Difference between revisions of "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. | ||
+ | |||
+ | ===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. |
Revision as of 10:56, 18 August 2008
|
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.