Cranial Nerve Palsies

• Definition: focal dysfunction of one or more cranial nerves anywhere along the axis from the brainstem nucleus, through the fascicle, subarachnoid space, cavernous sinus/skull base, to the peripheral branch. Localization along that axis — not the deficit itself — drives the workup.
• Why it matters: the same deficit can be benign or lethal depending on company kept. An isolated pupil-sparing CN III palsy in a diabetic is usually microvascular and self-limited; the same palsy with a dilated pupil is a posterior communicating artery aneurysm until proven otherwise. A partial CN VI palsy may be the false localizing sign of raised intracranial pressure preceding herniation.
• Most common single palsy: CN VII (Bell palsy), an acute idiopathic lower motor neuron facial paresis. Incidence is on the order of a few dozen cases per 100,000 person-years, distributed equally between sexes, peaking in young and middle adulthood.

Epidemiology worth recalling

• Pregnancy: Bell palsy risk rises substantially in the third trimester and early postpartum period — a stem detail examiners plant deliberately.
• Diabetes mellitus: the dominant risk state for both Bell palsy and microvascular ocular motor palsies (CN III, IV, VI) in patients over 50.
• Ocular motor palsies: CN VI is the most frequently affected ocular motor nerve across all ages because of its long, tethered intracranial course; CN IV palsy is the commonest single cause of isolated vertical diplopia and is frequently congenital with late decompensation.
• Children: post-viral or post-immunization benign CN VI palsy is described, but a new CN VI palsy in a child mandates imaging for posterior fossa tumor (pontine glioma, medulloblastoma).
• Multiple simultaneous palsies are never benign — they imply cavernous sinus, skull base, meningeal, or neuromuscular junction disease.

Microvascular / ischemic (most common cause of isolated CN III, IV, VI over age 50)

• Occlusion of the vasa nervorum in diabetes, hypertension, dyslipidemia, or smoking. Typically painful-but-pupil-sparing CN III.

Compressive

• Aneurysm: posterior communicating artery aneurysm → pupil-involving CN III.
• Uncal herniation: mass effect compresses CN III against the tentorial edge.
• Tumor: schwannoma (CN VIII), nasopharyngeal carcinoma invading skull base, parotid malignancy causing slowly progressive CN VII palsy, posterior fossa tumor in a child with CN VI palsy.
• Raised intracranial pressure (including idiopathic intracranial hypertension) stretching CN VI.

Inflammatory / infectious

• Presumed HSV-1 reactivation in the geniculate ganglion (Bell palsy); VZV reactivation (Ramsay Hunt syndrome, with vesicles in the ear canal); Lyme neuroborreliosis (classically bilateral CN VII); acute otitis media/cholesteatoma; HIV seroconversion; neurosarcoidosis; Guillain-Barré and its Miller Fisher variant (ophthalmoplegia, ataxia, areflexia).

Traumatic

• CN IV has the longest intracranial course and exits dorsally, making it the nerve most often injured by closed head trauma; temporal bone fracture injures CN VII.

Vascular/brainstem

• Lacunar or branch infarcts producing alternating syndromes (Weber, Benedikt, Claude, Foville, Raymond).

• Topography of CN III explains the pupil rule: the parasympathetic pupillomotor fibers travel superficially on the dorsomedial surface of the nerve and are supplied by pial vessels, whereas the somatic motor fibers occupy the core and depend on the vasa nervorum. An extrinsic compressive lesion (posterior communicating artery aneurysm, uncal herniation) squeezes the outside first → early mydriasis with poorly reactive pupil. Diabetic microvascular infarction destroys the ischemia-sensitive core → ptosis and ophthalmoplegia with a spared pupil, often with periorbital pain.
• "Down and out" eye: loss of medial, superior, and inferior recti plus inferior oblique leaves the unopposed lateral rectus (CN VI) and superior oblique (CN IV) — abduction and intorsion/depression. Levator palpebrae denervation adds ptosis.
• CN VI as a false localizer: the nerve ascends the clivus and bends over the petrous ridge through Dorello's canal, tethered at both ends. Any diffuse rise in intracranial pressure — regardless of location — stretches it, producing abduction failure that falsely suggests a pontine lesion.
• CN IV vulnerability: it is the thinnest nerve, has the longest intracranial course, and is the only cranial nerve to decussate and exit dorsally, so it is sheared by contrecoup head trauma. Superior oblique weakness causes hypertropia and excyclotorsion, hence the compensatory head tilt away from the lesion and the positive Parks-Bielschowsky three-step test.
• CN VII and the fallopian canal: viral or immune-mediated inflammatory edema of the nerve within a rigid bony canal produces a compartment syndrome → demyelination, conduction block, and, if severe, axonal loss (which predicts incomplete recovery and synkinesis).
• Forehead sparing in central lesions: the upper facial nucleus receives bilateral corticobulbar input; a unilateral upper motor neuron lesion therefore weakens only the contralateral lower face.
• Alternating brainstem syndromes arise because the exiting cranial nerve fascicle and the still-uncrossed corticospinal tract are adjacent, giving ipsilateral nerve plus contralateral body signs.

