Cranial Nerves
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)
Overview — The 12 Cranial Nerves
| CN | Name | Type | Skull Exit | Key Function(s) |
|---|---|---|---|---|
| I | Olfactory | S | Cribriform plate | Smell |
| II | Optic | S | Optic canal | Vision, pupillary light reflex (afferent) |
| III | Oculomotor | M | Superior orbital fissure | SR, IR, MR, IO, levator palpebrae; parasympathetic → pupil constriction |
| IV | Trochlear | M | Superior orbital fissure | Superior oblique (depression + intorsion) |
| V | Trigeminal | B | V1: SOF; V2: foramen rotundum; V3: foramen ovale | Facial sensation (V1/V2/V3); muscles of mastication (V3) |
| VI | Abducens | M | Superior orbital fissure | Lateral rectus (abduction) |
| VII | Facial | B | Internal acoustic meatus → stylomastoid foramen | Facial expression; taste anterior 2/3 tongue; lacrimation, salivation |
| VIII | Vestibulocochlear | S | Internal acoustic meatus | Hearing (cochlear); balance (vestibular) |
| IX | Glossopharyngeal | B | Jugular foramen | Taste posterior 1/3 tongue; pharyngeal sensation; parotid gland; carotid body/sinus |
| X | Vagus | B | Jugular foramen | Pharynx/larynx motor; visceral parasympathetic; gag reflex (efferent) |
| XI | Spinal Accessory | M | Jugular foramen | SCM and trapezius |
| XII | Hypoglossal | M | Hypoglossal canal | Tongue musculature |
Mnemonic (Type): Some Say Marry Money, But My Brother Says Big Brains Matter More → S, S, M, M, B, M, B, S, B, B, M, M
Cranial Nerve Nuclei by Brainstem Level
| Level | Cranial Nerves | Mnemonic Landmark |
|---|---|---|
| Midbrain | CN III (superior colliculus), CN IV (inferior colliculus) | "3 above 4" |
| Pons | CN V (mid-pons), CN VI & VII (lower pons), CN VIII (pontomedullary junction) | Facial colliculus = CN VI + VII loop |
| Medulla | CN IX, X, XII | Nucleus ambiguus (IX, X motor); hypoglossal nucleus (XII) |
| Spinal cord | CN XI (C1–C5/C6) | Only CN arising from spinal cord |
CN I — Olfactory Nerve
Anatomy
- Type: Special sensory (SVA) — smell
- Pathway: Olfactory epithelium → olfactory filaments traverse cribriform plate → olfactory bulb → olfactory tract → primary olfactory cortex (piriform cortex, amygdala, entorhinal cortex)
- Unique: Only cranial nerve that projects directly to cortex without a thalamic relay
- Unique: Olfactory receptor neurons are capable of neuronal regeneration throughout life
Clinical Correlates
- Anosmia causes:
- Head trauma — shearing of olfactory filaments at cribriform plate (most common neurological cause)
- Viral upper respiratory infection / COVID-19
- Neurodegenerative — Parkinson disease (early sign, precedes motor symptoms), Alzheimer disease
- Olfactory groove meningioma
- Kallmann syndrome — congenital anosmia + hypogonadotropic hypogonadism (failed GnRH neuron migration)
- Foster Kennedy syndrome: Unilateral anosmia + ipsilateral optic atrophy + contralateral papilledema → olfactory groove meningioma
- Pseudo-Foster Kennedy: Bilateral sequential AION — one disc swollen, other atrophic
- Uncinate seizures: Olfactory hallucinations (unpleasant burning smell) → mesial temporal lobe epilepsy
Board Pearl
CN I is the only cranial nerve without a thalamic relay. Anosmia is an early clinical feature of Parkinson disease and may precede motor symptoms by years.
CN II — Optic Nerve
Visual Pathway
- Retina (ganglion cells) → optic nerve → optic chiasm (nasal fibers cross) → optic tract → lateral geniculate nucleus (LGN, thalamus) → optic radiations → primary visual cortex (V1, calcarine cortex)
- Meyer loop (temporal lobe) carries inferior retinal fibers → represents superior visual field
- Baum loop (parietal lobe) carries superior retinal fibers → represents inferior visual field
- Pupillary light reflex pathway: Retina → optic nerve → chiasm → optic tract → pretectal olivary nucleus (pre-geniculate; bypasses LGN) → bilateral Edinger-Westphal nuclei via the posterior commissure → CN III → ciliary ganglion → pupillary sphincter
Visual Field Deficits by Lesion Location
| Lesion Site | Visual Field Deficit | Key Features / Causes |
|---|---|---|
| Optic nerve | Monocular vision loss / central scotoma | Optic neuritis, ischemic optic neuropathy, compressive lesion; RAPD present |
| Optic nerve–chiasm junction | Junctional scotoma | Ipsilateral central scotoma + contralateral superior temporal defect (Wilbrand knee fibers) |
| Optic chiasm (central) | Bitemporal hemianopia | Pituitary adenoma, craniopharyngioma, meningioma |
| Optic chiasm (lateral) | Binasal hemianopia (rare) | Bilateral ICA aneurysms compressing lateral chiasm |
| Optic tract | Contralateral incongruous homonymous hemianopia | Contralateral RAPD (more crossed nasal fibers) + "bowtie" (band) optic atrophy in the contralateral eye (nasal field loss → nasal disc pallor with temporal sparing) |
| LGN | Contralateral homonymous hemianopia (may be sectoral) | Anterior choroidal artery → lateral LGN; lateral posterior choroidal → medial LGN |
| Temporal lobe (Meyer loop) | Contralateral superior quadrantanopia ("pie in the sky") | MCA territory; temporal lobe surgery |
| Parietal lobe (Baum loop) | Contralateral inferior quadrantanopia ("pie on the floor") | MCA territory; associated OKN asymmetry |
| Occipital cortex (V1) | Congruous contralateral homonymous hemianopia | PCA infarct; macular sparing best explained by bilateral cortical macular representation + large cortical magnification (dual PCA/MCA supply teaching not well supported) |
| Bilateral occipital cortex | Cortical blindness | Bilateral PCA infarcts; Anton syndrome (denial of blindness); intact pupillary reflexes |
| Anterior ischemic optic neuropathy | Altitudinal field loss (usually inferior) | Arteritic (GCA) vs. non-arteritic; disc edema |
Papilledema vs. Optic Neuritis
| Feature | Papilledema | Optic Neuritis |
|---|---|---|
| Laterality | Bilateral | Usually unilateral |
| Vision | Initially preserved (enlarged blind spot) | Decreased acuity + central scotoma |
| Pain | Headache (positional, worse AM) | Retro-orbital pain with eye movement |
| RAPD | Absent (bilateral + symmetric) | Present (unilateral) |
| Color vision | Preserved early | Impaired (red desaturation) |
| Mechanism | Raised ICP transmitted via CSF sheath | Inflammatory demyelination (often MS-associated) |
RAPD (Marcus Gunn Pupil)
- Definition: Relative afferent pupillary defect — asymmetric pupillary light reflex indicating unilateral or asymmetric optic nerve (or extensive retinal) disease
- Swinging flashlight test: Light in normal eye → both pupils constrict; swing to affected eye → both pupils dilate (paradoxical dilation)
- Causes: Optic neuritis, ischemic optic neuropathy, optic nerve compression, extensive unilateral retinal disease, optic tract lesion (contralateral RAPD)
- Does NOT occur in: Cataracts, refractive errors, amblyopia, or symmetric bilateral optic neuropathies
- Board tip: A large optic tract lesion produces a contralateral RAPD (because more crossed nasal fibers are affected)
Board Pearl
Lesions posterior to the LGN (optic radiations, cortex) do NOT produce a RAPD because the pupillary reflex pathway exits at the pretectal nucleus, which is pre-geniculate. A RAPD always indicates pre-geniculate pathology.
