Cranial Nerves — What They Are and Why We Test Them
If you have ever sat in front of a clinician while they asked you to follow a moving finger with your eyes, stick out your tongue, or tell them whether you could smell something, there is a good chance you were being given a cranial nerve examination. It can feel a bit strange if nobody explains what is happening. So let me explain what is happening.
A quick bit of anatomy — bear with me
Most of the nerves in your body travel through the spinal cord. The cranial nerves are the exception. There are twelve pairs of them, and they emerge directly from the brain and brainstem, bypassing the spine entirely. They are numbered I to XII, largely in the order they come off the brain from front to back, and each one has a specific job to do.
Some are purely sensory — they carry information in. Some are purely motor — they carry instructions out. Several are both, which makes them particularly interesting and, frankly, occasionally confusing to revise for exams.
Their functions range from the obvious to the quietly remarkable. The olfactory nerve handles smell. The optic nerve handles vision. The vagus nerve — arguably the most important nerve in the body — wanders all the way down through your chest and abdomen, doing things that most people would find surprising: regulating heart rate, influencing gut motility, and playing a significant role in your stress and rest responses. It deserves its own article, and it will get one.
Why do osteopaths test cranial nerves?
Osteopathy is a whole-body approach to health. Part of that means we need to be reasonably confident that what we are looking at is a musculoskeletal problem rather than something that needs a different kind of attention entirely.
Cranial nerve testing is one of our neurological screening tools. It helps us answer a straightforward but important question: is the nervous system doing what it should be doing? If the answer is broadly yes, we can proceed with confidence. If something looks unusual, that tells us something worth investigating further — and in some cases, worth referring on.
It is not about finding something sinister. The vast majority of the time, everything checks out perfectly well. But it would not be good practice to skip the check entirely.
What we are actually looking at
Here is a brief tour of the twelve cranial nerves and what we are assessing when we test them.
I — Olfactory nerve (sensory) Smell. In practice, formal olfactory testing is not always part of a routine osteopathic assessment unless there is a specific reason — such as a head injury, post-viral symptoms, or reports of altered smell. COVID-19 brought this nerve significant public attention.
II — Optic nerve (sensory) Vision. We check visual acuity (can you see clearly?), visual fields (can you see to the sides without moving your eyes?), and sometimes pupillary responses to light. Changes here can indicate problems ranging from local eye issues to significant neurological events.
III, IV, VI — Oculomotor, Trochlear, Abducens (motor) These three nerves work together to move your eyes. We test them by asking you to follow a moving target — usually a finger — in an H-pattern. What we are looking for is smooth, coordinated tracking. If one eye lags, drifts, or if you report double vision at any point, that is useful clinical information. We are also checking for nystagmus — an involuntary rhythmic eye movement that can suggest problems with the vestibular system or cerebellum.
V — Trigeminal nerve (mixed) The trigeminal is the main sensory nerve of the face and also controls the muscles of chewing. We test sensation across the three divisions of the face — forehead, cheek, and chin — and check the corneal reflex and jaw muscles. Trigeminal neuralgia is one of the most painful conditions known to medicine, so this nerve tends to get people’s attention when things go wrong.
VII — Facial nerve (mixed) This nerve controls the muscles of facial expression and carries taste from the front of the tongue. We look at symmetry of the face — can you raise both eyebrows equally? Close both eyes tightly? Smile evenly? Asymmetry here can indicate a peripheral facial nerve palsy (such as Bell’s palsy) or, if accompanied by other signs, something more central.
VIII — Vestibulocochlear nerve (sensory) Hearing and balance. Clinically, we assess hearing roughly — can you hear a whispered number from a short distance? We might also use simple bedside tests like the Rinne and Weber tests if hearing loss is reported, to help distinguish between conductive and sensorineural causes. Balance and vestibular function connect closely to this nerve and to the cerebellum.
IX and X — Glossopharyngeal and Vagus (mixed) These two are often tested together. We check the gag reflex, observe palatal movement (does the soft palate rise symmetrically when you say “aah”?), and listen to the voice — hoarseness can indicate vagal involvement. The vagus nerve in particular has an enormous range of influence, from cardiovascular function to gut motility to breathing.
XI — Accessory nerve (motor) This one is close to home for osteopaths. The accessory nerve innervates the sternocleidomastoid and trapezius muscles — muscles we work with regularly. We test it by asking you to turn your head against resistance and shrug your shoulders. Weakness here can follow certain neck injuries or surgeries.
XII — Hypoglossal nerve (motor) Controls tongue movement. We simply ask you to stick your tongue out and observe whether it points straight or deviates to one side. If it deviates, it points toward the side of the lesion. Simple test, useful information.
What we are looking for overall
We are looking for symmetry, smoothness, and appropriate responses. One side behaving differently from the other is usually the first thing that catches our attention. A single isolated finding might mean very little. A cluster of findings affecting neighbouring cranial nerves starts to build a more specific picture — one that can help identify roughly where in the nervous system something might be happening.
It is worth noting that cranial nerve findings do not tell us the diagnosis. They tell us the location. A III, IV, and VI deficit together suggests something near the cavernous sinus. A VII and VIII deficit together suggests something near the cerebellopontine angle. The neurology points the map; further investigation fills in the detail.
When we refer
Any new, unexplained cranial nerve deficit warrants prompt medical review. This includes sudden visual changes, new facial asymmetry, unexplained swallowing difficulties, or significant hearing loss that has not been explained. Osteopaths are not the last stop in these situations — we are part of a wider healthcare network, and knowing when to pass the baton is just as important as knowing what to do with it.
A note for CPD purposes
For colleagues reading this — cranial nerve testing sits comfortably within scope of practice for osteopaths as part of a neurological screen. The key principles are documentation, appropriate clinical reasoning, and a clear referral pathway when findings fall outside the musculoskeletal remit. The Mast Cell Activation Syndrome and post-viral presentations of recent years have brought cranial nerve involvement — particularly vagal and vestibulocochlear — back into the foreground of manual therapy practice. Worth keeping sharp.
A mnemonic, because nobody actually memorises twelve nerves without one
On Old Olympus Towering Tops A Finn And German Viewed Some Hops.
The first letter of each word corresponds to each nerve: Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Auditory (vestibulocochlear), Glossopharyngeal, Vagus, Spinal accessory, Hypoglossal.
You are welcome.
Edward Bateman is a British-trained osteopath based in Gdańsk, Poland, offering English-language consultations in osteopathy, medical acupuncture, Pilates, and somatic movement therapy. If you have questions about your neurological health or would like to book an assessment, get in touch via the contact page.