Cranial Nerves

What You'll Learn

All 12 cranial nerves — type (sensory, motor, both), skull exit foramen, and key functions
Visual pathway from retina to cortex and visual field deficits by lesion location
CN III palsy — pupil-involving (compressive/aneurysm) vs. pupil-sparing (ischemic)
CN IV unique features — dorsal exit, full decussation, longest intradural/cisternal course (vs. CN VI longest subarachnoid course), Bielschowsky test
CN V divisions, trigeminal neuralgia, corneal reflex arc (afferent V1, efferent VII)
CN VII — UMN vs. LMN facial weakness, Bell palsy vs. Ramsay Hunt, nerve segment localization
CN VIII — Weber/Rinne interpretation, peripheral vs. central vestibular nystagmus
Tongue deviation rules (LMN → toward lesion; UMN → away from cortical lesion)
Cranial nerve syndromes — cavernous sinus, superior orbital fissure, orbital apex, CPA, jugular foramen
Brainstem cranial nerve fascicular syndromes (Weber, Benedikt, Millard-Gubler, Wallenberg)

HighYield Pearls

Pupil-involving CN III palsy: “down & out” eye + ptosis + dilated unreactive pupil → PCom aneurysm or uncal herniation until proven otherwise → emergent CTA/MRA. A complete, isolated, truly pupil-sparing CN III palsy in an older vasculopathic patient is usually microvascular (DM/HTN) and often resolves in weeks — but acute CN III palsy still often warrants CTA/MRA or MRI/MRA (partial palsies, relative pupil sparing, pain, age <50, or nonisolated findings should be imaged).
UMN vs. LMN facial weakness: UMN (stroke) spares forehead (bilateral cortical input to upper face); LMN (Bell palsy) involves entire ipsilateral face including forehead ± hyperacusis, dysgeusia, dry eye.
Ramsay Hunt syndrome: VZV reactivation → LMN CN VII palsy + vesicles in ear canal/auricle/palate ± CN VIII involvement → treat with antivirals + steroids; worse prognosis than Bell palsy.
Bilateral facial palsy: NOT Bell → think Lyme, sarcoid, GBS (esp. Miller Fisher / Bickerstaff), HIV seroconversion.
CN VI false-localizing sign: long subarachnoid course makes CN VI vulnerable to ↑ ICP (IIH, hydrocephalus, mass) — abducens palsy alone does NOT localize the lesion.
Trigeminal neuralgia in a young patient (≤ 50) or bilateral: MRI to rule out MS plaque at root entry zone or posterior fossa lesion — not just neurovascular conflict.
Corneal reflex arc: afferent V1 (nasociliary), efferent VII (orbicularis oculi) — absent reflex localizes to CN V or VII or pontine tegmentum (CPA tumors classically).
Tongue deviation rule: LMN CN XII → tongue deviates TOWARD lesion (weak genioglossus, opposite side pushes); + atrophy/fasciculations. UMN lesion → deviates AWAY (no atrophy).
Uvula deviation rule: CN X palsy → uvula deviates AWAY from lesion (weak side cannot elevate, contralateral pulls it over).
CPA syndrome: unilateral SNHL + tinnitus + ataxia ± CN V/VII involvement → vestibular schwannoma; BILATERAL vestibular schwannomas = NF2-related schwannomatosis (formerly neurofibromatosis type 2; chromosome 22 / NF2 / merlin) — pathognomonic.
Cavernous sinus syndrome: painful ophthalmoplegia involving CN III, IV, V1, V2, VI + Horner (sympathetics) — CN VI most vulnerable (runs through sinus, not in wall). Causes: thrombosis, Tolosa-Hunt, mucormycosis (diabetic/immunocompromised), pituitary apoplexy.
Jugular foramen (Vernet) syndrome: ipsilateral CN IX + X + XI palsy → glomus jugulare, schwannoma, metastasis. Collet-Sicard adds CN XII; Villaret adds Horner.
Parinaud (dorsal midbrain) syndrome: upgaze palsy + light-near dissociation + convergence-retraction nystagmus + lid retraction (Collier sign) → pineal mass (germinoma in young), hydrocephalus, MS.
Wallenberg (lateral medullary) syndrome: PICA/vertebral → ipsi CN V (face) + IX/X (dysphagia, hoarseness, ↓ gag) + Horner + cerebellar ataxia + crossed body pain/temp loss; NO weakness.
Anosmia: CN I damage — commonly post-traumatic (cribriform shear), viral (post-COVID), early Parkinson disease / Alzheimer (prodromal marker) — do NOT attribute solely to sinusitis without workup.

