The Cranial Nerve Examination

Twelve pairs of cranial nerves emerge from the brain and brainstem to wire the head and neck for sensation, movement, and the special senses. A disciplined cranial nerve examination is one of the most powerful localizing tools in clinical neurology: because each nerve has a known origin, course, and target, a single abnormal finding can place a lesion within millimeters along the neuraxis. The art lies not in memorizing isolated facts but in reasoning from the bedside finding back to the anatomy. This page walks nerve by nerve, from olfactory to hypoglossal, pairing how you test each one with the lesions it reveals.

Before the details, an orienting principle: the brainstem is organized so that nerves III and IV arise from the midbrain, V through VIII from the pons, and IX through XII from the medulla. Knowing roughly where a nerve is born tells you which neighboring structures a lesion is likely to involve. Always examine systematically and compare side to side.

The Classic Mnemonics

Generations of medical students have anchored the twelve names with the same verse. The order of the nerves:

• "On Old Olympus' Towering Tops, A Finn And German Viewed Some Hops" — Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Auditory (vestibulocochlear), Glossopharyngeal, Vagus, (spinal) Accessory, Hypoglossal.
• For whether each nerve is Sensory, Motor, or Both: "Some Say Marry Money But My Brother Says Big Brains Matter More" — S, S, M, M, B, M, B, S, B, B, M, M. So CN I and II are purely sensory; III, IV, VI, XI, XII are purely motor; and V, VII, IX, X carry mixed functions.

CN I — Olfactory (Smell)

The most frequently skipped nerve, and the one most worth testing when the history suggests it. Occlude one nostril and ask the patient to identify a familiar, non-irritant odor (coffee, vanilla, peppermint) with the other nostril, then repeat on the opposite side. Avoid irritants such as ammonia or alcohol — these stimulate the trigeminal nerve, not the olfactory pathway, and can mask a true anosmia.

• Localizing value: Unilateral or bilateral anosmia points to disease of the olfactory epithelium, bulb, or tracts.
• Classic causes: Head trauma (shearing of olfactory filaments as they pass through the cribriform plate), subfrontal masses such as an olfactory groove meningioma, and neurodegenerative disease — olfactory loss can be an early feature of Parkinson disease and Alzheimer disease.

CN II — Optic (Vision)

The optic nerve is examined in four parts: visual acuity (each eye separately, with the patient's correction), visual fields by confrontation, fundoscopy, and the afferent limb of the pupillary light reflex.

• Fundoscopy reveals the optic disc directly: swelling (disc edema, including papilledema from raised intracranial pressure) or pallor (suggesting prior optic atrophy).
• Relative afferent pupillary defect (RAPD), or Marcus Gunn pupil: on the swinging-flashlight test, light swung to the affected eye produces paradoxical pupillary dilation of both pupils because that eye's afferent signal is weaker. An RAPD indicates optic nerve or extensive retinal disease (for example, optic neuritis) and is one of the most reliable signs of asymmetric anterior visual pathway disease.

Visual field defects localize beautifully along the visual pathway:

• Monocular loss (one eye only) = lesion anterior to the chiasm — the retina or optic nerve.
• Bitemporal hemianopia (loss of both temporal fields) = lesion at the optic chiasm, classically a pituitary adenoma compressing the crossing nasal fibers.
• Homonymous defects (same-sided field loss in both eyes) = retrochiasmal lesion of the optic tract, radiations, or occipital cortex, contralateral to the field loss.

CN III, IV, VI — The Ocular Motor Nerves

These three are tested together by tracing the "H" pattern of conjugate gaze, watching for restricted movement, diplopia, and nystagmus. Remember the simple rule for the two outliers: the lateral rectus (abducts the eye) is CN VI and the superior oblique is CN IV; CN III supplies everything else, plus the levator (lid) and the pupillary constrictor. Assess the pupils and any anisocoria here — the light reflex has its afferent limb in CN II and its efferent (constrictor) limb in CN III.

• CN III (Oculomotor) palsy: ptosis and an eye resting "down and out," often with a dilated ("blown"), poorly reactive pupil. Because the pupilloconstrictor fibers run on the outer surface of the nerve, a compressive lesion (such as a posterior communicating artery aneurysm or uncal herniation) typically involves the pupil early. By contrast, ischemic/diabetic third nerve palsies, which infarct the central nerve, are often pupil-sparing. A painful, pupil-involving third nerve palsy is a neurosurgical emergency until an aneurysm is excluded.
• CN IV (Trochlear) palsy: the superior oblique normally depresses and intorts the adducted eye, so a palsy causes vertical diplopia worse on downgaze (descending stairs, reading). Patients adopt a compensatory head tilt away from the affected side to fuse the images. CN IV is the only cranial nerve to exit dorsally from the brainstem and the most slender, making it vulnerable to head trauma.
• CN VI (Abducens) palsy: impaired abduction with horizontal diplopia that is worse on gaze toward the lesion. Its long intracranial course makes CN VI especially vulnerable to raised intracranial pressure, producing a classic false-localizing sign — an isolated sixth nerve palsy that does not indicate where the actual pathology lies.

