Cranial Nerves

Cranial – Anatomy

Introduction

Examination of the cranial nerves allows one to “view” the brainstem all the way from its rostral to caudal extent. The brainstem can be divided into three levels, the midbrain, the pons and the medulla. The cranial nerves for each of these are: 2 for the midbrain (CN 3 & 4), 4 for the pons (CN 5-8), and 4 for the medulla (CN 9-12).

It is important to remember that cranial nerves never cross (except for one exception, the 4th CN) and clinical findings are always on the same side as the cranial nerve involved.

Cranial nerve findings when combined with long tract findings (corticospinal and somatosensory) are powerful for localizing lesions in the brainstem.

Cranial Nerve 1

Olfaction is the only sensory modality with direct access to cerebral cortex without going through the thalamus. The olfactory tracts project mainly to the uncus of the temporal lobes.

Cranial Nerve 2

This cranial nerve has important localizing value because of its “x” axis course from the eye to the occipital cortex. The pattern of a visual field deficit indicates whether an anatomical lesion is pre- or ostchiasmal, optic tract, optic radiation or calcarine cortex.

Cranial Nerve 3 and 4

These cranial nerves give us a view of the midbrain. The 3rd nerve in particular can give important anatomical localization because it exits the midbrain just medial to the cerebral peduncle. The 3rd nerve controls eye adduction (medial rectus), elevation (superior rectus), depression (inferior rectus), elevation of the eyelid (levator palpebrae superioris), and parasympathetics for the pupil. The 4th CN supplies the superior oblique muscle, which is important to looking down and in (towards the midline).

Pontine Level

Cranial nerves 5, 6, 7, and 8 are located in the pons and give us a view of this level of the brainstem.

Cranial Nerve 6

This cranial nerve innervates the lateral rectus for eye abduction. Remember that cranial nerves 3, 4 and 6 must work in concert for conjugate eye movements; if they don’t then diplopia (double vision) results. The medial longitudinal fasciculus (MLF) connects the 6th nerve nucleus to the 3rd nerve nucleus for conjugate movement.

Major Oculomotor Gaze Systems

Eye movements are controlled by 4 major oculomotor gaze systems, which are tested for on the neurological exam. They are briefly outlined here:

1. Saccadic (frontal gaze center to PPRF (paramedian pontine reticular formation) for rapid eye movements to bring new objects being viewed on to the fovea.
2. Smooth Pursuit (parietal-occipital gaze center via cerebellar and vestibular pathways) for eye movements to keep a moving image centered on the fovea.
3. Vestibulo-ocular (vestibular input) keeps image steady on fovea during head movements.
4. Vergence (optic pathways to oculomotor nuclei) to keep image on fovea predominantly when the viewed object is moved near (near triad- convergence, accommodation and pupillary constriction)

Cranial Nerve 5

The entry zone for this cranial nerve is at the mid pons with the motor and main sensory (discriminatory touch) nucleus located at the same level. The axons for the descending tract of the 5th nerve (pain and temperature) descend to the level of the upper cervical spinal cord before they synapse with neurons of the nucleus of the descending tract of the 5th nerve. Second order neurons then cross over and ascend to the VPM of the thalamus.

Cranial Nerve 7

This cranial nerve has a motor component for muscles of facial expression (and, don’t forget, the strapedius muscle which is important for the acoustic reflex), parasympathetics for tear and salivary glands, and sensory for taste (anterior two-thirds of the tongue). Central (upper motor neuron-UMN) versus Peripheral (lower motor neuron-LMN) 7th nerve weakness- with a peripheral 7th nerve lesion all of the muscles ipsilateral to the affected nerve will be weak whereas with a “central 7th”, only the muscles of the lower half of the face contralateral to the lesion will be weak because the portion of the 7th nerve nucleus that supplies the upper face receives bilateral corticobulbar (UMN) input.

Cranial Nerve 8

This nerve is a sensory nerve with two divisions- acoustic and vestibular. The acoustic division is tested by checking auditory acuity and with the Rinne and Weber tests. The vestibular division of this nerve is important for balance. Clinically it be tested with the oculocephalic reflex (Doll’s eye maneuver) and oculovestibular reflex (ice water calorics).

Medullary Level

Cranial nerves 9,10,11, and 12 are located in the medulla and have localizing value for lesions in this most caudal part of the brainstem.

Cranial Nerve 9 and 10

These two nerves are clinically lumped together. Motor wise, they innervate pharyngeal and laryngeal muscles. Their sensory component is sensation for the pharynx and taste for the posterior one-third of the tongue.

