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Cranial Nerves Overview
1. Olfactory – CN I – Smell
- Detects odors and sends smell information to the brain.
2. Optic – CN II – Vision
- Transmits visual information from the eyes to the brain.
3. Oculomotor – CN III – Eye movement, pupil constriction, eyelid control
- Moves the eye in most directions:
- Up – superior rectus
- Down – inferior rectus
- Inward toward the nose – medial rectus
- Up and out diagonally – inferior oblique
- Controls pupil constriction, making the pupil smaller in bright light.
- Lifts the eyelid, preventing drooping or ptosis.
4. Trochlear – CN IV – Eye movement down and inward
- Moves the eye down and inward toward the nose.
- Helps with looking at objects close to you, like reading a book.
5. Trigeminal – CN V – Facial sensation and chewing
- Feels touch, pain, and temperature from the face.
- Moves the muscles used for biting and chewing.
6. Abducens – CN VI – Eye movement side to side
- Moves the eye outward, away from the nose.
7. Facial – CN VII – Facial movement, taste, tear and saliva production
- Controls facial expressions like smiling, frowning, and closing eyes.
- Provides taste sensation from the front two-thirds of the tongue.
- Controls tear glands, keeping eyes moist.
- Controls salivary glands, keeping the mouth moist.
8. Vestibulocochlear – CN VIII – Hearing and balance
- Detects sound from the inner ear.
- Helps maintain balance and spatial orientation.
9. Glossopharyngeal – CN IX – Taste, swallowing, saliva, blood pressure reflex
- Provides taste sensation from the back one-third of the tongue.
- Helps move food down the throat during swallowing.
- Controls the parotid salivary gland.
- Senses blood pressure changes in the carotid artery.
10. Vagus – CN X – Autonomic control, swallowing, voice, gag reflex
- Controls automatic body functions like heart rate, breathing, and digestion.
- Assists with swallowing.
- Controls vocal cord movement, affecting the voice.
- Plays a role in the gag reflex to protect from choking.
11. Accessory – CN XI – Shoulder shrug and head turn
- Turns the head using the sternocleidomastoid muscle.
- Shrugs the shoulders using the trapezius muscle.
12. Hypoglossal – CN XII – Tongue movement
- Moves the tongue for speaking, eating, and swallowing.
Cranial Nerve Palsies – Overview & Definition
Definition
- Cranial nerve palsy – dysfunction or paralysis of one or more cranial nerves, leading to motor, sensory, or autonomic deficits.
- May be isolated or involve multiple nerves.
- Causes include vascular, traumatic, inflammatory, compressive, infectious, metabolic, or degenerative diseases.
Classification
Cranial nerve palsies are categorized based on their function and presentation:
- Motor nerve palsies – affect eye movements, swallowing, or facial expressions (CN III, IV, VI, VII, IX, X, XI, XII).
- Sensory nerve palsies – affect vision, hearing, or sensation (CN I, II, V, VIII).
- Mixed motor-sensory palsies – affect both movement and sensation (CN V, IX, X).
General Clinical Features
- Motor deficits – weakness, paralysis, atrophy of muscles
- Sensory deficits – numbness, pain, loss of smell, vision, or hearing
- Autonomic dysfunction – abnormal pupillary responses, dry eyes, excessive salivation or lacrimation
Common Causes of Cranial Nerve Palsies
- Vascular – stroke, giant cell arteritis
- Trauma – skull fractures, cribriform plate damage (CN I), direct nerve injury
- Tumors – acoustic neuroma (CN VIII), meningiomas, skull base tumors
- Infectious – meningitis, Lyme disease, herpes zoster, Ramsay Hunt syndrome
- Demyelinating – multiple sclerosis (CN II, V, VII involvement common)
- Neuromuscular disorders – myasthenia gravis (affecting CN III, IV, VI)
- Idiopathic – Bell’s palsy (CN VII), Tolosa-Hunt syndrome (CN III, IV, VI)
Diagnosis
- Clinical history and exam – pattern of weakness, sensory deficits, associated symptoms
- Neuroimaging – MRI/CT – rule out stroke, tumors, demyelination
- Electrophysiology – EMG, nerve conduction studies – if suspected neuropathy
- Blood tests – ESR, CRP, Lyme serology, glucose, B12, autoimmune panels
General Treatment Principles
Supportive care – physical therapy, eye protection, symptomatic treatment
Address underlying cause – vascular, infectious, inflammatory, neoplastic
Steroids – if inflammatory, Bell’s palsy or optic neuritis
Antibiotics/antivirals – if infectious, Lyme disease or herpes zoster
Surgical decompression – if compressive lesion or tumor
CN I – Olfactory Nerve Palsy
- Dysfunction of the olfactory nerve (CN I), responsible for smell perception.
