Every suspected traumatic brain injury warrants a thorough neurological examination. As you conduct your exam, it is important to remember that a brain injury may include structural or functional compromise of the nerves exiting the brain (or their nuclei within the brain). Thus, the cranial nerve exam will be an essential element of your neurological assessment.
Practitioners sometimes find the cranial nerve (CN) exam intimidating due to the complexity of the anatomy involved and the clinical sophistication that the exam demands from the clinician carrying it out. However, with a systematic approach to the exam — and a little bit of practice — it is possible to accurately and efficiently assess the function of each cranial nerve without difficulty.
Before we explain how to do that, let’s review some relevant anatomy.
The cranial nerves are bundles of sensory and/or motor fibres that 1) innervate muscles, 2) innervate glands, or 3) carry afferent nerve signals to the central nervous system from sensory receptors.
Of these nerves, there are twelve pairs. They are called “cranial” nerves because they emerge from the cerebrum, cerebellum, or brainstem and pass distally through openings in the cranium. Passing through the cranium, they are covered by sheaths derived from the cranial meninges.
Unlike spinal nerves, whose roots are neural fibers from the spinal grey matter, cranial nerves are composed of the neural processes associated with distinct brainstem nuclei and cortical structures.
As a general rule, the more posterior and lateral nuclei tend to be sensory, and the more anterior tend to be motor.
Cranial nerves I, II, and VIII are considered purely afferent (i.e., sensory).
Cranial nerves III, IV, VI, XI, and XII are purely efferent (i.e., motor).
The remaining cranial nerves, V, VII, IX, and X, are functionally mixed (i.e., sensory and motor).
It is essential that clinicians have a good understanding of the anatomical structures they are testing during clinical procedures like the CN exam. This ensures more precise assessments and more reliable results so that abnormalities causing neurological symptoms will be identified and promptly acted upon.
Before reading ahead, take a few minutes to review the following diagram:
Cranial nerves vector illustration.
Understanding how to test a particular nerve requires understanding what that particular nerve does. One must understand proper function to detect dysfunction in a physiological structure or system.
What specifically is the proper function of each cranial nerve? After reading through this list, run through it again and test yourself on each nerve’s number, name, and function(s):
| CN I | The Olfactory Nerve | Responsible for the sense of smell (olfaction) |
| CN II | The Optic Nerve | This is the “seeing” nerve as it carries visual signals from the retinae to the brain |
| CN III | The Oculomotor Nerve | Controls eye movements and pupil size |
| CN IV | The Trochlear Nerve | Controls eye movement and pupil size |
| CN V | The Trigeminal Nerve | Carries sensory information from the face and the mouth and innervates “chewing” muscles |
| CN VI | The Abducens Nerve | Carries motor signals responsible for lateral eye movement (eye abduction) |
| CN VII | The Facial Nerve | Carries signals that control facial muscles and the lacrimal gland, which produces tears |
| CN VIII | The Vestibulocochlear Nerve | Responsible for auditory and equilibrium sensations, or put more simply, hearing and balance |
| CN IX | The Glossopharyngeal Nerve | Carries signals from the throat and tongue and is responsible for things like taste at the back of the tongue and the gag reflex |
| CN X | The Vagus Nerve | Responsible for controlling the muscles of the pharynx, larynx, and diaphragm. It also carries afferent signals to the brain from many organs, including the heart and lungs |
| CN XI | The Spinal Accessory Nerve | Carries signals to the neck and shoulder muscles |
| CN XII | The Hypoglossal Nerve | Controls tongue movements |
Now, let’s look at how to test each nerve listed above. The following test descriptions are laid out according to the anatomical order of the cranial nerves, from CN I to XII.
NOTE: You do not necessarily need to follow this order when conducting your tests. Be creative! Think about how you can order your tests to 1) improve flow, 2) make the tests easier to remember, and 3) make your exam more efficient. (Here is one example of how you might administer the tests.)
The cranial nerve exam process includes the following:
Start by observing the face, neck, and torso for bruising, swelling, spasms, asymmetry, and/or other involuntary movements.
Test both nostrils independently.
Ask the patient if he has noticed any changes to his smell.
Assess the patient’s sense of smell by asking him to identify common smells (e.g., coffee, soap, cloves, mint) with his eyes closed. Have the patient cover each nostril and test each side independently for asymmetry.
A. Examine the patient’s visual acuity with a Snellen chart. The chart may be on a screen (via an eye exam app) or on paper. Keep the chart 30–40 cm from the patient and assess each eye’s acuity individually (as the patient covers each eye).
Eye test chart.
B. Test the visual field in all four quadrants. Cover the same eye as your patient. Tell your patient to stare at your nose during the visual field test. Wiggle your finger as you move it from the periphery toward the midline in all quadrants, and have your patient say “yes” whenever he sees your wiggling finger. Look for any glaring deficits or asymmetries in peripheral vision.
