Clinical Examination of the Oculomotor (CN III), Trochlear (CN IV), and Abducens (CN VI) Nerves

Functional Anatomy - Quick Reference

Step-by-Step Clinical Examination

1. Inspect at Rest (Primary Gaze)

• Ptosis: Look for drooping of the upper lid. Partial or complete ptosis points to CN III palsy (levator palpebrae) or sympathetic chain (Horner's - partial ptosis).
• Eye position: Note any resting deviation. A "down and out" eye at rest is the classic position of complete CN III palsy (unopposed CN IV + CN VI action). A medially deviated (esotropic) eye at rest suggests CN VI palsy.
• Head posture: A head tilt away from the affected side (compensatory) suggests CN IV palsy. Ask about old photographs to determine whether the tilt is longstanding (congenital palsy).

2. Pupil Examination

• Size: Measure in mm; compare both eyes. Note anisocoria.
• Direct and consensual light reflex: Shine a light in one eye; look for ipsilateral constriction (direct) and contralateral constriction (consensual). CN III carries the efferent (constrictor) limb.
• Swinging flashlight test: Detects an afferent pupillary defect (CN II problem, not CN III).
• Accommodation reflex: Ask the patient to look at a distant object, then quickly at your finger close up. Expect miosis and convergence. Loss of accommodation (cycloplegia) accompanies CN III palsy.

• Pupil-involving CN III palsy (dilated, non-reactive pupil): Strongly suggests compressive etiology - posterior communicating artery (PCOM) aneurysm until proven otherwise. Parasympathetic fibers lie superficially on the nerve, making them vulnerable to external compression. Requires urgent neuroimaging + CT/MR angiography.
• Pupil-sparing CN III palsy: More typical of microvascular ischemia (diabetes, hypertension). However, a pupil-sparing incomplete palsy still warrants urgent vascular imaging - some aneurysmal palsies lack pupil involvement initially and progress within 1 week.
• Rule: Any incomplete or pupil-involving CN III palsy = immediate evaluation for PCOM aneurysm. - Bradley and Daroff's Neurology in Clinical Practice

3. Ocular Motility - Six Cardinal Positions of Gaze

• Diplopia assessment: Ask if double vision persists with one eye covered. Binocular diplopia (resolves on cover) = extraocular muscle or nerve pathology. Monocular diplopia = corneal/lens problem or malingering.
• Diplopia from CN VI palsy is horizontal, worst on ipsilateral lateral gaze, and worst at distance.
• Diplopia from CN IV palsy is vertical/oblique, worst going down stairs or reading (down-and-in gaze), worse with ipsilateral head tilt.
• Diplopia from CN III palsy is multidirectional depending on which muscles are affected.

4. Cover and Alternate Cover Tests

• Cover-uncover test: Cover one eye; watch the uncovered eye - if it moves to take up fixation, a tropia is present.
• Alternate cover test: Alternately cover each eye rapidly. Allows latent deviations to manifest.
• The higher (hyperdeviated) eye in primary gaze - when uncovered, it moves downward to take fixation, indicating a hypertropia.

5. Park's Three-Step Test (for CN IV/Superior Oblique palsy)

1. Step 1: Identify which eye is higher (hypertropic) in primary gaze.
2. Step 2: Is the hyperdeviation greater on right gaze or left gaze?
3. Step 3: Is the hyperdeviation greater on right head tilt or left head tilt?

6. Convergence

• Loss of convergence suggests bilateral CN III involvement or a supranuclear (midbrain) problem.
• Light-near dissociation (pupils react to near but not light) = Parinaud's/dorsal midbrain syndrome or Argyll Robertson pupils.

7. Additional Tests When Indicated

• Double Maddox rod test: For suspected bilateral CN IV palsy or torsional diplopia; excyclotorsion >10 degrees total = bilateral superior oblique paresis.
• Ice test / rest test: If myasthenia gravis is in the differential (fatiguable ptosis and ophthalmoplegia that fluctuates, mimics CN III/IV/VI palsy).
• Prism bar: Measure vertical fusional amplitudes - >6 prism diopters suggests congenital (rather than acquired) CN IV palsy.

Clinical Findings in Palsy of Each Nerve

CN III Palsy - "Down and Out Eye"

• Complete: ptosis, eye deviated down and out, no elevation/depression/adduction, dilated non-reactive pupil (if pupil-involved), loss of accommodation
• Partial: any combination of the above
• Nuclear CN III palsy (rare): bilateral ptosis, contralateral superior rectus weakness (unique cross-innervation anatomy)

• Weber's syndrome: ipsilateral CN III palsy + contralateral hemiparesis (cerebral peduncle)
• Benedikt's syndrome: ipsilateral CN III palsy + contralateral tremor/chorea (red nucleus)
• Claude's syndrome: ipsilateral CN III palsy + contralateral cerebellar ataxia (superior cerebellar peduncle)
• Nothnagel's syndrome: ipsilateral CN III palsy + ipsilateral hemiataxia

CN IV Palsy - "Can't Read, Can't Go Downstairs"

• Vertical/torsional diplopia; worse going downstairs or reading (downgaze in adduction)
• Compensatory contralateral head tilt
• Bielschowsky sign positive
• Overaction of ipsilateral inferior oblique (excessive elevation on adduction)
• Most common cause of isolated symptomatic vertical diplopia
• Most common cause: congenital, trauma (CN IV decussates in the midbrain - both nerves vulnerable to head trauma), microvascular ischemia

CN VI Palsy - "Crossed Eyes Looking to One Side"

• Horizontal diplopia, worst on ipsilateral gaze, worst at distance
• Affected eye fails to abduct; at rest it may be medially deviated (esotropia)
• Patient may compensate with a face turn toward the affected side
• CN VI has the longest intracranial course - prone to raised intracranial pressure (false localizing sign), trauma, and microvascular ischemia
• Bilateral CN VI palsy: raised ICP (e.g., pseudotumor cerebri), Wernicke's encephalopathy, meningitis

Localizing the Lesion

Summary of Examination Sequence

1. Inspection at rest - ptosis, eye position, head tilt
2. Pupils - size, direct/consensual reflex, accommodation, swinging flashlight
3. Six cardinal positions of gaze - note restricted movement, diplopia
4. Cover/alternate cover test - identify and quantify tropia
5. Convergence test
6. Park's three-step test if vertical diplopia present
7. Head tilt test (Bielschowsky) for CN IV palsy
8. Maddox rod / double Maddox rod if torsional diplopia suspected
9. Ice test / fatigue test if myasthenia gravis in differential