The Twelve Cranial Nerves
Oh Oh Oh To Touch And Feel Very Good Velvet AH
Mnemonic for names: Oh Oh Oh To Touch And Feel Very Good Velvet AH = Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal.
Fiber type mnemonic: Some Say Marry Money But My Brother Says Big Brains Matter More = S S M M B M B S B B M M (I=Sensory, II=S, III=Motor, IV=M, V=Both, VI=M, VII=B, VIII=S, IX=B, X=B, XI=M, XII=M).
💡 Pupillary Reflexes and Horner Syndrome
Direct/consensual light reflex: Light → CN II (afferent) → pretectal nucleus → bilateral Edinger-Westphal nuclei → bilateral CN III → both pupils constrict.
RAPD (Marcus Gunn pupil): CN II damage. Swinging flashlight: light into damaged eye → both pupils DILATE (damaged afferent cannot drive constriction; consensual from normal eye is released). Key sign of optic nerve disease (optic neuritis, MS).
CN III palsy (efferent defect): Affected pupil dilated and unreactive to light in either eye. Opposite pupil responds normally.
Horner syndrome: Interrupted sympathetic pathway to eye (hypothalamus → ciliospinal center C8-T1 → superior cervical ganglion → eye). Triad: ptosis (Muller muscle), miosis (dilator pupillae lost), anhidrosis (facial sweating lost if preganglionic). Causes: Pancoast tumor, carotid dissection, lateral medullary infarction.
I-IV
CN I–IV: smell, vision, and eye movement
CN I (Olfactory): Smell. Sensory only. Passes through cribriform plate. Anosmia from: head trauma (shearing at cribriform plate), COVID-19, olfactory groove meningioma. First CN to show pathology in Parkinson and Alzheimer disease.
CN II (Optic): Vision. Sensory only. Not a true peripheral nerve — a CNS tract myelinated by oligodendrocytes. Papilledema = optic disc swelling from raised ICP. Optic neuritis (demyelination) = classic early MS presentation.
CN III (Oculomotor): Most extraocular muscles + levator palpebrae (lid) + pupillary constrictor (parasympathetic). Palsy: eye down-and-out, ptosis, blown pupil. Causes: posterior communicating artery aneurysm (pupil first — surgical emergency), uncal herniation, diabetes (pupil-sparing microvascular).
CN IV (Trochlear): Superior oblique — depresses, intorts the eye. Exits dorsal brainstem and crosses. Palsy: vertical diplopia worse going downstairs; patient tilts head away from affected side.
CN I = smell. CN II = vision (optic disc/papilledema). CN III = most eye muscles + pupil (blown pupil = surgical emergency). CN IV = superior oblique = head tilt palsy.
V-VIII
CN V–VIII: face, jaw, expression, and hearing
CN V (Trigeminal): Largest CN. V1 (ophthalmic — forehead, cornea), V2 (maxillary — cheek, upper teeth), V3 (mandibular — jaw, lower teeth, anterior tongue; motor to mastication). Trigeminal neuralgia: lancinating V2/V3 pain triggered by light touch; carbamazepine is first-line.
CN VI (Abducens): Lateral rectus — abducts eye. Longest intracranial course → most susceptible to raised ICP (false localizing sign). Palsy: medial deviation, lateral gaze diplopia.
CN VII (Facial): Facial expression, taste anterior 2/3 tongue, lacrimal and submandibular glands. Bell palsy: peripheral CN VII, complete ipsilateral facial paralysis, forehead INCLUDED. Central lesion: forehead SPARED (bilateral cortical upper-face innervation). Ramsay Hunt: zoster of CN VII → facial palsy + ear vesicles + hearing loss.
CN VIII (Vestibulocochlear): Cochlear division = hearing; vestibular = balance. Acoustic neuroma (vestibular schwannoma): unilateral SNHL, tinnitus, imbalance — at the CPA (cerebellopontine angle).
CN V = face sensation (V1/V2/V3) + mastication. CN VI = lateral rectus (abducts). CN VII = facial expression + taste anterior 2/3. Bell palsy = forehead down too. CN VIII = hearing + balance.
IX-XII
CN IX–XII: throat, voice, shoulder, and tongue
CN IX (Glossopharyngeal): Posterior 1/3 tongue taste, pharyngeal sensation (gag reflex afferent), parasympathetic to parotid gland, carotid body chemoreceptors.
