1. The Olfactory Nerve (CNI)

The olfactory nerve is responsible for the sense of smell. Although rarely tested in practice, alterations in smell can be caused by serious intracranial pathology (brain tumors, strokes, TBI), neurodegenerative diseases like Alzheimer’s, Parkinson’s, or MS, or benign and transient causes such as the common cold.

If both branches of the olfactory nerve are damaged, this can lead to permanent anosmia (loss of smell) and can lead to food tasting bland and decreased appetite. In most individuals, the sense of smell decreases over time, with up to 75% of individuals older than 80 have some degree of anosmia.

 How to test the Olfactory Nerve

The olfactory nerve is almost never tested within an acute care setting such as in the hospital. However, this is sometimes tested in outpatient neurology offices. To test the olfactory nerve, blindfold the patient and have them smell and identify common scents such as vanilla, cinnamon, coffee, or peppermint while covering up one nostril at a time. Do not use ammonia or alcohol as these can trigger intranasal trigeminal nerve receptors and bypass the olfactory nerve.

2. The Optic Nerve (CNII)

The optic nerve is the second of the cranial nerves and is responsible for vision. This nerve transmits signals perceived in the retina and cones of the retina to the occipital lobe. This is commonly tested within the clinical setting and for a variety of presentations.

Partial or complete loss of vision can be caused by conditions such as:

• Diabetes
• Intracranial pathology (ischemia, stroke, tumors)
• Inflammation or infection of the eye
• Toxicity

How to test the Optic Nerve

When testing the optic nerve, you need to examine the visual fields, the visual acuity, and the pupillary light reflex. All three are an important part of the cranial nerve assessment, although the pupillary light reflex involves cranial nerve 3 as well.

Testing the Visual Fields (II)

If the patient loses part of their vision on one side, it is termed partial hemianopia, and if they lose complete vision on one side it is complete hemianopia. There are a few different ways to test visual fields, but here is an easy way. Stand one arm length away from the patient and ask them to cover up one eye or do it for them if they are unable. Close your own eye on the same side. Now hold up numbers with your fingers at each of the four corners of their vision. Once satisfied, test the other eye.

Testing the Visual Acuity (II)

Nurses often assess visual acuity, and most emergency departments will have a Snellen eye chart to use. The distance the patient stands depends on the visual acuity chart (it should say). If you do not have easy access to this, there is an app you can get on your phone which is super helpful!

Have the patient stand the appropriate distance away and have them cover up one eye. Do not have them forcibly close the eye as this can somewhat inhibit their ability to see out of their open eye. Ask the patient to read the 20/20 line on the chart. On a standard Snellen eye chart, this would be 20 feet away. If the patient gets more than half wrong, move onto the line above. Stop once the patient gets over 50% right. Mark this down and test the other eye. If they wear glasses – have them wear their glasses for this as well!

Pupillary Light Reflex (II, III)

The pupillary light reflex tests both cranial nerves II and III. First, inspect both pupils and make sure they are equal in size and shape. Then dim the lights if possible and shine a penlight directly into the right eye. Both pupils should constrict and maintain symmetry. Note if they are brisk or sluggish and if they are symmetric. Remove the light source and watch both eyes dilate equally as well. Do the same for the left eye.

3. The Oculomotor Nerve (CNIII)

The oculomotor nerve controls the majority of the extraocular muscles. It is primarily responsible for eye movement, eyelid movement, and pupillary constriction. If there is any oculomotor nerve impairment, there will be a pupillary dilation, ptosis (drooping eyelid), and outward deviation of the eye – termed abduction. When a patient has diplopia (double vision), it is often due to a unilateral lesion on this cranial nerve. In most cases, third nerve palsy resolves over weeks to months.

Causes of oculomotor nerve palsy include:

• Intracranial aneurysm
• Microvascular ischemia (in diabetics especially)
• Trauma: Severe blows to head with skull fracture

Testing Extraocular Muscles (III, IV, VI)

To test the oculomotor nerve, you need to assess the EOMs. Testing the EOMs also tests cranial nerves IV and VI, as all three nerves are responsible for eye movement.

Hold your finger or a pen 2 feet in front of the patient’s eyes midline and have the patient focus on it with both eyes. Ask the patient to follow your finger or pen with only their eyes, moving the pen to the right, back to the midline, and then to the left and back again. Do this again for up and down. Lastly, do this again to the down-left diagonal angle, and then the down-right diagonal angle. You should have tested a total of 6 different directions – termed the “6 cardinal directions”.

Eye movement should be symmetric, smooth, and moving in all directions. At each extremity of vision, you should be observing excessive nystagmus. Nystagmus is repetitive uncontrolled eye movement.

Conjugate nerve palsy is when both eyes are unable to look in a specific direction during your testing. This most commonly occurs in the horizontal directions. This is usually due to a stroke within or near the brain stem.

The pupillary light reflex listed above is also used to assess the oculomotor nerve.

4. CN IV: The Trochlear Nerve

The fourth cranial nerve, the trochlear nerve, innervates the superior oblique muscle of the eyes. This means it controls the downward movement of the eyeball and prevents it from rolling upward. When there is a fourth nerve palsy, patients will often complain of vertical diplopia and/or tilting of objects. This may be most noticeable when in a downward gaze such as when going down the stairs. They may also have a head tilt, as the visual changes improve with tilting of the head. On exam, the eye will with deviated upward and rotated outward.

Testing the trochlear nerve involves evaluating the patient’s extra-ocular movements as described above.

5. CN V: The Trigeminal Nerve

The Trigeminal nerve is the 5th cranial nerve and responsible for facial sensation, as well as moving the muscles involved with biting and chewing. This has three branches including the ophthalmic V1, maxillary V2, and Mandibular V3. Compression of this nerve root can cause trigeminal neuralgia – a rare but painful condition.

