Cranial Nerves and Common Lesions: Anatomy & Clinical Correlations

The cranial nerves (CN) are 12 pairs of nerves emerging from the brainstem, responsible for sensory, motor, and autonomic functions. Their lesions present with distinct clinical signs, making them a crucial topic in neurology and clinical practice.

1. Overview of Cranial Nerves

2. Common Cranial Nerve Lesions and Clinical Presentation

(A) CN I – Olfactory Nerve Lesion

• Cause: Head trauma (cribriform plate fracture), neurodegenerative diseases (Parkinson’s, Alzheimer’s)
• Presentation: Anosmia (loss of smell), altered taste perception

(B) CN II – Optic Nerve Lesion

• Cause: Optic neuritis (MS), glaucoma, tumors
• Presentation:
  • Monocular vision loss (optic nerve lesion)
  • Bitemporal hemianopia (chiasm compression, e.g., pituitary tumor)
  • Afferent pupillary defect (Marcus Gunn pupil)

(C) CN III – Oculomotor Nerve Lesion

• Cause: Posterior communicating artery aneurysm, uncal herniation
• Presentation:
  • “Down and out” eye (loss of medial rectus, superior & inferior rectus)
  • Ptosis (levator palpebrae weakness)
  • Dilated pupil (mydriasis) (loss of parasympathetic function)

(D) CN IV – Trochlear Nerve Lesion

• Cause: Trauma, congenital
• Presentation:
  • Vertical diplopia, worsens when looking downstairs
  • Head tilt opposite side to compensate

(E) CN V – Trigeminal Nerve Lesion

• Cause: Trigeminal neuralgia, brainstem lesions
• Presentation:
  • Loss of facial sensation (V1, V2, V3)
  • Jaw deviation to side of lesion
  • Absent corneal reflex

(F) CN VI – Abducens Nerve Lesion

• Cause: Increased ICP, cavernous sinus thrombosis
• Presentation:
  • Horizontal diplopia
  • Inability to abduct eye (unopposed medial rectus)

(G) CN VII – Facial Nerve Lesion

• Cause: Bell’s palsy (LMN), stroke (UMN)
• Presentation:
  • LMN Lesion (Bell’s Palsy): Complete ipsilateral facial paralysis
  • UMN Lesion (Stroke): Spares forehead, affects lower face

(H) CN VIII – Vestibulocochlear Nerve Lesion

• Cause: Acoustic neuroma, Meniere’s disease
• Presentation:
  • Sensorineural hearing loss
  • Vertigo, nystagmus

(I) CN IX – Glossopharyngeal Nerve Lesion

• Cause: Brainstem stroke, compression
• Presentation:
  • Loss of gag reflex
  • Impaired taste (posterior 1/3 of tongue)

(J) CN X – Vagus Nerve Lesion

• Cause: Brainstem stroke, surgery (thyroidectomy)
• Presentation:
  • Dysphagia, hoarseness
  • Uvula deviates opposite side

(K) CN XI – Accessory Nerve Lesion

• Cause: Neck trauma, surgery
• Presentation:
  • Shoulder droop (trapezius paralysis)
  • Weak head turning (SCM weakness)

(L) CN XII – Hypoglossal Nerve Lesion

• Cause: Medullary stroke, ALS
• Presentation:
  • Tongue deviates toward lesion

3. Clinical Mnemonics for Cranial Nerves

(A) Names of the Cranial Nerves

Mnemonic: “Oh, Oh, Oh, To Touch And Feel Very Green Vegetables, AH!”

• Olfactory
• Optic
• Oculomotor
• Trochlear
• Trigeminal
• Abducens
• Facial
• Vestibulocochlear
• Glossopharyngeal
• Vagus
• Accessory
• Hypoglossal

(B) Cranial Nerve Function (Sensory, Motor, Both)

Mnemonic: “Some Say Money Matters, But My Brother Says Big Brains Matter More!”

• S = Sensory, M = Motor, B = Both
• I (S), II (S), III (M), IV (M), V (B), VI (M), VII (B), VIII (S), IX (B), X (B), XI (M), XII (M)

4. Diagnostic Approach

5. Key Takeaways

✅ Cranial nerves control vision, facial movement, swallowing, and autonomic functions.
✅ UMN lesions spare the forehead, LMN lesions affect the entire face (Bell’s Palsy).
✅ Horner’s syndrome (ptosis, miosis, anhidrosis) involves sympathetic dysfunction, not CN III.
✅ Stroke patients often present with CN IX and X deficits (dysphagia, uvula deviation).

Further Reading

• NHS Overview on Cranial Nerve Disorders: NHS UK
• NICE Guidelines on Neurological Disorders: NICE