Clinical Pearl — Macular Sparing
Macular sparing in occipital lobe (PCA) strokes is most likely explained by bilateral cortical representation of the macula combined with the very large cortical magnification of macular vision at the occipital pole. The classic "dual PCA/MCA blood supply to the macula" teaching is widely taught but not well supported by anatomic or angiographic studies (Osborn; Caplan). Either way, the finding helps distinguish cortical from optic tract/LGN lesions (which typically lack macular sparing).
CN III, IV, VI — Ocular Motor Nerves
CN III — Oculomotor Nerve
Nuclear Organization
- Location: Midbrain, ventral periaqueductal gray, at level of superior colliculus
- Exit: Interpeduncular fossa (between cerebral peduncles)
- Course: Between PCA and SCA → lateral wall of cavernous sinus → superior orbital fissure → orbit
| Subnucleus | Innervation | Key Feature |
|---|---|---|
| Superior rectus subnucleus | Contralateral superior rectus | Only subnucleus that crosses |
| Inferior rectus subnucleus | Ipsilateral inferior rectus | — |
| Medial rectus subnucleus | Ipsilateral medial rectus | — |
| Inferior oblique subnucleus | Ipsilateral inferior oblique | — |
| Central caudal nucleus | Bilateral levator palpebrae superioris | Single midline nucleus → bilateral ptosis in nuclear lesion |
| Edinger-Westphal nucleus | Pupillary sphincter + ciliary muscle (parasympathetic) | Preganglionic fibers → ciliary ganglion → short ciliary nerves |
Divisions in the Orbit
- Superior division: Levator palpebrae superioris + superior rectus
- Inferior division: Medial rectus + inferior rectus + inferior oblique + parasympathetic fibers (pupil + accommodation)
- Parasympathetic preganglionic fibers travel with the branch to inferior oblique → synapse in ciliary ganglion → short ciliary nerves → pupillary sphincter + ciliary muscle (accommodation)
CN III Palsy — Clinical Features
- Complete palsy: Eye "down and out" (unopposed SO and LR), ptosis, mydriasis (fixed dilated pupil)
- Pupil-involving (compressive): Parasympathetic fibers run superficially/peripherally on nerve → compressed first by mass lesions
- PCom aneurysm — #1 cause of pupil-involving CN III palsy → emergent CTA/MRA/DSA
- Uncal herniation, posterior fossa tumors
- Pupil-sparing (ischemic): Microvascular ischemia affects interior of nerve (vasa nervorum) — spares peripheral parasympathetic fibers
- Diabetes mellitus, hypertension, vasculitis
- Typically self-resolves in 8–12 weeks
Board Pearl
Pupil-involving CN III palsy = aneurysm until proven otherwise. Requires emergent vascular imaging (CTA/MRA). The rule of the pupil: parasympathetic fibers travel superficially → compressed by external mass before ischemia affects them.
Midbrain Fascicular Syndromes (CN III +)
| Syndrome | Structures Involved | Findings |
|---|---|---|
| Weber | CN III fascicle + cerebral peduncle | Ipsilateral CN III palsy + contralateral hemiparesis |
| Benedikt | CN III fascicle + red nucleus | Ipsilateral CN III palsy + contralateral tremor/ataxia (rubral tremor) |
| Nothnagel | CN III fascicle + superior cerebellar peduncle | Ipsilateral CN III palsy + ipsilateral cerebellar ataxia |
| Claude | CN III fascicle + red nucleus + SCP | Ipsilateral CN III palsy + contralateral ataxia + contralateral tremor |
CN IV — Trochlear Nerve
- Nucleus: Midbrain, at level of inferior colliculus
- Unique features (3 board-tested facts):
- Only CN that exits dorsally (from posterior brainstem)
- Only CN that fully decussates (left nucleus → right SO muscle)
- CN IV has the longest INTRACRANIAL course (exits dorsally, wraps around brainstem); CN VI is most vulnerable to STRETCH from ↑ICP (classic false-localizing sign as it ascends the clivus).
- Innervation: Trochlear nucleus → contralateral superior oblique (fibers decussate within the brainstem before exiting dorsally); the trochlear nerve itself (post-decussation) supplies the SO on the side it ultimately reaches. Clinical implication: a nuclear lesion causes contralateral SO palsy, whereas a nerve lesion (the more common scenario) causes ipsilateral SO palsy. SO action = depresses (in adduction), intorts, abducts the eye.
CN IV Palsy — Clinical Features
- Symptoms: Vertical diplopia, worse looking down (reading, descending stairs)
- Compensatory head tilt: Away from affected side (tilts toward healthy side to reduce diplopia)
- Hypertropia: Affected eye is higher (defective depression in adduction)
- Three-step test (Parks-Bielschowsky):
- Which eye is hypertropic? → identifies 4 possible muscles
- Hypertropia worse on left or right gaze? → narrows to 2 muscles
- Worse on head tilt to which side? → identifies the paretic muscle
- Causes: Trauma (#1), congenital (decompensated), microvascular, rarely tumor
Board Pearl
CN IV is the only cranial nerve that decussates and exits dorsally. Bilateral CN IV palsies are common after head trauma (contrecoup injury at anterior medullary velum). Suspect bilateral CN IV palsy when alternating hypertropia is present on lateral gaze or large V-pattern esotropia.
CN VI — Abducens Nerve
- Nucleus: Dorsal pons, beneath floor of 4th ventricle at the facial colliculus
- Course: Longest subarachnoid course up the clivus → Dorello canal (beneath petroclinoid ligament) → cavernous sinus (runs THROUGH sinus, not in wall) → SOF → orbit
- Innervation: Ipsilateral lateral rectus (LR6)
CN VI Palsy — Clinical Features
- Presentation: Horizontal diplopia, worse at distance and looking toward affected side; esotropia
- False localizing sign: CN VI palsy from raised ICP (nerve stretched over petrous apex) — does NOT indicate a pontine lesion
- Gradenigo syndrome: Petrous apicitis → CN VI palsy + facial pain (V) + otitis media
- CN VI nuclear lesion: Causes ipsilateral horizontal gaze palsy (not just LR weakness) because the nucleus contains both LR motor neurons AND internuclear neurons projecting to contralateral CN III (MR) via MLF
Extraocular Muscles — Innervation Summary
| Muscle | Nerve | Primary Action | Testing Position |
|---|---|---|---|
| Superior rectus | CN III | Elevation (best in abduction) | Up and out |
| Inferior rectus | CN III | Depression (best in abduction) | Down and out |
| Medial rectus | CN III | Adduction | Toward nose |
| Inferior oblique | CN III | Elevation in adduction + extorsion | Up and in |
| Superior oblique | CN IV | Depression in adduction + intorsion | Down and in |
| Lateral rectus | CN VI | Abduction | Laterally (temporally) |
Mnemonic: LR6SO4 — all the rest CN III
Clinical Pearl — Cavernous Sinus Syndrome
CN III, IV, V1, V2, and VI travel through or along the wall of the cavernous sinus. A cavernous sinus lesion (thrombosis, tumor, fistula, Tolosa-Hunt syndrome) can cause painful ophthalmoplegia with variable CN involvement. CN VI is most vulnerable (runs freely through the sinus, not in the wall). The sympathetic plexus surrounding the ICA is also at risk → Horner syndrome.
