How do I test cranial nerves (CN) 3, 4, and 6 and document the findings?

Testing Cranial Nerves 3,4, and 6

Test CN 3,4, and 6 together by assessing extraocular movements in all directions of gaze, pupillary responses, and eyelid position, then document specific deficits with directional notation and associated findings.

Functions of CN 3,4, and 6

These three cranial nerves work together to control eye movements and are best examined as a functional unit 1, 2:

• CN 3 (Oculomotor): Controls superior rectus (upward gaze), inferior rectus (downward gaze), medial rectus (adduction), inferior oblique (upward and outward), levator palpebrae (eyelid elevation), and pupillary constriction 2, 3
• CN 4 (Trochlear): Controls superior oblique muscle (downward and inward gaze, intorsion) 2, 3
• CN 6 (Abducens): Controls lateral rectus muscle (abduction/outward gaze) 2, 3

Examination Technique

Eye Movement Testing

Ask the patient to follow your finger in an "H" pattern while keeping their head still 4:

• Test horizontal gaze (right and left) - assesses CN 6 primarily
• Test vertical gaze (up and down) - assesses CN 3 primarily
• Test oblique movements (up-right, up-left, down-right, down-left) - assesses CN 3 and CN 4 4
• Observe for smooth pursuit, full range of motion, and conjugate movements 4

Pupillary Examination (CN 3)

• Direct light reflex: Shine light in each eye and observe ipsilateral pupil constriction 4
• Consensual light reflex: Observe contralateral pupil constriction when light is shone in opposite eye 4
• Accommodation: Ask patient to focus on distant object then near object; pupils should constrict with near focus 4

Eyelid Position (CN 3)

• Observe for ptosis (drooping eyelid) at rest, which indicates CN 3 dysfunction 4, 5

Documentation Format

Normal Findings

Document as: "CN 3,4,6: Extraocular movements intact in all directions. Pupils equal, round, reactive to light and accommodation (PERRLA). No ptosis. No nystagmus." 4

Abnormal Findings - Be Specific

CN 3 palsy 1, 5:

• "Ptosis of [right/left] eye"
• "Dilated [right/left] pupil, non-reactive to light" (if pupil-involving)
• "Limited [upward/downward/medial] gaze [right/left] eye"
• "Eye rests in 'down and out' position at rest"

CN 4 palsy 1, 2:

• "Limited downward gaze when eye is adducted [right/left]"
• "Vertical diplopia worse when looking down and toward [opposite] side"
• "Head tilt away from affected side" (compensatory)

CN 6 palsy 1, 2:

• "Limited abduction [right/left] eye"
• "Horizontal diplopia worse with [right/left] lateral gaze"
• "Esotropia (inward deviation) of [right/left] eye at rest"

Critical Clinical Pitfalls

Distinguish peripheral from supranuclear lesions: Peripheral, nuclear, or infranuclear cranial nerve lesions produce ipsilateral symptoms, while supranuclear (cortical) lesions produce contralateral symptoms 6. Document the side of deficit carefully.

CN 6 palsy can be a false localizing sign: Isolated CN 6 palsy may occur with increased intracranial pressure without direct nerve compression, so do not assume the lesion is at the nerve itself 6.

Cavernous sinus pathology affects multiple nerves: Lesions in the cavernous sinus or orbital apex affect CN 3,4, and 6 together on the same side 6. Document all three if multiple deficits are present.

Pupil-sparing vs. pupil-involving CN 3 palsy matters: Pupil-sparing CN 3 palsy suggests microvascular ischemia (often diabetic), while pupil-involving palsy suggests compressive lesion (aneurysm, tumor) requiring urgent imaging 1.

When to Image

MRI with contrast at 3.0T is the gold standard for evaluating cranial neuropathy when dysfunction is identified on examination 7, 6. The American College of Radiology recommends MRI head and orbit/face/neck with and without IV contrast for CN 3,4, or 6 involvement 7.