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Diseases of the Brainstem and Cranial Nerves of the Horse: Relevant Examination Techniques and Illustrative Video Segments

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Diseases of the Brainstem and Cranial Nerves of the Horse: Relevant Examination Techniques and Illustrative Video Segments IN-DEPTH: NEUROLOGY Diseases of the Brainstem and Cranial Nerves of the Horse: Relevant Examination Techniques and Illustrative Video Segments Robert J. MacKay, BVSc (Dist), PhD, Diplomate ACVIM Author’s address: Alec P. and Louise H. Courtelis Equine Teaching Hospital, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610; e-mail: mackayr@ufl.edu. © 2011 AAEP. 1. Introduction (pons and cerebellum) and myelencephalon (me- This lecture focuses on the functions of the portions dulla oblongata). Because the diencephalon was of the brainstem caudal to the diencephalon. In discussed in the previous lecture under Forebrain addition to regulation of many of the homeostatic Diseases, it will not be covered here. mechanisms of the body, this part of the brainstem controls consciousness, pupillary diameter, eye 3. Functions (Location) movement, facial expression, balance, prehension, mastication and swallowing of food, and movement Pupillary Light Response, Pupil Size (Midbrain, Cranial and coordination of the trunk and limbs. Dysfunc- Nerves II, III) tion of the brainstem and/or cranial nerves therefore In the normal horse, pupil size reflects the balance of manifests in a great variety of ways including re- sympathetic (dilator) and parasympathetic (con- duced consciousness, ataxia, limb weakness, dys- strictor) influences on the smooth muscle of the phagia, facial paralysis, jaw weakness, nystagmus, iris.2–4 Preganglionic neurons for sympathetic and strabismus. Careful neurologic examination supply to the head arise in the gray matter of the in the field can provide accurate localization of first four thoracic segments of the spinal cord and brainstem and cranial nerve lesions. Recognition subsequently course rostrally in the cervical sympa- of brainstem/cranial nerve dysfunction is an impor- thetic nerve within the vagosympathetic trunk. tant step in the processes of diagnosis and treat- After synapse in the cranial cervical ganglion adja- ment. cent to the guttural pouch, the post-ganglionic sym- pathetic neurons continue to the smooth muscle of 2. Anatomy and Nomenclature the orbit and act to cause pupillary dilation. Emo- The brainstem includes the diencephalon, mesen- tional and other influences on sympathetic pupillary cephalon (midbrain), and rhombencephalon (hind- tone are governed by hypothalamic centers that act brain).1 With the exception of the olfactory nerves through upper motor neuron (UMN) tracts descend- (I), all cranial nerves are arrayed along the brains- ing from the midbrain. Interruption of pre- or post- tem. The hindbrain is divided into metencephalon ganglionic sympathetic nerves to the eye causes NOTES AAEP PROCEEDINGS ր Vol. 57 ր 2011 353 IN-DEPTH: NEUROLOGY Horner’s syndrome, with miosis of the pupil, ptosis component of the facial nerve contains fibers from (reflecting hypotonia of the dorsal tarsal (Mueller’s) the tongue (taste) and middle ear. muscle), and spontaneous sweating and vasodilata- tion over the side of the face. Parasympathetic Balance and Equilibrium (Medulla, Cranial Nerve VIII), preganglionic neurons arise in the midbrain and exit Hearing (Medulla, Cranial Nerve VIII, Forebrain) the skull in the oculomotor nerve (III). These neu- The vestibular system is responsible for orientation rons synapse behind the eye in the ciliary ganglion. of the horse relative to gravity. The receptor is in Post-ganglionic neurons pass along the optic nerve the bony labyrinth of the inner ear. The membra- to innervate the ciliary muscle and constrictor of the nous labyrinth includes 3 semicircular ducts con- pupil. The afferent part of the pupillary light reflex taining endolymph that connect to vestibular nerve passes via the optic nerves and optic tracts, past the endings at the cristae ampullares. Vestibular neu- thalamus, to terminate in the midbrain. There is rons pass centrally through the internal acoustic extensive decussation of these tracts both in the meatus to penetrate the rostral medulla and termi- chiasm and midbrain. nate in 4 vestibular nuclei. These nuclei have nu- merous projections to the nuclei controlling Eye Position (Midbrain, Pons, Cranial Nerves III, IV, VI) extraocular muscles, the cerebellum, and the spinal From nuclei in the midbrain and pons, the oculomo- cord. The vestibular system controls the conjugate tor, trochlear, and abducens nerves exit the cranial movements of the eyes during movement of the head cavity through the orbital fissure and ramify in the through extensive connections with the nuclei of periorbital tissues to innervate the muscles of the cranial nerves III, IV, and VI. Vestibular-cerebel- eye. The