Oculomotor (CN III)

• Ptosis, mydriasis, and an eye resting down-and-out, with binocular diplopia that disappears when either eye is covered. A painful pupil-involving palsy in a patient with sudden severe headache is the aneurysm/subarachnoid stem. The classic microvascular stem is a 55-year-old with poorly controlled type 2 diabetes with ptosis and a normally reactive pupil.

Trochlear (CN IV)

• Vertical or oblique diplopia worse on downgaze — trouble reading and descending stairs — with a compensatory head tilt to the opposite shoulder. Stem cue: recent closed head injury, or a lifelong photograph showing head tilt.

Abducens (CN VI)

• Horizontal diplopia worse at distance and on gaze toward the lesion, esotropia at rest, and an adopted face turn. Stem cues: headache, papilledema, and pulsatile tinnitus in an obese woman of childbearing age (idiopathic intracranial hypertension), or a malnourished/alcohol-using patient with confusion and ataxia (thiamine deficiency).

Facial (CN VII, lower motor neuron)

• Acute onset over hours to a day of unilateral complete hemifacial weakness including the forehead, inability to close the eye with upward globe rotation (Bell phenomenon), effaced nasolabial fold, drooling, hyperacusis (stapedius denervation), decreased tearing, and loss of taste on the anterior two-thirds of the tongue (chorda tympani). Postauricular pain often precedes the weakness.
• Vesicles in the auditory canal or on the pinna with severe otalgia and vertigo = Ramsay Hunt syndrome.
• Bilateral facial palsy = think Lyme neuroborreliosis, neurosarcoidosis (Heerfordt syndrome with uveitis and parotid enlargement), Guillain-Barré, or HIV.

Red-flag combinations

• Proptosis, chemosis, fever, and CN III/IV/V1/V2/VI involvement = cavernous sinus thrombosis.
• Fluctuating ptosis and diplopia that worsen through the day with a normal pupil = myasthenia gravis, not a nerve palsy.

Step 1 — bedside localization

• Examine the pupil (size, reactivity, anisocoria in light versus dark), levator function, and ductions in all nine positions; perform alternate cover testing and the Parks-Bielschowsky three-step test for suspected CN IV palsy.
• For facial weakness, the single decisive question is forehead involvement: sparing means central (stroke pathway, urgent imaging and stroke-team activation per AHA/ASA acute stroke guidance); full involvement means peripheral.
• Grade facial palsy severity with the House-Brackmann scale, the instrument used in the AAO-HNS Bell's palsy guideline, which also predicts recovery.
• Fundoscopy for papilledema in any CN VI palsy.

Step 2 — who gets imaged

• Any pupil-involving CN III palsy is a neurovascular emergency. If subarachnoid hemorrhage is suspected, obtain noncontrast head CT first, followed immediately by CT angiography (or MRA) of the circle of Willis to exclude posterior communicating artery aneurysm; if CT is negative and suspicion persists, proceed to lumbar puncture for xanthochromia.
• Observation applies only to a complete, pupil-sparing CN III palsy in a vasculopathic adult over 50, alongside vascular risk-factor evaluation. Any partial or incomplete external ophthalmoplegia with a spared pupil should be imaged urgently, because aneurysmal compression can present that way. Re-examine the pupil daily for the first week — pupil involvement may be delayed. Failure to improve by roughly three months, progression, or any new sign mandates MRI with contrast.
• Isolated pupil-sparing CN IV or VI palsy in a vasculopathic adult may likewise be observed with the same three-month rule.
• MRI brain and orbits with contrast for any multiple, progressive, bilateral, or pain-with-numbness presentation, or for facial palsy that evolves over weeks (parotid or perineural tumor).
• MR venography when cavernous sinus thrombosis is suspected.

