Diseases Associated With Clinical Signs Originating From Cranial Nerves

Chapter 83

Diseases Associated With Clinical Signs Originating From Cranial Nerves

Caroline Hahn

The nerves emanating from the brainstem (the midbrain, pons, medulla, and cerebellum), whether they are sensory or motor, autonomic or somatic, are called cranial nerves. The brainstem and cranial nerves coordinate and control unconscious sensory, proprioceptive, and motor functions. Disorders of these structures are characterized by one to all of a set of distinctive abnormal neurologic signs.

Twelve pairs of cranial nerves innervate the head and extend into the body, and each nerve may carry motor, sensory, or both types of nerve fibers, and may mediate somatic, autonomic, or both types of functions (Table 83-1). Knowledge of the location and action of individual cranial nerves is critical for interpretation of the neurologic examination (see the sixth edition of Current Therapy in Equine Medicine, Chapter 130). Cranial nerves and their central nervous system (CNS) components are numbered sequentially from rostral to caudal on the basis of site of attachment to the brain: cranial nerve I (CN I; olfactory) and CN II (optic) attach to the forebrain; CNs III (oculomotor) and IV (trochlear) are associated with the midbrain (rostral brainstem); CNs V (trigeminal), VI (abducens), VII (facial), VIII (ves­tibulocochlear), IX (glossopharyngeal), X (vagus), XI (spinal accessory), and XII (hypoglossal) are associated with the mid and caudal portions of the brainstem. All but the optic and olfactory nerves have a peripheral portion that is ensheathed or myelinated by Schwann cells. Cranial nerves I and II are not peripheral “nerves” at all, but are actually extensions of the CNS; early anatomists assumed that these two structures were peripheral nerves, and the nomenclature has persisted. Cranial nerve nuclei can have two names, depending on whether their function is somatic or autonomic (parasympathetic). For example, the oculomotor nucleus of CN III supplies some of the extraocular muscles, whereas the parasympathetic nucleus of CN III innervates smooth muscle of the eye and orbit. Some nuclei contain neurons of multiple cranial nerves, yet all those neurons have a similar function. For example, the nucleus ambiguus, comprising neurons associated with CN IX, X, and XI, innervates the striated muscle of the larynx and pharynx.

TABLE 83-1

Clinical Signs and Cranial Nerve Testing

Function Innervation Clinical Testing Dysfunction
Vision II, IIa, VIIe Menace response
Pupillary light reflex—IIa and IIIe (parasympathetic)
Blindness, dilated pupil (although this can also be related to CN III dysfunction)
Pupil size IIa, IIIe, parasympathetic, sympathetic Pupillary light reflex
Pharmacologic testing
Anisocoria, mydriasis (↓ CN II or parasympathetic innervation)
Miosis (↓ sympathetic innervation)
(cerebellar lesions and primary ophthalmic lesions can also affect pupil size)
Eyeball position VIIIa
IIIe, IVe, VIe
Eyeball position in different head positions
Vestibulo-ocular reflex VIIIa and IIIe, IVe, VIe
Strabismus—static (LMN), dynamic (vestibular)
Facial sensation Va—all three branches Tactile stimulation Va—different regions of the face: nasal septum, ventral eyelid, dorsal eyelid
Palpebral reflex—Va and VIIe (dorsal and ventral eyelids)
Auriculopalpebral reflex Va and VIIe (stimulate just in front of the external ear canal; sensation to the inside of the pinna is principally provided by CN VII)
Facial hypoalgesia
Mastication Ve—mandibular Jaw tone
Assess the bulk of the masticatory (temporalis and masseter muscles)
Muscle atrophy
Dropped jaw, if bilateral dysfunction
Facial expression and movement VIIe Facial symmetry
Palpebral reflex Va and VIIe
Movement and position of muzzle, external nares, eyelids, ears
Facial paresis/paralysis
Vestibular function VIIIa Head position
Eyeball position (also involves III, IV, VI)
Vestibulo-ocular reflex (VIIIa and IIIe, IVe, VIe)
Gait and movement
Head tilt, circling, rolling
Spontaneous nystagmus
Deranged body posture and ataxia
Pharyngeal function IX and X, a and e Swallowing Dysphagia
Salivation (ptyalism)
Laryngeal function X and XI, a and e Phonation
Respiratory stridor—laryngeal obstruction, aspiration
Tongue XIIe Observation—LMN signs, usage
Withdrawal from tactile stimulus
Atrophy, paresis/paralysis—unilateral or bilateral


a, Afferent; e, efferent, LMN, lower motor neuron.

An understanding of the relative position of the peripheral portion of the cranial nerves (Figure 83-1), as well as the location of their nuclei (Figure 83-2), is important in establishing whether a particular combination of cranial nerve signs could be caused by a single lesion or has to involve a multifocal disease process. A lesion affecting a particular cranial nerve could be affecting the peripheral nerve or its soma in the brainstem, and a thorough neurologic examination and ancillary tests may be required to establish the exact location—brainstem lesions can be expected to additionally affect other cranial nerves as well as upper motor neuron and proprioceptive tracts and, if the ascending reticular activating system is additionally affected, abnormalities in levels of arousal. Neurons of CNs III to XII are arranged in nuclei in the brainstem.

Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Diseases Associated With Clinical Signs Originating From Cranial Nerves
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