Brainstem Ocular Motor Syndromes
| Syndrome | Lesion | Clinical Findings |
|---|---|---|
| INO (Internuclear Ophthalmoplegia) | Ipsilateral MLF (between CN VI and CN III nuclei) | On contralateral gaze: ipsilateral eye fails to adduct + contralateral eye shows abducting nystagmus. Convergence preserved (rules out medial rectus weakness). Bilateral & young → MS; unilateral & older → pontine stroke. |
| One-and-a-half syndrome | Ipsilateral PPRF (or CN VI nucleus) plus ipsilateral MLF, in dorsal pons | Ipsilateral horizontal gaze palsy (the "one") + INO on contralateral gaze (the "half"). Only intact horizontal movement = contralateral eye abduction. Causes: pontine stroke, MS, tumor. |
| Eight-and-a-half syndrome | One-and-a-half + ipsilateral CN VII (facial colliculus involvement in dorsal pontine tegmentum) | One-and-a-half findings + ipsilateral LMN-pattern facial palsy (forehead involved). Localizes to a more extensive dorsal pontine tegmental lesion. |
| WEBINO (Wall-Eyed Bilateral INO) | Bilateral MLF + medial rectus subnuclei of CN III, in midbrain/upper pons | Bilateral INO + exotropia at rest ("wall-eyed"); convergence often impaired (helps distinguish from typical INO). Young → MS; older → midbrain stroke or tumor. |
| Ocular Tilt Reaction (OTR) | Utriculo-ocular pathway: utricle → vestibular nucleus → MLF → interstitial nucleus of Cajal (INC, rostral midbrain) | Triad: head tilt + skew deviation + conjugate ocular torsion, all toward the same side. Peripheral (labyrinth/CN VIII): tilt toward the lesion (lower eye on lesion side). Central (brainstem above vestibular nucleus): tilt away from lesion. Useful localizer in posterior-fossa stroke. |
Skew Deviation vs CN IV (Trochlear) Palsy
| Feature | Skew Deviation | CN IV Palsy |
|---|---|---|
| Mechanism | Vertical misalignment from utriculo-ocular pathway dysfunction (brainstem/cerebellum) | Superior oblique weakness from CN IV lesion (nuclear, fascicular, or peripheral nerve) |
| Park’s 3-step test | Does NOT consistently fit a single muscle pattern (often "inconclusive") | Hyperdeviation worse on (1) contralateral gaze, (2) ipsilateral head tilt (Bielschowsky positive), (3) downgaze |
| Upright vs supine (key bedside test) | Hyperdeviation improves (≥50%) when supine (gravity removes otolithic input) | Hyperdeviation does NOT change with position |
| Ocular torsion | Higher eye incyclotorted, lower eye excyclotorted (toward the side of head tilt) | Affected eye excyclotorted; patient adopts head tilt to OPPOSITE side to compensate |
| Associated findings | Brainstem signs (other CN palsies, ataxia, dysarthria); often part of the ocular tilt reaction triad | Isolated; trauma is most common etiology in adults; congenital forms may show facial asymmetry |
Board Pearl — Vertical Diplopia
- Upright-supine test: the single most useful bedside maneuver to separate skew (improves ≥50% supine) from CN IV palsy (no change). Skew almost always carries associated brainstem signs (cerebellar ataxia, vertigo, other CN palsies); CN IV palsy is usually isolated.
- OTR triad (head tilt + skew + ocular torsion) localizes the lesion along the utriculo-ocular pathway from labyrinth → vestibular nucleus → MLF → INC.
- WEBINO = wall-eyed bilateral INO; bilateral MLF + medial rectus subnuclei; young → MS, older → midbrain stroke or tumor.
- Eight-and-a-half = one-and-a-half + ipsilateral peripheral-pattern CN VII; tells you the lesion is in dorsal pontine tegmentum involving the facial colliculus.
CN V — Trigeminal Nerve
Anatomy — Three Divisions
| Division | Foramen | Sensory Territory | Motor? |
|---|---|---|---|
| V1 — Ophthalmic | Superior orbital fissure | Forehead, upper eyelid, cornea, bridge of nose, tip of nose | No |
| V2 — Maxillary | Foramen rotundum | Cheek, upper lip, upper teeth, nasal cavity, palate | No |
| V3 — Mandibular | Foramen ovale | Lower face, jaw, lower teeth, anterior 2/3 tongue (general sensation only), ear | Yes — muscles of mastication |
Mnemonic — "Standing Room Only": V1 = Superior orbital fissure; V2 = foramen Rotundum; V3 = foramen Ovale.
Sensory Nuclei (Board-High-Yield)
- Mesencephalic nucleus: Proprioception (jaw, teeth) — unique because it contains primary sensory cell bodies within the CNS
- Principal (chief) sensory nucleus: Light touch — in pons
- Spinal trigeminal nucleus: Pain and temperature — extends from pons into upper cervical spinal cord
- Somatotopic "onion-skin" pattern: perioral → rostral nucleus, lateral face → caudal nucleus
- Board tip: Lateral medullary (Wallenberg) syndrome affects the spinal trigeminal nucleus → ipsilateral facial pain/temperature loss in "onion-skin" distribution
Motor Component
- Motor nucleus: Pons (medial to principal sensory nucleus)
- Muscles of mastication: Masseter, temporalis, medial pterygoid, lateral pterygoid
- Also innervates: Tensor tympani, tensor veli palatini, mylohyoid, anterior belly of digastric
- Jaw deviation: Toward the weak side (ipsilateral pterygoid weakness → unopposed contralateral pterygoid pushes jaw toward weak side)
- Jaw jerk reflex: Afferent AND efferent = CN V (mesencephalic nucleus); brisk jaw jerk → bilateral UMN lesion above pons (pseudobulbar palsy)
Trigeminal Neuralgia (Tic Douloureux)
- Classic features: Sudden, severe, lancinating/electric shock-like facial pain in V2 and/or V3 distribution
- Triggers: Chewing, talking, brushing teeth, wind on face, light touch
- Duration: Seconds to <2 minutes; refractory period between attacks
- Etiology: Neurovascular compression (usually SCA) at root entry zone → focal demyelination
- Red flags for secondary cause: Age <40, bilateral, V1 involvement, sensory loss, abnormal neuro exam → consider MS, tumor, or other structural lesion
- Treatment: First-line = carbamazepine or oxcarbazepine; surgical = microvascular decompression (Jannetta procedure)
Corneal Reflex Arc
- Afferent: CN V1 (ophthalmic division) — nasociliary branch
- Efferent: CN VII (facial nerve) — temporal and zygomatic branches → orbicularis oculi
- Direct response: Blink of stimulated eye
- Consensual response: Blink of contralateral eye
- Board tip: Absent corneal reflex with intact facial nerve function → V1 lesion; absent bilateral corneal reflexes when stimulating one side → afferent (V1) lesion on that side
Board Pearl
The mesencephalic nucleus of CN V is unique — it is the only place in the CNS containing primary sensory neuron cell bodies (proprioception for jaw). All other primary sensory neurons reside in peripheral ganglia.