oculomotor nerve also supplies the levator lar pathways pass through the caudal cerebellar palpebrae and pupillary constrictor muscles, and peduncle. These pathways function to smoothly co- the abducens nerve innervates the retractor bulbi ordinate the movements of the eyeballs, trunk, and muscle. Lesions in these nerves (or nuclei) cause limbs with those of the head. Vestibulospinal true strabismus. tracts descend ipsilaterally to synapse on LMN and facilitate extensor muscles of the limbs while inhib- Mastication (Pons, Cranial Nerve V) iting flexor muscles. Some vestibulospinal tracts cross and reduce extensor tonus in contralateral The lower motor neurons of the trigeminal nerve limbs. arise in the pons and pass through the petrous tem- Unilateral disease involving the peripheral part of poral bone in the foramen ovale adjacent to sensory the vestibular system causes asymmetric ataxia trigeminal neurons and are distributed to the mus- with preservation of strength. The poll rotates to- cles of mastication: masseters, pterygoids, tempo- ward the side of the lesion, and the head and neck rals, and rostral digastricus. With unilateral may be turned toward the lesion. The body leans, damage to the trigeminal nucleus (or nerve), there is falls, or rolls toward the side of the lesion, and the deviation of the lower jaw toward the normal side. horse may stagger in tight circles. Because there is By 2 weeks after injury, there is obvious muscular some visual compensation for vestibular ataxia, atrophy. Bilateral severe involvement of the tri- blindfolding exacerbates the signs. In horses with geminal nuclei (or nerves) causes a dropped jaw, central vestibular disease, head tilt may be either weak jaw tone, slight tongue protrusion, and inabil- toward or away from the side of the lesion. The ity to prehend or chew feed. latter presentation is known as paradoxical central vestibular disease and usually follows involvement Facial Expression and Movement (Medulla, Cranial Nerve of vestibular connections within the cerebellum. VII) Unilateral vestibular disease often causes spontane- The facial nerves arise from nuclei in the rostral ous or positional nystagmus, and physiological (ves- medulla and exit the calvarium with CN VIII via the tibular) nystagmus may be absent or abnormal internal acoustic meatus. The nerve courses when the head is moved toward the side of the through the facial canal in the petrous temporal lesion. In peripheral disease, the nystagmus is al- bone adjacent to the middle ear and emerges ways horizontal, rotatory, or arc-shaped, with the through the stylomyastoid foramen. The facial fast phase away from the lesion. With central in- nerve is distributed to the muscles of facial expres- volvement of the vestibular system, nystagmus also sion including those of the ear, eyelid, nose, and lips may be vertical. Typically, the eye on the affected and the caudal belly of the digastric muscle. With side rotates ventrally in the orbit, whereas the eye involvement of the nucleus or proximal nerve, there on the normal side rotates dorsally (especially when is drooping of the ear and lip, ptosis, collapse of the the head is extended). This abnormal eye position nostril, and the muzzle is pulled toward the normal is termed vestibular strabismus. Bilateral vestib- side. Saliva often drools from the affected side of ular disease is characterized by severe symmetric the mouth, and the horse has difficulty prehending ataxia and wide, sweeping movements of the head food, especially grain. There may be exposure ker- from side to side. Neurons of the cochlear division atitis. The facial nerve also contributes parasym- of cranial nerve VIII pass from receptors in the pathetic neurons to lacrimal glands. The sensory middle/inner ear to auditory centers in the midbrain 354 2011 ր Vol. 57 ր AAEP PROCEEDINGS IN-DEPTH: NEUROLOGY and thalamus. A variety of local reflexes are Tongue Movement (Cranial Nerve XII, Medulla) initiated by stimulation of the cochlear nerve. In Neurons of the hypoglossal nerve originate in the addition, there is projection of conscious pathways hypoglossal nucleus in the caudal aspect of the me- for hearing from the thalamus to the cortex (tempo- dulla and emerge from the medulla as a horizontal ral lobe?). Deafness is congenital in some row of rootlets, which combine to form the nerve as “splashed white” blue-eyed horses of several differ- it enters the hypoglossal foramen. After emerging ent breeds; otherwise, deafness is rarely recognized from this foramen, the hypoglossal nerve runs for- in horses. ward and ventrally in association with the guttural pouch and stylohyoid bone to innervate the geniohy- Taste (Cranial Nerves VII, IX, X, Medulla, Forebrain) oideus and muscles of the tongue. Interruption of Taste buds are found on the surface of the tongue the hypoglossal pathways causes hemiparesis of the and also in the soft palate, pharynx,
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