There are 12 paired cranial nerves, named and numbered according to the rostral-caudal order of attachment to the brain. They serve a variety of functions and predominantly provide the motor and sensory innervation to the head. The effects of a mononeuropathy depend on where in its pathway the nerve is affected and the etiology. The signs and symptoms of a cranial nerve mononeuropathy vary depending on which nerve is affected.
Olfaction begins with transduction of odorants from the air into the nasal mucosa. These odorants diffuse or are transported to bipolar receptor cells located in the olfactory neuroepithelium in the roof of the nasal chamber. Action potentials are induced in these cells, which synapse with olfactory bulb glomeruli. The receptor cell axons project through the cribriform plate of the ethmoid bone and synapse within the glomerular layer of the olfactory bulb. The paired olfactory bulbs are located at the base of the frontal lobe overlying the cribriform plate. The second-order neurons leave the olfactory bulb to synapse on the primary olfactory cortex. These areas encode characteristics of odor quality, identity, familiarity, and emotion.
Function and disorders
Changes in olfactory function frequently go unnoticed and often do not present to a clinician. Patients may notice altered taste, rather than a loss of sense of smell. Olfaction is critically important for safety, nutritional status, and quality of life. Disorders can manifest as a total loss of smell (anosmia), partial loss of smell (hyposmia), distortions (dysosmias), or spontaneous olfactory hallucinations (phantosmias). Infrequently, olfactory dysfunction can be the presenting sign/symptom of neurodegenerative disorders (such as idiopathic Parkinson disease), or an intracranial mass lesion.
The diagnosis can usually be made clinically. Commercial odor identification tests are available, which require patients to identify several predefined smells. These may be useful to confirm olfactory dysfunction. Psychophysical tests are useful to validate and classify olfactory dysfunction, but establishing the cause of olfactory loss relies heavily on the history. Olfactory evoked potentials are available in specialist centers.
Axons making up the optic nerve arise from retinal ganglion cells. These axons run toward the lamina cribrosa and merge in the optic papilla. At this point, they form the optic nerve. In the orbital apex, the nerve passes through the muscle origins and enters the optic canal. The nerve continues to course upward and inward until it meets with the contralateral nerve to form the optic chiasm superior to the sella and pituitary gland. Action potentials are then carried to the lateral geniculate body. The intraorbital portion is surrounded by the subarachnoid space and dura that extends from the intracranial cavity. The central retinal artery and vein course through the middle of the nerve.
Function and disorders
Humans have a highly developed visual system, which transmits information from the environment. The optic nerve carries millions of fibers from the retina into the central nervous system (CNS). Vision is critical for human function and, as such, optic nerve pathology can severely affect quality of life. Optic nerve lesions typically produce monocular visual loss, which can be sudden or gradual, and may or may not be associated with pain. The potential causes of optic neuropathy are diverse and include vascular, toxic, metabolic, traumatic, compressive, infectious, inflammatory, and idiopathic etiologies.
Symptoms of optic nerve damage can represent changes in visual acuity, contrast, brightness, or color. A detailed description of visual dysfunction is essential and can narrow the differential. To define the degree of optic nerve dysfunction, the following tests are frequently performed.
Visual acuity: this can be tested using a Snellen chart. Optic nerve damage may result in central visual loss.
Color vision: this can be assessed with a series of color plates. Patients with unilateral optic nerve impairment have great difficulty identifying colors between eyes (dyschromatopsia), and this is more affected than visual acuity.
Pupillary testing: a relative afferent pupillary defect (RAPD) is the only objective test of optic nerve dysfunction.
Visual fields testing: a basic visual fields test can be performed at the bedside by comparing the patient's peripheral vision with the clinician's. If a defect is identified, formal testing may be required, for example with Goldmann perimetry.
Direct ophthalmoscopy: visualizing the optic nerve as it enters the back of the eye can reveal pallor (optic atrophy) or disk swelling (papillitis or papilledema).
