Cranial Nerves — Anatomy and Palsies

Nuclear vs. Infranuclear vs. Supranuclear Lesions

• Nuclear lesions affect the motor nucleus (CN III, IV, VI, VII, IX, X, XI, XII) within the brainstem, causing ipsilateral weakness with loss of reflexes and fasciculations; associated brainstem signs (crossed syndromes) implicate contralateral limb motor/sensory findings
• Infranuclear lesions occur distal to the brainstem nucleus, involving the fascicle as it traverses the brainstem or after exiting through foramina; characterized by ipsilateral weakness without contralateral neurological signs; Weber's syndrome (CN III + contralateral hemiparesis) exemplifies a fascicular/nuclear boundary lesion
• Supranuclear lesions affect cortical or corticobulbar tract inputs (e.g., CN VII upper face) sparing lower face due to bilateral cortical innervation; CN XII and CN VII lower face show contralateral weakness (opposite side of body lesion)

Anatomical Basis of Cranial Nerve Dysfunction

• Axonal degeneration from compression (tumor, fracture), ischemia (stroke), inflammation (Bell's palsy, Guillain-Barré syndrome), or demyelination causes conduction block; severity ranges from neurapraxia (conduction block, intact axons, excellent prognosis) to axonotmesis (axonal loss, Wallerian degeneration, weeks-months recovery) to neurotmesis (complete transection, poor spontaneous recovery)
• Brainstem syndromes produce characteristic contralateral deficits: Weber (CN III + contralateral hemiplegia), Claude (CN III + contralateral ataxia), Foville (CN VI + VII + contralateral hemiplegia), lateral medullary/Wallenberg (CN V, IX, X + ipsilateral Horner, contralateral spinothalamic loss)
• Nuclear involvement compromises the motor nucleus pool controlling muscles; damage to CN VII nucleus affects entire hemifacial musculature including forehead (contrast with supranuclear lesion sparing upper face), whereas CN XII nucleus damage causes ipsilateral tongue atrophy and fasciculations

Vascular Causes (Most Common Overall)

• Acute ischemic stroke (thrombotic or embolic): Weber's, Claude's, Foville's, Wallenberg's, locked-in syndromes; risk factors include age, hypertension, diabetes, atrial fibrillation, hyperlipidemia, smoking
• Hemorrhagic stroke: acute onset, severe presentation, brainstem involvement with rapid deterioration
• Posterior circulation insufficiency: TIA prodrome preceding completed stroke

Traumatic Causes

• Temporal bone fracture: CN VII (most common), CN VIII injuries; longitudinal fractures more commonly injure CN VII
• Basilar skull fracture: immediate palsies suggest nerve transection (poor prognosis); delayed onset suggests edema (better prognosis)
• Penetrating injury, birth trauma: forceps delivery causing CN VII, CN VI injuries

Neoplastic Causes

• Primary brainstem glioma: insidious onset, multiple CN involvement, imaging abnormality
• Skull base tumors (acoustic neuroma, meningioma, nasopharyngeal carcinoma): CN V, VII, VIII, IX, X, XI, XII in variable combinations; progressive course
• Metastases: brain, skull base, leptomeningeal involvement; cancer history
• Pituitary apoplexy: sudden multiple CN palsies (III, IV, VI, V1/V2) with ophthalmoplegia

Infectious Causes

• Bell's palsy (idiopathic): presumed viral (HSV-1, VZV), CN VII only, acute onset, 70% recover fully within 3 months
• Ramsey Hunt syndrome: VZV with vesicles in external auditory canal or soft palate; CN VII + VIII involvement; more severe than idiopathic Bell's
• Lyme disease (Borrelia burgdorferi): bilateral CN VII palsies, meningitis; endemic areas (Northeast US)
• Otitis media/mastoiditis: CN VII injury from middle ear inflammation/infection
• Meningitis (bacterial, viral, TB, fungal): multiple CN palsies (CN III, IV, VI, VII, VIII, IX, X), meningeal signs, CSF abnormalities
• HIV/AIDS: CN palsies from opportunistic infections (CMV, TB, cryptococcal meningitis) or direct viral involvement
• Syphilis: tertiary—gummas; neurosyphilis—CN palsies with positive serology
• Botulism: CN III, IV, VI, VII, IX, X, XII involvement; exposure history (foodborne, wound, infant)

