A cranial nerve exam tests all 12 pairs of nerves that emerge directly from the brain, each responsible for a specific function like smell, vision, facial movement, or swallowing. The exam follows a systematic sequence from CN I through CN XII, using simple bedside tools and specific maneuvers to isolate each nerve’s function. Here’s how each one is tested.
Equipment You’ll Need
A full cranial nerve exam requires surprisingly low-tech tools. The standard checklist includes a pen flashlight, a Snellen chart (distance) and near-vision card, a tuning fork, a cotton ball, sterile cotton swabs, a reflex hammer, a tongue depressor, two pencils, and a mechanical ticking watch or timer. You’ll also need substances for smell and taste testing: vials of coffee, vanilla extract, and lemon juice, plus sugar and salt packets or sterile solutions with a dropper. Two test tubes (for warm and cold water) round out the kit.
CN I: Smell
The olfactory nerve carries smell signals from the nose to the brain. Test it one nostril at a time by having the patient close their eyes and block the opposite nostril. Hold a familiar, non-irritating substance under the open nostril: coffee, vanilla, or orange peel all work well. Avoid anything with a sharp chemical sting like ammonia, which stimulates pain fibers rather than true smell receptors. Detection matters more than identification. If the patient can tell something is there, even if they can’t name it, the nerve is likely functioning.
CN II: Vision
The optic nerve is tested through three components: visual acuity, visual fields, and pupil responses.
Visual Acuity
Use a Snellen chart at 20 feet (or a near-vision card at reading distance) and test each eye separately. The patient covers one eye and reads the smallest line they can. This gives a baseline for optic nerve function.
Visual Fields by Confrontation
Sit directly across from the patient at arm’s length. Have them cover one eye and look straight at your nose. Close your own corresponding eye so you can use your own visual field as a reference. Hold up fingers in each of the four quadrants of their visual field (upper left, upper right, lower left, lower right) and ask the patient to count them. Test each eye separately. A deficit in one quadrant can point to damage at a specific location along the visual pathway.
Pupil Responses and the Swinging Flashlight Test
Shine a penlight into one eye and watch both pupils constrict. Then swing the light to the other eye. Normally, both pupils stay constricted as the light moves back and forth. If one pupil dilates when the light swings to it, instead of staying constricted, that’s a relative afferent pupillary defect, sometimes called a Marcus Gunn pupil. It signals a problem with the optic nerve or retina on that side. In mild cases the pupil constricts briefly before widening. In severe cases it dilates immediately as the light arrives.
CN III, IV, and VI: Eye Movements
Three nerves control the muscles that move each eye. CN III (oculomotor) handles most eye movements plus eyelid elevation and pupil constriction. CN IV (trochlear) controls the muscle that rotates the eye downward and inward. CN VI (abducens) controls the muscle that moves the eye outward.
Test all three together by asking the patient to follow your finger or a penlight through an H-shaped pattern. Start at center, move horizontally to one side, then trace up and down, return to center, move horizontally to the other side, and trace up and down again. This isolates each muscle in sequence. Watch for smooth, symmetrical tracking. Any lag, double vision, or inability to move in a direction localizes to a specific nerve.
While you’re here, check the eyelids for drooping (ptosis), which can indicate a CN III problem, and observe for any involuntary rhythmic eye movements (nystagmus).
CN V: Facial Sensation and Jaw Strength
The trigeminal nerve has three sensory branches covering different zones of the face, plus a motor branch that powers the chewing muscles. The three sensory zones are: the forehead and upper face (ophthalmic division), the cheek and upper jaw area (maxillary division), and the lower jaw and chin (mandibular division).
Test light touch by dabbing a cotton wisp on each zone, comparing left to right. Ask the patient if it feels the same on both sides. For sharp sensation, use the pointed and blunt ends of a broken cotton swab or a safety pin, alternating randomly, and ask the patient to distinguish “sharp” from “dull” with eyes closed. Test all three divisions on both sides.
For motor function, ask the patient to clench their jaw and feel the masseter and temporalis muscles contract on each side. They should feel firm and symmetrical. Then have them open their mouth against resistance. If the motor branch is damaged on one side, the jaw will deviate toward the weak side when opening.