CN VII — Facial Nerve
Four Functional Components
| Component | Fiber Type | Nucleus | Function |
|---|---|---|---|
| Branchial motor | SVE | Facial motor nucleus (pons) | Muscles of facial expression, stapedius, posterior belly of digastric, stylohyoid |
| Visceral motor (parasympathetic) | GVE | Superior salivatory nucleus | Lacrimal gland (via greater petrosal nerve); submandibular & sublingual glands (via chorda tympani) |
| Special sensory (taste) | SVA | Nucleus solitarius (rostral/gustatory) | Taste — anterior 2/3 tongue (via chorda tympani) |
| General sensory | GSA | Spinal trigeminal nucleus | Small area of external ear (Ramsay Hunt zone) |
Facial Nerve Segments & Lesion Localization
| Segment | Location | Branches Given Off | Lesion at This Level |
|---|---|---|---|
| Intracranial | Pons → CPA → IAM | None | All CN VII functions lost + may involve CN VIII (CPA tumors) |
| Labyrinthine | IAM → geniculate ganglion | Greater petrosal nerve (lacrimation) | All functions lost + dry eye (absent tearing) |
| Tympanic | Geniculate ganglion → across middle ear | Nerve to stapedius | Facial weakness + hyperacusis + loss of taste + reduced salivation |
| Mastoid | After second genu | Chorda tympani (taste + salivation) | Facial weakness + loss of taste + reduced salivation; hearing normal, no hyperacusis |
| Extracranial | After stylomastoid foramen | Terminal motor branches | Facial weakness only — no taste/lacrimation/hyperacusis deficits |
UMN vs. LMN Facial Weakness
| Feature | UMN (Central) | LMN (Peripheral) |
|---|---|---|
| Forehead | Spared (bilateral cortical innervation of upper face) | Involved (cannot raise eyebrow or wrinkle forehead) |
| Distribution | Contralateral lower face | Entire ipsilateral face |
| Eye closure | Usually possible | Weak or absent (Bell phenomenon — eye rolls up) |
| Taste | Intact | May be lost (if lesion proximal to chorda tympani) |
| Emotional facial movement | May be preserved (emotional pathways bypass cortex) | Lost |
| Causes | Stroke, tumor, MS | Bell palsy, Ramsay Hunt, parotid tumor, otitis media |
Bell Palsy
- Definition: Acute idiopathic unilateral LMN facial paralysis
- Most common cause of acute unilateral facial nerve palsy
- Proposed etiology: HSV-1 reactivation in geniculate ganglion → nerve edema within the bony fallopian canal
- Clinical: Rapid onset (hours to days), may have preceding viral illness, retroauricular pain, hyperacusis, taste loss
- Prognosis: ~85% recover completely; poor prognostic signs: complete paralysis, no recovery by 3 weeks, age >60
- Treatment: Corticosteroids (prednisone 60–80 mg/day × 7 days, started within 72 hours); eye care (lubricant, taping at night)
- Antivirals: Adding valacyclovir to steroids — may benefit severe cases (controversial; no clear benefit for mild/moderate)
Ramsay Hunt Syndrome (Herpes Zoster Oticus)
- Cause: VZV reactivation in geniculate ganglion
- Classic triad: Ipsilateral facial nerve palsy + vesicular eruption in ear (external ear, ear canal, tympanic membrane) + otalgia
- May also involve: CN VIII (vertigo, sensorineural hearing loss), taste loss, decreased lacrimation
- Prognosis: Worse than Bell palsy — only ~50% full recovery
- Treatment: Antivirals (valacyclovir/acyclovir) + corticosteroids
Board Pearl
UMN facial weakness spares the forehead because the upper face portion of the facial motor nucleus receives bilateral cortical input. In LMN lesions, the entire ipsilateral face is weak. Hyperacusis in facial palsy localizes the lesion proximal to the nerve to stapedius (within the facial canal).
Clinical Pearl — Bilateral Facial Weakness
Bilateral LMN facial weakness (facial diplegia) has a distinct differential: Guillain-Barré syndrome (most common cause), Lyme disease, sarcoidosis (Heerfordt syndrome), HIV, Möbius syndrome (congenital). Always think GBS when bilateral facial weakness develops acutely.
CN VIII — Vestibulocochlear Nerve
Cochlear Division — Hearing
- Pathway: Hair cells (organ of Corti) → spiral ganglion → cochlear nerve → cochlear nuclei (pons) → bilateral superior olivary complex → lateral lemniscus → inferior colliculus → medial geniculate nucleus (thalamus) → primary auditory cortex (Heschl gyrus, superior temporal)
- Mnemonic — "SSLIMA": Spiral ganglion → Superior olivary complex → Lateral lemniscus → Inferior colliculus → Medial geniculate → Auditory cortex (Heschl).
- BAEP wave I = cochlear nerve (CN VIII, extra-axial); waves II–V are intra-axial brainstem generators. Persistence of wave I with loss of all later waves on BAEP is consistent with brain-death physiology (wave I is generated peripheral to the brainstem) — though per the 2023 AAN/AAP/CNS/SCCM guideline, BAEP is NOT one of the accepted ancillary tests for brain-death determination (acceptable: nuclear CBF, 4-vessel angiography, TCD in adults).