Inflammatory/Autoimmune Causes

• Guillain-Barré syndrome: ascending paralysis including CN involvement (CN VII, IX, X, XII common); areflexia, CSF albuminocytologic dissociation
• Miller Fisher syndrome: CN palsies (III, IV, VI), ataxia, areflexia (variant of GBS)
• Myasthenia gravis: CN III, IV, VI (ptosis, ophthalmoplegia), VII, IX involvement; ocular or bulbar presentation; positive edrophonium/antibodies
• Sarcoidosis: Heerfordt syndrome (fever, parotitis, uveitis, CN VII palsy); hilar lymphadenopathy on imaging

Metabolic/Toxic Causes

• Diabetes mellitus: CN III (pupil-sparing ischemic), CN VI palsies; microvascular ischemia
• Melkersson-Rosenthal syndrome: recurrent CN VII palsy, lip swelling, oral fissures; rare
• Alcohol abuse: CN palsies from Wernicke's encephalopathy (thiamine deficiency), direct toxic effect
• Tetanus: trismus, CN VII ("risus sardonicus"), dysphagia

Congenital/Hereditary Causes

• CHARGE syndrome: CN VII, IX, X involvement; congenital heart disease, choanal atresia
• Möbius syndrome: bilateral CN VII (and CN VI) paralysis from birth; developmental brainstem anomaly
• Hemifacial spasm: involuntary facial grimacing; compression of CN VII at root entry zone (usually vascular)

Iatrogenic/Procedure-Related

• Bell's palsy post-vaccination: temporal association with viral vaccines or COVID-19 vaccination (rare)
• CN injury during surgery: parotid surgery (CN VII), skull base surgery
• Intracranial pressure elevation: CN III compression ("blown pupil"), CN VI (false localizing sign with elevated ICP)

Masses at Skull Base/Jugular Foramen

• Jugular foramen syndrome: CN IX, X, XI involvement (glomus tumor, meningioma)
• Hypoglossal canal syndrome: CN XII isolated injury

CN I (Olfactory)

• Loss of smell (anosmia) with preserved taste
• Common causes: head trauma, upper respiratory viral infection, neurodegenerative diseases (Parkinson's, Alzheimer's)
• Usually asymptomatic unless olfactory groove meningioma or anterior fossa fracture
• Distinguish from ageusia (taste loss via CN VII, IX, X) and hyposmia (reduced smell)

CN II (Optic)

• Monocular vision loss (amaurosis fugax if transient), visual field defects, optic disc pallor or edema
• Afferent pupillary defect (APD): pupil appears dilated on affected side because of impaired light perception
• Relative: flashlight test shows slower constriction on affected side; Marcus Gunn pupil
• Optic neuritis, retinal artery occlusion, cavernous sinus thrombosis
• No motor component (CN II purely sensory)

CN III (Oculomotor)

• Ptosis: drooping eyelid from levator palpebrae paralysis
• "Down and out" eye position: superior oblique (CN IV) unopposed, lateral rectus (CN VI) unopposed
• Ophthalmoplegia: impaired eye movements (medial, superior, inferior rectus; inferior oblique)
• Dilated pupil: parasympathetic fibers (preganglionic) affected in "blown pupil"—emergency sign suggesting transtentorial herniation with uncal compression
• Pupil-sparing CN III palsy: ischemic (diabetes, hypertension); parasympathetic fibers in outer layer of nerve preserved; excellent prognosis
• Pupil-involving CN III palsy: compressive lesion (tumor, aneurysm, herniation); medical emergency
• Weber's syndrome: CN III nucleus + contralateral hemiplegia (substantia nigra infarct)

