Cranial nerve lesions are detected by assessing the function of the organs they innervate.
Function of cranial nerve I, the olfactory nerve, is assessed by determining the horse’s ability to locate food through smell or by assessing the horse’s reaction to a strong, unpleasant odour such as cloves. The function of cranial nerve II, the optic nerve, is determined by the menace response and the pupil’s contraction response to light. The function of cranial nerve III, the oculomotor nerve, is assessed by evaluation of the pupil’s contraction response to light as well as by looking for strabismus and ptosis. The function of cranial nerve IV, the trochlear nerve, is assessed by the absence or presence of dorsomedial strabismus. Examining the tone and function of the muscles of mastication indicates the function of cranial nerve V, the trigeminal nerve. Medial strabismus, globular retraction and prolapse of the third eyelid are indicative of dysfunction of cranial nerve VI, the abducent nerve. The function of cranial nerve VII, the facial nerve, is assessed by examining the tone and function of the muscles of facial expression. o Facial Nerve Paralysis - The function of cranial nerve VIII, the vestibulochoclear nerve, is assessed by gauging the horse’s response to loud noise. Dysphagia and alterations in voice are indicative of cranial nerve IX, the glossopharyngeal nerve, dysfunction. Lesions in cranial nerve X, the vagus nerve, result in coma or death. Lesions in cranial nerve XI, the accessory nerve, result in flaccid paralysis of the muscles of the neck. Lesions in cranial nerve XII, the hypoglossal nerve, result in abnormal movement and function of the tongue, which is most notable during drinking.
Peripheral Vestibular Disease
Infection or trauma to the structures of the
causes inflammation of the ear
canals, which inhibits the function of the vestibular system.
Peripheral vestibular disease can be caused by trauma to the region of the skull, which houses the small bones of the inner ear, causing fracture of these bones or inflammation of the soft tissue of the inner ear canal. Infection of the inner ear is usually bacterial or fungal. Infection can spread from an upper respiratory tract infection, a guttural pouch infection, or can be related to the introduction of foreign objects, such as ear plugs, into the ear canal. The clinical presentations of peripheral vestibular disease include ataxia, head tilt, circling, head or ear rubbing, and signs of impairment of the cranial nerves. If the cranial nerves are affected there may be nystagmus and paralysis of the nerve, which usually presents as drooping of the affected ear.
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