The Corneal Reflex
This tests the sensory limb of CN V and the motor limb of CN VII together. Lightly touch the cornea (not just the white of the eye) with a fine wisp of cotton, approaching from the side so the patient doesn’t blink from seeing it coming. A normal response is a brisk blink in both eyes.
CN VII: Facial Movement
The facial nerve controls the muscles of facial expression. Test it by asking the patient to perform a series of movements and watching for symmetry:
- Raise your eyebrows (or look up at the ceiling): tests the forehead muscles
- Close your eyes tightly: you should not be able to pry them open easily
- Smile or show your teeth: watch for one side of the mouth lagging
- Puff out your cheeks: press gently on each side to check if air escapes
The distinction between upper and lower face weakness matters. The forehead receives nerve input from both sides of the brain, so a stroke (upper motor neuron lesion) typically spares the forehead while weakening the lower face. Damage to the facial nerve itself, as in Bell’s palsy, affects the entire half of the face including the forehead.
CN VIII: Hearing and Balance
The vestibulocochlear nerve carries both hearing and balance signals. Screen hearing by rubbing your fingers together next to each ear, or by holding a ticking watch at arm’s length and slowly bringing it closer until the patient hears it. Test each ear separately.
If hearing seems reduced, two tuning fork tests help distinguish whether the problem is in the outer/middle ear (conductive) or the inner ear and nerve (sensorineural). For the Rinne test, strike the tuning fork and place its base on the bone behind the ear (mastoid process), then hold it next to the ear canal. Normally, air conduction sounds louder than bone conduction. For the Weber test, place the vibrating fork on the top center of the head. Normally the sound is heard equally on both sides. If it lateralizes to one side, the pattern helps identify the type of hearing loss.
CN IX and X: Swallowing and Palate
The glossopharyngeal (IX) and vagus (X) nerves are tested together because they share responsibility for throat sensation, swallowing, and voice.
Start by asking the patient to say “ahh” while you watch the soft palate with a light. It should rise symmetrically, and the uvula should stay midline. If one side is weak, the palate gets pulled toward the stronger (unaffected) side and the uvula deviates away from the damaged nerve. Listen to the patient’s voice as well. Hoarseness or a nasal quality can indicate vagus nerve dysfunction.
The gag reflex provides additional information. Gently touch one palatal arch with a cotton swab or tongue blade, then the other, waiting for a gag response each time. However, this reflex is absent in some healthy people and present in others, so its absence alone doesn’t confirm nerve damage. The soft palate elevation test is more reliable.
CN XI: Shoulder and Neck Strength
The spinal accessory nerve powers two muscles: the trapezius (which shrugs the shoulders and stabilizes the shoulder blade) and the sternocleidomastoid (which turns the head). Test the trapezius by asking the patient to shrug both shoulders upward while you press down on them. Compare strength on each side. Test the sternocleidomastoid by placing your hand against one side of the patient’s jaw and asking them to turn their head toward your hand against resistance. The sternocleidomastoid on the opposite side is doing the work, so turning the head to the right tests the left muscle, and vice versa.
CN XII: Tongue Movement
The hypoglossal nerve controls the tongue muscles. Ask the patient to stick out their tongue and observe whether it protrudes straight or deviates to one side. If the nerve is damaged, the tongue deviates toward the weak side, because the functioning muscle on the healthy side pushes it over. Also ask the patient to push their tongue into each cheek while you press against it from outside, testing strength on both sides. Look at the tongue at rest inside the mouth for any wasting or involuntary twitching (fasciculations) on one side, which can indicate nerve or lower motor neuron damage.
Red Flags During the Exam
Certain findings during a cranial nerve exam warrant urgent investigation. Sudden onset of double vision can signal anything from a nerve compression to an aneurysm. A new drooping eyelid (ptosis) paired with a severe, sudden headache on the same side raises concern for an intracranial aneurysm pressing on CN III. Headache with vision loss, especially if the optic disc appears swollen, suggests dangerously elevated pressure inside the skull. Other warning signs include bulging of one eye (proptosis) associated with pain or vision loss, and any combination of cranial nerve deficits with vomiting, seizures, or changes in alertness or mental state.