CN IV (Trochlear)

• Vertical diplopia worse looking downward or when reading
• Head tilt away from lesion: patient compensates by tilting head to opposite side
• Intorsion loss: superior oblique affected; images appear rotated
• Most common traumatic CN palsy; longest intracranial course makes vulnerable
• Bilateral CN IV involvement possible from midbrain lesions (e.g., pineal region tumors compressing dorsal midbrain)

CN V (Trigeminal)

• Three divisions (V1 ophthalmic, V2 maxillary, V3 mandibular); both sensory and motor
• Facial sensory loss in distribution of affected division (corneal sensory for reflex arc)
• Motor dysfunction: jaw weakness, jaw deviation toward weak side (masseter, temporalis, pterygoid paralysis)
• Corneal reflex loss: V1 sensory, CN VII motor component—afferent/efferent deficit indicates CN V and/or CN VII involvement
• Trigeminal neuralgia: lancinating pain (V2 or V3), not true nerve palsy; medial lemniscus syndrome: contralateral facial sensory + ipsilateral body sensory
• Sensory level in medulla: Wallenberg's syndrome causes ipsilateral "onion-skin" facial sensory loss (CN V nucleus damaged)

CN VI (Abducens)

• Impaired lateral eye movement (abduction): inability to move affected eye temporally
• Ipsilateral eye deviation medially at rest (unopposed medial rectus)
• Horizontal diplopia worse looking toward affected side
• "False localizing sign": CN VI palsy can indicate elevated intracranial pressure from any cause (long intracranial course, easily stretched)
• Most common CN palsy in increased ICP; Foville's syndrome includes CN VI + CN VII
• Wernicke's encephalopathy: bilateral CN VI (and CN III, IV) involvement

CN VII (Facial)

• Facial weakness: asymmetric smile, inability to close eye on affected side, drooping corner of mouth
• Complete hemifacial palsy: forehead involved (distinguishes infranuclear from supranuclear—supranuclear spares upper face due to bilateral innervation)
• Bell's phenomenon: upward deviation of eye when attempting eye closure (normal reflex)
• Loss of taste anterior 2/3 tongue (chorda tympani branch)
• Hyperacusis: weakness of stapedius; loud sounds uncomfortable
• Loss of corneal reflex (motor component of blink reflex); tear production decreased
• Crocodile tears: aberrant regeneration causing tearing with mastication
• Ramsey Hunt syndrome: vesicles in external auditory canal/soft palate + CN VII + VIII
• Bilateral CN VII palsies: Lyme disease (Bell's palsy typically unilateral, making bilateral presentation highly suggestive of Borrelia)
• Central CN VII palsy (supranuclear): forehead sparing, contralateral to lesion; suggests stroke affecting corticobulbar tract

CN VIII (Vestibulocochlear)

• Vestibular component: vertigo, nystagmus (ipsilateral), oscillopsia, ataxia, nausea/vomiting
• Cochlear component: sensorineural hearing loss, tinnitus, hyperacusis
• Caloric test abnormality: warm caloric produces nystagmus toward affected side; cold caloric away (mnemonic: "COWS"—Cold Opposite, Warm Same)
• Acoustic neuroma: progressive unilateral sensorineural hearing loss + CN VII involvement (cerebellopontine angle syndrome)
• Ramsey Hunt: CN VII + VIII involvement

CN IX & X (Glossopharyngeal & Vagus)

• Dysphagia (difficulty swallowing): aspiration risk
• Dysphonia (voice change): breathy, hoarse quality from vocal cord paralysis
• Dysarthria: slurred speech
• Gag reflex loss: CN IX sensory, CN X motor; asymmetric uvula deviation away from lesion (CN X weak side pulls less)
• Ipsilateral palatal droop
• CN IX isolated injury: rare; causes loss of taste posterior 1/3 tongue, loss of carotid sinus reflex
• Nasal regurgitation: soft palate weakness during swallowing
• Wallenberg's syndrome: ipsilateral CN IX, X involvement + contralateral spinothalamic loss
• Jugular foramen syndrome: CN IX, X, XI palsies together (skull base pathology)
• Vagal bradycardia: CN X parasympathetic involvement (heart rate regulation)