Oculomotor (III), trochlear (IV), and abducens (VI)
The third cranial nerve emerges from the midbrain nucleus that lies ventral to the sylvian aqueduct. One unpaired and 4 paired subnuclei can be distinguished. The most dorsal subnucleus contains the visceral Edinger-Westphal nucleus and the levator palpebrae nucleus. The Edinger-Westphal nucleus mediates pupillary constriction. Laterally the dorsal, intermediate, and ventral subnuclei provide innervation to the ipsilateral inferior rectus, inferior oblique, and medial rectus, respectively. The third nerve fascicles leave the nucleus and pass ventrally through the red nucleus before exiting just medial to the cerebral peduncles. In the subarachnoid space the third nerve passes between the superior cerebellar and posterior cerebral arteries. The nerve then enters the lateral wall of the cavernous sinus and divides into a superior and inferior branch as it enters the orbit through the superior orbital fissure.
The trochlear nucleus is located in the midbrain tegmentum at the level of the inferior colliculus. The nerve fascicles course posteroinferiorly to decussate at the anterior medullary velum before exiting from the dorsal aspect of the midbrain. The trochlear nerve is the only nerve to arise from the dorsal aspect of the brainstem. The fourth nerve traverses the brainstem cisterns close to the undersurface of the tentorial edge and pierces the dura to enter the lateral cavernous sinus. The trochlear nerve enters the orbit through the superior orbital fissure to innervate the superior oblique muscle.
The abducens nucleus contains motor neurons for the lateral rectus and interneurons traveling through the medial longitudinal fasciculus to the contralateral third nerve nucleus (to allow simultaneous movement of the contralateral medial rectus muscle). The nerve fascicles leave the nucleus and travel within the pontine tegmentum to leave the brainstem in the horizontal sulcus between the pons and medulla. The nerve enters the subarachnoid space and courses vertically along the clivus over the petrous apex of the temporal bone, where it is tethered in the Dorello canal. It then enters the cavernous sinus lateral to the internal carotid artery and finally enters the orbit through the superior orbital fissure.
Function and disorders
The third, fourth, and sixth cranial nerves are responsible for eye movements.
The third cranial nerve controls most extraocular muscles, including the superior, inferior, and medial recti, and the inferior oblique muscles. In addition, it innervates the levator palpebrae superioris, which elevates the eyelid, and carries parasympathetic innervation to the pupil. Patients often present with paralysis of adduction, elevation, and depression, and when the pupil is involved a large unreactive pupil is noted. This presentation can suggest serious neurologic disorders, namely subarachnoid hemorrhage, cerebral aneurysms, uncal herniation, or meningitis, so prompt recognition and evaluation is needed.
The fourth cranial nerve innervates the superior oblique muscle, which controls depression, intorsion, and adduction of the eye. It is the most common cause of vertical diplopia. The frequency of fourth nerve palsy is difficult to accurately report, but in one large series it was more common than both oculomotor and abducens palsies. The abducens nerve innervates the lateral rectus muscle and controls abduction. Patients typically present with horizontal double vision. It may be an isolated finding or part of a systemic disease.
Simple bedside testing of eye movements can be performed to elicit a third, fourth, or sixth nerve palsy. The patient is asked to keep his or her head still and follow the examiner's index finger with the eyes. The examiner slowly moves his or her finger up and down and from side to side at eye level and observes eye movements. The patient should report any diplopia. Diplopia is maximal in the direction of action of the paralyzed muscle. The outer image is the false image and disappears when the abnormal (paretic) eye is covered.
The trigeminal nerve has 3 main branches: ophthalmic (V1), maxillary (V2), and mandibular (V3). V1 enters the cranial cavity through the superior orbital fissure, V2 through the foramen rotundum, and V3 through the foramen ovale. V1 and V2 traverse the cavernous sinus. The first-order cell bodies carrying modalities of pain, temperature, pressure, and light touch in all 3 branches are located in the trigeminal (gasserian) ganglion in the Meckel cave (near the petrous apex of the temporal bone). Proprioceptive fibers have their first-order cell bodies in the mesencephalic nucleus of the brainstem. From the trigeminal ganglion, the nerve fibers enter the pons and synapse in multiple trigeminal nuclei. From there, second-order neurons carry afferent information to the ventral posteromedial nucleus of the thalamus. Finally, third-order neurons relay to the primary sensory cortex. Efferent motor fibers originate in the motor nucleus of the trigeminal nerve in the midpons and travel with V3 through the foramen ovale to supply the muscles of mastication (masseter, temporalis, mylohyoid, medial and lateral pterygoid, and anterior belly of the digastric), as well as the tensor tympani and tensor veli palatini. The trigeminal nerve and its branches also mediate the afferent limbs of the corneal blink and lacrimal reflexes, and both afferent and efferent limbs of the jaw-jerk reflex.