Cranial Nerve 11

This nerve is a motor nerve for the sternocleidomastoid and trapezius muscles. The UMN control for the sternocleidomastoid (SCM) is an exception to the rule of the ipsilateral cerebral hemisphere controls the movement of the contralateral side of the body. Because of the crossing then recrossing of the corticobulbar tracts at the high cervical level, the ipsilateral cerebral hemisphere controls the ipsilateral SCM muscle. This makes sense as far as coordinating head movement with body movement if you think about it (remember that the SCM turns the head to the opposite side). So if I want to work with the left side of my body I would want to turn my head to the left so the right SCM would be activated.

Cranial Nerve 12

The last of the cranial nerves, CN 12 supplies motor innervation for the tongue.

Potential Traps

A 6th nerve palsy may be a “false localizing sign”. The reason for this is that it has the longest intracranial route of the cranial nerves, therefore it is the most susceptible to pressure that can occur with any cause of increased intracranial pressure.

Clinical Pearls

Rules of Diplopia

1. Diplopia is maximum in the direction of action of the paretic muscle
2. The most peripherally seen image is the false image and comes from the eye with the paretic muscle.
3. The diplopia is horizontal if the medial or lateral recti are involved and vertical if the elevator or depressor muscles are involved.

Intranuclear ophthalmoplegia (INO)

A lesion of the MLF causes nystagmus of the abducting eye with absent adduction of the other eye. The lesion is on the side of the eye that should be adducting. There can be a bilateral INO in which case neither eye adducts with horizontal gaze.

Cranial – Normal Exam

Cranial Nerve 1 – Olfaction

This CN is tested one nostril at a time by using a nonirritating smell such as tobacco, orange, vanilla, coffee, etc. Detection of the smell is more important than the actual identification.

Cranial Nerve 2 – Visual acuity

The first step in assessing the optic nerve is testing visual acuity. This can be done with a standard Snellen chart or with a pocket chart (Rosenbaum). Have the patient use their glasses if needed to obtain best-corrected vision. Have the patient hold the pocket chart at the focal length that is best for them which is usually 14 inches. Have them recite the line with the smallest letters that they can read and record the acuity.

Cranial Nerve 2 – Visual fields

There are several different screening tests that can be used to assess visual fields at the bedside. First hold up both hands superiorly and inferiorly and ask the patient if they can see both hands and do they look symmetric. Then test each eye individually using your fingers in the four quadrants of the visual field and ask the patient to count fingers held up or point to the hand when a finger wiggles using yourself as a control. A second screening test is to use a grid card. Have the patient focus on the dot in the center of the grid then ask if any part of the grid is missing or looks different. A third method is to use a cotton tip applicator. Testing one eye at a time ask the patient to say “now” as soon as they see the applicator come into their side vision as they focus on the examiner’s nose. All of these tests are screening tests. Formal perimetry is the most accurate way of assessing visual fields.

Cranial Nerve 2 – Fundoscopy

Direct visualization of the optic nerve head is an important and valuable part of assessing CN 2. Systematically look at the optic disc, vessels, retinal background and fovea.

Cranial Nerves 2 & 3 – Pupillary Light Reflex

The afferent or sensory limb of the pupillary light reflex is CN2 while the efferent or motor limb is the parasympathetics of CN3. Shine a flashlight into each eye noting the direct as well as the consensual constriction of the pupils. The swinging flashlight test is used to test for a relative afferent pupillary defect or a Marcus Gunn pupil. Swinging the flashlight back and forth between the two eyes identifies if one pupil has less light perception than the other. Shine the flashlight at one eye noting the size of both pupils. Then swing the flashlight to the other eye. If both pupils now dilate then that eye has perceived less light stimulus (a defect in the sensory or afferent pathway) than the opposite eye.

Cranial Nerves 3, 4 & 6 – Inspection and Ocular Alignment

Before checking ocular movements it is important to inspect the eyes. Look for ptosis. Note the appearance of the eyes and check for ocular alignment (the reflection of your light source should fall on the same location of each eyeball).

Cranial Nerves 3, 4 & 6 – Versions

Testing extraocular range of motion with both eyes open and following the target (conjugate gaze) is called versions. The patient is asked to follow a target through the six principle positions of gaze. Note any misalignment of the eyes or complaint of diplopia (double vision).

Cranial Nerves 3, 4 & 6 – Ductions

If there is any misalignment of the eyes or diplopia on versions it is important to then examine each eye with the other covered (this is called ductions). The patient should follow an object through the six principle positions of gaze so each extraocular muscle’s function is tested.

