Overview of Cranial Nerve Testing
Cranial nerve testing is a check-up that doctors use to assess the nerves that come from the brain and connect to the head, neck, and trunk. This testing is useful in many medical situations, and it can usually be done quickly and easily in most healthcare settings, such as hospitals or outpatient clinics.
If anything unusual is found during the test, it can indicate neurological problems like brain masses or disease progression that needs quick treatment. For example, an expanding brain aneurysm, which is a weak area in a blood vessel in the brain that swells up.
This type of testing is especially helpful for monitoring patients who can’t communicate because they are not aware of their surroundings. These might be patients recovering from traumatic brain injury, stroke, or other issues inside their skulls. The test helps doctors to keep track of any worsening function or condition in these patients.
In turn, if anything unusual is spotted during the cranial nerve testing, more advanced tests (like brainstem auditory evoked potentials or BAEP) may be necessary. These tests require a referral to a specialized healthcare center.
Anatomy and Physiology of Cranial Nerve Testing
Cranial nerves are responsible for sensation and movement in your head, neck and trunk. The first two cranial nerves, the olfactory (smell) and optic (vision) nerves, come out from the brain, while the rest come out from the brainstem. If these nerves stop working right, it helps doctors figure out where the problem is in the brain or brainstem.
These cranial nerves control different functions, from movement and sensation to automatic actions. Typically, a problem with only one cranial nerve usually means there’s an issue with the peripheral nerves. However, a problem in the brainstem may result in multiple cranial nerves being affected, along with motor and sensory issues in the limbs.
Cranial Nerve I: The olfactory nerve helps us smell and contributes to how we taste our food. Almost all of our ability to sense chemicals comes from our sense of smell, with taste contributing only a small part. Chemical receptors in the lining of our noses help us identify different smells by capturing the molecules of the smells around us and sending signals to the olfactory bulb in our brain, which then sends these signals to the areas of the brain responsible for processing smell.
Cranial Nerve II: The optic nerve connects our eyes to our brain and helps us see. This nerve carries visual information from the retina in our eyes to our brain. Different parts of the visual field are organized in specific ways within the optic nerve, and some fibers are also responsible for the reaction of our pupils to light (the pupillary light reflex).
Cranial Nerves III, IV, and VI: The oculomotor, trochlear, and abducens nerves work together to move the eye. Each nerve starts from its corresponding area in the brainstem and travels to the eye through the superior orbital fissure, a gap in the bony structure protecting the eye. These nerves control different eye muscles to enable smooth and coordinated eye movements.
Cranial Nerve V: The trigeminal nerve helps us feel sensation on our face. It is divided into three sections: the ophthalmic, maxillary, and mandibular nerves, which feel different parts of the face. This nerve also controls some muscles involved in chewing.
Cranial Nerve VII: The facial nerve controls the muscles that allow us to make facial expressions. Damage to this nerve can cause facial paralysis on one side. It also helps regulate salivation and taste in the front two-thirds of the tongue. The nerve also senses a small external portion of the ear and canal.
Cranial Nerve VIII: The vestibulocochlear nerve helps with hearing and balance. This nerve carries information from the inner ear to the brain about the vibrations that make up sound and the motion and position of our head.
Cranial Nerve IX: The glossopharyngeal nerve controls the muscle that helps with swallowing and secretes saliva. It also senses taste from the back third of the tongue and monitors blood pressure and blood composition (oxygen, carbon dioxide).
Cranial Nerve X: The vagus nerve plays a significant role in automatic (involuntary) functions in the body. It controls the part of the nervous system that helps your body rest and digest food and helps control your heart rate. It is also involved in swallowing, talking, and digesting food.
Why do People Need Cranial Nerve Testing
Cranial nerve (CN) testing is often used by healthcare professionals in settings such as hospitals, clinics, and emergency medical services. It’s needed when patients present with various medical conditions and symptoms, and here’s why:
If a person is experiencing neurologic symptoms like headaches, seizures, or issues with their senses or movement, CN testing can be used to help find out what’s going wrong.
In cases of traumatic brain injury, which could occur from falls, accidents, or during sports, CN testing can be essential to understand the extent and location of the injury.
There are instances where blood might leak into the brain spaces (a condition known as Intracranial hemorrhage), CN testing could be helpful to find the source of the bleeding and to guide treatment.
In a scenario where a person has a cerebral aneurysm (an abnormal bulging or ballooning in the wall of a blood vessel in the brain), CN testing can be critical in understanding the effects of the aneurysm on the brain.
When the brain has unusual growths, referred to as intracranial masses, CN testing can be used to assist in identifying their location and impact on brain function.
CN testing is crucial during the diagnosis or treatment of cerebrovascular accidents — more commonly known as strokes, where blood flow to the brain is interrupted.
If a patient becomes unconscious or falls into a coma, CN testing becomes vital in assessing brain function and the prospect of recovery. Similarly, in the sensitive task of evaluating brain death, which means no brain activity or any chance of recovery, these tests can provide the necessary evidence.
When a Person Should Avoid Cranial Nerve Testing
There are some situations where testing of the cranial nerves (CN), which are nerves in your brain that control things like your senses and face movements, might not be possible or safe. These include:
If a person has really bad facial injuries or a lot of swelling on their face, it might be hard to do the test properly and completely.
If a person has a severe neck injury or if there’s a big concern that they might have injured their neck, it’s usually not safe to do a specific test called vestibulo-ocular reflex testing. This is a test that checks how well your eyes and ears work together to help with balance.