Function and disorders
The trigeminal nerve is the biggest cranial nerve. It carries sensation from the face and mucosal surfaces, cornea, and supratentorial dura, as well as providing motor innervations to the muscles of mastication. The differential for a trigeminal neuropathy is very broad. Intra-axial pathology, particularly of the pons, can result in trigeminal dysfunction, but only rarely does this result in a mononeuropathy. Extra-axial lesions are more likely to affect the trigeminal nerve or its branches alone. Symptoms of trigeminal neuropathy depend on the location and etiology of the lesion and may include loss of sensation in the distribution of 1 or more trigeminal nerve branches, neuropathic pain, or weakness of the muscles of mastication.
Facial sensation can be tested by asking the patient to close his or her eyes and report where a stimulus is felt. Light touch with a cotton wool stick, pinprick with the end of a sterile needle, and warm and cold stimuli can be tested on each side of the face. Contraction of the masseter and temporal muscles can be examined by visual inspection, and palpation of the masseter muscles can be examined when the patient is chewing.
The jaw jerk can be tested as follows: with the patient's mouth slightly open, the mandible is tapped just below the lips in a downward direction. The masseter will move the mandible upward. Normally this reflex is weak, but it may be pronounced with upper motor neuron lesions.
The strength of the pterygoid muscles may be tested by asking the patient to open the jaw against resistance.
The corneal reflex can be tested with cotton wool (afferent-trigeminal, efferent-facial) and elicits an ipsilateral and contralateral blink response in normal individuals.
The facial nerve is composed of both motor and sensory roots (nervus intermedius) and has a long intracranial course with 3 bends and multiple branches. The motor root has neuronal cell bodies in the facial nucleus of the lateral caudal pons. Fibers from the nucleus course posteriorly and form a sharp loop around the sixth nerve nucleus, forming the facial colliculus. The seventh cranial nerve then exits the brainstem at the pontomedullary junction, traverses the cerebellopontine cistern, and enters the facial canal through the meatus of the internal auditory canal. The nervus intermedius carries general somatic afferent and special visceral efferent fibers, and is separate from the motor root only between the brainstem and the facial canal. The geniculate ganglion, containing the cell bodies of general somatic afferent and special visceral efferent neurons, is located in the temporal bone within the facial canal.
The first branch of the seventh cranial nerve is the greater superficial petrosal nerve, which travels to the sphenopalatine and pterygopalatine ganglion, and carries parasympathetic fibers to innervate the lacrimal gland of the eye. The second branch innervates the stapedius muscle. The chorda tympani (third branch) carries taste sensation from the anterior two-thirds of the tongue, as well as parasympathetic innervation to the sublingual and submandibular glands (through the submandibular ganglion). The facial nerve exits the cranium through the stylomastoid foramen and enters the parotid gland, where it splits into 5 terminal branches (temporal, zygomatic, buccal, mandibular, and cervical), which innervate the muscles of facial expression, and digastric and stylohyoid muscles.
Function and disorders
This is the most common cranial nerve mononeuropathy. It can affect people of all ages. There are many etiologies, and the most important initial step is to rule out central causes of facial weakness, including ischemic stroke and pontine neoplasms. The most common cause of a peripheral facial palsy is Bell palsy.
Muscles of facial expression can be tested to determine facial nerve function. Patients are asked to screw their eyes up tightly and resist opening, raise their eyebrows against resistance, show their teeth, and purse their lips. With an upper motor neuron lesion, only the lower half of the face on the contralateral side is affected, due to bilateral innervation of the upper facial muscles. With a lower motor neuron lesion, there is upper and lower weakness on the ipsilateral side.