Supranuclear gaze systems- Introduction

The purpose of supranuclear control of gaze is to insure that the image that is being looked at is centered or maintained on the fovea of the retina. The following maneuvers test the major systems that control eye movements.

Saccades

Saccades are tested by holding up your two hands about three feet apart and instructing the patient to look at the finger that is wiggling without moving their head. The patient’s eyes should be able to quickly, smoothly and accurately jump from target to target.

Smooth Pursuit

To test Smooth Pursuit ask the patient to keep watching the target without moving their head. Then move the target slowly from side to side and up and down. The eyes should be able to follow the target smoothly without lagging behind or jerking to catch up with the target.

Optokinetic Nystagmus

Optokinetic Nystagmus is a test of smooth pursuit with quick resetting saccades. Use a tape with repeating shapes on it and ask the patient to look at each new object as it appears as you run the tape between your fingers to the right, left, up, and down. The patient will have brief pursuit eye movements in the direction of the tape movement with quick saccades or jerks in the opposite direction. The resetting saccades are easier to observe than the brief pursuit movement.

Vestibulo-ocular reflex

The vestibulo-ocular reflex is obtained by having the patient visually fixate on an object straight ahead, then rapidly turning the patient’s head form side to side and up and down. The eyes should stay fixed on the object and turn in the opposite direction of the head movement.

Vergence

Vergence eye movements occur when the eyes move simultaneously inward (convergence) or outward (divergence) in order to maintain the image on the fovea that is close up or far away. Most often convergence is tested as part of the near triad. When a patient is asked to follow an object that is brought from a distance to the tip of their nose the eyes should converge, the pupil will constrict and the lens will round up (accommodation).

Cranial Nerve 5 – Sensory

Test for both light touch (cotton tip applicator) and pain (sharp object) in the 3 sensory divisions (forehead, cheek, and jaw) of CN 5.

Cranial Nerves 5 & 7 – Corneal reflex

The ophthalmic division (V1) of the 5th nerve is the sensory or afferent limb and a branch of the 7th nerve to the orbicularis oculi muscle is the motor or efferent limb of the corneal reflex. The limbal junction of the cornea is lightly touched with a strand of cotton. The patient is asked if they feel the touch as well as the examiner observing the reflex blink.

Cranial Nerve 5 – Motor

The motor division of CN 5 supplies the muscles of mastication (temporalis, masseters, and pterygoids). Palpate the temporalis and masseter muscles as the patient bites down hard. Then have the patient open their mouth and resist the examiner’s attempt to close the mouth. If there is weakness of the pterygoids the jaw will deviate towards the side of the weakness. The last test for this nerve is testing for a jaw jerk, which is a stretch reflex. Have the patient slightly open their mouth then place your finger on their chin and strike your finger with a reflex hammer. Normally there is no movement. If there is a jaw jerk it is said to be positive and this indicates an upper motor neuron lesion.

Cranial Nerve 7 – Motor

The motor division of CN 7 supplies the muscles of facial expression. Start from the top and work down. Have the patient wrinkle forehead (frontalis muscle), close eyes tight (orbicularis oculi) show their teeth (buccinator), and purse lips or blow a kiss (orbicularis oris). If there is weakness especially in a bilateral upper motor neuron distribution, get the patient to smile by telling a joke or funny story. With a pseudobulbar palsy automatic or emotional facial expression will be more complete than movements to command.

Cranial Nerve 7 – Sensory, Taste

Taste is the sensory modality tested for the sensory division of CN 7. The examiner can use a cotton tip applicator dipped in a solution that is sweet, salty, sour, or bitter. Apply to one side then the other side of the extended tongue and have the patient decide on the taste before they pull their tongue back in to tell you their answer.

Cranial Nerve 8 – Auditory Acuity, Weber & Rinne Tests

The cochlear division of CN 8 is tested by screening for auditory acuity. This can be done by the examiner lightly rubbing their fingers by each ear or by using a ticking watch. Compare right versus left. Further screening for conduction versus neurosensory hearing loss can be accomplished by using the Weber and Rinne tests. The Weber test consists of placing a vibrating tuning fork on the middle of the head and asking if the patient feels or hears it best on one side or the other. The normal patient will say it is the same in both ears. The patient with unilateral neurosensory hearing loss will hear it best in the normal ear while the patient with a unilateral conductive hearing loss will hear it best in the abnormal ear. The Rinne test consists of comparing bone conduction (placing the tuning fork on the mastoid process) versus air conduction (placing the tuning fork in front of the pinna). Normally, air conduction is greater than bone conduction. For neurosensory hearing loss air conduction is still greater than bone conduction but for conduction hearing loss bone conduction will be greater than air conduction.

