Neurology Neurology · Clinical

Cranial Nerve Lesions

The four most commonly examined cranial nerve lesions — CN III, VI, VII, and XII — each have a characteristic clinical presentation that enables bedside diagnosis without imaging.

✦ The Mnemonic

"Three Drops Down-Out; Six Faces Lateral; Seven Faces Fall; Twelve Tongues Turn"

CN III (down+out+ptosis) · CN VI (lateral gaze palsy) · CN VII (facial palsy) · CN XII (tongue deviation)

III CN III Palsy Eye 'down and out'; ptosis; if complete: fixed dilated pupil (surgical) or spared pupil (medical/diabetic)

VI CN VI Palsy Failure of lateral gaze (abduction); convergent squint; horizontal diplopia worse looking to affected side

VII CN VII Palsy (LMN) Entire ipsilateral face affected including forehead; eye cannot close (Bell's palsy); taste lost ant. 2/3 tongue

VII CN VII Palsy (UMN) Forehead SPARED (bilateral cortical representation); contralateral lower face only

XII CN XII Palsy Tongue deviates toward the side of the lesion on protrusion; ipsilateral wasting + fasciculation

📚 Clinical Breakdown

CN III — surgical vs medical palsy: the critical distinction. Surgical causes (aneurysm, herniation, tumour) compress the outer parasympathetic fibres first → dilated pupil + ophthalmoplegia. Medical causes (diabetic, ischaemic mononeuropathy) destroy the vasa nervorum centrally → ophthalmoplegia with a spared (round, reacting) pupil. A painful CN III palsy with a dilated pupil = PComm aneurysm until proven otherwise.

CN VI — raised ICP palsy: the abducens nerve has the longest intracranial course, making it a 'false localising sign' in raised ICP — it can be stretched without a structural lesion at the level of the pons. Always consider this before attributing a VI palsy to a pontine lesion.

CN XII lesions: LMN lesion (nucleus, nerve, or hypoglossal canal) = tongue deviates toward the side of the lesion + atrophy + fasciculation. UMN lesion (corticobulbar tract above the nucleus) = tongue deviates away from the lesion with increased tone, no atrophy. The majority of clinical hypoglossal lesions are LMN (skull base tumours, neck dissection, carotid endarterectomy).

CN III surgical key sign Dilated fixed pupil

CN III medical key sign Pupil SPARED

CN VII LMN Entire face including forehead

Tongue deviation LMN: toward lesion; UMN: away from lesion

⭐ Clinical Pearl

Horner's syndrome: ptosis (partial — superior tarsal muscle), miosis (small pupil), anhidrosis (loss of sweating), enophthalmos (apparent). Caused by interruption of the sympathetic chain: central (hypothalamus → T1 — Pancoast tumour, lateral medullary syndrome), preganglionic (T1 → superior cervical ganglion — cervical rib, thyroid carcinoma, carotid dissection), postganglionic (carotid siphon → orbit).

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