- Key point: Bilateral cortical representation after superior olivary complex → unilateral cortical lesions do NOT cause deafness
Weber and Rinne Tests
| Test | Technique | Conductive Hearing Loss | Sensorineural Hearing Loss |
|---|---|---|---|
| Weber | Tuning fork on vertex/forehead | Lateralizes to affected ear (bone conduction bypasses middle ear pathology) | Lateralizes to unaffected ear |
| Rinne | Compare air (next to ear) vs. bone (mastoid) conduction | Bone > air (Rinne negative) | Air > bone (Rinne positive, same as normal but quieter) |
Vestibular Division — Balance
- Pathway: Semicircular canals + otolith organs → vestibular (Scarpa) ganglion → vestibular nerve → vestibular nuclei (pontomedullary junction) → projections to cerebellum, MLF, spinal cord, cortex
Peripheral vs. Central Vestibular Nystagmus
| Feature | Peripheral | Central |
|---|---|---|
| Direction | Unidirectional (fast phase away from lesion) | May be direction-changing or purely vertical/torsional |
| Fixation | Suppressed by visual fixation | Not suppressed by fixation |
| Vertigo severity | Severe | Mild or absent |
| Hearing loss/tinnitus | Common | Uncommon |
| Head impulse test (HIT) | Abnormal (corrective saccade) | Normal |
| Skew deviation | Absent | May be present |
| HINTS exam | Abnormal HIT (catch-up saccade); unidirectional nystagmus; no skew (reassuring/benign pattern; INFARCT mnemonic flags the opposite/central findings: Impulse Normal, Fast-phase Alternating, Refixation on Cover Test) | Any one of: normal HIT, direction-changing nystagmus, vertical skew deviation — stroke until proven otherwise |
Cerebellopontine Angle (CPA) Tumors
- Vestibular schwannoma (acoustic neuroma): Most common CPA tumor (~80%)
- Arises from Schwann cells of vestibular division of CN VIII (typically superior vestibular nerve)
- Presentation: Progressive unilateral sensorineural hearing loss, tinnitus, imbalance; may compress CN V (facial numbness) and CN VII (facial weakness) as it enlarges
- Imaging: MRI with contrast — enhancing mass at IAM/"ice cream cone" sign
- Epidermoid cyst: 2nd most common CPA mass (non-enhancing, DWI bright)
- Meningioma: 3rd most common CPA mass (dural-based, enhancing, calcification)
- NF2-related schwannomatosis (formerly neurofibromatosis type 2): Bilateral vestibular schwannomas — pathognomonic; chromosome 22 / NF2 / merlin (schwannomin) gene
Board Pearl
HINTS exam (Head Impulse, Nystagmus, Test of Skew) has higher sensitivity than early MRI for posterior fossa stroke in acute vestibular syndrome. A normal head impulse test in acute vertigo should raise concern for a central (stroke) cause. Weber lateralizes to the affected ear in conductive loss (counterintuitive but board-tested).
CN IX & X — Glossopharyngeal & Vagus Nerves
CN IX — Glossopharyngeal Nerve
Components
- Sensory: Posterior 1/3 tongue (general sensation + taste), oropharynx, middle ear, carotid body (chemoreceptor) and carotid sinus (baroreceptor) → CN IX — afferent signal goes via Hering's nerve to the nucleus tractus solitarius. Distinguish from the aortic arch baroreceptor + aortic bodies, which travel via CN X (aortic nerve of Cyon / depressor nerve). Both terminate in the NTS → ventrolateral medulla for autonomic blood-pressure control.
- Motor: Stylopharyngeus (only muscle — elevates pharynx during swallowing and speech)
- Parasympathetic: Inferior salivatory nucleus → lesser petrosal nerve → otic ganglion → parotid gland
- Nuclei: Nucleus ambiguus (motor), nucleus solitarius (taste + visceral afferents), spinal trigeminal nucleus (general sensation)
Glossopharyngeal Neuralgia
- Severe lancinating pain in throat, tonsillar fossa, ear; triggered by swallowing, coughing, talking
- May cause syncope via carotid sinus reflex (bradycardia/asystole) — glossopharyngeal neuralgia with syncope
- Treatment: Carbamazepine/oxcarbazepine; microvascular decompression for refractory cases
CN X — Vagus Nerve
Components
- Motor (branchiomotor): Nucleus ambiguus → pharyngeal muscles (swallowing), laryngeal muscles (voice)
- Recurrent laryngeal nerve: All intrinsic laryngeal muscles except cricothyroid
- Superior laryngeal nerve (external branch): Cricothyroid muscle
- Parasympathetic: Dorsal motor nucleus of vagus → thoracoabdominal viscera (heart, lungs, GI to splenic flexure)
- Sensory: External ear (Arnold nerve — cough reflex from ear), larynx, viscera
Clinical Correlates
- Gag reflex: Afferent = CN IX, Efferent = CN X
- Uvula deviation: Palate and uvula deviate AWAY from the lesion (weak side drops, intact side pulls uvula toward itself)
- "Curtain sign" (Vernet sign): Posterior pharyngeal wall moves toward intact side when patient says "ah"
- Unilateral vocal cord paralysis: Hoarseness, breathy voice; left recurrent laryngeal nerve more commonly affected (longer course, looping under aortic arch)
- Bilateral vocal cord paralysis: Stridor, respiratory distress (cords in paramedian position) — emergency
- Causes of recurrent laryngeal nerve palsy: Thyroid surgery, lung cancer (left), aortic aneurysm, mediastinal tumor, post-intubation
Jugular Foramen Syndrome
- Contents: CN IX, X, XI, internal jugular vein, inferior petrosal sinus
- Vernet syndrome: CN IX + X + XI involvement → dysphagia, hoarseness, loss of gag reflex, trapezius/SCM weakness
- Collet-Sicard syndrome: CN IX + X + XI + XII — jugular foramen + hypoglossal canal
- Villaret syndrome: Collet-Sicard + sympathetic chain → adds Horner syndrome
- Causes: Glomus jugulare tumor, metastases, skull base fracture, meningioma, infection
Board Pearl
Gag reflex: afferent = CN IX, efferent = CN X. The uvula deviates AWAY from the lesion side (toward the intact side). The left recurrent laryngeal nerve has a longer course (loops under the aortic arch), making it more vulnerable to mediastinal pathology.
Clinical Pearl — Lateral Medullary Syndrome (Wallenberg)
The lateral medulla contains CN IX/X nuclei, spinal trigeminal nucleus, vestibular nuclei, inferior cerebellar peduncle, and descending sympathetics. Wallenberg syndrome (PICA or vertebral artery occlusion) causes: ipsilateral CN IX/X palsy (dysphagia, hoarseness), ipsilateral facial pain/temperature loss, ipsilateral Horner syndrome, ipsilateral cerebellar ataxia, contralateral body pain/temperature loss, vertigo, and nystagmus. Motor spared (pyramids are medial).
CN XI — Spinal Accessory Nerve
Anatomy
- Origin: Spinal accessory nucleus in anterior horn of spinal cord (C1–C5/C6)
- Course: Rootlets ascend through foramen magnum → briefly join CN X in jugular foramen → exit jugular foramen → descend in posterior triangle of neck to innervate SCM and trapezius
- Type: Pure motor (branchial motor — SVE)
Muscles & Testing
| Muscle | Action | Testing | Weakness Presentation |
|---|---|---|---|
| Sternocleidomastoid (SCM) | Turns head to opposite side; tilts head ipsilaterally; bilateral → neck flexion | Have patient turn head against examiner’s hand; palpate contralateral SCM | Difficulty turning head to contralateral side |
| Trapezius | Shoulder elevation, scapular retraction, arm abduction >90° | Shrug shoulders against resistance | Shoulder droop, scapular winging (lateral), difficulty abducting arm above horizontal |
Clinical Correlates
- Iatrogenic injury: Most common cause — posterior triangle lymph node biopsy, carotid endarterectomy, neck dissection
- Jugular foramen lesions: Combined CN IX, X, XI palsy
- SCM vs trapezius cortical innervation: The SCM receives predominantly ipsilateral corticobulbar input, whereas the trapezius receives predominantly contralateral input; both receive some bilateral projections. Consequence: a unilateral cortical/corticobulbar lesion causes weakness of the ipsilateral SCM (impaired head turn toward the contralateral / hemiparetic side) but weakness of the contralateral trapezius (shoulder shrug weakness on the hemiparetic side).
