Cranial Nerves Overview
Modalities · nuclei · brainstem exits · skull-base foramina · examination · localization
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Smell; receptor axons cross the cribriform plate to bulb, tract, and primary olfactory cortex.
Test each nostril with a recognizable non-irritating odor.
Anosmia: sinonasal disease, cribriform trauma, olfactory-groove lesion, or neurodegeneration.
Retina → nerve → chiasm → tract → LGN → radiations → occipital cortex; afferent light-reflex limb.
Acuity, color, fields, pupils/RAPD, and fundus.
Chiasm: bitemporal loss. Retrochiasmal: homonymous defect.
Levator, SR, IR, MR, IO; Edinger–Westphal parasympathetics constrict pupil and accommodate.
Ptosis, pupils, ductions, accommodation.
Complete palsy: ptosis, “down and out,” ± mydriasis. Painful pupil involvement requires urgent aneurysm evaluation.
Superior oblique: depresses the adducted eye and intorts. Exits dorsally and decussates.
Depression in adduction; vertical/torsional diplopia and head tilt.
Nuclear lesion affects contralateral SO; nerve lesion affects ipsilateral SO.
V1/V2/V3 facial sensation; V3 motor to mastication and related muscles. V1 is corneal-reflex afferent limb.
V1–V3 sensation, masseter/temporalis, jaw opening, corneal reflex when indicated.
Meckel cave, cavernous sinus, peripheral divisions, or brainstem; jaw deviates toward weak V3 side.
Lateral rectus abduction; nucleus also drives horizontal conjugate gaze.
Abduction and horizontal diplopia by gaze direction.
Nerve palsy causes ipsilateral abduction deficit; nuclear lesion causes ipsilateral horizontal-gaze palsy.
Facial expression, stapedius; anterior 2/3 taste; lacrimal and submandibular/sublingual secretion. Corneal-reflex efferent limb.
Brow raise, eye closure, smile, cheek puff.
Peripheral palsy affects forehead and lower face; supranuclear palsy usually spares forehead.
Cochlear hearing and vestibular head-motion, position, and balance signals; travels with VII in the IAC.
Hearing, Weber/Rinne, nystagmus, head impulse, gait, positional symptoms.
Combined VII/VIII findings suggest CPA or IAC pathology.
Posterior 1/3 taste/sensation, pharynx, carotid sinus/body; stylopharyngeus; parotid secretion. Gag afferent limb.
Voice, swallow, palatal/pharyngeal sensation selectively.
Isolated IX palsy is uncommon; IX–XI combination suggests jugular-foramen disease.
Motor to palate/pharynx/larynx; visceral sensory and parasympathetic supply to thoracoabdominal viscera. Gag efferent limb.
Voice, swallow/aspiration, palatal elevation, cough; laryngoscopy when needed.
Palate droops and uvula deviates away; consider medulla, jugular foramen, carotid space, or recurrent laryngeal course.
Upper cervical spinal roots ascend via foramen magnum and exit the jugular foramen to SCM and trapezius.
Shoulder shrug and head rotation against resistance.
Weak shrug/shoulder droop; weak SCM impairs turning toward the opposite side. Vulnerable in posterior-triangle surgery.
Intrinsic and extrinsic tongue muscles except palatoglossus; exits hypoglossal canal.
Atrophy, fasciculations, protrusion, movement, articulation, and oral-phase swallowing.
LMN lesion: tongue deviates toward lesion with ipsilateral atrophy/fasciculations.
Multiple ipsilateral cranial neuropathies with contralateral long-tract findings favor an intra-axial brainstem process; grouped deficits without long-tract signs often localize to a cistern, skull-base foramen, cavernous sinus, or extracranial space.