Cranial Nerves

Function: Special sensory (smell)

Pathway: Olfactory epithelium → cribriform plate → olfactory bulb → primary olfactory cortex (piriform cortex, uncus)

Testing: Each nostril separately with non-irritating odors (coffee, vanilla)

Clinical:

• Anosmia: Head trauma (shearing of olfactory filaments), COVID-19, Parkinson’s disease, Alzheimer’s disease
• Olfactory groove meningioma: Unilateral anosmia + ipsilateral optic atrophy + contralateral papilledema (Foster-Kennedy syndrome)
• Uncinate seizures: Olfactory aura (typically unpleasant/burning smell) → medial temporal lobe
• Kallmann syndrome: Congenital anosmia + hypogonadotropic hypogonadism
• Frontal lobe lesions / orbitofrontal cortex damage: Impaired odor discrimination (not just threshold)
• Olfactory hallucinations (phantosmia): Migraine aura, temporal lobe epilepsy, psychiatric disorders

Function: Special sensory (vision)

Pathway: Retina → optic nerve → optic chiasm → optic tract → lateral geniculate nucleus (LGN) → optic radiations → primary visual cortex (V1, occipital lobe)

Testing: Visual acuity, visual fields, fundoscopy, pupillary light reflex (afferent), color vision

Visual Field Defects

Visual Field Defect	Localization & Causes
Monocular vision loss	Retina or optic nerve (ischemic optic neuropathy, optic neuritis, retinal detachment)
Central scotoma	Macular disease or optic nerve (optic neuritis, toxic/nutritional neuropathy)
Cecocentral scotoma	Leber hereditary optic neuropathy, methanol/ethambutol toxicity
Junctional scotoma	Optic nerve–chiasm junction → ipsilateral central scotoma + contralateral superior temporal defect
Bitemporal hemianopsia	Optic chiasm compression (pituitary adenoma, craniopharyngioma)
Binasal hemianopsia	Bilateral lateral chiasmal lesions (carotid aneurysms, glaucoma)
Incongruous homonymous hemianopsia	Optic tract or LGN (more asymmetric fields)
Congruous homonymous hemianopsia	Optic radiations or occipital cortex (more symmetric fields)
Superior quadrantanopia (“pie in the sky”)	Temporal lobe (Meyer’s loop)
Inferior quadrantanopia (“pie on the floor”)	Parietal lobe optic radiations
Homonymous hemianopsia with macular sparing	Occipital cortex (PCA infarct with MCA collateral supply)
Homonymous hemianopsia with macular splitting	Larger occipital lesion involving both PCA + MCA regions
Altitudinal visual field loss	Ischemic optic neuropathy (AION), retinal artery occlusion
Enlarged blind spot	Papilledema, optic disc drusen, increased intracranial pressure

Functions: Somatic motor (4 EOM + levator) + parasympathetic (pupil constriction, accommodation)

Location: Midbrain (level of superior colliculus), exits interpeduncular fossa

Nuclear Organization

Nucleus	Location	Muscles Supplied
Medial rectus nucleus	Midbrain (ventral)	Ipsilateral medial rectus
Inferior rectus nucleus	Midbrain (ventral)	Ipsilateral inferior rectus
Inferior oblique nucleus	Midbrain (ventral)	Ipsilateral inferior oblique
Superior rectus nucleus	Midbrain (dorsal)	Contralateral superior rectus
Central caudal nucleus	Midbrain (midline, caudal)	Bilateral levator palpebrae
Edinger-Westphal nucleus	Midbrain (dorsal, midline)	Pupillary sphincter + ciliary muscle (parasympathetic)

Nerve Divisions (in the orbit)

Division	Muscles Innervated
Superior division	Levator palpebrae + Superior rectus
Inferior division	Medial rectus, Inferior rectus, Inferior oblique + Parasympathetic fibers

Testing: H-pattern EOMs, ptosis, pupil light + accommodation responses

Clinical:

• Complete palsy: “Down & out,” ptosis, dilated nonreactive pupil
• Pupil-involving palsy (compressive): PCom aneurysm (parasympathetic fibers superficial → compressed first)
• Pupil-sparing palsy (ischemic): Diabetes/HTN (vasa nervorum → somatic fibers injured first)
• Uncal herniation: Early blown pupil → CN III palsy → coma
• Midbrain fascicular syndromes:
  • Weber: CN III palsy + contralateral hemiparesis
  • Benedikt: CN III palsy + contralateral tremor/ataxia
  • Nothnagel: CN III palsy + ipsilateral cerebellar ataxia

Function: Motor (superior oblique muscle)

Nucleus: Midbrain (inferior colliculus level)

Action: Depression and intorsion of the eye (best seen when adducted)

Testing: Ask patient to look down and in; Parks-Bielschowsky head tilt test

Clinical:

• Vertical diplopia: Worse when looking down (reading, stairs)
• Head tilt: Away from affected side to compensate
• Causes: Trauma (most common), microvascular, congenital

Function: Sensory (face) + Motor (mastication)

Divisions:

• V1 (Ophthalmic): Forehead, cornea, tip of nose
• V2 (Maxillary): Cheeks, upper lip, maxillary teeth
• V3 (Mandibular): Lower jaw, mandibular teeth, anterior 2/3 tongue (sensation only)