Cranial Nerve 8 – Vestibular

The vestibular division of CN 8 can be tested for by using the vestibulo-ocular reflex as already demonstrated or by using ice water calorics to test vestibular function. The later test is usually reserved for patients who have vertigo or balance problems or in the comatose patient when one is testing brainstem function.

Cranial Nerves 9 & 10 – Motor

The motor division of CN 9 & 10 is tested by having the patient say “ah” or “kah”. The palate should rise symmetrically and there should be little nasal air escape. With unilateral weakness the uvula will deviate toward the normal side because that side of the palate is pulled up higher. With bilateral weakness neither side of the palate will elevate and there will be marked nasal air escape.

Cranial Nerves 9 & 10 – Sensory and Motor: Gag Reflex

The gag reflex tests both the sensory and motor components of CN 9 & 10. This involuntary reflex is obtained by touching the back of the pharynx with the tongue depressor and watching the elevation of the palate.

Cranial Nerve 11 – Motor

CN 11 is tested by asking the patient to shrug their shoulders (trapezius muscles) and turn their head (sternocleidomastoid muscles) against resistance.

Cranial Nerve 12 – Motor

The 12th CN is tested by having the patient stick out their tongue and move it side to side. Further strength testing can be done by having the patient push the tongue against a tongue blade. Inspect the tongue for atrophy and fasciculations. If there is unilateral weakness, the protruded tongue will deviate towards the weak side. By having the patient say lah-pah-kah, the examiner is testing the motor components of CN 12, 7, and 9&10

Cranial – Abnormal

Cranial Nerve 1 – Olfaction

This patient has difficulty identifying the smells presented. Loss of smell is anosmia. The most common cause is a cold (as in this patient) or nasal allergies. Other causes include trauma or a meningioma effecting the olfactory tracts. Anosmia is also seen in Kallman syndrome because of agenesis of the olfactory bulbs.

Cranial Nerve 2 – Visual acuity

This patient’s visual acuity is being tested with a Rosenbaum chart. First the left eye is tested, then the right eye. He is tested with his glasses on so this represents corrected visual acuity. He has 20/70 vision in the left eye and 20/40 in the right. His decreased visual acuity is from optic nerve damage.

Cranial Nerve 2 – Visual fields

The patient’s visual fields are being tested with gross confrontation. A right sided visual field deficit for both eyes is shown. This is a right hemianopia from a lesion behind the optic chiasm involving the left optic tract, radiation or striate cortex.

Cranial Nerve 2 – Fundoscopy

The first photograph is of a fundus showing papilledema. The findings of papilledema include:

1. Loss of venous pulsations
2. Swelling of the optic nerve head so there is loss of the disc margin
3. Venous engorgement
4. Disc hyperemia
5. Loss of the physiologic cup
6. Flame shaped hemorrhages

This photograph shows all the signs except the hemorrhages and loss of venous pulsations.

The second photograph shows optic atrophy, which is pallor of the optic disc resulting form damage to the optic nerve from pressure, ischemia, or demyelination.

Cranial Nerves 2 & 3 – Pupillary Light Reflex

The swinging flashlight test is used to show a relative afferent pupillary defect or a Marcus Gunn pupil of the left eye. The left eye has perceived less light stimulus (a defect in the sensory or afferent pathway) then the opposite eye so the pupil dilates with the same light stimulus that caused constriction when the normal eye was stimulated.

Cranial Nerves 3, 4 & 6 – Inspection and Ocular Alignment

This patient with ocular myasthenia gravis has bilateral ptosis, left greater than right. There is also ocular misalignment because of weakness of the eye muscles especially of the left eye. Note the reflection of the light source doesn’t fall on the same location of each eyeball.

Cranial Nerves 3, 4 & 6 – Versions

• The first patient shown has incomplete abduction of her left eye from a 6th nerve palsy.
• The second patient has a left 3rd nerve palsy resulting in ptosis, dilated pupil, limited adduction, elevation, and depression of the left eye

Cranial Nerves 3, 4 & 6 – Ductions

Each eye is examined with the other covered (this is called ductions). The patient is unable to adduct either the left or the right eye. If you watch closely you can see nystagmus upon abduction of each eye. When both eyes are tested together (testing versions) you can see the bilateral adduction defect with nystagmus of the abducting eye. This is bilateral internuclear ophthalmoplegia often caused by a demyelinating lesion effecting the MLF bilaterally. The adduction defect occurs because there is disruption of the MLF (internuclear) connections between the abducens nucleus and the lower motor neurons in the oculomotor nucleus that innervate the medial rectus muscle.