How to test the Trigeminal Nerve

To test the trigeminal nerve, you are testing their facial sensation. Lightly touch both sides of the forehead and ask if they felt the same. Do this on the cheek, and then the chin. If the patient is uncooperative, you can test their corneal reflex. Do this by having the patient look right, then touch their left cornea with a whisp of cotton. They should blink. Do this on both sides.

6. CN VI: The Abducens Nerve

The sixth cranial nerve, the abducents nerve innervates the lateral rectus muscle of the eye. This means its responsible for outward movement of the eyes. Patients with dysfunction of this nerve will be unable to outwardly move their eyes. This causes horizontal diplopia, where the double images are side-by-side, which is worse at far distances.

This nerve is often the first nerve compressed when there is any increased intracranial pressure (ICP). However, more common causes include vascular disease (diabetes, hypertension, atherosclerosis) or trauma.

To test the abducents nerve, test the EOMs as described above.

7. CN VII: The Facial Nerve

Cranial nerve VII is the facial nerve, which controls the muscles of facial expression, as well as the sensation of taste of the front of the tongue. Facial nerve palsy can occur for various reasons, the most common being Bell’s palsy. Some other common causes include stroke, Lymes disease, trauma, or even diabetes.

How to test the Facial Nerve

To test the facial nerve, you must assess the patient’s facial expressions. Have the patient close their eyes tightly, then have them open their eyes. Ask them to frown, looking for symmetry in the forehead muscles. Have them smile and look for any drooping or asymmetry.

Clinical Tip: To differentiate Bell’s palsy from stroke, assess the patient’s use of their forehead muscles. Peripheral nerve lesions (such as with Bell’s palsy) cause paresis of the entire side of the face. Central lesions tend to only effect the lower portion of the face. This is not always the case though, so you must use clinical judgement. Bell’s palsy should have no other associated neuro deficits. Check out my Infographic for more information!

8. CN VIII: The Vestibulocochlear Nerve

The vestibulocochlear nerve, also called the auditory vestibular nerve, is responsible for hearing and balance. Vestibular neuritis is when the nerve becomes inflamed and can cause vertigo, dizziness, and balancing difficulties – most likely from a viral infection.

How to test the Vestibulocochlear Nerve

While not routinely tested within the hospital, the vestibulocochlear nerve involves testing both hearing and balance. Hearing is tested by holding your fingers a few inches away from their ears and rubbing them together. If they can hear, then that is a pass. Test their balance by assessing their gait while walking. The presence of nystagmus can also indicate vestibular dysfunction.

9. CN IX: The Glossopharyngeal Nerve

The glossopharyngeal nerve is partially responsible for the sensation of taste, pharyngeal sensation, as well as for the gag reflex. A damaged glossopharyngeal nerve can cause a loss of taste in part of the tongue and cause trouble swallowing.

How to test the Glossopharyngeal Nerve

Palatal Movement (IX, X)

Instead of doing the gag reflex which can be very uncomfortable for patients, you can instead assess palatal movement. Do this by having the patient yawn or say “ahh”, and observe their palate movement for symmetry. If this is abnormal, consider testing the gag reflex.

Gag Reflex (IX, X)

When performing the cranial nerve assessment, the easiest way to test the glossopharyngeal nerve is to test their gag reflex, however, this is usually not necessary in the clinical setting. Remember that approximately 20% of people will not have a gag reflex at baseline. Check both sides of the pharyngeal wall by gently poking the pharynx with a cotton swab.

Dysarthria (IX, X, XII)

There is no specific test for this but listen to the patient’s speech. Assess for any slurred speech or abnormality of the voice. Ask the patient or the family if it sounds different than normal.

10. CN X: The Vagus Nerve

The Vagus nerve innervates the hearts, lungs, and digestive tract, along with a few muscles. Most noticeably, it controls the heart rate, GI motility, sweating, and speech. It is also partially responsible for the gag reflex (along with cranial nerve IX).

Overstimulation of the vagal nerve can drop the heart rate and cause syncope, termed vasovagal syncope. Activities that stimulate the vagal nerve include bearing down, holding breath, carotid massage, or extreme fear or stress.

There are even implantable vagus nerve stimulators that can help slow down the firing of neurons within the brain and thus help manage seizures.

How to test the Vagus Nerve

The only real way to test the vagus nerve is via the gag reflex as described above.

11. CN XI: The Accessory Nerve

The accessory nerve innervates the sternocleidomastoid and trapezius muscles. This means it is responsible for tilting/rotating the head as well as shrugging the shoulders. This nerve can be damaged after neck surgery or blunt force trauma.

How to test the Accessory Nerve

To test the trapezius muscle, ask the patient to shrug both of their shoulders at the same time. Then apply some downward pressure with both hands and ask them to shrug both shoulders against the resistance.

To test the sternocleidomastoid, place a hand against their cheek and ask them to rotate their head against resistance in each direction. If you notice weakness, this indicates the opposite side is the weaker muscle.

12. CN XII: The Hypoglossal Nerve

The hypoglossal nerve controls most of the movement of the tongue. This means it is highly responsible for speech and swallowing. Damage to the hypoglossal nerve is rare, but if so are likely to be caused by tumors or gunshot wounds. Other causes include stroke or neurodegenerative disease.

How to test the Hypoglossal Nerve

To test the hypoglossal nerve, ask the patient to stick out their tongue. If the tongue deviates to one side, this indicates hypoglossal nerve dysfunction on the side of deviation. Then ask them to move their tongue from side to side rapidly. Additionally, listen for dysarthria when the patient is speaking as described above.