Cell bodies of the vestibular division reside in the vestibular (Scarpa) ganglion in the internal acoustic meatus. Their dendrites project to the hair cells of the vestibular sensory organs (hair cells in the ampullae of the 3 semicircular canals, and hair cells in the maculae of the utricle and saccule) and axons project to the lateral, medial, superior, and inferior vestibular nuclei in the caudal pons. The cochlear division of the eighth cranial nerve has cell bodies in the spiral (auditory) ganglion with dendrites projecting to the hair cells of the auditory sensory organ (the organ of Corti within the cochlea). Axons of the cochlear division exit the internal acoustic meatus and course with the vestibular portion to enter the brainstem at the junction of the pons and medulla (cerebellopontine angle) and synapse in the ventral and dorsal cochlear nuclei of the rostral medulla.
Function and disorders
The vestibulocochlear nerve is a purely special sensory afferent nerve consisting of vestibular and cochlear divisions. Their axons run together through the internal acoustic meatus (which also transmits the facial nerve) and the brainstem. Symptoms of dysfunction include hearing loss, tinnitus, and vertigo.
Simple bedside hearing tests such as whispering a word or number in one ear with the other covered and having the patient repeat the word can be used to assess the degree of hearing impairment.
Rinne test: a tuning fork is placed on the mastoid bone (bone conduction) until the sound can no longer be heard. The tuning fork is then placed next to the external ear (air conduction). Usually air conduction is better than bone conduction, so the sound can still be heard; this is a positive Rinne test. If bone conduction is better than air conduction, this is a negative Rinne test and indicates conductive hearing loss in that ear.
Weber test: the tuning fork is placed on the forehead. The patient is asked in which ear the sound is louder. If the patient hears the sound equally in each ear or cannot localize, this is normal and is termed a midline Weber. The Weber lateralizes toward a conductive hearing loss and away from a sensorineural hearing loss. For example, if the patient hears the sound louder in the right ear, then this indicates either a right conductive or a left sensorineural hearing loss.
The glossopharyngeal nerve exits the rostral medulla at the pontomedullary junction, crosses the cerebellopontine cistern, and exits the cranial cavity through the jugular foramen. It contains general somatic afferent, general visceral efferent, special visceral afferent, and parasympathetic fibers that innervate the tongue and pharynx. Visceral and taste fibers within the nerve end in the nucleus solitarius of the medulla, which also receives afferent fibers from the carotid body and carotid sinus. Preganglionic parasympathetic fibers travel through the tympanic nerve to the lesser petrosal nerve and synapse in the otic ganglion before supplying the parotid gland.
Function and disorders
It is predominantly a sensory nerve but also contains some motor and parasympathetic fibers. Isolated glossopharyngeal neuropathy is rare, as lesions often involve other cranial nerves in close proximity (VIII, X, XI, and XII). Additionally, isolated palsy of the glossopharyngeal nerve can often be asymptomatic, due to redundant innervation of target structures by other cranial nerves. The nerve innervates the tongue and pharynx, including pain, temperature, and tactile sensation from the posterior third of the tongue, the tonsils, medial tympanic membrane, and Eustachian tube. It also innervates the stylopharyngeus muscle, involved in swallowing, mediates taste from the posterior third of the tongue, and sends parasympathetic innervation to the parotid gland.
The gag reflex is absent if a nerve palsy is present, as the afferent impulse is carried by the glossopharyngeal nerve.
The vagus nerve exits the brainstem just below the glossopharyngeal nerve, at the pontomedullary junction, traverses the cerebellopontine angle, and exits the cranium through the jugular foramen. The first main branch of the vagus nerve is the pharyngeal branch, which runs in the carotid sheath between the internal and external carotid arteries. It innervates the levator veli palatini, salpingopharyngeus, and palatopharyngeus muscles, and the uvula. The superior laryngeal nerve descends lateral to the pharynx, its external branch innervating the cricothyroid muscle. The recurrent laryngeal nerve is the third motor branch of the vagus and supplies all intrinsic muscles of the larynx except the cricothyroid. The right recurrent laryngeal nerve loops around the right subclavian artery, while the left loops under the aortic arch before ascending in the tracheoesophageal groove to the larynx.
