Cranial Nerves

A patient's pupil is fixed and dilated — but which cranial nerve is involved, and how do you systematically test all twelve before things escalate? The assessment technique matters as much as the finding.

Cranial nerve (CN) assessment is a systematic bedside evaluation of twelve paired nerves, each tested with a specific technique. CN I (olfactory): have the patient identify a familiar scent with eyes closed, one nostril at a time. CN II (optic): test visual acuity and visual fields by confrontation. CN III, IV, VI (oculomotor, trochlear, abducens): assess together using the six cardinal fields of gaze and PERRLA — pupils equal, round, reactive to light and accommodation. CN V (trigeminal): test facial sensation in three branches (forehead, cheek, jaw) with light touch and assess jaw clench strength. CN VII (facial): ask the patient to smile, frown, raise eyebrows, and puff cheeks — asymmetry is the key finding. CN VIII (acoustic/vestibulocochlear): whisper test or finger rub at 1–2 feet from the ear; Weber and Rinne if hearing loss suspected. CN IX and X (glossopharyngeal, vagus): test together — listen for hoarseness, check gag reflex, observe uvula midline rise when the patient says "ah." CN XI (spinal accessory): have the patient shrug shoulders and turn the head against resistance. CN XII (hypoglossal): ask the patient to stick out the tongue — it deviates toward the weak side. Document bilaterally, comparing side to side, and always establish a baseline so changes are caught early.

Key Distinctions

Don't confuse CN VII (facial motor — smile, close eyes tightly) with CN V (trigeminal — facial sensation and jaw strength). Students mix up CN IX and X because they're tested together; IX is primarily sensory (taste, gag afferent), X is primarily motor (swallow, voice, gag efferent). CN III controls pupil constriction and most eye movements — a blown pupil implicates III, not II.

Clinical Pearl

Always test CN III first in any acute neuro change — a fixed, dilated pupil is an emergency suggesting uncal herniation. And remember: compare every CN finding bilaterally — a unilateral change is pathology until proven otherwise.

Cranial Nerve Deficits

Cranial nerve deficits are functional losses tied to specific nerves, and the clinical value lies in localizing damage. Upper motor neuron (UMN) lesions — like stroke — affect the lower face contralaterally because the upper face receives bilateral cortical input. Lower motor neuron (LMN) lesions — like Bell's palsy (CN VII) — paralyze the entire ipsilateral face, forehead included. This UMN vs. LMN distinction is among the highest-yield testable patterns. Key deficit patterns nurses must recognize: CN III (oculomotor) — fixed, dilated pupil with ptosis and eye deviated down and out, a red flag for uncal herniation. CN V (trigeminal) — loss of corneal reflex or facial sensation, placing the eye at injury risk. CN VII (facial) — inability to close the eye or manage oral secretions, creating aspiration and corneal drying risks. CN IX and X (glossopharyngeal/vagus) — absent gag reflex, dysphagia, hoarse voice; the patient cannot protect their airway. CN XII (hypoglossal) — tongue deviates toward the side of the lesion on protrusion, impairing speech and swallowing. Nursing priority is always safety: airway protection for lower CN deficits (IX, X, XII) and corneal protection for CN V or VII deficits.

Key Distinctions

UMN facial weakness (stroke) spares the forehead; LMN facial weakness (Bell's palsy) affects the entire half — students constantly reverse this. Don't confuse absent gag reflex (CN IX/X deficit) with generalized decreased LOC; a fully alert patient post-stroke can still lack a gag. Tongue deviation points toward the lesion side, not away — the weak side cannot push.

Clinical Pearl

Forehead spared = brain (UMN, stroke). Forehead involved = nerve (LMN, Bell's). This one distinction drives every facial weakness question on the exam.