CN XI (Accessory)

• Shoulder weakness: trapezius paralysis causes difficulty elevating shoulder, weakness abducting arm above horizontal
• Neck weakness: sternocleidomastoid paralysis causes weakness turning head toward opposite side, head deviation toward weak side
• Sternocleidomastoid testing: patient turns head against resistance; ipsilateral muscle contracts
• Trapezius testing: shoulder shrug against resistance; ipsilateral trapezius contracts
• Nuclear lesion: ipsilateral weakness (CN XI nucleus in medulla)
• Polio, Grisel's syndrome (atlantoaxial subluxation)

CN XII (Hypoglossal)

• Unilateral tongue atrophy (fasciculations visible in acute denervation)
• Tongue weakness: deviates toward weak (paralyzed) side on protrusion (opposite to facial droop direction)
• Dysarthria from tongue dysfunction
• Dysphagia (milder than CN IX/X involvement)
• Fasciculations visible within days of acute injury; indicates lower motor neuron lesion
• Supranuclear lesion (corticobulbar): weakness contralateral, no fasciculations, no atrophy
• ALS causes progressive CN XII involvement bilaterally with fasciculations
• Gradenigo's syndrome: CN VI + CN XII from petrous bone infection (mastoiditis)

Multiple CN Involvement (Polyradiculopathy/Polycranial Neuropathy Patterns)

• CN III, IV, VI together: orbital pathology, cavernous sinus thrombosis, carvernous sinus syndrome (Tolosa-Hunt syndrome—granulomatous inflammation, responsive to steroids)
• CN V, VII together: Melkersson-Rosenthal, parotitis, skull base tumor

Step 1 — Localize at the bedside before ordering anything

• Supranuclear vs. infranuclear: forehead sparing (CN VII) or absent atrophy/fasciculations (CN XII) points above the nucleus and mandates stroke-pathway evaluation rather than a peripheral work-up.
• Pupil examination in any CN III palsy: pupil-involving palsy implies external compression of the superficially placed parasympathetic fibers; pupil-sparing palsy with intact motility elsewhere suggests microvascular infarction of the nerve core.
• Grading scales: House-Brackmann grade I–VI is the standard facial-nerve severity scale referenced by the AAO-HNS Bell's palsy guideline; NIH Stroke Scale is used when a brainstem syndrome is suspected.

Step 2 — Emergent imaging when the picture is compressive or vascular

• Pupil-involving CN III palsy: urgent CTA or MRA of the head to exclude a posterior communicating artery aneurysm; catheter angiography remains the gold standard when noninvasive studies are negative but suspicion persists. Add noncontrast CT and, if needed, lumbar puncture for xanthochromia if subarachnoid hemorrhage is possible.
• Suspected brainstem stroke (crossed findings, acute multiple palsies): noncontrast head CT first to exclude hemorrhage, then vessel imaging and MRI with diffusion-weighted sequences, per the AHA/ASA acute ischemic stroke guideline. DWI is far more sensitive than CT for small pontine and medullary lesions.
• Progressive, multiple, or skull-base palsies: MRI brain and skull base with gadolinium and thin cuts through the cavernous sinus and foramina.

