Cranial Nerves II, III, IV, VI, and VIII

Cranial Nerves Overview

• Cranial Nerves II, III, IV, and VI control the visual system and eye movements.
• Cranial Nerve VIII controls the auditory and vestibular system.

Learning Objectives

• Describe pathways for cranial nerves II, III, IV, VI, and VIII.
• Include cell body locations in the CNS and where synapses occur.
• Describe the function of cranial nerves II, III, IV, VI, and VIII.
• Understand clinical tests for cranial nerves II, III, IV, and VI integrity.
• Describe the pupillary, consensual light reflex, and accommodation reflex.
• Describe signs/symptoms of damage to these pathways.
• Describe lesion locations based on clinical case deficits.
• List/describe signs/symptoms of cranial nerve lesions or nuclei of II, III, IV, VI, and VIII.

Outline of Topics Covered

• Cranial Nerve II (Optic Nerve) and its pathway will be covered.
• Cranial Nerve III (Oculomotor Nerve), including its pathway, oculomotor pathway, pupillary light reflex, consensual light reflex, accommodation, clinical correlations, and testing will be covered.
• Cranial Nerve IV (Trochlear Nerve), including its pathway and clinical correlation and testing is discussed.
• Cranial Nerve VI (Abducens Nerve), including its pathway and clinical correlation and testing is discussed.
• Cranial Nerve VIII (Vestibulocochlear Nerve), specifically auditory and vestibular pathways, will be covered.

Cranial Nerve II: Optic Nerve

• Visual system converts light energy into visual information.
• Light energy is transduced to rods, cones, then bipolar, and ganglion cells of the retina.
• Axons of ganglion cells project to the optic nerve.
• The optic nerve extends to the optic chiasm.
• They synapse onto the lateral geniculate nucleus of the thalamus.
• Projections lead to the visual cortex in the occipital lobe.

Clinical Testing for CN II

• Visual acuity testing uses a Snellen's chart.
• Visual field testing involves the patient closing one eye while the examiner tests the visual field of the other eye.
• With the arm fully extended, an index finger is brought from the periphery to the center of the visual field.
• The patient indicates when the finger appears.
• Use an ophthalmoscope to visualize the fundus: A sharp optic disc indicates no vascular or CNS issues, while a disc that is not sharp may indicate a vascular or CNS problem such as papilledema, which indicates CSF buildup.
• Ensure to examine the retina and macula.

CN III - Oculomotor Nerve

• The oculomotor nerve includes somatic motor and parasympathetic visceral motor components.
• Somatic motor component innervates 4 of the 6 extrinsic ocular muscles for eye movement.
• Cell bodies are in the oculomotor nucleus in the rostral midbrain.
• Parasympathetic visceral motor component innervates intrinsic ocular muscles for pupillary constriction.
• Cell bodies lie in the Edinger-Westphal nucleus.
• The oculomotor nerve also innervates the muscle that elevates the upper eyelid.

Pupillary Light Reflex

• Light reaches the optic nerve.
• Synapses occur in the pretectal area.
• Cell bodies in the pretectal area synapse on the Edinger-Westphal nucleus.
• Edinger-Westphal is the parasympathetic nucleus of CN III.

Consensual Light Reflex

• Light shone in one eye causes constriction in the other eye.
• The optic nerve uses the posterior commissure to cross and innervate cell bodies in the pretectal area.
• These synapse upon the contralateral Edinger-Westphal nucleus.

Accommodation Reflex

• Key events include increased curvature in the lens, pupillary constriction, and convergence of the eyes.
• Efferents from the Edinger-Westphal nucleus signal the ciliary muscle to contract.
• Tension on the suspensory ligament of the lens is released, allowing curvature to increase.
• Edinger-Westphal neurons cause sphincter-like pupillary constrictor muscles to contract and sharpen the image.
• The oculomotor nucleus projects axons to contract both medial recti muscles, causing the eyes to converge.

Clinical Considerations and Testing of CN III

• Lesions to the oculomotor division cause lower motor neuron symptoms such as lateral strabismus followed by diplopia and ptosis.
• Lesions to the Edinger-Westphal division result in dilated pupils and paralysis of accommodation.
• Testing involves checking pupillary and consensual light reflexes with a light in each eye and the accommodation reflex by having a patient focus on an object as it is brought towards the patient's nose.

CN IV: Trochlear Nerve

• The trochlear nerve has only a somatic motor component.
• Innervates the superior oblique muscle of the eye.
• Cell bodies are in the trochlear nucleus in the caudal midbrain.
• The trochlear nucleus axons exit dorsally and cross the midline.
• It innervates the contralateral superior oblique eye muscle, ipsilateral to CN IV.
• The superior oblique muscle causes inward rotation and downward movement of the eye.

Clinical Considerations for CN IV

• Patients with a lesion of the Trochlear nucleus/nerve tilt their head to the unaffected side to correct diplopia.
• Vascular lesions like aneurysms of the posterior cerebral or superior cerebellar arteries can damage CN IV.
• Symptoms of CN IV lesions include outward rotation of the eye, diplopia, and weakness of downward gaze.

CN VI: Abducens Nerve

• The abducens nerve innervates the lateral rectus muscle of the eye to rotate the eye laterally.
• The caudal pons section shows the abducens nucleus and its axon projecting to innervate the ipsilateral lateral rectus eye muscle.

Clinical Considerations for CN VI

• Deficits in CN VI can be caused by aneurysms of the PICA, basilar, or internal carotid artery and pathological conditions in the cavernous sinus.
• Lesions to CN VI exhibit lower motor neuron symptoms like medial (internal) strabismus and diplopia.
• Clinical testing involves instructing the patient to follow a finger with both eyes in vertical and horizontal directions.

CN VIII: Vestibulocochlear Nerve

• The vestibulocochlear nerve is a special sensory afferent.
• Necessary for vestibular (balance, posture) and auditory (hearing) processing.
• Lesions in the cochlear division lead to ipsilateral hearing loss.
• Lesions in the vestibular division cause nystagmus, vertigo, ataxia, loss of balance, nausea, and vomiting.
• Lesions affecting both divisions cause all symptoms.
• A vestibular schwannoma (acoustic neuroma) is a benign tumor affecting myelin-forming Schwann cells of CN VIII in the cerebellopontine angle.
• The tumor can impinge on CN VIII, causing related symptoms.
• A large tumor can also affect CN VII (Bell's Palsy) and CN V (tic doloreaux).