Step 3 — targeted labs

• Glucose/HbA1c and lipids for microvascular palsy; ESR and CRP in anyone over 50 with diplopia to screen for giant cell arteritis (ACR criteria; temporal artery biopsy confirms).
• Two-tier Lyme serology (EIA then immunoblot, or modified two-tier per CDC/IDSA) for facial palsy with exposure history or bilateral involvement.
• Acetylcholine receptor antibodies and ice-pack test when myasthenia is in the differential; lumbar puncture showing albuminocytologic dissociation for Guillain-Barré/Miller Fisher; ACE level and chest imaging for sarcoidosis.
• The AAO-HNS guideline explicitly recommends against routine laboratory testing or imaging in typical new-onset Bell palsy.

Immediate triage first

• Pupil-involving CN III palsy → emergent vascular imaging and neurosurgical/neurointerventional consultation for aneurysm clipping or endovascular coiling; ruptured aneurysm is managed per AHA/ASA aneurysmal subarachnoid hemorrhage guidance.
• Forehead-sparing facial weakness or any accompanying long-tract sign → acute stroke pathway (AHA/ASA): noncontrast CT, IV thrombolysis with a fibrinolytic (alteplase or tenecteplase) if within the window, thrombectomy for large-vessel occlusion.
• Cavernous sinus thrombosis → broad-spectrum IV antibiotics covering Staphylococcus aureus plus streptococci and anaerobes, with anticoagulation used case-by-case.

Bell palsy (AAO-HNS guideline)

• Oral corticosteroids (prednisone or prednisolone) started within 72 hours of onset in patients 16 and older — the strongest recommendation, supported by the AAN practice parameter, because they reduce inflammatory edema within the fallopian canal.
• Antivirals (a nucleoside analogue such as valacyclovir) may be offered in addition to, never instead of, steroids; benefit is modest at best. For Ramsay Hunt syndrome, antiviral plus steroid is standard.
• Eye protection is mandatory — ocular lubricant during the day, ointment and taping or a moisture chamber at night. This is the intervention that prevents the one truly vision-threatening complication.
• Electrodiagnostic testing is reserved for complete paralysis; physical therapy and facial retraining help chronic cases.

Ocular motor palsies

• Microvascular palsies are managed by risk-factor control (glycemic control per ADA Standards of Care, blood pressure and statin therapy per ACC/AHA) plus symptomatic monocular occlusion or Fresnel prisms; most recover spontaneously within about three months.
• Persistent, stable strabismus beyond six months → strabismus surgery or botulinum toxin to the antagonist muscle.
• Lyme facial palsy: oral doxycycline per IDSA/AAN/ACR 2020. Doxycycline is acceptable in children of any age for short courses (≤21 days) under that guideline.

Contraindicated or wrong

• Do not patch a child's eye without ophthalmology input — amblyopia risk.
• Do not treat with steroids alone when herpes zoster vesicles are present.
• Avoid doxycycline in pregnancy; substitute amoxicillin or cefuroxime. Age alone is not a reason to withhold it for Lyme disease.
• Do not withhold imaging in a pupil-involving CN III palsy because the patient is diabetic.

Emergencies

• Aneurysmal rupture with subarachnoid hemorrhage — a pupil-involving CN III palsy can be a sentinel sign; thunderclap headache, meningismus, and depressed consciousness signal rupture.
• Uncal herniation — a fixed dilated pupil with progressive obtundation in a patient with a mass lesion; requires immediate osmotic therapy and decompression.
• Cavernous sinus thrombosis extension — worsening proptosis, contralateral eye involvement, sepsis, or septic emboli.
• Exposure keratopathy progressing to corneal ulcer and perforation in facial palsy — a red, painful eye with a fluorescein-staining defect and reduced acuity; this is the reason eye care is non-negotiable.

Disease complications

• Synkinesis from aberrant facial nerve regeneration — involuntary eye closure with smiling; crocodile tears (gustatory lacrimation) when secretomotor fibers misroute to the lacrimal gland.
• Aberrant regeneration of CN III — lid elevation on attempted downgaze or adduction (pseudo-von Graefe sign) and pupillary constriction on adduction; its presence in a non-traumatic palsy suggests a compressive lesion rather than microvascular ischemia.
• Persistent diplopia and torticollis from incomplete ocular motor recovery.
• Amblyopia in a child whose strabismus or ptosis occludes the visual axis during the critical period.
• Optic atrophy and permanent vision loss if the CN VI palsy reflects untreated raised intracranial pressure with chronic papilledema.
• Facial contracture, hemifacial spasm, and psychosocial morbidity after incomplete Bell palsy recovery — more likely with complete paralysis at presentation and advanced age.

