Clinical meaning
The twelve cranial nerves emerge from the brainstem (except CN I and II from the cerebrum) and serve sensory, motor, or mixed functions. CN I (olfactory) passes through the cribriform plate; fractures here cause anosmia and CSF rhinorrhea. CN II (optic) transmits visual information via the optic chiasm where nasal fibers decussate; lesions produce characteristic visual field defects (bitemporal hemianopia from chiasmal compression). CN III (oculomotor) carries parasympathetic fibers on its surface, making pupil dilation the earliest sign of compression (uncal herniation). The corneal reflex tests CN V (afferent) and CN VII (efferent). CN VII upper vs lower motor neuron distinction is critical: central lesions spare the forehead (bilateral cortical innervation), peripheral lesions (Bell palsy) affect the entire half of face.
Diagnosis & workup
Diagnostics & workup: - CN I: test each nostril separately with non-irritating substances (coffee, vanilla); anosmia may indicate frontal lobe lesion or cribriform plate fracture - CN II: visual acuity (Snellen chart), visual fields by confrontation, pupillary light reflex (afferent limb), fundoscopy (papilledema, optic atrophy) - CN III/IV/VI: extraocular movements in H pattern; CN III palsy = 'down and out' eye with ptosis and mydriasis; CN IV = difficulty looking down and inward; CN VI = inability to abduct eye - CN V: facial sensation (V1 forehead, V2 cheek, V3 jaw), corneal reflex (afferent V1), jaw clench (motor V3) - CN VII: facial symmetry, forehead wrinkling, eye closure, smile; distinguish UMN (forehead spared) from LMN (entire face) - CN VIII: Weber (lateralizes to conductive loss side), Rinne (air>bone = normal or sensorineural; bone>air = conductive) - CN IX/X: gag reflex, palate elevation (uvula deviates AWAY from lesion), voice quality - CN XI: sternocleidomastoid (turns head to OPPOSITE side), trapezius (shoulder shrug) - CN XII: tongue protrusion (deviates TOWARD lesion side), fasciculations (LMN lesion)