Clinical Presentation
- Loss of smell – anosmia, unilateral or bilateral
- Altered taste perception – often due to smell impairment
- Parosmia – distorted smells, e.g., burnt or rotten odors
- Cacosmia – perceiving foul odors that aren’t present
Physical Exam
- Olfactory testing – identify non-irritating scents like coffee or vanilla
- Nasal exam – assess for obstructions, polyps, or sinus disease
Labs, Studies, and Imaging
- MRI brain – rule out tumor, neurodegenerative disease, or trauma
- CT sinuses – if chronic sinus disease or nasal polyps suspected
- COVID-19 testing – if acute anosmia without nasal congestion
Common Causes
- Head trauma
- Neurodegenerative diseases – Parkinson’s, Alzheimer’s
- Viral infections – including COVID-19
Treatment
- Address underlying cause – sinus disease, neurodegeneration
- Olfactory training therapy – daily exposure to different scents
- Supportive care for post-viral anosmia – smell often returns over time
Key Takeaways
- Anosmia can be an early sign of neurodegenerative disease (Parkinson’s, Alzheimer’s).
- Head trauma can permanently damage CN I.
- Post-viral anosmia (e.g., COVID-19) may last weeks to months.
CN II – Optic Nerve Palsy
- Dysfunction of the optic nerve (CN II), which transmits visual signals from the retina to the brain.
Clinical Presentation
- Unilateral or bilateral vision loss
- Decreased visual acuity – blurry or dim vision
- Loss of color vision – red desaturation, early sign of optic neuritis
- Relative afferent pupillary defect (RAPD) – abnormal light reflex, pupil doesn’t constrict normally
Physical Exam
- Visual acuity testing – Snellen chart
- Fundoscopic exam
- Optic disc swelling (papilledema) – increased ICP
- Pale optic disc (optic atrophy) – chronic optic neuropathy
- Blurred disc margins (optic neuritis) – inflammatory cause, often MS
- Swinging flashlight test
- Normal – light in either eye causes both pupils to constrict equally
- RAPD (CN II defect) – light in the affected eye causes both pupils to dilate instead of constricting
Labs, Studies, and Imaging
- MRI brain with contrast – rule out optic neuritis (MS), tumor, or stroke
- Lumbar puncture with oligoclonal bands – if MS-related optic neuritis suspected
- ESR, CRP, temporal artery biopsy – if giant cell arteritis suspected
- Fluorescein angiography – if vascular optic neuropathy suspected
Common Causes
- Optic neuritis – MS, autoimmune diseases – young female, painful vision loss
- Ischemic optic neuropathy – GCA, diabetes, hypertension – sudden, painless vision loss
- Papilledema – increased ICP (tumor, pseudotumor cerebri) – bilateral optic disc swelling
- Glaucoma – optic nerve damage from high intraocular pressure
- Toxic optic neuropathy – methanol poisoning, ethambutol, amiodarone
- Trauma – direct optic nerve injury
Treatment
- Optic neuritis (MS-related) → IV steroids (methylprednisolone)
- Giant cell arteritis → High-dose steroids immediately (to prevent blindness)
- Papilledema → Treat increased ICP – acetazolamide (↓ CSF production), VP shunt, weight loss in obese patients
- Glaucoma → Pressure-lowering medications – latanoprost, timolol, acetazolamide
- Toxic optic neuropathy → Remove offending agent, supportive care
Key Takeaways
- Painful vision loss? Think optic neuritis (MS until proven otherwise).
- Painless, sudden vision loss? Think ischemic optic neuropathy (GCA, stroke, diabetes).
- Bilateral optic disc swelling = papilledema (increased ICP, rule out brain mass).
CN III – Oculomotor Nerve Palsy
- Dysfunction of the oculomotor nerve (CN III), leading to eye movement impairment, ptosis, and pupillary dysfunction.