Equipment used for Cranial Nerve Testing
If you are going to have cranial nerve (CN) testing, don’t worry, the equipment needed is straightforward and it’s usually available at your doctor’s office or the hospital. One key tool used is a tuning fork, specifically the 256-hertz or 512-hertz type, which is used for hearing tests known as the Rinne and Weber tests.
Ishihara or Hardy-Rand-Ritter plates may be used to test how well you see colors. A special visual examination, known as a fundoscopy, is also used to check the health of your optic nerve, which sends visual signals from your eyes to your brain.
The University of Pennsylvania Smell Identification Test or “Sniffin’ Sticks” are potentially used to check your sense of smell. If these specific tests can’t be found, common items like coffee, soap, or peanut butter might be used instead.
Other tools used for these tests are simple and portable: a pen light or flashlight, cotton wisps, safety pins or blunt tip needles, and a small eye chart known as a hand-held Snellen chart. These can easily be carried in a doctor’s white coat. Additionally, sweet, sour, salty, and bitter solutions are used to test how well you can taste on the front two-thirds of your tongue.
How is Cranial Nerve Testing performed
The olfactory nerve, also known as Cranial Nerve I, helps you smell. Usually, doctors don’t test this nerve as much as the others. If your nose gets blocked, it could mean you can’t smell as well. There’s a simple test doctors can do called the ‘cold spatula test’. They hold a chilled metal spatula under your nose and ask you to breathe normally. If the spatula gets foggy, it means your breath has moisture. If one side of the spatula has less mist, it could suggest a blockage in your nose.
To check the functioning of your olfactory nerve, doctors might have you shut your eyes and block one nostril. Then, they’ll bring a familiar scent, like coffee or soap, close to your open nostril, and ask you to sniff and name the smell. Then they’ll do the same with the other nostril. The whole process is done very carefully to make sure you have no other clues that the smell is present except for the scent itself. If you confuse the smells or can’t identify either one, it might suggest a loss of smell, also known as anosmia.
There are also standardized kits available which helps doctors to conduct the test in a consistent manner. Some examples are the University of Pennsylvania Smell Identification Test (UPSIT), the Connecticut Chemosensory Clinical Research Center (CCCRC) test, the Brief Smell Identification Test (B-SIT), and Sniffin’ Sticks Test (SST).
The optic nerve, Cranial Nerve II, is responsible for vision. To test it, doctors will check various things like your ability to see near and far, your visual fields (how much you can see on the sides without moving your eyes), light reflexes in your pupils, and the accommodation reflex (how your eye adjusts for distance). They might use a Snellen chart which has several lines of letters in reducing size. You’ll be asked to read each line from a distance of 20 feet. If you can’t, you’ll be asked to move closer until you can. This test is repeated for each eye separately. You may be asked to wear any corrective lenses or glasses you use for distance vision during this test.
Furthermore, your near vision will be tested using the same Snellen chart, but this time held 30-40 cm away from your face. To test the accommodation reflex, they will ask you to focus on a pen or the examiner’s finger, and observe the movement and constriction of your eyes and pupils.
Doctors also test the field of vision, which tells them how much you can see on the side without having to move your eyes. They’ll ask you to cover one eye at a time and focus on a point while they move a pen or a finger from your periphery towards the center. You’ll need to tell them when you can see the object. If you are unconscious, doctors will quickly move a hand toward your eye to see if you blink in response.
Lastly, the pupillary light reflex is checked by shining a flashlight into your eyes and checking how your pupils change size in response to light. In a healthy person, both pupils will constrict (get smaller) when a light shines on them. This is known as the light reflex. If one pupil doesn’t constrict as much, it could suggest damage to the optic nerve.
What Else Should I Know About Cranial Nerve Testing?
Checking the cranial nerves is a very important part of any neurological check-up. It is even more critical in cases where the person is unable to respond, like in a coma, or in a situation where brain death is being evaluated. Some of the cranial nerves that can be assessed in someone who is comatose include the optic and oculomotor nerves (which helps us see how the pupils react to light), the trigeminal nerve (which is checked through the corneal reflex, or how the eye blinks when something comes close to it), the vestibulocochlear nerve (how the eyes reacts to head movement), and the glossopharyngeal and vagus nerves (checked through the gag reflex).
If the pupillary light reflex is decreased or missing, this can be a sign of complications like an increase in brain pressure, the brain swelling, or the brain suffering a secondary injury. Comparing findings from these tests can help doctors better understand the neurological condition a patient has.
Damage to Cranial Nerve I can cause a loss of smell, and it normally occurs in 5% to 17% of cases where a head injury has taken place. Prolonged loss of smell can also be associated with disorders like Lewy body dementia and Parkinson’s disease. There are no known treatments to improve the chances or speed up the recovery of the sense of smell after traumatic injury.
Cranial Nerve II is responsible for vision. Damage to this nerve can result in visual field defects or vision loss. For example, damage made to the optic nerve before its connection to the eye can result in blindness in that eye. Damage to this nerve can also affect the pupil’s automatic response to light, and optic neuropathies like glaucoma or optic neuritis may be linked to an abnormal pupillary light reflex.
Cranial nerve III controls many eye movements, including the raising of the upper eyelid. Complete loss of function in this nerve may cause the affected eye to droop, with the pupil often unresponsive and dilated. The causes may be due to health conditions like high blood pressure or diabetes, or because of formation of tumors or aneurysms. Treatments include medication or surgical intervention, depending on the cause and severity of the condition.
Cranial nerve IV is responsible for the movement of only one muscle in the eye. When this nerve is damaged, it can result in double vision, where the duplicate image appears above and to the side of the actual image. These symptoms can be especially noticeable when looking down or during close-visual activities such as reading. The most common cause of this condition is often unknown or congenital.