C.(Optional) If you have one on hand, You can inspect the optic disc with an ophthalmoscope.
A. Observe the patient’s pupils for size, shape, and symmetry. Note any irregularities.
B. Tests the pupils’ direct and indirect reactions to light. Shine a penlight into one eye and note the dilation—the “direct response.” At the same time, note whether the other eye also dilates—the “indirect response.” Test both eyes.
C. Conduct an H-pattern test for eye movement:
D. Test convergence by moving a penlight toward the bridge of the patient’s nose at the end of the H-pattern test, noting any asymmetries or aberrations as eyes move medially.
E. (Optional) You may decide to test saccades. Do this by facing your patient, standing no more than 1 meter away. Place your fingers 1.5 feet from the midline (3 feet apart). Ask the patient to keep their head still while looking back and forth from one finger to another as quickly as possible, ten times in a row. The head should remain still. Note any involuntary head movements, any over or undershooting of the target finger, or any latency.
Recall that the trigeminal nerve has motor and sensory functions.
A. Test facial sensations using two simple tools: a dull object (e.g., eraser tip or ball) and a sharp object (e.g., paper clip or tweezers). Be sure to test all three divisions of the trigeminal nerve — ophthalmic, maxillary, and mandibular — with both objects.
Ask the patient to close their eyes. As you apply the objects to all three regions of the face, have the patient indicate whether the object is “sharp” or “soft.”
Trigemial nerve illustration. Ophtalmic, Maxillary and mandibular nerve
A. Test corneal reflex by having the patient hold their eyes open while you gently touch each eye with a cotton swab; note if the patient blinks.
B. Test motor function of the trigeminal nerve by having the patient open mouth against resistance and assess for weakness. Also, look for jaw deviation on opening. Finally, palpate for masseter asymmetry.
A. Observe the symmetry of the face at rest and during movement.
B. Test facial muscles by asking the patient to perform various facial expressions, such as smiling, frowning, raising eyebrows, puffing out cheeks, showing teeth, and closing eyes tightly.
C. (Optional) Test taste on the anterior 2/3 of the tongue with sweet, sour, or salty solutions.
A. Screen for hearing loss by standing behind the patient and rubbing fingers together lightly on the right or left side of the patient’s head while asking the patient to locate which side they hear the rubbing. Do this enough times to determine whether one side or the other is inhibited (rule out the possibility of lucky guesses!). A slight variation is to whisper something on each side and have the patient repeat it back to you.
B. To further test hearing, perform the Weber and Rinne tests using a 512 Hz tuning fork:
If bone conduction is heard longer than air conduction, this indicates that the patient has conductive hearing loss.
If air conduction is heard longer than bone conduction – but not twice as long – then it may indicate sensorineural hearing loss.
B. (Optional) You may also check for nystagmus. If you suspect vestibular pathology, consider performing the head thrust and/ or the Dix-Hallpike maneuvers. Keep in mind: symptoms like nystagmus and nausea may also be indicative of stroke. If you are concerned about the possibility of a stroke, you may do a Head Impulse-Nystagmus-Test of Skew (or HINTS) exam.
Note: you should never have to perform a HINTS and a Dix-Hallpike on the same visit.
Injury to the spinal accessory nerve can lead to dysfunction of the trapezius and sternocleidomastoid. As a major scapular stabilizer, dysfunction of the trapezius can lead to pain and disability.
Conducting a thorough and systematic cranial nerve assessment is essential in evaluating the health of an individual’s central nervous system. It can provide valuable information about potential underlying conditions and help identify specific areas for further evaluation and treatment. In the case of concussion, any cranial nerve irregularities discovered in the post-injury exam should be thoroughly evaluated, and when necessary, prompt referral should be arranged.
Remember that many pathologies may explain cranial neuropathies. Aside from traumatic head injury, other possible causes are diabetes, high blood pressure, infection, and tumors. They can also be idiopathic. Always consider these when formulating your differential diagnoses.
Moreover, it is crucial to consider the patient’s history, medications, and overall health when assessing cranial nerve function. Certain conditions and medications can affect the functioning of the nerves, so it is important to have a comprehensive understanding of the patient’s medical background.
Furthermore, it is important to remember that while the tests described above can help with initial screening, they should not be used as the sole basis for diagnosis. Not every positive test means underlying clinically significant pathology. A thorough history, careful physical examination, and appropriate medical imaging are all crucial in making an accurate diagnosis and determining the appropriate treatment plan. Consultation with a neurologist may be necessary for more complex cases.
Though perhaps daunting in scope and complexity, you will inevitably master the cranial nerve examination with careful and consistent practice. Few aspects of the physical examinations provide so much information to the clinician so quickly, and you will find this exam incredibly useful in your everyday practice and not merely with your concussion patients. After all is said and done, acquiring the art of the cranial nerve exam is undoubtedly worth your time, effort, and the inevitable pains that come with learning hard things.
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