Step 3 — Targeted laboratory and electrophysiologic testing

• Glucose and HbA1c for microvascular palsy; ESR and CRP in older patients with diplopia or ischemic-appearing palsy to screen for giant cell arteritis.
• Two-tier Lyme serology (EIA followed by immunoblot or a second EIA) per CDC/IDSA in endemic exposure, especially bilateral CN VII palsy.
• AChR and anti-MuSK antibodies, ice-pack test, and repetitive nerve stimulation for fatigable ptosis/ophthalmoparesis; CSF showing albuminocytologic dissociation supports GBS; CSF cytology (often repeated) for leptomeningeal carcinomatosis.
• Electroneuronography/EMG after facial palsy quantifies degeneration and informs prognosis; the AAO-HNS advises against routine labs or imaging in typical new-onset unilateral Bell's palsy.

Immediate/emergency actions

• Pupil-involving CN III palsy: treat as an aneurysm or herniation until excluded — emergent vascular imaging and neurosurgical/neuro-interventional consultation; secure the airway and manage intracranial pressure if there is depressed consciousness.
• Acute brainstem stroke: IV thrombolysis (alteplase, or tenecteplase as an alternative) within the eligible window and endovascular thrombectomy for large-vessel occlusion, per the AHA/ASA acute ischemic stroke guideline.
• Neuromuscular respiratory failure (GBS, myasthenic crisis, botulism): serial forced vital capacity and negative inspiratory force; intubate on trend, not on blood gas.

First-line therapy by etiology

• Bell's palsy: oral corticosteroids (prednisone) started within 72 hours of onset for patients 16 and older — a strong recommendation of the AAO-HNS guideline. Antivirals (valacyclovir) may be offered only as an adjunct to steroids; the AAO-HNS recommends against antiviral monotherapy.
• Eye protection is mandatory in any lagophthalmos: preservative-free artificial tears by day, ointment plus taping or a moisture chamber at night.
• Ramsay Hunt syndrome: corticosteroid plus antiviral (acyclovir or valacyclovir), started as early as possible.
• Lyme-associated facial palsy: oral doxycycline is first-line for Lyme cranial neuropathy and is also an accepted option for Lyme meningitis per the IDSA/AAN/ACR 2020 Lyme guideline; parenteral ceftriaxone (or cefotaxime/penicillin G) is preferred for parenchymal CNS involvement (encephalitis, myelitis) or when oral therapy is not tolerated.
• Botulism: early equine heptavalent botulism antitoxin (BabyBIG for infant botulism) plus airway and ventilatory support; antitoxin arrests further paralysis but does not reverse established deficits — do not delay for confirmatory testing, and involve the health department/CDC for release.
• Myasthenia gravis: acetylcholinesterase inhibitor (pyridostigmine) plus immunotherapy; IVIG or plasma exchange for crisis. GBS: IVIG or plasma exchange — corticosteroids are not effective and are not recommended.
• Trigeminal neuralgia: carbamazepine (or oxcarbazepine) is first-line per AAN/EFNS; microvascular decompression or gamma-knife for refractory disease.

Escalation and definitive management

• Microvascular CN III/IV/VI palsy: risk-factor control and observation, with prisms or monocular occlusion for diplopia; most resolve over months. Persistent, stable strabismus beyond about six months is corrected surgically.
• Chronic exposure keratopathy: gold-weight lid loading or tarsorrhaphy; late facial reanimation for non-recovering palsy. Hemifacial spasm: botulinum toxin injection.
• Contraindicated/avoid: antiviral monotherapy for Bell's palsy, steroids as sole therapy for GBS, and aminoglycosides, fluoroquinolones, and magnesium in myasthenia.

Disease-related — emergencies first

• Ruptured posterior communicating artery aneurysm: a pupil-involving CN III palsy is a sentinel sign of aneurysmal expansion; sudden thunderclap headache, meningismus, or depressed consciousness signals subarachnoid hemorrhage. Emergency.
• Uncal herniation: progressive ipsilateral pupillary dilation with a "down and out" eye plus declining consciousness reflects midbrain compression. Emergency.
• Neuromuscular respiratory failure in GBS, myasthenic crisis, or botulism: bulbar palsies plus a falling forced vital capacity, staccato speech, and paradoxical abdominal breathing precede hypercapnia. Emergency.
• Exposure keratopathy and corneal ulceration: lagophthalmos with a lost corneal reflex (CN V1 afferent, CN VII efferent) desiccates the cornea; a red, painful eye with a fluorescein-staining defect requires same-day ophthalmology. Emergency for vision.
• Aspiration pneumonia: CN IX/X palsy abolishes airway protection; wet voice, cough with swallowing, and a new infiltrate are the tip-offs. Screen swallowing before any oral intake.