Treatment complications

• Corticosteroids: hyperglycemia (particularly relevant since diabetics are over-represented), insomnia, mood disturbance or frank psychosis, gastritis, and with repeated courses, avascular necrosis of the femoral head.
• Doxycycline: photosensitivity and pill esophagitis; dental staining is a concern with prolonged or repeated tetracycline exposure, not with the short courses (≤21 days) used for Lyme disease.
• Botulinum toxin for strabismus: induced ptosis or a new vertical deviation from diffusion into adjacent muscles.
• Endovascular coiling or clipping: periprocedural stroke, rebleeding, and vasospasm.

• The pupil decides the emergency. A CN III palsy with a dilated, sluggish pupil = posterior communicating artery aneurysm until excluded; single best next step is urgent vascular imaging (noncontrast CT first if SAH is suspected, then CTA/MRA of the circle of Willis), not observation and not an HbA1c. Only a complete, pupil-sparing palsy in a vasculopath is observable — and recheck the pupil daily for a week, because involvement can be delayed.
• Forehead sparing = central. Bilateral corticobulbar innervation of the upper face means a stroke spares the forehead; Bell palsy does not. This is the most frequently tested discriminator on the exam.
• **Thiamine deficiency causes CN VI palsy — and CN VI supplies the lateral rectus. The deficit is failure of abduction** (with horizontal diplopia and, when bilateral, a horizontal gaze palsy), not medial rectus weakness. (Editorial note: the QUICK FACTS line attributing Wernicke-related CN VI palsy to medial rectus weakness requires correction.)
• Bilateral CN VII palsy is never idiopathic — think Lyme neuroborreliosis first, then neurosarcoidosis (Heerfordt), Guillain-Barré, and HIV.
• **CN VI is the *false localizing sign***: its long tethered course over the petrous ridge means raised intracranial pressure from any cause produces abduction failure. Look for papilledema before you localize to the pons.
• Vesicles change the diagnosis and the drug: facial palsy with ear vesicles and vertigo = Ramsay Hunt, requiring antiviral plus steroid, with worse prognosis than Bell palsy.
• Steroids within 72 hours are the highest-value intervention in Bell palsy (AAO-HNS); antivirals are adjunctive at best, and eye lubrication with nighttime taping is what actually saves the cornea.
• The classic distractor is myasthenia gravis: fatigable ptosis and variable diplopia with a normal pupil — order acetylcholine receptor antibodies, not an angiogram.
• Head tilt away from the lesion with vertical diplopia worse on reading or descending stairs = CN IV; confirm with the Parks-Bielschowsky three-step test.

• Bell's palsy (CN VII) is the most common CN palsy; diagnosis of exclusion
• CN III palsy (oculomotor) → ipsilateral ptosis, "down and out" eye, pupil dilation (if complete)
• CN VI palsy (abducens) → inability to abduct eye; most common CN palsy in children
• Wernicke-Korsakoff syndrome causes CN VI palsies bilaterally due to medial rectus weakness
• Weber syndrome = CN III + contralateral hemiparesis (midbrain infarct)

Mnemonic - CN Palsies by Level

• Brainstem syndromes = alternating (ipsilateral CN + contralateral motor signs)
• CN III → Weber, Benedict, Claude syndromes
• CN VI → Foville, Raymond syndromes

1. 1.Confusing Bell's palsy with central facial droop: Central stroke spares forehead (bilateral UMN innervation); Bell's palsy affects entire face including forehead.
2. 2.Missing pupil involvement in CN III palsy: A dilated pupil = surgical emergency (aneurysm); pupil-sparing CN III palsy may be medically managed (microvascular ischemia).
3. 3.Over-investigating Bell's palsy: ~75% idiopathic; neuroimaging not indicated unless atypical features (bilateral, recurrent, progressive, or neurologic deficits).

• Bell's palsy: Corticosteroids (prednisone 1mg/kg/day × 7 days) + eye care (lubricating drops, eye taping, protective glasses to prevent corneal abrasion)
• CN III/IV/VI palsies: Treat underlying cause (stroke → tPA/thrombectomy; aneurysm → neurosurgery); prism or eye patch for diplopia
• Infectious causes (Lyme): Doxycycline 100mg BID × 14-21 days
• Cavernous sinus thrombosis: IV antibiotics + anticoagulation (controversial)