CN V — Trigeminal (Facial Sensation and Mastication)

The trigeminal nerve is the great sensory nerve of the face, divided into three territories: V1 (ophthalmic), V2 (maxillary), and V3 (mandibular). Test light touch and pinprick in each division and compare sides.

• Motor function (carried in V3) supplies the muscles of mastication. Palpate the masseter and temporalis as the patient clenches, and watch jaw opening: with a unilateral motor lesion the jaw deviates toward the weak side (the intact pterygoid pushes it across).
• Corneal reflex: lightly touch the cornea with a wisp of cotton — a normal response is a bilateral blink. The afferent limb is V1 and the efferent (blink) limb is CN VII, so testing it probes two nerves at once.
• Clinical note: trigeminal neuralgia produces brief, lancinating pain in one or more divisions, often triggered by light touch, chewing, or a cold breeze.

CN VII — Facial (Facial Movement, Taste, Hearing Modulation)

Test the facial nerve by asking the patient to raise the eyebrows, close the eyes tightly against resistance, smile/show teeth, and puff out the cheeks. The single most important distinction in all of cranial nerve examination is the pattern of facial weakness:

• Upper motor neuron (central) facial weakness — as in a hemispheric stroke — spares the forehead. The upper face (frontalis, orbicularis oculi) receives bilateral corticobulbar innervation, so a one-sided cortical lesion leaves forehead wrinkling and eye closure relatively intact while the lower face droops.
• Lower motor neuron (peripheral) facial weakness — as in Bell's palsy — weakens the entire hemiface, including the forehead, on the affected side, because the lesion is at or distal to the facial nucleus where upper and lower face fibers travel together.

CN VII also carries taste from the anterior two-thirds of the tongue (via the chorda tympani) and innervates the stapedius muscle, so a proximal lesion may produce hyperacusis (sounds seem uncomfortably loud) and altered taste alongside the facial weakness.

CN VIII — Vestibulocochlear (Hearing and Balance)

The eighth nerve has two divisions: cochlear (hearing) and vestibular (balance). Screen hearing by rubbing fingers or whispering near each ear, then characterize any loss with a 512 Hz tuning fork:

• Weber test: place the vibrating fork on the vertex. Sound lateralizes to the affected ear in conductive loss and to the normal ear in sensorineural loss.
• Rinne test: compare air conduction (fork by the ear canal) to bone conduction (fork on the mastoid). Normally air conduction is louder and lasts longer than bone conduction (a positive Rinne). When bone conduction is heard as well as or better than air conduction (a negative Rinne), a conductive hearing loss is present.
• Vestibular function is assessed by observing for nystagmus, testing gait and balance, and — when indicated — the head impulse test and positional maneuvers.

CN IX, X — Glossopharyngeal and Vagus (Palate, Swallow, Voice)

These two are examined together because they share the pharyngeal plexus. Ask the patient to open wide and say "aah," watching the soft palate and uvula:

• Palate elevation: with a unilateral vagal lesion the weak side fails to rise, so the uvula deviates away from the weak side, pulled toward the intact musculature.
• Gag reflex: touching the posterior pharynx triggers a gag — the afferent limb is CN IX and the efferent limb is CN X.
• Vagus (X) also governs the voice and swallow; a lesion may produce hoarseness (recurrent laryngeal involvement) and dysphagia. Listen to speech quality and ask about choking on liquids.

CN XI — Accessory (Neck and Shoulder)

The spinal accessory nerve supplies two muscles. Test the sternocleidomastoid by having the patient turn the head against your hand — note that the right SCM turns the head to the left (each muscle turns the head to the opposite side). Test the trapezius with a shoulder shrug against resistance. Weakness and atrophy of these muscles, with a drooping shoulder, suggest an accessory nerve lesion, often in the posterior triangle of the neck.

CN XII — Hypoglossal (Tongue)

Inspect the tongue at rest for atrophy and fasciculations, then ask the patient to protrude it straight out. The genioglossus pushes the tongue forward and to the opposite side, so with a lower motor neuron lesion the tongue deviates toward the side of the lesion (the weak side cannot push, and the intact side wins), accompanied by ipsilateral wasting and fasciculations. ("The tongue points to the lesion.")

Summary Table: The Twelve Cranial Nerves

Bringing It Together

The cranial nerve examination rewards pattern recognition. A few combinations recur often enough to memorize: an RAPD with reduced acuity points to the optic nerve; a "down-and-out" eye with a dilated pupil demands urgent exclusion of a compressive third nerve lesion; forehead sparing separates a central from a peripheral facial palsy; the uvula deviates away from a vagal lesion while the tongue deviates toward a hypoglossal one. When several adjacent nerves fail together, think of a single lesion at their shared anatomic crossroads — a cavernous sinus, a cerebellopontine angle, or a brainstem segment — and let the anatomy guide the imaging.