Which extraocular muscle is supplied by CN VI, and what happens when it is paralyzed?

The lateral rectus muscle is supplied by CN VI (the abducens nerve). When paralyzed, the eye cannot abduct (turn outward), causing an esotropia (inward deviation) and horizontal diplopia that is worst when looking toward the side of the affected eye. CN VI palsy is the most common isolated cranial nerve palsy affecting ocular motility.

What is the primary action of the superior oblique?

The primary action of the superior oblique is intorsion (inward rotation of the 12 o'clock position of the globe). Its secondary actions are depression and abduction. This is why CN IV (trochlear nerve) palsy, which paralizes the superior oblique, causes difficulty with activities requiring downward gaze, such as reading or descending stairs.

What does LR6SO4 mean?

LR6SO4 is a mnemonic for remembering extraocular muscle innervation: Lateral Rectus is innervated by CN 6 (abducens), Superior Oblique by CN 4 (trochlear), and all other extraocular muscles (medial rectus, superior rectus, inferior rectus, inferior oblique, and levator palpebrae) by CN 3 (oculomotor nerve).

What is Hering's Law of Equal Innervation?

Hering's Law states that conjugate (yoke) muscles receive equal and simultaneous innervation during eye movements. For example, when looking right, the right lateral rectus and left medial rectus receive equal signals. This maintains conjugate gaze. It is clinically relevant in strabismus: weakening one muscle of a yoke pair affects the fellow eye's position.

How does a CN III palsy present clinically?

A complete CN III palsy causes ptosis (levator palpebrae paralysis), the eye deviating down and out (lateral rectus and superior oblique still functioning), and a dilated, non-reactive pupil (loss of parasympathetic fibers). The pupil-involving feature suggests compression (such as a posterior communicating artery aneurysm) rather than ischemia, which typically spares the pupil -- making it an urgent neurological emergency.

Extraocular Muscle Anatomy

The Six Extraocular Muscles

Six extraocular muscles (EOMs) control the movement of each eye. They originate from the annulus of Zinn at the orbital apex and insert on the sclera. Knowing their actions and nerve supply is essential for interpreting motility tests, understanding diplopia, and assisting with strabismus patients.

The Four Rectus Muscles

The four rectus muscles run straight from the apex to the globe:

Muscle	Primary Action	Secondary Actions	Nerve
Medial rectus (MR)	Adduction (in)	None significant	CN III
Lateral rectus (LR)	Abduction (out)	None significant	CN VI (abducens)
Superior rectus (SR)	Elevation	Intorsion, adduction	CN III
Inferior rectus (IR)	Depression	Extorsion, adduction	CN III

The Two Oblique Muscles

The obliques have more complex courses and actions:

Muscle	Primary Action	Secondary Actions	Nerve
Superior oblique (SO)	Intorsion	Depression, abduction	CN IV (trochlear)
Inferior oblique (IO)	Extorsion	Elevation, abduction	CN III

💡 Clinical Tip: The superior oblique wraps around the trochlea (a pulley on the orbital rim) before inserting on the globe. This changes its pull direction. CN IV palsy (trochlear nerve palsy) causes vertical diplopia worse when looking down and toward the nose -- patients often tilt their head away from the affected side to compensate.

Cranial Nerve Summary

A useful mnemonic for EOMs and their nerves: LR6SO4 (remaining muscles CN3):

Lateral Rectus = CN 6 (abducens)
Superior Oblique = CN 4 (trochlear)
Everything else = CN 3 (oculomotor)

Yoke Muscles and Hering's Law

Hering's Law of Equal Innervation states that equal and simultaneous innervation goes to the yoke muscles (paired muscles responsible for conjugate gaze in each direction). The six cardinal positions of gaze each have a designated yoke pair:

Gaze Direction	Right Eye Muscle	Left Eye Muscle
Right	Lateral rectus	Medial rectus
Left	Medial rectus	Lateral rectus
Right and up	Superior rectus	Inferior oblique
Right and down	Inferior rectus	Superior oblique
Left and up	Inferior oblique	Superior rectus
Left and down	Superior oblique	Inferior rectus

Clinical Testing

Motility testing assesses EOM function in the nine cardinal positions of gaze (primary, plus eight surrounding positions). The CPOA often performs this by having the patient follow a target (typically a penlight or fixation stick) while the examiner observes for:

Full, symmetrical movements
Nystagmus (rhythmic oscillation)
Diplopia in any direction
Incomitant deviation (misalignment that changes size with gaze direction)

⚠️ Common Mistake: Do not confuse the primary action of the superior oblique (intorsion) with that of the superior rectus (elevation). When the eye is adducted, the superior oblique becomes the primary depressor. Knowing when to test each muscle in its "diagnostic" position is key to proper motility assessment.

Common EOM Palsies

EOM palsies are named by which cranial nerve is affected:

CN III palsy: eye is "down and out" (ptosis + dilated pupil if compressive; pupils spared in ischemic). Most EOMs paralyzed except LR and SO.
CN IV palsy: vertical diplopia, worse looking down. Head tilt away from affected eye.
CN VI palsy: inability to abduct (turn outward). Horizontal diplopia greatest on ipsilateral gaze. Often from elevated ICP or microvascular disease.

Key Takeaways

Six EOMs control eye movement: 4 rectus and 2 oblique muscles
Lateral rectus = CN VI; superior oblique = CN IV; all others = CN III
The superior oblique's primary action is intorsion; it also depresses and abducts
Hering's Law: equal innervation to yoke muscles in conjugate gaze
Motility testing in 9 positions detects paresis, restriction, and diplopia patterns
CN VI palsy (inability to abduct) is the most common isolated EOM palsy

Extraocular Muscle Anatomy | CPOA Exam