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Six extraocular muscles work in precise coordination to move each eye through a full range of motion, maintain alignment, and enable the binocular vision that allows depth perception. Understanding these muscles — their names, actions, and nerve supply — is essential for every paraoptometric. It underpins your ability to assist with cover testing, document eye movement findings, recognize red flags like sudden-onset diplopia, and understand why patients with certain cranial nerve palsies present the way they do.
Three cranial nerves supply all six muscles: the oculomotor nerve (CN III) controls four muscles, the trochlear nerve (CN IV) controls one, and the abducens nerve (CN VI) controls one. A useful mnemonic is LR6SO4 — Lateral Rectus is CN 6, Superior Oblique is CN 4, and all others are CN 3. This pattern is tested frequently on both the CPO and CPOA certification exams.
The four rectus muscles (superior, inferior, medial, lateral) all originate from the annulus of Zinn at the apex of the orbit and insert directly onto the sclera in front of the equator of the eye. The two oblique muscles take different paths, which explains why their actions are less intuitive than their names suggest.
Primary action: adduction (moving the eye toward the nose). The strongest of the four rectus muscles. Weakness or palsy produces an exotropia (eye drifts outward). Paralysis of the medial rectus is a feature of internuclear ophthalmoplegia (INO), caused by a lesion in the medial longitudinal fasciculus, seen in multiple sclerosis.
Primary action: abduction (moving the eye away from the nose). The only muscle innervated by CN VI. Palsy causes esotropia (eye turns in) with inability to abduct. CN VI has the longest intracranial course and is especially vulnerable to raised intracranial pressure — a new CN VI palsy warrants urgent investigation.
Primary action: elevation (especially when the eye is abducted). Secondary actions: intorsion and adduction. The superior rectus is best tested when the eye is abducted 23° (the angle of the muscle's pull). When abducted, it becomes a pure elevator.
Primary action: depression (especially when the eye is abducted). Secondary actions: extorsion and adduction. Trauma can damage the inferior rectus or entrap it in orbital floor fractures ("blowout fracture"), restricting upward gaze and causing vertical diplopia — a finding that should be flagged immediately.
Primary action: intorsion (rotating top of eye toward nose). Secondary: depression and abduction. The superior oblique passes through the trochlea (a fibrocartilage pulley at the medial orbital wall), which redirects its pull. This is why its actions seem counterintuitive — it depresses the eye, especially when adducted. CN IV palsy causes vertical diplopia worse on downgaze; patients often tilt their head to compensate.
Primary action: extorsion (rotating top of eye away from nose). Secondary: elevation and abduction. The inferior oblique is best tested when the eye is adducted — in this position it becomes a pure elevator. Inferior oblique overaction is a common finding in patients with strabismus and produces a characteristic "V-pattern" (divergence on upgaze).
Testing eye movements in the nine cardinal positions allows systematic evaluation of each extraocular muscle. The six diagnostic positions isolate muscle pairs: each position is the primary testing position for two yoke muscles (one in each eye that work together for that direction of gaze).
Right Gaze
Right LR + Left MR
Left Gaze
Left LR + Right MR
Up-Right
Right SR + Left IO
Up-Left
Left SR + Right IO
Down-Right
Right IR + Left SO
Down-Left
Left IR + Right SO
The cover test is the primary clinical tool for detecting and measuring misalignment. Paraoptometrics commonly assist with or perform cover testing as part of the comprehensive exam workup.
Hirschberg Corneal Light Reflex
A penlight held at 33cm. Corneal reflections should be symmetric and slightly nasal to the pupil center (due to positive angle kappa). Asymmetric reflexes indicate misalignment: nasal displacement of reflex = exotropia; temporal = esotropia. Each 1mm of reflex displacement ≈ 15 prism diopters of deviation.
Cover-Uncover Test (Detects Tropias)
Cover one eye. Watch the uncovered eye — any movement to pick up fixation indicates a manifest deviation (tropia) in that eye. Uncover and watch the previously covered eye return. Movement indicates it was deviated under the cover. Esotropia: eye moves outward when covered fellow eye is uncovered. Exotropia: eye moves inward.
Alternating Cover Test (Detects Phorias + Tropias)
Alternate the occluder rapidly between eyes, preventing fusion. Any eye movement as the cover moves reveals latent or manifest deviation. More sensitive than cover-uncover. Used to measure total deviation with prism bars.
When the eyes are misaligned, each eye sends a different image to the brain, producing diplopia (double vision). The brain can suppress one image (especially in children during the critical period), which can lead to amblyopia if persistent. Understanding the terminology is essential for accurate documentation.
