Extraocular Muscles and Innervation for the CPOA Exam

The Six Extraocular Muscles

Each eye is moved by six extraocular muscles (EOMs). Four are rectus muscles (straight muscles) that pull the eye along the four cardinal directions, and two are oblique muscles that add rotational components to eye movement.

💡 Clinical Tip: A classic mnemonic for cranial nerve innervation: LR6 SO4 rest 3. Lateral Rectus = CN 6 (Abducens). Superior Oblique = CN 4 (Trochlear). All remaining muscles = CN 3 (Oculomotor). This single sentence covers all six extraocular muscles.

Cranial Nerve Roles

CN III (Oculomotor Nerve)

CN III innervates four of the six extraocular muscles (medial rectus, superior rectus, inferior rectus, inferior oblique) plus the levator palpebrae superioris (which raises the upper eyelid) and the pupillary constrictor (via parasympathetic fibers).

A complete CN III palsy causes:

• Ptosis (drooping upper eyelid from levator paralysis)
• Eye deviated "down and out" (unopposed action of lateral rectus pulling temporally and superior oblique pulling downward)
• Dilated, non-reactive pupil (if parasympathetic fibers are involved)

CN IV (Trochlear Nerve)

CN IV innervates only one muscle: the superior oblique. It is the longest cranial nerve and the only one that exits the brainstem dorsally (from the back). CN IV palsy results in:

• Vertical diplopia (double vision, especially when looking down and nasally -- such as reading)
• Head tilt away from the affected eye (compensatory, to reduce torsional diplopia)
• Inferior oblique overaction (the antagonist muscle is unopposed)

CN VI (Abducens Nerve)

CN VI innervates only the lateral rectus. CN VI palsy produces:

• Esotropia (eye turns inward, toward the nose) because the medial rectus is unopposed
• Horizontal diplopia (worsens when looking in the direction of the affected lateral rectus)
• Inability to abduct (move eye outward) on the affected side

Testing Extraocular Muscle Function

The doctor tests EOM function by asking the patient to follow a target through the six cardinal positions of gaze (also called the "H" test or cover test positions): right, right-up, right-down, left, left-up, left-down. Each position isolates a specific muscle pair.

The CPOA may be asked to:

• Provide the fixation target (penlight or accommodative target) while the doctor observes.
• Document the doctor's findings in the chart (e.g., "EOM full OU," or "LR OD restricted -- 50% abduction").
• Explain the test to the patient: "Follow my finger/light with your eyes, without moving your head."

⚠️ Common Mistake: Moving the fixation target too quickly during motility testing. Slow, deliberate movements allow the doctor to observe the full range of motion in each muscle. A target moved too quickly may overshoot the point of restriction, missing subtle motility deficits.

Ductions vs. Versions

• Ductions: Movement of one eye independently (monocular movements). Used to assess each eye individually.
• Versions: Conjugate movements of both eyes together in the same direction (e.g., both eyes move right, left, up, down).

Key Takeaways

• Six extraocular muscles move each eye: 4 rectus (medial, lateral, superior, inferior) and 2 oblique (superior, inferior).
• Mnemonic: LR6 SO4 rest 3 -- Lateral Rectus = CN 6, Superior Oblique = CN 4, all others = CN 3.
• CN III palsy: ptosis + eye down and out + possible dilated pupil.
• CN IV palsy: vertical diplopia + head tilt away from affected eye.
• CN VI palsy: esotropia + inability to abduct the affected eye.
• The CPOA provides the fixation target and documents findings during motility testing.