Board Pearl
SCM cortical control is predominantly ipsilateral; trapezius is predominantly contralateral (both with bilateral component). A right cortical lesion weakens the right SCM → impaired head turn to the left (R SCM normally pulls the head to the left) AND weakens the left trapezius (shoulder shrug weakness on the hemiparetic side). Net result of a hemispheric stroke: the head/eyes deviate toward the lesion (away from the hemiparetic side). A destructive frontal lesion causes head/eye deviation toward the lesion; an irritative (seizure) lesion drives head/eye deviation away.
CN XII — Hypoglossal Nerve
Anatomy
- Nucleus: Hypoglossal nucleus in the dorsal medulla (floor of 4th ventricle, near midline)
- Exit: Between pyramid and olive of medulla → hypoglossal canal
- Innervation: All intrinsic tongue muscles + 3 of 4 extrinsic tongue muscles (genioglossus, hyoglossus, styloglossus) — palatoglossus is CN X
- Key muscle: Genioglossus — protrudes tongue forward and to the contralateral side
Tongue Deviation Rules
| Lesion Type | Tongue Deviation | Other Signs | Explanation |
|---|---|---|---|
| LMN (CN XII or nucleus) | Deviates toward the lesion | Atrophy + fasciculations on affected side | Weak genioglossus on lesion side → intact contralateral genioglossus pushes tongue toward weak side |
| UMN (cortex/corticobulbar) | Deviates away from the cortical lesion (toward body weakness side) | No atrophy, no fasciculations; may have spastic tongue | Corticobulbar fibers to CN XII nucleus are predominantly crossed → contralateral genioglossus weak |
Clinical Correlates
- Medial medullary syndrome (Dejerine): Ipsilateral CN XII palsy + contralateral hemiparesis (pyramid) + contralateral proprioceptive loss (medial lemniscus) — ASA territory
- Causes of CN XII palsy:
- Nuclear/fascicular: Medullary stroke, syringobulbia, motor neuron disease (ALS)
- Skull base: Hypoglossal canal tumor, metastases, skull base fracture, Collet-Sicard syndrome
- Extracranial: Carotid dissection, carotid endarterectomy, neck surgery, infection
- Bilateral CN XII palsy: Tongue atrophy + fasciculations bilaterally; consider ALS, skull base metastases, or bilateral carotid pathology
- Pseudobulbar palsy vs. bulbar palsy:
- Bulbar (LMN): Tongue atrophy, fasciculations, flaccid dysarthria; ALS, polio
- Pseudobulbar (UMN): Brisk jaw jerk, spastic dysarthria, emotional lability; bilateral cortical/subcortical strokes, MS, ALS
Board Pearl
LMN CN XII lesion: tongue deviates toward the lesion ("licks the wound"). UMN lesion: tongue deviates away from the cortical lesion (toward the weak body side). LMN hallmarks = atrophy + fasciculations; UMN hallmarks = no atrophy, spastic tongue, brisk jaw jerk.
CN ↔ Muscle Quick Reference (Easily-Confused Pairs)
These muscle → cranial nerve pairings are scattered through the prose above; this table consolidates the high-yield "trap" muscles that boards reliably test.
| Muscle | Cranial Nerve | Function / Clinical Significance |
|---|---|---|
| Stylopharyngeus | CN IX (Glossopharyngeal) | The ONLY motor muscle supplied by CN IX; elevates the pharynx during swallowing. Common board trap. |
| Anterior belly of digastric | CN V3 (Mandibular div. of trigeminal), via the mylohyoid nerve | Same nerve as the mylohyoid; depresses the mandible / elevates the hyoid. Branchial arch 1 derivative. |
| Posterior belly of digastric | CN VII (Facial) | Branchial arch 2 derivative. The two digastric bellies have different CN supplies — classic trap. |
| Stylohyoid | CN VII (Facial) | Same group as the posterior belly of digastric; both are arch-2 derivatives. |
| Mylohyoid | CN V3 (mylohyoid nerve) | Same nerve as anterior belly of digastric. |
| Tensor tympani | CN V3 | Dampens loud sounds by tensing the tympanic membrane. |
| Stapedius | CN VII | Dampens loud sounds at the stapes. CN VII lesion proximal to the stapedial branch → hyperacusis. The tensor tympani vs stapedius pair (V3 vs VII) is a high-frequency board distractor. |
| Tensor veli palatini | CN V3 | The ONLY palatal muscle NOT innervated by CN X. Tenses the soft palate; opens the Eustachian tube on yawning/swallowing. |
| Levator veli palatini, palatoglossus, palatopharyngeus, musculus uvulae | CN X (Vagus), via the pharyngeal plexus | All other palatal muscles are vagal. CN X palsy → uvular deviation AWAY from the lesion (intact side pulls the uvula). |
| Intrinsic + extrinsic tongue muscles | CN XII (Hypoglossal) — except palatoglossus = CN X | Classic exception — palatoglossus is named "glossus" but is innervated by vagus, not hypoglossal. |
| Cricothyroid | CN X (Vagus), via the external laryngeal branch of the superior laryngeal nerve | The ONLY laryngeal muscle NOT innervated by the recurrent laryngeal nerve. Tenses the vocal cord (pitch). |
| All other intrinsic laryngeal muscles | CN X, via the recurrent laryngeal nerve | Recurrent laryngeal lesion (thyroid surgery, mediastinal tumor) → vocal cord paralysis, hoarseness. |
| Sternocleidomastoid (SCM) + trapezius | CN XI (Spinal accessory) | SCM cortical control predominantly ipsilateral; trapezius predominantly contralateral (see CN XI section above). |
💎 Board Pearl — Most-Tested CN/Muscle Traps
- Stylopharyngeus = CN IX (only motor muscle of IX).
- Anterior belly of digastric = V3; posterior belly = VII (two bellies, two arches, two nerves).
- Tensor tympani = V3; stapedius = VII (CN VII lesion proximal to stapedial branch → hyperacusis).
- Tensor veli palatini = V3; all other palatal muscles = X.
- Palatoglossus = CN X (the “glossus” that is NOT hypoglossal).
- Cricothyroid = external laryngeal (superior laryngeal) branch of X — the only laryngeal muscle NOT by the recurrent laryngeal nerve.
