Why Cranial Nerves Trip Students Up
The cranial nerve examination is vast — there are 12 nerves, each with distinct anatomy and clinical tests. In an OSCE, you will rarely be asked to examine all 12 in 8 minutes. More commonly you will be directed: "Examine the cranial nerves supplying this patient's eye movements" or "This patient has noticed weakness of the right side of their face — please examine." You must be able to examine each nerve in isolation or as part of a focused assessment, and interpret your findings confidently.
The framework below covers all 12 nerves with the clinical tests and most important pathology for each.
💡 Tip
OSCE reality: The most commonly examined cranial nerve stations are CN II (visual fields, acuity), CN III/IV/VI (eye movements), CN V (facial sensation, jaw jerk), CN VII (facial nerve palsy), and CN IX/X (speech, swallowing, palate). Focus your revision on these five areas.
The 12 Cranial Nerves: Tests and Key Pathology
🧠 Mnemonic
Names: "Oh Oh Oh To Touch And Feel Very Good Velvet, Ah Heaven!"
CN I Olfactory, II Optic, III Oculomotor, IV Trochlear, V Trigeminal, VI Abducens, VII Facial, VIII Vestibulocochlear, IX Glossopharyngeal, X Vagus, XI Accessory, XII Hypoglossal
CN I — Olfactory (Smell)
Test: Occlude one nostril; ask the patient to sniff a familiar substance (coffee, peppermint — not ammonia as that stimulates CN V). Compare sides.
Pathology: Anosmia — trauma (shearing of olfactory filaments at cribriform plate), olfactory groove meningioma, normal ageing, COVID-19, zinc deficiency.
CN II — Optic (Vision)
This is a full examination in itself. Cover systematically:
- 1Visual acuity: Snellen chart at 6 metres. Record as 6/6, 6/12, etc. If reduced, use a pinhole — improvement suggests a refractive error, not optic nerve pathology.
- 2Colour vision: Ishihara plates — colour desaturation is an early sign of optic neuritis (MS)
- 3Visual fields: Confrontation testing — sit 1 metre from patient, cover one eye each, use a wiggling finger or red pin in each quadrant. Compare to your own fields as control.
- 4Pupils: PERLA (Pupils Equal Round Reactive to Light and Accommodation). Assess direct and consensual reflex. Swinging torch test for relative afferent pupillary defect (RAPD) — in optic neuritis, the affected pupil paradoxically dilates when light swings to it.
- 5Fundoscopy: Offer to examine. Mention you would dilate pupils with tropicamide. Look for papilloedema (blurred disc margins, loss of venous pulsation), optic atrophy (pale disc), or diabetic/hypertensive retinopathy.
💎 Clinical Pearl
RAPD (Marcus Gunn pupil): Swing the torch from the normal to the affected eye. The affected eye's pupil dilates (rather than remaining constricted) because less afferent signal is reaching the midbrain. This is a sign of severe unilateral optic nerve disease — optic neuritis, optic nerve compression, severe glaucoma.
Visual field defects and their localisation:
| Defect | Lesion |
|---|---|
| Monocular blindness | Optic nerve (ipsilateral) |
| Bitemporal hemianopia | Optic chiasm (e.g. pituitary adenoma) |
| Homonymous hemianopia | Optic tract / radiation (e.g. stroke) |
| Superior quadrantanopia | Temporal lobe (Meyer's loop) |
| Inferior quadrantanopia | Parietal lobe |
| Macular sparing | Occipital cortex (posterior cerebral artery) |
CN III, IV, VI — Oculomotor, Trochlear, Abducens (Eye Movements)
These three nerves control extraocular movements. Examine together:
- 1Inspect the eyes: Ptosis (CN III), proptosis, squint (strabismus)
- 2Pupils: Size and reactivity (CN III carries parasympathetic fibres)
- 3Eye movements: Ask the patient to follow your finger in an H pattern covering all 6 directions. Look for:
- Limitation of movement in any direction
- Diplopia: Ask "do you ever see double?" and in which direction is it worst
- Nystagmus: Fast and slow components — note the direction of fast phase
| CN | Muscle(s) | Movement tested |
|---|---|---|
| CN III (oculomotor) | SR, IR, MR, IO, levator palpebrae | All except lateral and inferolateral |
| CN IV (trochlear) | Superior oblique | Inframedial (looking down and in — going down stairs) |
| CN VI (abducens) | Lateral rectus | Pure abduction (lateral gaze) |
🧠 Mnemonic
LR₆SO₄ — Lateral Rectus CN VI, Superior Oblique CN IV, everything else CN III
CN III palsy (complete): Ptosis + "down and out" eye + fixed dilated pupil (mydriasis). A surgical CN III palsy (e.g. posterior communicating artery aneurysm) is pupil-involving — emergency CT angiography. A medical CN III palsy (e.g. diabetes, hypertension) often spares the pupil initially.
CN IV palsy: Diplopia on downward gaze (going down stairs). Head tilt away from the side of the lesion to compensate.
CN VI palsy: Failure of abduction; diplopia worst on lateral gaze to the affected side. CN VI has the longest intracranial course — a false localising sign in raised ICP.
CN V — Trigeminal (Facial Sensation + Jaw)
- 1Sensation: Test light touch and pin prick in all three divisions bilaterally:
- V1 (ophthalmic): Forehead
- V2 (maxillary): Cheek
- V3 (mandibular): Jaw
- 1Corneal reflex: Touch the cornea (not the sclera) with a wisp of cotton wool — afferent V1, efferent CN VII (blink). Reduced or absent in V1 lesion.