- Pupil-involving – emergent, usually compressive
- Pupil-sparing – less concerning, often ischemic
Clinical Presentation
- “Down and out” eye – lateral and downward deviation (unopposed CN IV & VI)
- Ptosis – weakness of levator palpebrae superioris
- Diplopia – double vision
- Mydriasis (blown pupil) if pupil-involving – suggests aneurysm or mass
- Loss of accommodation – blurry near vision
Physical Exam
- EOM testing – limited upward, downward, and medial movement
- Ptosis
- Pupil involvement:
- Dilated & non-reactive → EMERGENT – aneurysm, herniation, mass
- Normal pupil → Likely ischemic – diabetes, hypertension
Labs, Studies, and Imaging
- CT or MRI with contrast – rule out aneurysm, especially posterior communicating artery aneurysm
- MRA or CTA – if vascular cause suspected
- Blood glucose, A1C – if pupil-sparing, consider diabetic microvascular disease
- ESR, CRP – if giant cell arteritis suspected, especially in elderly
Treatment
- Compressive (pupil-involving causes like aneurysm, tumor, herniation) → NEUROSURGICAL EMERGENCY
- Ischemic (pupil-sparing, often diabetic) → Observation, glycemic & BP control
- Strabismus correction – prism glasses if persistent diplopia
- Steroids – if inflammatory cause (giant cell arteritis, Tolosa-Hunt syndrome)
Key Takeaways
- Pupil-involving CN III palsy = life-threatening until proven otherwise.
- Most common ischemic cause = diabetes.
- Most common compressive cause = posterior communicating artery aneurysm.
CN V – Trigeminal Nerve Palsy
- Dysfunction of the trigeminal nerve (CN V), which provides sensory innervation to the face and motor control of mastication muscles.
- Can affect one or more of its three divisions:
- V1 – Ophthalmic → Forehead, cornea, upper eyelid sensation
- V2 – Maxillary → Cheek, upper lip, nasal mucosa sensation
- V3 – Mandibular → Jaw movement, lower lip sensation
Clinical Presentation
- Facial numbness or paresthesia – depends on affected division
- Jaw deviation toward the affected side – loss of V3 motor function
- Trigeminal neuralgia – nerve irritation rather than true palsy
- Severe, episodic, electric-shock-like facial pain
- Triggered by light touch, chewing, cold air
- Most commonly affects V2 or V3
- Corneal reflex absent – if V1 lesion
- Difficulty chewing – if V3 affected
Physical Exam
- Sensory testing – loss of sensation in V1, V2, or V3 distribution
- Jaw testing – weakness in mastication, jaw deviates toward lesion
- Corneal reflex – absent blink reflex on affected side (V1 involvement)
- Trigger zones – if trigeminal neuralgia, light touch may trigger severe pain
Labs, Studies, and Imaging
- MRI brain with contrast – rule out mass (e.g., cerebellopontine angle tumor) or multiple sclerosis
- CT head – if trauma or skull fracture suspected
- ESR, CRP – if giant cell arteritis suspected, especially in elderly
- Lumbar puncture – if concern for infection like herpes zoster or meningitis
Common Causes
- Trigeminal neuralgia – most common idiopathic cause
- Multiple sclerosis – consider in young adults with bilateral symptoms
- Cerebellopontine angle tumors – acoustic neuroma, meningioma
- Herpes zoster – Ramsay Hunt syndrome if CN VII also involved
- Brainstem stroke – lateral medullary syndrome (Wallenberg)
- Trauma – fractures affecting trigeminal branches
Treatment
- Trigeminal neuralgia → Carbamazepine (1st line), oxcarbazepine
- MS-related → Disease-modifying therapy – interferon-beta, natalizumab
- Post-herpetic neuralgia → Gabapentin, pregabalin, TCAs
- Tumor-related → Neurosurgical evaluation
- Pain refractory to meds → Surgical decompression – microvascular decompression, radiofrequency ablation
Key Takeaways
- Trigeminal neuralgia = shock-like, unilateral facial pain (V2, V3).
- MS should be suspected in young patients with bilateral symptoms.
- Absent corneal reflex suggests V1 involvement.
- Jaw deviation occurs toward the side of the lesion.