Smooth Pursuit

The patient shown has progressive supranuclear palsy. As part of this disease there is disruption of fixation by square wave jerks and impairment of smooth pursuit movements. Saccadic eye movements are also impaired. Although not shown in this video, vertical saccadic eye movements are usually the initial deficit in this disorder.

Optokinetic Nystagmus

This patient has poor optokinetic nystagmus when the tape is moved to the right or left. The patient lacks the input from the parietal-occipital gaze centers to initiate smooth pursuit movements therefore her visual tracking of the objects on the tape is inconsistent and erratic. Patients who have a lesion of the parietal-occipital gaze center will have absent optokinetic nystagmus when the tape is moved toward the side of the lesion.

Vestibulo-ocular reflex

The vestibulo-ocular reflex should be present in a comatose patient with intact brainstem function. This is called intact “Doll’s eyes” because in the old fashion dolls the eyes were weighted with lead so when the head was turned one way the eyes turned in the opposite direction. Absent “Doll’s eyes” or vestibulo-ocular reflex indicates brainstem dysfunction at the midbrain-pontine level.

Vergence

Light-near dissociation occurs when the pupils don’t react to light but constrict with convergence as part of the near reflex. This is what happens in the Argyll-Robertson pupil (usually seen with neurosyphilis) where there is a pretectal lesion affecting the retinomesencephalic afferents controlling the light reflex but sparing the occipitomesencephalic pathways for the near reflex.

Cranial Nerve 5 – Sensory

There is a sensory deficit for both light touch and pain on the left side of the face for all divisions of the 5th nerve. Note that the deficit is first recognized just to the left of the midline and not exactly at the midline. Patients with psychogenic sensory loss often identify the sensory change as beginning right at the midline.

Cranial Nerves 5 & 7 – Corneal reflex

A patient with an absent corneal reflex either has a CN 5 sensory deficit or a CN 7 motor deficit. The corneal reflex is particularly helpful in assessing brainstem function in the unconscious patient. An absent corneal reflex in this setting would indicate brainstem dysfunction.

Cranial Nerve 5 – Motor

• The first patient shown has weakness of the pterygoids and the jaw deviates towards the side of the weakness.
• The second patient shown has a positive jaw jerk which indicates an upper motor lesion affecting the 5th cranial nerve.

Cranial Nerve 7 – Motor

• The first patient has weakness of all the muscles of facial expression on the right side of the face indicating a lesion of the facial nucleus or the peripheral 7th nerve
• The second patient has weakness of the lower half of his left face including the orbicularis oculi muscle but sparing the forehead. This is consistent with a central 7th or upper motor neuron lesion

Cranial Nerve 7 – Sensory, Taste

The patient has difficulty correctly identifying taste on the right side of the tongue indicating a lesion of the sensory limb of the 7th nerve.

Cranial Nerve 8 – Auditory Acuity, Weber & Rinne Tests

This patient has decreased hearing acuity of the right ear. The Weber test lateralizes to the right ear and bone conduction is greater than air conduction on the right. He has a conductive hearing loss.

Cranial Nerve 8 – Vestibular

Patients with vestibular disease typically complain of vertigo – the illusion of a spinning movement. Nystagmus is the principle finding in vestibular disease. It is horizontal and torsional with the slow phase of the nystagmus toward the abnormal side in peripheral vestibular nerve disease. Visual fixation can suppress the nystagmus. In central causes of vertigo (located in the brainstem) the nystagmus can be horizontal, upbeat, downbeat, or torsional and is not suppressed by visual fixation.

Cranial Nerves 9 & 10 – Motor

When the patient says “ah” there is excessive nasal air escape. The palate elevates more on the left side and the uvula deviates toward the left side because the right side is weak. This patient has a deficit of the right 9th & 10th cranial nerves.

Cranial Nerves 9 & 10 – Sensory and Motor: Gag Reflex

Using a tongue blade, the left side of the patient’s palate is touched which results in a gag reflex with the left side of the palate elevating more then the right and the uvula deviating to the left consistent with a right CN 9 & 10 deficit.

Cranial Nerve 11 – Motor

When the patient contracts the muscles of the neck the left sternocleidomastoid muscle is easily seen but the right is absent. Looking at the back of the patient, the left trapezius muscle is outlined and present but the right is atrophic and hard to identify. These findings indicate a lesion of the right 11th cranial nerve.

Cranial Nerve 12 – Motor

Notice the atrophy and fasciculation of the right side of this patient’s tongue. The tongue deviates to the right as well because of weakness of the right intrinsic tongue muscles. These findings are present because of a lesion of the right 12th cranial nerve.