🔍 Quick ReferenceCN / function · Foramen / course · Clinical lesion

CN / function

CN I (olfactory) → special visceral afferent — smell via cribriform plate
CN II (optic) → special somatic afferent — vision; afferent limb of pupillary light reflex
CN III (oculomotor) — Edinger-Westphal nucleus → parasympathetic to sphincter pupillae + ciliary muscle (accommodation)
CN IV (trochlear) → only CN exiting DORSAL brainstem; only CN that fully DECUSSATES; supplies CONTRALATERAL superior oblique
CN V (trigeminal) → LARGEST cranial nerve; sensation of face + muscles of mastication (V3); afferent limb of corneal/jaw-jerk reflex
CN VII (facial) — chorda tympani → taste anterior 2/3 tongue + submandibular/sublingual salivation
CN IX (glossopharyngeal) → afferent limb of gag reflex + carotid sinus baroreceptor + parotid parasympathetic (via otic ganglion)
CN X (vagus) → efferent gag + soft palate/pharynx/larynx motor; recurrent laryngeal → vocal cords
CN XI (accessory) → sternocleidomastoid (turns head AWAY) + trapezius (shoulder shrug)
CN XII (hypoglossal) → all tongue muscles EXCEPT palatoglossus (CN X)

Foramen / course

Cribriform plate → CN I
Optic canal → CN II + ophthalmic artery
Superior orbital fissure → CN III, IV, V1, VI + sympathetics + ophthalmic veins
Foramen rotundum → V2 (maxillary)
Foramen ovale → V3 (mandibular) + accessory meningeal artery
Internal acoustic meatus → CN VII + VIII + labyrinthine artery
Stylomastoid foramen → CN VII exits skull
Jugular foramen → CN IX, X, XI + internal jugular vein + sigmoid sinus
Hypoglossal canal → CN XII
Cavernous sinus lateral wall → CN III, IV, V1, V2; CN VI runs WITHIN sinus (most vulnerable)
Rule of 4 of the brainstem → 4 midline structures (motor, MLF, medial lemniscus, motor nucleus of CN); 4 CN nuclei in medulla (IX, X, XI, XII), 4 in pons (V, VI, VII, VIII), 4 above (I, II, III, IV)

Clinical lesion

“Down & out” eye + ptosis + dilated pupil → compressive CN III palsy — PCom aneurysm / uncal herniation
Complete, isolated, truly pupil-sparing CN III in an older vasculopathic patient → microvascular (DM/HTN) ischemic mononeuropathy (but acute CN III palsy often still warrants CTA/MRA or MRI/MRA — image if partial palsy, relative pupil sparing, pain, age <50, or nonisolated findings)
Vertical diplopia worse on downgaze/contralateral tilt + compensatory head tilt → CN IV (trochlear) palsy — Bielschowsky positive
Lancinating electric facial pain triggered by light touch/chewing/cold → trigeminal neuralgia (SCA neurovascular conflict; MS if young/bilateral)
Horizontal diplopia + impaired abduction with ↑ ICP → CN VI false-localizing sign
Acute peripheral facial palsy + hyperacusis + dysgeusia + dry eye → Bell palsy (HSV reactivation)
Facial palsy + vesicles in ear canal/palate ± SNHL/vertigo → Ramsay Hunt (VZV, geniculate ganglion)
Bilateral facial diplegia → Lyme, sarcoid (Heerfordt: uveitis + parotitis + fever + facial palsy), GBS, HIV
Unilateral SNHL + tinnitus + ataxia ± absent corneal reflex → vestibular schwannoma in CPA
Bilateral vestibular schwannomas → NF2-related schwannomatosis (formerly neurofibromatosis type 2; chromosome 22 / NF2 / merlin) — pathognomonic
Throat/ear paroxysms triggered by swallowing/coughing/talking → glossopharyngeal neuralgia (image for posterior fossa lesion, MS)
Hoarse voice + dysphagia + uvula deviates AWAY + loss of gag → CN X (vagus) / recurrent laryngeal nerve palsy (Pancoast, aortic arch, thyroid surgery)
Shoulder droop + weak shrug + weak head turn AWAY from lesion → CN XI (accessory) injury (radical neck dissection, jugular foramen)
Tongue deviates TOWARD lesion + atrophy + fasciculations → LMN CN XII (hypoglossal) palsy
Painful ophthalmoplegia (CN III/IV/V1/V2/VI) ± Horner → cavernous sinus syndrome (thrombosis, Tolosa-Hunt, mucormycosis, pituitary apoplexy)
Ipsilateral CN III palsy + contralateral hemiparesis → Weber syndrome (midbrain base)
Ipsilateral CN III palsy + contralateral ataxia/tremor → Benedikt syndrome (midbrain tegmentum, red nucleus)
Ipsilateral CN VI + VII palsy + contralateral hemiparesis → Millard-Gubler (ventral pons)
Ipsilateral CN V/IX/X + Horner + cerebellar signs + crossed pain/temp loss → Wallenberg / lateral medullary (PICA, vertebral)
Upgaze palsy + light-near dissociation + convergence-retraction nystagmus + Collier sign → Parinaud (dorsal midbrain) — pineal mass, hydrocephalus, MS
Vernet syndrome → jugular foramen — CN IX, X, XI (glomus jugulare)
Collet-Sicard syndrome → CN IX, X, XI, XII (jugular foramen + hypoglossal canal)
Villaret syndrome → CN IX, X, XI, XII + Horner (parapharyngeal/retroparotid space)
Post-traumatic anosmia or hyposmia in early PD/AD → CN I dysfunction (cribriform shear vs. neurodegenerative prodrome)

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