Esotropia
Eye turns inward (crossed eyes). May be accommodative (hyperopia-driven) or non-accommodative.
Exotropia
Eye turns outward (wall-eyed). Often intermittent initially, especially on distance viewing or fatigue.
Hypertropia
Eye turns upward. Often indicates superior oblique palsy or inferior rectus restriction.
Comitant
Deviation is the same in all positions of gaze — suggests the problem is not a specific muscle but a central or refractive cause.
Incomitant
Deviation varies with gaze direction — suggests a specific muscle or nerve palsy. Always investigate further.
Phoria
Latent deviation controlled by fusion. Only revealed when fusion is disrupted (cover test). Esophoria, exophoria, hyperphoria.
Red Flag: New-Onset Diplopia in Adults
New sudden double vision in an adult — especially with a dilated pupil, ptosis, or severe headache — requires urgent evaluation. A "blown pupil" (dilated, unreactive) with CN III palsy may indicate a posterior communicating artery aneurysm compressing the nerve, which is a neurological emergency. Always alert the doctor immediately and do not proceed with routine testing.
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Types, testing, and clinical role in managing ocular misalignment.
Snellen, LogMAR, and measuring VA in clinical practice.
Browse all CPO and CPOA study topics by category.
The oculomotor nerve (CN III) is by far the most dominant, controlling four of the six extraocular muscles: the superior rectus, inferior rectus, medial rectus, and inferior oblique. It also innervates the levator palpebrae superioris (which lifts the upper eyelid) and carries parasympathetic fibers that constrict the pupil via the sphincter pupillae. A complete CN III palsy produces a characteristic triad: the eye drifts down and out (due to unopposed action of CN IV and CN VI muscles), ptosis (drooping eyelid), and a dilated, non-reactive pupil (if the parasympathetic fibers are also affected). The trochlear nerve (CN IV) controls only the superior oblique, and the abducens nerve (CN VI) controls only the lateral rectus.
The superior oblique's primary action is intorsion (rotating the top of the eye toward the nose), with secondary actions of depression and abduction. It is the most frequently misunderstood extraocular muscle because its name suggests it moves the eye upward, but it actually depresses the eye — particularly when the eye is in an adducted (inward) position. To test the superior oblique (CN IV) in isolation, ask the patient to look in (adduct) first, then have them look down. Any weakness in this position indicates a superior oblique palsy. CN IV palsy classically causes vertical diplopia that worsens on the affected side and when looking down (e.g., reading or descending stairs), and patients often compensate with a head tilt away from the affected eye.
The lateral rectus is innervated by the abducens nerve (CN VI), the longest intracranial course of any cranial nerve. A CN VI palsy produces an esotropia (the eye turns inward) because the medial rectus is unopposed. The patient has horizontal diplopia that worsens on gaze toward the affected side, and the eye cannot fully abduct (move outward). CN VI palsy is the most common isolated ocular motor nerve palsy. Common causes include: increased intracranial pressure (CN VI is vulnerable due to its long course — it gets compressed against the petrous ridge), microvascular disease from diabetes or hypertension (most common in adults over 50), trauma, and rarely tumors. New-onset CN VI palsy always warrants further investigation.
These are two different tests that evaluate different conditions. The cover-uncover test detects a tropia (manifest misalignment present even without covering): cover one eye and watch the uncovered eye — if it moves to pick up fixation, a tropia is present in the uncovered eye. Then uncover and watch the covered eye return — movement indicates a tropia in that eye. The alternating cover test is more sensitive and detects both tropias AND phorias (latent misalignment controlled by fusion): rapidly alternate the cover between eyes so fusion is never allowed. Any movement of the eye as the cover moves reveals misalignment. The alternating cover test dissociates fusion and reveals the total deviation, which is always equal to or greater than the deviation found on cover-uncover.
The nine cardinal positions systematically test each extraocular muscle pair: primary position (straight ahead), right gaze (right lateral rectus, left medial rectus), left gaze (left lateral rectus, right medial rectus), right-up (right superior rectus, left inferior oblique), left-up (left superior rectus, right inferior oblique), right-down (right inferior rectus, left superior oblique), left-down (left inferior rectus, right superior oblique), up (both superior recti, both inferior obliques), and down (both inferior recti, both superior obliques). By observing movement in each position, the examiner can isolate which muscle or nerve is underperforming. When a patient has diplopia, the image separation is greatest in the field of action of the paretic muscle, which helps localize the problem. Testing all nine positions is essential in any patient presenting with double vision or asymmetric eye position.