Cranial Nerve Syndromes — Localization by Anatomic Site
Major Cranial Nerve Syndrome Table
| Syndrome / Site | Cranial Nerves Affected | Key Features | Common Causes |
|---|---|---|---|
| Cavernous sinus | III, IV, V1, V2, VI + sympathetic plexus | Painful ophthalmoplegia; CN VI most vulnerable (runs through sinus); Horner syndrome possible; proptosis, chemosis if CCF | Tolosa-Hunt (granulomatous inflammation), thrombosis, CCF, pituitary apoplexy, tumor, infection |
| Superior orbital fissure | III, IV, V1, VI | Similar to cavernous sinus but NO V2 involvement and no Horner; orbital pain | Tumor, inflammation, trauma |
| Orbital apex | III, IV, V1, VI + CN II (optic nerve) | SOF syndrome PLUS visual loss (optic neuropathy); proptosis | Tumor, granulomatosis with polyangiitis, sarcoidosis, infection (mucormycosis — especially in diabetic/immunocompromised) |
| Cerebellopontine angle (CPA) | VII, VIII (primarily) ± V, VI | Unilateral sensorineural hearing loss, tinnitus, facial weakness/numbness; large lesions → cerebellar signs | Vestibular schwannoma (#1), meningioma, epidermoid cyst |
| Jugular foramen (Vernet) | IX, X, XI | Dysphagia, hoarseness, loss of gag, trapezius/SCM weakness | Glomus jugulare, metastases, meningioma, skull base fracture |
| Collet-Sicard | IX, X, XI, XII | Vernet + tongue deviation/atrophy | Skull base tumors, carotid dissection, trauma |
| Villaret | IX, X, XI, XII + sympathetics | Collet-Sicard + Horner syndrome | Retroparotid/retropharyngeal space lesions |
| Garcin (half-base) | Multiple unilateral CNs (up to all 12) | Progressive unilateral cranial neuropathies without raised ICP or long-tract signs | Skull base malignancy (nasopharyngeal carcinoma, metastatic), carcinomatous meningitis |
| Gradenigo | V, VI | Lateral rectus palsy + facial pain + otitis media (petrous apicitis) | Complicated otitis media, petrous bone osteomyelitis |
Brainstem Cranial Nerve Syndromes (Summary)
| Syndrome | Level | Artery | Ipsilateral CN Deficit | Contralateral Findings |
|---|---|---|---|---|
| Weber | Midbrain (ventral) | PCA perforators | CN III | Hemiparesis (cerebral peduncle) |
| Benedikt | Midbrain (tegmentum) | PCA perforators | CN III | Tremor/ataxia (red nucleus) |
| Claude | Midbrain (dorsal tegmentum) | PCA perforators | CN III | Ataxia (SCP) + tremor (red nucleus) |
| Millard-Gubler | Ventral pons | Basilar perforators | CN VI + VII | Hemiparesis (corticospinal tract) |
| Foville | Dorsal pons | Basilar perforators | CN VI (gaze palsy) + VII | Hemiparesis (variable — only when corticospinal in basis pontis is involved) |
| Wallenberg | Lateral medulla | PICA / vertebral | CN IX, X, Horner, cerebellar ataxia, facial pain/temp loss | Body pain/temp loss (spinothalamic) |
| Dejerine (medial medullary) | Medial medulla | ASA | CN XII | Hemiparesis (pyramid) + proprioceptive loss (medial lemniscus) |
Clinical Pearl — Crossed Brainstem Rule
The hallmark of a brainstem lesion is crossed findings: ipsilateral cranial nerve deficit + contralateral long-tract signs (motor and/or sensory). This pattern distinguishes brainstem stroke from hemispheric stroke, where all deficits are on the same (contralateral to lesion) side.
Additional High-Yield Cranial Nerve Topics
Cranial Nerve Reflexes — Quick Reference
| Reflex | Afferent | Efferent | Clinical Use |
|---|---|---|---|
| Pupillary light reflex | CN II | CN III (parasympathetic) | RAPD detection; coma assessment |
| Corneal reflex | CN V1 | CN VII (bilateral) | Brainstem integrity; trigeminal/facial nerve assessment |
| Jaw jerk | CN V (mesencephalic) | CN V (motor) | Brisk → bilateral UMN lesion above pons (pseudobulbar) |
| Gag reflex | CN IX | CN X | Bulbar function; intubation assessment |
| Oculocephalic ("doll's eyes") | CN VIII (vestibular) | CN III, VI (via MLF) | Brainstem integrity in comatose patients |
| Vestibulo-ocular (cold calorics) | CN VIII (vestibular) | CN III, VI (via MLF) | "COWS" — Cold Opposite, Warm Same (fast phase direction in conscious patient) |
Multiple Cranial Neuropathies — Differential Diagnosis
- Infectious: Lyme disease, HIV, tuberculosis, fungal meningitis, syphilis
- Inflammatory: Sarcoidosis, GPA (Wegener), Tolosa-Hunt, IgG4-related disease
- Neoplastic: Leptomeningeal carcinomatosis (#1 consideration in progressive multiple cranial neuropathies), skull base tumors, lymphoma, nasopharyngeal carcinoma
- Autoimmune: GBS (Miller Fisher variant — ophthalmoplegia, ataxia, areflexia), myasthenia gravis (mimics cranial nerve palsies)
- Vascular: Cavernous sinus thrombosis, carotid dissection, diabetic cranial neuropathy
- Other: Paget disease, trauma, Chiari malformation
Board Pearl
Progressive, painless, multiple cranial neuropathies in a cancer patient = leptomeningeal carcinomatosis until proven otherwise. Diagnosis requires CSF cytology (may need repeat LP) and/or MRI with gadolinium showing leptomeningeal enhancement. Miller Fisher syndrome (anti-GQ1b antibodies) is the classic autoimmune mimic with ophthalmoplegia + ataxia + areflexia.
Pupil Pathology — High-Yield
Argyll-Robertson Pupil
- Small, irregular, bilateral pupils
- Accommodates but does not react to light (light-near dissociation)
- Causes: Neurosyphilis (classic), diabetes mellitus, multiple sclerosis, midbrain tumors
- Lesion: rostral midbrain periaqueductal region (precise site debated); distinct from the pretectal lesion of Parinaud (which produces mid-dilated, not miotic, pupils) — interrupts the light reflex but spares the near (accommodation-convergence) reflex, which travels via a separate ventral pathway
- Mnemonic: "Argyll-Robertson Prostitute pupil — accommodates but doesn't react"
Adie (Holmes-Adie) Tonic Pupil
- Large, sluggish pupil with tonic re-dilation
- Light-near dissociation (better near response — light reflex impaired)
- Vermiform movements of the iris on slit-lamp examination
- Reduced deep tendon reflexes (= Holmes-Adie syndrome when combined)
- Lesion: Postganglionic parasympathetic damage (ciliary ganglion / short ciliary nerves)
- Pharmacology: Supersensitive to dilute (0.125%) pilocarpine → constricts (normal pupil does not respond at this dilution) — denervation hypersensitivity
- Demographics: Young women, idiopathic most common
Horner Syndrome — 3-Neuron Sympathetic Pathway
| Neuron | Pathway | Common Lesions | Anhidrosis Distribution |
|---|---|---|---|
| 1st-order (central) | Hypothalamus → descends through brainstem → ciliospinal center of Budge-Waller (C8–T2) | Brainstem stroke (Wallenberg), MS, brainstem tumor, syringomyelia | Entire ipsilateral hemibody |
| 2nd-order (preganglionic) | Ciliospinal center → exits T1 ventral root → ascends through stellate ganglion → superior cervical ganglion (SCG) | Pancoast tumor (apical lung), cervical rib, brachial plexus injury, thyroid/neck surgery | Face and neck only |
| 3rd-order (postganglionic) | SCG → carotid plexus on ICA → cavernous sinus → orbit via long ciliary nerves | Carotid artery dissection (classic — pain + Horner in young adult), cavernous sinus disease, cluster headache | No anhidrosis (sweat fibers travel with ECA branches) |
Pharmacologic Testing for Horner Syndrome
| Agent | Purpose | Normal Pupil | Horner Pupil |
|---|---|---|---|
| Cocaine 4–10% | Screening (confirms Horner) | Dilates (blocks NE reuptake; requires intact sympathetic tone) | Fails to dilate (no tonic NE release) |
| Apraclonidine 0.5–1% | Modern screening alternative to cocaine | No effect / mild miosis | Paradoxical dilation due to denervation supersensitivity (α1 upregulation) |
| Hydroxyamphetamine 1% | Localizes lesion (after ≥24h from cocaine test) | Dilates (releases stored NE from postganglionic terminals) | Dilates in 1st and 2nd order (preganglionic); fails to dilate in 3rd order (postganglionic — no NE stores) |
Board Pearl
Horner + neck pain in a young adult = carotid dissection until proven otherwise. Order CTA/MRA of the neck. The dissection injures the postganglionic (3rd-order) sympathetic fibers traveling on the ICA — no facial anhidrosis because sweat fibers follow the ECA. Cocaine confirms Horner; hydroxyamphetamine localizes.
