Cranial Nerves — Function and Lesions

CN I (Olfactory Nerve) – Pure Sensory

• Olfactory receptor neurons in the nasal epithelium transmit signals via the cribriform plate to the olfactory bulb
• Lesions cause anosmia (loss of smell); commonly occur with head trauma, smoking, or olfactory groove meningiomas

CN II (Optic Nerve) – Pure Sensory

• Retinal ganglion cells form the optic nerve; transmits visual information from photoreceptors through bipolar cells
• Lesions cause visual field defects depending on location: optic nerve (monocular vision loss), optic chiasm (bitemporal hemianopsia), optic tract/lateral geniculate/optic radiations (homonymous hemianopsia)
• Afferent limb of pupillary light reflex

CN III (Oculomotor Nerve) – Motor

• Motor innervation to medial rectus, superior rectus, inferior rectus, and inferior oblique extraocular muscles; levator palpebrae superioris (eyelid elevation); and parasympathetic preganglionic fibers to ciliary ganglion (pupil constriction, accommodation)
• Nuclear/fascicular lesions cause ipsilateral CN III palsy with contralateral hemiparesis (Weber syndrome: CN III palsy + contralateral body weakness)
• Peripheral compression (e.g., posterior communicating artery aneurysm, tentorial herniation) causes "down and out" eye position with pupillary dilation

CN IV (Trochlear Nerve) – Motor

• Innervates superior oblique muscle; only CN with decussation (crosses contralateral side before exiting)
• Lesions cause vertical diplopia (worse with downward gaze and head tilt toward side of lesion); head tilt away from affected nerve
• Most common cause is microvascular ischemia; classic teaching point: longest intracranial course of any CN (vulnerable to trauma)

CN V (Trigeminal Nerve) – Mixed (Sensory and Motor)

• Three divisions: V1 (ophthalmic—forehead, upper eyelid), V2 (maxillary—upper face), V3 (mandibular—lower face, anterior tongue)
• Sensory: corneal reflex (afferent limb), facial sensation
• Motor: muscles of mastication, tensor tympani, anterior belly of digastric, mylohyoid
• Lesions cause facial pain/numbness, loss of corneal reflex, or jaw weakness; trigeminal neuralgia presents as severe paroxysmal facial pain (pathophysiology unclear but associated with vascular compression)

CN VI (Abducens Nerve) – Motor

• Innervates lateral rectus muscle (abduction of eye)
• Longest intracranial CN segment; vulnerable to increased intracranial pressure, ischemia, and basilar meningitis
• Lesion causes "medial deviation" of eye (unopposed medial rectus) with inability to abduct

CN VII (Facial Nerve) – Mixed

• Motor: muscles of facial expression, stapedius, posterior belly of digastric, stylohyoid; parasympathetic to lacrimal and salivary glands
• Sensory: taste (anterior 2/3 of tongue via chorda tympani), sensation to external ear
• Passes through temporal bone (petrous portion); vulnerable to Bell's palsy (viral inflammation—most common CN VII lesion)
• Peripheral facial palsy (CN VII lesion distal to brainstem) spares forehead because forehead muscles receive bilateral cortical innervation; central facial palsy (upper motor neuron lesion) spares forehead but affects lower face

CN VIII (Vestibulocochlear Nerve) – Pure Sensory

• Cochlear division: hearing; damage causes sensorineural hearing loss
• Vestibular division: balance and head position; damage causes vertigo, nystagmus, ataxia
• Lesion causes ipsilateral hearing loss and vestibular dysfunction; acoustic neuroma (schwannoma of CN VIII) classically presents with unilateral hearing loss and tinnitus

CN IX (Glossopharyngeal Nerve) – Mixed

• Sensory: posterior 1/3 of tongue (taste and general sensation), pharynx, carotid sinus (baroreceptors)
• Motor: stylopharyngeus muscle; parasympathetic to parotid gland
• Afferent limb of gag reflex
• Lesion causes ipsilateral loss of posterior tongue sensation and weak pharyngeal contraction; glossopharyngeal neuralgia similar to trigeminal neuralgia but rarer

CN X (Vagus Nerve) – Mixed

• Sensory: epiglottis, larynx, pharynx; visceral afferents from thoracic and abdominal organs
• Motor: pharyngeal and laryngeal muscles (phonation, swallowing), parasympathetic to heart (↓ HR), lungs, and GI tract
• Longest CN; exits cranium and travels through neck and thorax
• Lesion causes ipsilateral vocal cord paralysis (hoarseness), dysphagia, and loss of gag reflex (efferent limb); parasympathetic loss may cause tachycardia
• Bilateral CN X lesion causes severe dysphagia and aspiration risk