CN X (Vagus): Most widespread CN. Parasympathetic to heart, lungs, and GI tract. Motor to pharynx and larynx (voice). Left recurrent laryngeal nerve loops under the aortic arch — vulnerable to mediastinal masses → hoarseness. Uvula deviates AWAY from CN X lesion.
CN XI (Spinal Accessory): Motor to SCM (turns head contralaterally) and trapezius (shoulder shrug). Palsy: cannot turn head away from lesion, shoulder drop. Often damaged during cervical lymph node biopsy.
CN XII (Hypoglossal): Motor to tongue muscles. LMN palsy: tongue deviates TOWARD the lesion on protrusion (ipsilateral genioglossus weak → contralateral pushes tongue toward weak side). Ipsilateral tongue wasting and fasciculations.
CN IX = gag reflex afferent + posterior tongue taste. CN X = vagus = viscera everywhere + voice. Uvula AWAY from lesion. CN XI = SCM + trapezius. CN XII = tongue motor → tongue TOWARD LMN lesion.
🔬 Clinical Scenario — Cranial Nerve Palsies in Practice
Cranial nerve examination localizes lesions from nose to medulla:
A
PComm aneurysm — CN III compression. Berry aneurysm at the posterior communicating artery junction lies adjacent to CN III as it exits the midbrain. Parasympathetic fibers run on the outside of CN III → compressed first → blown pupil before eye movement palsy. Unilateral blown pupil + headache = aneurysm until proven otherwise. CT angiography immediately.
B
Acoustic neuroma (vestibular schwannoma). Benign Schwann cell tumor of CN VIII vestibular division in the internal auditory canal, expanding into the CPA. Early: unilateral SNHL, tinnitus, balance difficulties. Enlarging: compresses CN VII (facial palsy), CN V (facial numbness, lost corneal reflex), eventually cerebellum/brainstem. MRI with gadolinium is diagnostic.
C
Trigeminal neuralgia. Severe, brief, lancinating electric-shock pain in V2 or V3, triggered by light touch (eating, talking, wind). Caused by vascular compression of CN V at the root entry zone (usually the superior cerebellar artery). Treatment: carbamazepine first line. Surgery: microvascular decompression (Janetta procedure) — placing a sponge between the artery and nerve root. Highly effective.
D
Horner syndrome from carotid dissection. Traumatic or spontaneous carotid artery dissection → mural hematoma → compresses sympathetic fibers along carotid → Horner triad (ptosis, miosis, anhidrosis). Accompanied by ipsilateral neck pain/headache. Ischemic stroke risk from thromboembolism. MRI/MRA of neck confirms. Treatment: antiplatelet or anticoagulation.
✓ Quick Self-Test
1. Mnemonic for the 12 cranial nerves?
2. Signs of complete CN III palsy and main causes?
3. What is the RAPD and what does it indicate?
4. Uvula with left CN X lesion vs tongue with left CN XII lesion?
5. What is Horner syndrome and its three findings?
Answers:
1. Oh Oh Oh To Touch And Feel Very Good Velvet AH = Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, (Spinal) Accessory, Hypoglossal.
2. Complete CN III palsy: ptosis (levator palpebrae paralysis), eye deviated down and out (loss of SR/IR/MR/IO; unopposed SO/CN IV and LR/CN VI), and mydriasis/blown pupil (loss of pupillary sphincter parasympathetic innervation). Causes: posterior communicating artery aneurysm (pupil affected first), uncal herniation, diabetes mellitus (typically pupil-sparing microvascular).
3. RAPD (relative afferent pupillary defect/Marcus Gunn pupil): detected on swinging flashlight test. When light swings to the eye with damaged optic nerve (CN II), both pupils DILATE — because the damaged CN II cannot generate adequate consensual constriction, and the consensual response from the normal eye is released. Indicates optic nerve disease: optic neuritis (MS), optic neuropathy.
4. Left CN X lesion: right vagus (intact) pulls uvula to the RIGHT — uvula deviates AWAY from the left lesion. Left CN XII LMN lesion: right genioglossus (intact) pushes tongue to the LEFT — tongue deviates TOWARD the left lesion. Uvula: away. Tongue: toward.
5. Horner syndrome: interruption of the sympathetic pathway to the eye. Three classic findings: (1) Ptosis — partial, from loss of superior tarsal (Muller) muscle; (2) Miosis — from loss of iris dilator pupillae, leaving constrictor unopposed; (3) Anhidrosis — loss of ipsilateral facial sweating (if preganglionic lesion). Common causes: Pancoast tumor (apical lung), carotid artery dissection, lateral medullary infarction.