- Weakness
- Withdrawal reflex in recumbent animals may be reduced.
- Lower motor neuron signs
- Pelvic limbs
- Ataxia
- Upper motor neuon weakness
- For example, swaying and stumbling.
- Withdrawal reflex in recumbent animals may be exaggerated.
- Thoracic limbs
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.
- The skin of the longissimus musculature is firmly stimulated, and observed for:
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.
- Affect the anal and perineal reflexes.
- 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.
- Parasympathetic fibres leave the spinal cord in the sacral segments and combine to form the pelvic nerve.
- Pudendal nerve
- The pudendal nerve innervates the striated muscle of the bladder neck.
- Pelvic nerve
- 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.
- In horses, problems are usually caused by defects in the lower motor neurons in the pelvic nerve.