CN XI (Accessory Nerve) – Motor

• Innervates sternocleidomastoid (head turning) and trapezius (shoulder shrug)
• Lesion causes ipsilateral inability to turn head (SCM weakness) and shoulder droop (trapezius weakness); head rotates away from lesion
• Unique: has both cranial and spinal rootlets

CN XII (Hypoglossal Nerve) – Motor

• Innervates tongue muscles (genioglossus, hyoglossus, styloglossus, intrinsic muscles)
• Lesion causes ipsilateral tongue atrophy and weakness; tongue deviates toward affected side (unopposed contralateral genioglossus pulls tongue away)
• Can be affected in lateral medullary syndrome (Wallenberg) or as isolated lesion (lingual artery damage, skull base tumors)

Key Mechanisms of CN Lesions

• Vascular (stroke, aneurysm compression): sudden onset; Weber, Claude, Wallenberg syndromes
• Inflammatory/Infectious (Bell's palsy, meningitis, syphilis): gradual onset with systemic symptoms
• Neoplastic (skull base tumors, acoustic neuroma, meningioma): progressive symptoms; may affect multiple CNs
• Traumatic (temporal bone fracture, blunt head injury): variable onset based on severity
• Metabolic/Toxic (diabetes, vitamin B12 deficiency): often bilateral and insidious

CN I (Olfactory) Lesion

• Anosmia (loss of smell) with preserved taste (taste = CN VII, IX, X)
• Patients may report loss of appetite (actually loss of retronasal olfaction)
• Unilateral anosmia suggests ipsilateral lesion (olfactory nerve, bulb, or tract); bilateral suggests bilateral pathology or central cause

CN II (Optic) Lesion

• Monocular vision loss (optic nerve lesion proximal to optic chiasm)
• Optic neuritis (inflammation, often MS-related): painful vision loss, afferent pupillary defect (APD), normal fundus initially
• **Central ret

The pupil decides the workup in CN III palsy

• Pupil-involving CN III palsy = surgical until proven otherwise. Parasympathetic fibers travel superficially in the nerve, so external compression (posterior communicating artery aneurysm, uncal herniation) hits them first → dilated, poorly reactive pupil plus ptosis and a down-and-out eye. Single best next step: urgent noninvasive vascular imaging of the head (CTA or MRA), consistent with American Academy of Ophthalmology Preferred Practice Pattern guidance.
• Pupil-sparing CN III palsy in an older diabetic/hypertensive patient reflects microvascular infarction of the nerve core via the vasa nervorum — often painful, but not aneurysmal. Common distractor: ordering surgery/angiography reflexively for every third nerve palsy.

Upper vs lower motor neuron CN VII — the single most tested contrast

• Peripheral (LMN) CN VII lesion weakens the entire hemiface, including forehead and eye closure (Bell's palsy, Ramsay Hunt, parotid tumor).
• Central (UMN) lesion spares the forehead because the frontalis receives bilateral corticobulbar input — forehead wrinkling preserved, lower face weak; look for accompanying limb findings.
• Bell's palsy management: the AAN and AAO-HNS Bell's palsy guidelines support oral corticosteroids (prednisone) begun within 72 hours, with antivirals (valacyclovir) as optional add-on in severe cases; eye lubrication and taping prevent exposure keratopathy. Vesicles in the ear canal → Ramsay Hunt (VZV) → steroids plus antiviral. Bilateral palsy → think Lyme disease, sarcoidosis, GBS — not idiopathic Bell's.

Localization one-liners

• CN VI palsy is the classic false localizing sign of raised intracranial pressure; in the cavernous sinus it lies free in the lumen and is affected earliest.
• CN XII: tongue deviates toward the LMN lesion, with atrophy and fasciculations; CN X: uvula deviates away from the lesion; hoarseness after thyroidectomy = recurrent laryngeal branch of vagus.
• Reflex arcs: corneal = V1 in, VII out; gag = IX in, X out; light = II in, III out.
• Cerebellopontine angle mass (vestibular schwannoma) → CN VIII, then VII and V (lost corneal reflex); bilateral = NF2. Best test: MRI with gadolinium of the internal auditory canals.