Motor: Muscles of mastication (masseter, temporalis, pterygoids)

Testing: Light touch/pinprick in all three divisions, corneal reflex (afferent), jaw jerk, masseter strength

Clinical:

• Trigeminal neuralgia: Lancinating facial pain in V2/V3 distribution, triggered by touch/chewing
• Jaw deviation: Toward side of weakness (unopposed pterygoid)
• Onion-skin pattern: Lateral medullary lesion affects descending trigeminal tract

Function: Motor (lateral rectus muscle)

Nucleus: Pons (facial colliculus)

Action: Eye abduction (look laterally)

Testing: Horizontal eye movements (failure to abduct eye)

Clinical:

• CN VI palsy: Horizontal diplopia, worse at distance and looking toward affected side
• False localizing sign: Can occur with increased ICP (compresses nerve along skull base)
• Dorsal pontine syndrome: CN VI palsy + horizontal gaze palsy (PPRF involvement)

Functions: Motor (facial expression) + Sensory (taste anterior 2/3 tongue) + Parasympathetic (lacrimal, salivary glands)

Nucleus: Pons

Testing: Facial symmetry, eye closure, smile, taste (anterior 2/3 tongue), Schirmer test (tears)

UMN vs LMN:

• UMN (cortical): Forehead spared (bilateral innervation), lower face weak
• LMN (peripheral): Entire hemifacial weakness including forehead

Clinical:

• Bell’s palsy: Acute LMN facial palsy, often post-viral, hyperacusis (stapedius muscle)
• Ramsay Hunt syndrome: Facial palsy + vesicles in ear (VZV, geniculate ganglion)
• Acoustic neuroma: CN VII + CN VIII involvement

Function: Special sensory (hearing and balance)

Components:

• Cochlear: Hearing (spiral ganglion → cochlear nuclei → superior olive → inferior colliculus → medial geniculate → auditory cortex)
• Vestibular: Balance (vestibular ganglion → vestibular nuclei → cerebellum, MLF, spinal cord)

Testing: Weber, Rinne tests; nystagmus evaluation; head impulse test

Clinical:

• Acoustic neuroma: Unilateral hearing loss, tinnitus, imbalance; MRI shows CPA mass
• Meniere’s disease: Episodic vertigo, hearing loss, tinnitus, aural fullness
• Vestibular neuritis: Acute vertigo, abnormal head impulse test, nystagmus

Functions: Sensory (posterior 1/3 tongue, pharynx) + Motor (stylopharyngeus) + Parasympathetic (parotid gland)

Nucleus: Medulla

Testing: Gag reflex (afferent), taste posterior tongue, palatal elevation

Clinical:

• Glossopharyngeal neuralgia: Severe pain in throat/ear, triggered by swallowing
• Often affected with CN X: Jugular foramen syndrome

Functions: Motor (pharynx, larynx) + Sensory (larynx, viscera) + Parasympathetic (thoracoabdominal viscera)

Nucleus: Medulla (nucleus ambiguus for motor)

Testing: Gag reflex (efferent), voice quality, palatal elevation (“ah”)

Clinical:

• Unilateral palsy: Uvula deviates away from lesion, hoarseness, dysphagia
• Bilateral palsy: Severe dysphagia, aspiration risk, respiratory compromise
• Lateral medullary syndrome: CN IX, X affected with Horner’s, ataxia, crossed sensory loss

Function: Motor (sternocleidomastoid, trapezius)

Origin: Spinal cord (C1-C5) → exits jugular foramen

Testing: Shoulder shrug (trapezius), head turn against resistance (SCM turns head to opposite side)

Clinical:

• Iatrogenic injury: Lymph node biopsy, carotid surgery
• SCM weakness: Difficulty turning head to opposite side
• Trapezius weakness: Shoulder droop, difficulty elevating arm above horizontal

Function: Motor (tongue muscles)

Nucleus: Medulla (hypoglossal nucleus)

Testing: Tongue protrusion, lateral movements, strength (push against cheek)

Clinical:

• LMN lesion: Tongue deviates toward weak side, atrophy, fasciculations
• UMN lesion: Tongue deviates away from lesion (toward weak body side)
• Medial medullary syndrome: CN XII palsy + contralateral hemiparesis + contralateral proprioception loss

Cranial Nerve Nuclei Locations

Location	Cranial Nerves
Midbrain	CN III (superior colliculus), CN IV (inferior colliculus)
Pons	CN V, VI, VII, VIII
Medulla	CN IX, X, XII
Spinal Cord	CN XI (C1-C5)

High-Yield Examination Tips

Brainstem Syndromes Quick Reference

Syndrome	Location	Features
Weber	Midbrain (ventral)	Ipsi CN III palsy + contra hemiparesis
Benedikt	Midbrain (tegmentum)	Ipsi CN III palsy + contra tremor/ataxia
Wallenberg	Lateral medulla	Ipsi CN IX/X, Horner’s, ataxia + contra pain/temp loss
Medial medullary	Medial medulla	Ipsi CN XII + contra hemiparesis + contra proprioception loss