Overview
This lecture explains how to systematically assess each cranial nerve during a neurologic examination, emphasizing key signs, techniques, and diagnostic clues.
General Principles of Cranial Nerve Assessment
- Cranial nerves originate in the brainstem; dysfunction suggests pathology in the brainstem or nerve pathway.
- Specific cranial nerve signs can help localize disease at the base of the skull.
Cranial Nerve I: Olfactory
- Test smell only if head trauma, anterior fossa lesion, or reported smell/taste issues.
- Use identifiable odors for each nostril; avoid irritants unless malingering is suspected.
Cranial Nerve II: Optic
- Assess visual acuity in each eye separately using a Snellen or handheld chart.
- Test color perception with Ishihara or Hardy-Rand-Ritter plates.
- Check visual fields in all four quadrants by confrontation.
- Test pupillary responses and perform a funduscopic exam.
Cranial Nerves III, IV, VI: Oculomotor, Trochlear, Abducens
- Observe eye movement symmetry, lid droop (ptosis), and abnormal movements.
- Test extraocular movements in all directions; detect nystagmus and muscle palsies.
- Check pupils for anisocoria and light response.
Cranial Nerve V: Trigeminal
- Test facial sensation in ophthalmic, maxillary, and mandibular regions using pinprick and corneal reflex with cotton.
- Assess motor function by palpating masseter while clenching teeth and jaw deviation when opening.
Cranial Nerve VII: Facial
- Assess for hemifacial weakness during spontaneous movement and deliberate actions.
- Distinguish central vs. peripheral causes by forehead and eye muscle involvement.
- Test taste on anterior two-thirds of tongue and check for hyperacusis with tuning fork.
Cranial Nerve VIII: Vestibulocochlear
- Test hearing in each ear using whispered voice; confirm abnormality with audiologic tests.
- Use Weber and Rinne tests for hearing loss differentiation if needed.
- Evaluate vestibular function through nystagmus testing.
Vertigo and Nystagmus Evaluation
- No central causes of unilateral hearing loss; CNS signs with vertigo indicate central pathology.
- Use nystagmus characteristics and maneuvers (Frenzel lenses, head thrust, Dix-Hallpike) to differentiate central vs. peripheral vertigo.
- BPPV: Dix-Hallpike maneuver positive for latency, fatigable nystagmus; Epley maneuver may resolve symptoms.
Cranial Nerves IX, X: Glossopharyngeal, Vagus
- Observe palatal elevation and uvula movement during "ah" and test gag reflex for symmetry.
- Hoarseness with normal gag/palate warrants search for compressive lesions.
Cranial Nerve XI: Spinal Accessory
- Test sternocleidomastoid by head turn against resistance; test trapezius by shoulder shrug against resistance.
Cranial Nerve XII: Hypoglossal
- Ask patient to stick out tongue; inspect for atrophy, fasciculations, or deviation toward lesion side.
Key Terms & Definitions
- Nystagmus — Involuntary, rhythmic eye movement with slow and quick phases.
- Anosmia — Loss of smell.
- Anisocoria — Unequal pupil sizes.
- Ptosis — Drooping of the upper eyelid.
- Hemifacial weakness — Weakness on one side of the face.
- BPPV — Benign paroxysmal positional vertigo, a peripheral cause of vertigo.
Action Items / Next Steps
- Practice maneuvers for cranial nerve exams.
- Review relevant figures for the Epley and Dix-Hallpike maneuvers.
- Prepare to differentiate central vs. peripheral vertigo in clinical cases.