What is the LR6SO4AO3 mnemonic and why is it useful?

LR6SO4AO3 is a memory aid for the cranial nerve innervation of the extraocular muscles: Lateral Rectus is innervated by CN 6 (abducens), Superior Oblique by CN 4 (trochlear), and All Others by CN 3 (oculomotor). This is useful because on the CPO exam and in clinical practice, you need to quickly identify which nerve is affected based on the pattern of eye movement deficit. CN III controls four muscles plus lid elevation and pupil constriction.

What eye movement pattern would you expect with a right CN VI palsy?

A right CN VI palsy paralyzes the right lateral rectus. Without the lateral rectus to abduct the eye, the medial rectus pulls the right eye inward, causing a right esotropia (right eye turns toward the nose). The patient has horizontal diplopia that worsens when they look to the right (in the direction of the weakened lateral rectus). The diplopia resolves when looking to the left, because in that direction the right eye is adducting (medial rectus function), not requiring the lateral rectus.

Why do patients with CN IV palsy tilt their head?

CN IV palsy weakens the superior oblique, which is responsible for intorsion (rotating the top of the eye inward) and depression when the eye is adducted. To compensate for the loss of intorsion, patients tilt their head toward the opposite shoulder, which effectively intorts the affected eye using the head and neck. This head tilt is a classic sign of CN IV palsy and is confirmed by the three-step test (Parks-Bielschowsky test) used to identify the paretic muscle.

What is the difference between a duction and a version?

A duction is the movement of one eye alone. For example, adduction is the inward rotation of one eye toward the nose. A version is a binocular conjugate eye movement in which both eyes move together in the same direction. Dextroversion means both eyes look right; levoversion means both eyes look left. Vergences, by contrast, are binocular disconjugate movements in opposite directions, such as convergence (both eyes rotate inward for near focus).

Why is a CN III palsy with a dilated pupil considered a medical emergency?

The pupillary sphincter is innervated by parasympathetic fibers that travel on the outside of the CN III nerve bundle. When an aneurysm (most commonly a posterior communicating artery aneurysm) compresses CN III, the external pupillary fibers are affected first, causing a dilated, non-reactive pupil along with ptosis and eye deviation. This presentation requires urgent neuroimaging to rule out a life-threatening aneurysm. A microvascular CN III palsy from diabetes or hypertension typically spares the pupil because the internal ischemia affects the central motor fibers while the external pupillary fibers remain intact.

Extraocular Muscle Anatomy for the CPO Exam

The six extraocular muscles (EOMs) work in precise coordination to move each eye in any direction. Understanding their individual actions, origins, insertions, and nerve supply is fundamental anatomy for any ophthalmic professional. The CPO exam tests this knowledge directly, often through clinical scenarios involving eye movement deficits.

The Six Extraocular Muscles

Six muscles control each eye: four rectus muscles and two oblique muscles. All six muscles originate from the bony orbit and insert onto the sclera.

Muscle	Abbreviation	Primary Action	Cranial Nerve
Medial rectus	MR	Adduction (turns eye inward)	CN III
Lateral rectus	LR	Abduction (turns eye outward)	CN VI
Superior rectus	SR	Elevation, intorsion, adduction	CN III
Inferior rectus	IR	Depression, extorsion, adduction	CN III
Superior oblique	SO	Intorsion, depression (when adducted), abduction	CN IV
Inferior oblique	IO	Extorsion, elevation (when adducted), abduction	CN III

The LR6SO4AO3 Mnemonic

The classic mnemonic for remembering which cranial nerve innervates which muscle is LR6SO4AO3:

LR6: Lateral Rectus, CN 6 (abducens)
SO4: Superior Oblique, CN 4 (trochlear)
AO3: All Others, CN 3 (oculomotor)

CN III (oculomotor) is by far the busiest cranial nerve for eye movement, controlling four of the six EOMs plus the levator palpebrae superioris (upper eyelid elevation) and the pupillary sphincter.

Only three cranial nerves control eye movement. CN VI is the simplest (one muscle, one action: abduction). CN IV is the only cranial nerve that exits the dorsum of the brainstem and has the longest intracranial course, making it vulnerable to head trauma.

The Ring of Zinn (Annulus of Zinn)

Four of the six EOMs (both obliques are exceptions) originate from a common fibrous ring at the orbital apex called the annulus of Zinn (or common tendinous ring). These four muscles form a cone of tissue around the optic nerve called the muscle cone. The optic nerve, ophthalmic artery, and CN III, IV, and VI all pass through or near this ring.

Cranial Nerve Palsy Patterns

When an extraocular muscle is weakened or paralyzed, the eye deviates in the direction opposite to its normal action. Understanding these deviation patterns is essential for the CPO exam.

CN VI (Abducens) Palsy

The most common cranial nerve palsy. The lateral rectus cannot abduct the eye, so the medial rectus pulls it inward. The result is an esotropia (eye turns inward, nasally). Patients have horizontal diplopia that worsens when looking in the direction of the affected lateral rectus. Causes include elevated intracranial pressure, microvascular disease (hypertension, diabetes), and trauma.

CN IV (Trochlear) Palsy

The superior oblique cannot depress the eye when it is adducted, and cannot intort it. The result is a hypertropia (eye turns upward) that worsens when looking down and inward. Patients often tilt their head to compensate (characteristic head tilt to the opposite shoulder). CN IV palsy is frequently caused by head trauma (even minor).

CN III (Oculomotor) Palsy

The most dramatic palsy. Loss of CN III causes ptosis (levator palpebrae), a down-and-out eye position (only the lateral rectus and superior oblique still function), and a dilated, non-reactive pupil (loss of parasympathetic input to the sphincter). This presentation is a medical emergency, as a compressive CN III palsy (often from a posterior communicating artery aneurysm) is life-threatening.

A CN III palsy with a dilated pupil is a surgical emergency until proven otherwise. It may indicate a compressing aneurysm. A "pupil-sparing" CN III palsy (normal pupil size) is more likely microvascular in etiology (diabetes, hypertension) and is less immediately dangerous.

Ductions and Versions

Eye movements are described in standardized terms:

Ductions: Monocular movements of one eye (adduction, abduction, elevation, depression, intorsion, extorsion).
Versions: Binocular conjugate movements (both eyes moving together in the same direction). Dextroversion means both eyes look right; levoversion means both look left.
Vergences: Binocular disconjugate movements (eyes move in opposite directions). Convergence (both eyes turn in) and divergence (both eyes turn out) are the primary vergence movements.

Key Takeaways

Six muscles move each eye: medial, lateral, superior, and inferior recti, plus superior and inferior obliques.
LR6SO4AO3: Lateral Rectus = CN VI, Superior Oblique = CN IV, All Others = CN III.
CN VI palsy causes esotropia (eye turns in); CN IV palsy causes hypertropia with head tilt; CN III palsy causes ptosis, down-and-out eye, and dilated pupil.
A CN III palsy with a dilated pupil is a medical emergency.
Ductions are monocular movements; versions are conjugate binocular movements; vergences are disconjugate movements.