Function and disorders
The vagus nerve contains both visceral efferent and afferent fibers and has 3 main motor branches. It innervates all striated muscles of the larynx and pharynx, except the stylopharyngeus muscle (innervated by IX) and the tensor veli palatini muscle (mandibular branch of V). Sensory input from the larynx, pharynx, external auditory canal, lateral tympanic membrane, and posterior fossa meningeal layers are mediated by the vagus. Visceral afferent information is also conveyed by the vagus nerve from the thoracic and abdominal viscera, and it delivers parasympathetic fibers to these regions as well, in addition to the larynx and pharynx.
Tenth nerve palsy can result in hoarseness, dysphagia, and dyspnea, as well as palatal droop and deviation of the uvula to the contralateral side. Lesions distal to pharyngeal branches, or a lesion of the recurrent laryngeal nerve itself, present with isolated hoarseness. The gag reflex is absent, as the efferent limb is formed by the vagus nerve.
The cranial part of the accessory nerve originates from the nucleus ambiguus of the medulla and emerges from the lateral aspect of the medulla. On reaching the jugular foramen, these fibers join the vagus nerve and supply the muscles of the soft palate, larynx, and pharynx. The spinal part of the accessory nerve originates in the rostral spinal cord at C1 to C5 levels through a series of rootlets that emerge between the dorsal and ventral roots. It ascends through the foramen magnum and briefly joins the cranial portion of the accessory nerve before separating again as the nerve exits the skull (with the vagus) through the jugular foramen. The fibers of the spinal accessory nerve emerge through the posterior border of the sternocleidomastoid (SCM) muscle and supply both the SCM and the trapezius muscles. As it crosses the posterior triangle of the neck it is closely related to superficial cervical lymph nodes.
Function and disorders
The accessory nerve is purely a motor nerve and supplies the SCM and trapezius muscles. It consists of a cranial and spinal part. It is not commonly injured, but due to its long, superficial extracranial course, it is susceptible to iatrogenic injury.
The accessory nerve can be assessed by testing the strength of trapezius and SCM muscles. Trapezius weakness results in a drooping shoulder at rest and mild scapular winging with attempted shoulder elevation and arm abduction >90°. When the patient shrugs his or her shoulders against resistance, unilateral weakness may be detected. SCM weakness results in difficulty when turning the head in the opposite direction to the injury. To test, the patient is asked to turn his or her head to the side against resistance. Proximal lesions of the nerve produce weakness of both the SCM and trapezius muscles.
The hypoglossal nucleus is located in the dorsal aspect of the caudal medulla, just below the floor of the fourth ventricle. General somatic efferent fibers from the nuclei course ventrally to exit the brainstem as 2 bundles of rootlets at the ventrolateral sulcus of the medial medulla. These rootlets exit the cranium through the hypoglossal canal just rostral to the foramen magnum and unite on the extracranial side. The hypoglossal nerve descends lateral to the internal carotid artery and vagus nerve, and then courses anteriorly to supply the ipsilateral intrinsic and extrinsic (genioglossus, styloglossus, and hyoglossus) muscles of the tongue. Additionally, efferent motor fibers from C1 course along the hypoglossal nerve for a short distance before leaving to innervate the geniohyoid and thyrohyoid muscles, and form the superior root of the ansa cervicalis. Meningeal branches from C1 and C2 innervating the dura mater of the posterior fossa are also carried by the hypoglossal nerve. Finally, the hypoglossal nerve receives sympathetic fibers from the superior cervical ganglion and communicates with the lingual branch of the mandibular nerve, which mediates tactile sensation from the anterior two-thirds of the tongue.
Function and disorders
The 12th nerve is purely motor in function. It moves and alters the shape of the tongue by providing ipsilateral motor innervation to the intrinsic and extrinsic tongue muscles. Lesions of the medulla (nuclear lesions) can cause 12th nerve dysfunction but are usually associated with other neurologic symptoms and cranial nerve deficits.
A 12th nerve mononeuropathy can be due to nuclear (medullary) or infranuclear lesions. Close examination of the tongue at rest and in motion can help establish this diagnosis. Symptoms of a 12th nerve palsy typically include unilateral or bilateral tongue weakness with deviation toward the affected side on tongue protrusion, tongue atrophy (with scalloping or accentuation of the midline groove), fasciculation of the tongue at rest, tongue flaccidity, or the inability to rapidly move the tongue side to side or vertically.
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