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 Strabismus 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 Respiration	Dysphonia 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.