What is the mnemonic LR6SO4 and what does it mean?

LR6SO4 (rest CN3) is a mnemonic for extraocular muscle innervation: LR = Lateral Rectus is innervated by CN VI (6), SO = Superior Oblique is innervated by CN IV (4), and all remaining extraocular muscles (medial rectus, superior rectus, inferior rectus, and inferior oblique) are innervated by CN III (3). This is one of the highest-yield facts for ophthalmic certification exams.

What are the signs of a CN VI palsy?

CN VI (abducens) palsy weakens the lateral rectus muscle, causing limited abduction (inability to fully turn the eye outward). The affected eye is esotropic (turned inward) at primary gaze due to the unopposed action of the medial rectus. Patients experience horizontal diplopia that is worst when looking toward the side of the affected muscle and at distance.

Why does a patient with a CN IV palsy tilt their head?

CN IV palsy weakens the superior oblique, whose primary action is intorsion (rotating the top of the eye inward). Without this intorsion, the eye extorts, causing cyclovertical diplopia. The patient tilts their head toward the opposite shoulder to use the other eye's intorsion to compensate for the affected eye's torsional imbalance, reducing the diplopia. The head tilt is toward the opposite side of the palsy.

Extraocular Muscles and Innervation: CPO Exam Guide

The Six Extraocular Muscles

Six extraocular muscles (EOMs) control eye movement. Four rectus muscles and two oblique muscles work together to move each eye in all directions of gaze. As a CPO, you will perform extraocular muscle testing (versions and ductions) and document findings. Understanding which muscle moves the eye in which direction and which nerve controls it allows you to localize defects when a patient reports double vision or limited movement.

The Muscles and Their Primary Actions

Muscle	Primary Action	Secondary Actions	Nerve
Medial rectus (MR)	Adduction (in)	None significant	CN III
Lateral rectus (LR)	Abduction (out)	None significant	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 mnemonic LR6SO4 (rest CN3) summarizes the cranial nerve innervation: Lateral Rectus = CN VI, Superior Oblique = CN IV, all other muscles = CN III. This is one of the most tested facts in ophthalmic certification exams.

Cardinal Positions of Gaze

Eye movements are tested in the nine cardinal positions of gaze. In each position, a specific muscle pair is primarily responsible:

Right gaze: right lateral rectus + left medial rectus
Left gaze: left lateral rectus + right medial rectus
Up-right: right superior rectus + left inferior oblique
Up-left: left superior rectus + right inferior oblique
Down-right: right inferior rectus + left superior oblique
Down-left: left inferior rectus + right superior oblique

This is why version testing (having the patient follow a target through the H-pattern) systematically evaluates all six muscles in both eyes simultaneously.

Cranial Nerve Palsies

CN VI (Abducens) Palsy

Weakens the lateral rectus, causing inability to fully abduct the eye. The eye is esotropic (turned in) at primary gaze. Horizontal diplopia is worse at distance and in the direction of the palsied lateral rectus.

CN IV (Trochlear) Palsy

Weakens the superior oblique. Patients present with vertical diplopia, head tilt to the opposite side (to compensate for the loss of intorsion), and inability to depress the eye in adduction.

CN III (Oculomotor) Palsy

Weakens most EOMs. The eye assumes a "down and out" position from the unopposed action of the lateral rectus (CN VI) and superior oblique (CN IV). Associated ptosis and a fixed, dilated pupil distinguish a complete CN III palsy.

A patient with a head tilt to the left and vertical diplopia likely has a right CN IV palsy (or left CN IV palsy depending on specific findings). Head tilt away from the affected side is the classic compensatory mechanism for superior oblique palsy, used to reduce excyclotorsion of the eye.

Confusing the superior oblique's primary action (intorsion) with depression. The superior oblique is most effective as a depressor only when the eye is already adducted. In primary position, intorsion is its main role. Testing SO function specifically requires looking at depression in adduction.

Key Takeaways

LR6SO4 (rest CN3): lateral rectus = CN VI, superior oblique = CN IV, all others = CN III
Medial rectus adducts; lateral rectus abducts; superior/inferior rectus elevate/depress
Superior oblique: intorsion + depression in adduction; inferior oblique: extorsion + elevation in adduction
CN VI palsy: esotropia, horizontal diplopia worse at distance
CN IV palsy: vertical diplopia, head tilt away from affected side
CN III palsy: down-and-out eye, ptosis, dilated fixed pupil