Twelve pairs of nerves, known as cranial nerves, emerge directly from the brain and brainstem. These specialized nerves control most of the motor and sensory functions of the head and neck. A cranial nerve examination is a fundamental part of a neurological assessment, designed to systematically check the integrity of these pathways. Health professionals evaluate neurological function by observing specific responses to identify the location of any potential issues.
Examining the Special Senses CN I and CN II
The first two cranial nerves are dedicated entirely to the special senses of smell and sight. To test the Olfactory nerve (CN I), the patient closes their eyes and occludes one nostril. They are then presented with a familiar, non-irritating scent, such as coffee or soap, and asked to identify it. This is repeated for the other nostril to check for symmetry.
The Optic nerve (CN II) assessment involves three main components related to vision. Visual acuity is measured using a Snellen chart, where the patient reads progressively smaller lines of letters from a distance. Visual fields are checked next using the confrontation method, comparing peripheral vision in four quadrants. Finally, a fundoscopic examination visualizes the back of the eye, checking the optic disc where the nerve fibers exit the retina.
Assessing Eye Movement CN III IV and CN VI
Three separate nerves, the Oculomotor (CN III), Trochlear (CN IV), and Abducens (CN VI), work together to control the precise movements of the eyes. These nerves are tested simultaneously by evaluating the six cardinal fields of gaze. The examiner moves a target in a distinct “H” pattern, asking the patient to follow it with their eyes only while keeping their head still.
The Oculomotor nerve (CN III) is responsible for raising the eyelid and moving the eye in most directions, including upward, downward, and inward. It also carries fibers that control the constriction of the pupil. Dysfunction can cause the eye to deviate downward and outward, alongside a droopy eyelid (ptosis).
The Trochlear nerve (CN IV) supplies the superior oblique muscle, which is responsible for downward and inward rotation of the eyeball. The Abducens nerve (CN VI) controls the lateral rectus muscle, which moves the eye outward, away from the nose. Failure of CN VI results in the eye being pulled inward, as the medial rectus muscle acts without opposition.
CN III function is also tested by assessing pupillary responses. The pupillary light reflex involves shining a light into one eye, which should cause both pupils to constrict (direct and consensual responses). The accommodation reflex is checked by having the patient shift focus from a distant object to a close object, causing the eyes to converge and the pupils to constrict slightly.
Evaluating Facial Function CN V and CN VII
The Trigeminal nerve (CN V) and the Facial nerve (CN VII) manage the motor and sensory functions of the face. The Trigeminal nerve’s motor function controls the muscles used for chewing (mastication). Motor strength is tested by asking the patient to clench their teeth tightly while the examiner palpates the masseter and temporalis muscles. The patient is also asked to open their mouth against resistance to test the strength of the pterygoid muscles.
The sensory component of CN V provides sensation to the entire face through three divisions: ophthalmic, maxillary, and mandibular. The examiner tests light touch and sharp sensation across these three areas on both sides of the face, comparing the patient’s perception. The corneal reflex is a specific test where a wisp of cotton is gently touched to the edge of the cornea. This should elicit a blink response, relying on CN V for sensation and CN VII for the motor response.
The Facial nerve (CN VII) is primarily motor, controlling all the muscles of facial expression. Its function is evaluated by asking the patient to perform several symmetrical movements. These actions include wrinkling the forehead by raising the eyebrows, closing the eyes tightly against resistance, smiling, and puffing out the cheeks. The examiner looks for any asymmetry or weakness during these expressions.
Testing Pharyngeal and Auditory Function CN VIII IX and CN X
CN VIII, CN IX, and CN X are involved in functions related to hearing, balance, and the throat. CN VIII has two branches: the cochlear branch for hearing and the vestibular branch for balance. Hearing is screened by the whispered voice test, where the examiner whispers into one ear while occluding the other. Balance function is assessed by observing the patient’s gait and performing the Romberg test, where the patient stands still with feet together and eyes closed.
The Glossopharyngeal nerve (CN IX) and the Vagus nerve (CN X) are often tested together due to their shared control over the pharynx and larynx. Voice quality is assessed by noting any hoarseness or nasal tone, which suggests vocal cord weakness from CN X dysfunction. Deficits in these nerves can also lead to difficulties in swallowing and managing secretions.
To check the muscular control of the throat, the patient is asked to open their mouth and say “Ah.” The examiner observes the soft palate and uvula, which should rise symmetrically and move upward in the midline. The gag reflex tests the sensory component of CN IX and the motor component of CN X, and is typically reserved for cases where impairment is suspected.
Checking Motor Control CN XI and CN XII
The final two cranial nerves are purely motor, controlling muscles in the neck, shoulder, and tongue. The Accessory nerve (CN XI) supplies motor function to the sternocleidomastoid and trapezius muscles. The strength of the sternocleidomastoid muscle is tested by asking the patient to turn their head against the resistance of the examiner’s hand. The Trapezius muscle is evaluated by asking the patient to shrug their shoulders against the downward resistance applied by the examiner.
Both tests are performed bilaterally to compare strength and symmetry, as weakness indicates a potential issue with CN XI function. The Hypoglossal nerve (CN XII) controls the intrinsic and extrinsic muscles of the tongue. This is tested by asking the patient to protrude their tongue straight out, looking for any deviation. Tongue strength is also assessed by asking the patient to press the tip of their tongue firmly against the inside of their cheek while the examiner provides external resistance.