Internuclear Ophthalmoplegia (INO)
- Lesion: MLF between the CN VI nucleus (pons) and the contralateral CN III nucleus (midbrain)
- Side of INO = side of MLF lesion = side of adduction deficit
- Findings: Ipsilateral medial rectus fails to adduct on CONTRALATERAL lateral gaze (gaze AWAY from the side of the MLF lesion); contralateral eye shows abducting nystagmus
- Convergence preserved (distinguishes INO from a CN III nuclear/fascicular lesion)
- Bilateral INO + young patient → multiple sclerosis (classic)
- Unilateral INO + older patient → brainstem stroke (small-vessel pontine infarct)
- WEBINO (Wall-Eyed Bilateral INO): rostral midbrain MLF lesion with bilateral exotropia in primary gaze
One-and-a-Half / Eight-and-a-Half Syndromes
- One-and-a-half syndrome: Ipsilateral PPRF (or CN VI nucleus) plus ipsilateral MLF → ipsilateral conjugate gaze palsy ("one") + ipsilateral INO ("half"). Only remaining horizontal movement = abduction of the contralateral eye (with nystagmus).
- Eight-and-a-half syndrome: One-and-a-half + ipsilateral peripheral CN VII palsy (facial colliculus involvement) — "7 + 1½ = 8½"
- Etiology: Pontine stroke (most common) or MS
Parinaud Syndrome (Dorsal Midbrain Syndrome)
- Classic tetrad:
- Upgaze palsy
- Light-near dissociation
- Convergence-retraction nystagmus (on attempted upgaze)
- Eyelid retraction (Collier sign)
- Setting-sun sign in infants (forced downward gaze with eyelid retraction)
- Causes: Pineal region tumor (germinoma classic in adolescent boys), aqueductal stenosis with hydrocephalus, midbrain stroke, MS
Nystagmus Types — Localization
| Nystagmus Type | Localization | Common Etiologies |
|---|---|---|
| Downbeat | Cervicomedullary junction / flocculus | Chiari I malformation, SCA6, alcohol/lithium toxicity |
| Upbeat | Caudal medulla or pontomesencephalic junction | Brainstem stroke, MS, Wernicke encephalopathy |
| See-saw | Parasellar / chiasm | Large pituitary tumor, craniopharyngioma |
| Periodic alternating | Cervicomedullary / cerebellar (nodulus) | Chiari, cerebellar disease — baclofen-responsive |
| Pendular | Oculopalatal myoclonus (Guillain-Mollaret triangle: dentate → red nucleus → inferior olive) | Brainstem/cerebellar stroke; MS |
| Convergence-retraction | Dorsal midbrain | Parinaud syndrome (pineal tumor, aqueductal hydrocephalus) |
Cavernous Sinus Syndrome — Complete Picture
- Contents: CN III, IV, V1, V2 (lateral wall) + CN VI + ICA with surrounding sympathetic plexus (within the sinus itself)
- Clinical lesion: Painful ophthalmoplegia + V1/V2 sensory loss + postganglionic Horner (sympathetics on ICA) + variable proptosis/chemosis
- CN VI is most vulnerable (runs freely through the sinus, not protected within the wall)
| Etiology | Key Features |
|---|---|
| Cavernous sinus thrombosis | Chemosis, proptosis, fever; often bilateral via intercavernous sinuses; source often facial/sinus infection (danger triangle of face) |
| Tolosa-Hunt syndrome | Granulomatous inflammation; painful ophthalmoplegia; steroid-responsive; diagnosis of exclusion |
| Carotid-cavernous fistula (CCF) | Pulsatile proptosis + orbital bruit; arterialized conjunctival vessels; high-flow (direct, often post-traumatic) or low-flow (dural, spontaneous) |
| Meningioma | Slowly progressive cranial neuropathies; dural-based enhancement on MRI |
| Pituitary apoplexy | Sudden severe headache + ophthalmoplegia + visual loss; endocrine collapse; surgical emergency |
| Mucormycosis | Diabetics (DKA) or immunocompromised; rhino-orbital-cerebral; black eschar; angioinvasive — emergent debridement + amphotericin B |
Board Pearl
Cavernous sinus lesion = painful ophthalmoplegia + V1/V2 sensory loss + postganglionic Horner. Superior orbital fissure lesion has the same picture minus V2 (V2 exits via foramen rotundum). Orbital apex syndrome adds optic neuropathy (CN II). The triad of pulsatile proptosis + chemosis + orbital bruit → carotid-cavernous fistula.
References
- Blumenfeld H. Neuroanatomy Through Clinical Cases. 3rd ed. Sinauer Associates; 2021.
- Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 8th ed. Wolters Kluwer; 2022.
- Campbell WW, Barohn RJ. DeJong’s The Neurologic Examination. 8th ed. Wolters Kluwer; 2020.
- Ropper AH, Samuels MA, Klein JP, Prasad S. Adams and Victor’s Principles of Neurology. 12th ed. McGraw Hill; 2023.
- Biller J, Gruener G, Brazis PW. DeMyer’s The Neurologic Examination: A Programmed Text. 7th ed. McGraw Hill; 2017.
- Aminoff MJ, Josephson SA. Aminoff’s Neurology and General Medicine. 6th ed. Academic Press; 2021.
- Kattah JC, Talkad AV, Wang DZ, Hsieh YH, Newman-Toker DE. HINTS to diagnose stroke in the acute vestibular syndrome. Stroke. 2009;40(11):3504–3510.
- Gronseth GS, Paduga R. Evidence-based guideline update: steroids and antivirals for Bell palsy. Neurology. 2012;79(22):2209–2213.
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