Late/structural sequelae

• Aberrant regeneration: misdirected axonal sprouting produces facial synkinesis and crocodile tears (gustatory lacrimation) after CN VII injury, and lid elevation on downgaze (pseudo-von Graefe sign) after CN III injury — aberrant CN III regeneration argues for a compressive, not ischemic, cause.
• Permanent weakness and contracture after axonotmesis or neurotmesis; hemifacial spasm may follow incomplete recovery.

Treatment-related

• Corticosteroids: hyperglycemia (notable in the diabetic patient with an ischemic palsy), insomnia, mood change, and with repeated courses osteonecrosis.
• Carbamazepine: hyponatremia via SIADH, agranulocytosis/aplastic anemia, and Stevens-Johnson syndrome linked to **HLA-B*15:02** — screen patients of Asian ancestry before starting.
• IVIG: thrombosis, aseptic meningitis, renal injury, and anaphylaxis in IgA deficiency; plasma exchange: hypotension, citrate-induced hypocalcemia, catheter complications.
• Botulinum toxin: local diffusion causing ptosis or dysphagia.
• Thrombolysis: symptomatic intracranial hemorrhage — sudden neurologic worsening mandates stopping infusion and emergent CT.

• **Pupil-involving CN III palsy is a PCom aneurysm until proven otherwise: parasympathetic fibers run superficially and are crushed first by compression, whereas diabetic microvascular infarction damages the core and spares the pupil**. Single best next step: urgent CTA/MRA, not observation.
• Forehead tells you the level: forehead spared = supranuclear/central (stroke, contralateral to the lesion, because the upper face has bilateral corticobulbar input); forehead involved = peripheral CN VII (Bell's, Ramsay Hunt).
• Bilateral facial palsy is almost never idiopathic — it mandates a work-up for Lyme disease, sarcoidosis (Heerfordt syndrome), GBS, or HIV before accepting a diagnosis of Bell's palsy. Send two-tier Lyme serology in an endemic exposure and treat with doxycycline per IDSA/AAN/ACR.
• Deviation rules: the tongue deviates toward the lesioned CN XII (unopposed contralateral genioglossus); the uvula deviates away from the lesioned CN X. Fasciculations and hemiatrophy of the tongue mark a lower motor neuron lesion.
• **CN VI palsy is the classic *false localizing sign*** of raised intracranial pressure because of its long intradural course over the petrous ridge — do not localize the mass to the pons on that finding alone.
• Gradenigo syndrome = petrous apicitis with the triad of CN VI palsy, deep retro-orbital/facial pain (CN V), and otorrhea complicating otitis media — the pairing is CN VI with CN V, not CN XII (the CN XII reference in Clinical Presentation is flagged for editorial correction).
• CN IV palsy: vertical diplopia worse on downgaze (descending stairs, reading), compensatory head tilt away from the lesion, and worsening on tilt toward the lesion — the basis of the Parks–Bielschowsky three-step test.
• The examiner's favorite mimic: fatigable, variable ptosis and ophthalmoparesis with a normal pupil is myasthenia gravis, not a CN III palsy; confirm with the ice-pack test and AChR antibodies. Likewise, failure of adduction with abducting nystagmus in the other eye and preserved convergence is internuclear ophthalmoplegia (MLF), not CN III.
• The Bell's palsy distractor: antiviral monotherapy. The AAO-HNS recommends oral corticosteroids within 72 hours, antivirals only as an adjunct, and eye protection for everyone with incomplete lid closure.