- 2Jaw jerk: Place your finger on the patient's relaxed, slightly open jaw and tap — brisk jaw jerk = UMN lesion above the pons (bilaterally). Normal jaw jerk is absent or just present.
- 3Motor: Palpate the masseters and temporalis as the patient clenches their teeth.
CN VII — Facial (Facial Muscles + Taste)
- 1Inspect: Facial asymmetry at rest, nasolabial fold flattening, eye closure
- 2Test:
- Raise eyebrows (frontalis)
- Screw eyes tightly shut — try to open them
- Puff out cheeks (buccinator)
- Show teeth / smile (orbicularis oris)
- Whistle
💎 Clinical Pearl
Upper vs Lower Motor Neuron Facial Palsy — the most important distinction:
| Feature | UMN (e.g. stroke) | LMN (e.g. Bell's palsy) |
|---|---|---|
| Forehead sparing | YES — upper face spared | NO — whole face affected including forehead |
| Cause | Stroke, tumour (contralateral cortex) | Bell's palsy, Ramsay Hunt, parotid tumour, Lyme disease |
| Why spared | Upper face has bilateral cortical representation | LMN lesion affects all ipsilateral fibres |
Memory trick: "A stroke spares the forehead because both sides of the brain cover the top."
CN VIII — Vestibulocochlear (Hearing + Balance)
- 1Hearing: Whispered voice test — stand 60 cm away, occlude the opposite ear, whisper a number/word. If abnormal:
- Rinne's test: Vibrating 512 Hz tuning fork on mastoid then held in front of the ear.
- Normal / sensorineural loss: AC > BC (Rinne positive)
- Conductive loss: BC > AC (Rinne negative)
- Weber's test: Tuning fork on vertex of skull.
- Conductive loss: Lateralises to the affected ear
- Sensorineural loss: Lateralises to the unaffected ear
- 1Balance: Romberg's test — stand with feet together, arms by sides. Eyes open then closed. Falls with eyes closed (positive) = sensory ataxia (posterior column, proprioceptive loss). Falls with eyes open = cerebellar ataxia.
CN IX and X — Glossopharyngeal and Vagus (Swallowing + Speech)
- 1Palate: Ask the patient to say "Ahh" — the soft palate should rise symmetrically. In a unilateral CN X lesion, the uvula deviates away from the side of the lesion (pulled by the intact contralateral side).
- 2Speech: Listen for dysarthria (slurred, nasal, or scanning speech)
- 3Gag reflex: Afferent CN IX, efferent CN X — test only if clinically indicated (rarely in OSCE)
- 4Swallowing: Ask if they have difficulty
CN XI — Accessory (Sternocleidomastoid + Trapezius)
- 1Sternocleidomastoid: Place hand on patient's cheek and ask them to turn against resistance. The contralateral SCM contracts — test each side.
- 2Trapezius: Ask the patient to shrug their shoulders against downward resistance.
Weakness may indicate a lesion at the jugular foramen (alongside CN IX and X — "jugular foramen syndrome").
CN XII — Hypoglossal (Tongue)
- 1Inspect: At rest in the mouth — wasting or fasciculations (LMN)
- 2Protrusion: "Stick out your tongue" — deviates toward the side of the lesion (the weak side)
- 3Power: Ask the patient to push their tongue against the inside of their cheek while you press from outside
Common Examiner Follow-Up Questions
"This patient has a right-sided facial weakness affecting the whole face including the forehead. What is the most likely diagnosis and how would you manage it?"
"Whole-face involvement including the forehead indicates a lower motor neuron lesion — a peripheral CN VII palsy. The most common cause is Bell's palsy, which is idiopathic and thought to be related to HSV-1 reactivation. I would examine for vesicles in the external auditory canal (Ramsay Hunt syndrome — HSV zoster), parotid swelling, and Lyme disease exposure. Management of Bell's palsy: oral prednisolone 50 mg daily for 10 days within 72 hours of onset to improve recovery rates. Aciclovir may be added if Ramsay Hunt is suspected. Eye care is essential — artificial tears, eye patch at night, and urgent ophthalmology review if corneal exposure is at risk."
"A patient presents with sudden onset severe headache and a complete right CN III palsy with a dilated pupil. What is the most important diagnosis to exclude?"
"A posterior communicating artery aneurysm — this is a neurosurgical emergency. The parasympathetic fibres of CN III run on the outside of the nerve and are compressed first by an expanding aneurysm, causing a pupil-involving CN III palsy. The sudden severe headache may represent a sentinel bleed or subarachnoid haemorrhage. I would arrange immediate CT head, and if negative, a lumbar puncture at 12 hours to look for xanthochromia. CT angiography or conventional catheter angiography confirms the aneurysm. This requires immediate neurosurgical referral for clipping or endovascular coiling."
"What is the difference between internuclear ophthalmoplegia and a CN VI palsy?"
"Both cause horizontal gaze problems but the mechanism differs. A CN VI palsy causes pure failure of abduction of the ipsilateral eye, with intact adduction. Internuclear ophthalmoplegia (INO) is caused by a lesion in the medial longitudinal fasciculus (MLF) — the pathway connecting the CN VI nucleus to the contralateral CN III nucleus. An INO causes failure of adduction of the ipsilateral eye on lateral gaze, with nystagmus in the abducting eye, but normal convergence. INO in a young patient is highly suggestive of multiple sclerosis; in an older patient, brainstem stroke."