Extraocular Muscles and Cranial Nerves in Eye Care

Q: What is the Parks 3-Step test used for?

Parks 3-Step localizes the paretic muscle in vertical strabismus from a suspected single-muscle palsy. It uses three observations — which eye is hyper, worse gaze, and worse head tilt — to identify one of eight muscles, most commonly identifying a CN IV palsy of the superior oblique.

Q: Which cranial nerve controls the superior oblique?

The superior oblique is innervated by cranial nerve IV (trochlear). It's the only muscle with this innervation. The mnemonic LR6 SO4 R3 captures the rule: LR by CN VI, SO by CN IV, the rest by CN III.

Q: What does the LR6 SO4 R3 mnemonic mean?

LR6 SO4 R3 says: lateral rectus by CN VI, superior oblique by CN IV, all the rest by CN III. CN III also controls the levator (lid), pupil sphincter, and ciliary muscle, so a complete CN III palsy shows ptosis, a 'down and out' eye, and a fixed dilated pupil.

Q: When does the Parks 3-Step test fail or give a wrong answer?

Parks fails in skew deviation, restrictive strabismus (TED, floor fracture, post-surgical), long-standing DVD, prior strabismus surgery, and multi-muscle palsies like myasthenia gravis. Use forced ductions for restrictive disease, upright-supine for skew, and ice/fatigability for MG.

Q: What is the difference between a CN IV palsy and a skew deviation?

CN IV palsy is a peripheral lesion with extorsion of the higher eye and a head tilt away. Skew deviation is supranuclear, typically shows intorsion of the higher eye, is part of the ocular tilt reaction, and typically improves supine. Skew requires urgent imaging.

Q: Which extraocular muscle is most commonly affected in thyroid eye disease?

Inferior rectus is the most commonly affected EOM in thyroid eye disease, followed by medial rectus. TED is restrictive (fibrosis), not paretic — so forced ductions are positive. A fibrotic IR can mimic a contralateral SR palsy on Parks.

Q: What does it mean if a CN III palsy involves the pupil?

A pupil-involving CN III palsy is a neurosurgical emergency — concern for posterior communicating artery aneurysm. Urgent CTA or MRA is indicated. Pupil-sparing CN III palsies in older patients with vascular risk usually reflect microvascular ischemia and typically resolve in 8–12 weeks.

Eye muscle actions, CN III/IV/VI/VII innervation, and Parks 3-Step Test

Anatomy and Clinical Interpretation of Ocular Motility

The six extraocular muscles and their actions

Each EOM has primary, secondary, and tertiary actions when starting from primary gaze. The reference table on this page lists only the primary action — the secondary and tertiary actions are what make Parks 3-Step work and are summarized here.

The four rectus muscles have intuitive primary actions: superior rectus elevates, inferior rectus depresses, medial rectus adducts, lateral rectus abducts. The two vertical recti are most efficient as elevators or depressors when the eye is abducted, because that is when the muscle's pull aligns with the vertical axis. In adduction, the vertical recti add minor torsional and horizontal components but contribute little vertical movement.

The two oblique muscles are less intuitive. The superior oblique tendon passes through the trochlea and inserts behind the equator of the globe. Its primary action is intorsion, secondary action is depression, and tertiary action is abduction. The SO depresses most strongly when the eye is adducted — the reason an SO palsy localizes to the gaze position where the affected eye is adducted (an RSO palsy shows greatest hyperdeviation on left gaze, when the right eye is adducted).

The inferior oblique mirrors the SO with opposite vertical action: primary extorsion, secondary elevation, tertiary abduction. The IO elevates most strongly in adduction. This adduction dependence is the entire reason gaze direction matters in Parks — in primary gaze, four muscles share vertical work, but in side gaze only two muscles do the lifting and a paretic oblique becomes unmistakable.

Cranial nerve innervation: the LR6 SO4 R3 mnemonic

Three cranial nerves drive eye movement: III (oculomotor), IV (trochlear), and VI (abducens). The standard mnemonic LR6 SO4 R3 summarizes the assignment:

LR6 — lateral rectus is innervated by CN VI (abducens).
SO4 — superior oblique is innervated by CN IV (trochlear).
R3 — every other muscle (medial rectus, inferior rectus, superior rectus, inferior oblique) is innervated by CN III (oculomotor).

CN III also supplies the levator palpebrae superioris (lid elevation), the iris sphincter (parasympathetic pupillary constriction), and the ciliary muscle (accommodation). A complete CN III palsy therefore presents with a clinically unmistakable triad: ptosis, a “down and out” eye, and a fixed dilated pupil.

CN III is anatomically subdivided into superior and inferior divisions at the orbital apex. The superior division supplies SR and levator only. The inferior division supplies MR, IR, IO, and the parasympathetics to the pupil and ciliary body. A partial CN III palsy can therefore mimic an isolated SR or IR weakness. Always evaluate eyelid position and pupil reactivity before assigning a vertical deviation to a single muscle.

CN VI has a long subarachnoid course and is uniquely sensitive to elevated intracranial pressure because the nerve can be stretched over the petrous ridge of the temporal bone. A unilateral or bilateral abducens palsy in a patient with new headache, papilledema, or other neurologic symptoms is a red flag for raised ICP and warrants imaging.

Why CN IV (trochlear) palsies are the most common

Trochlear palsy is the single most common diagnosis from Parks 3-Step, and the reasons are anatomical. CN IV is the longest intracranial cranial nerve, the only one to exit the brainstem dorsally, and the only one that decussates before innervating its target. That long, exposed, dorsal course makes it especially vulnerable to closed head trauma — even relatively minor head injuries can produce a unilateral or bilateral CN IV palsy.

Many SO palsies that present in adulthood are actually congenital with late decompensation, having been clinically silent for years thanks to a compensatory head tilt and unusually large vertical fusional amplitudes. A useful clinical pearl: ask the patient to bring in old photographs. A visible head tilt in childhood photos confirms congenital decompensation, which doesn't require neuroimaging beyond ruling out the obvious mimics.

In adults without trauma, a sudden new SO palsy often reflects microvascular ischemia in the setting of hypertension or diabetes. These typically resolve spontaneously over 8–12 weeks. Demyelinating disease should be considered in younger patients, especially when associated with other neurologic signs.

How Parks 3-Step works

Parks 3-Step narrows eight possible vertical deviations down to a single paretic muscle using three sequential observations. The logic builds on which muscles can produce hyperdeviation, which act on side gaze, and how head tilt isolates the obliques from the recti.

Step 1 halves the candidate pool. A right hyperdeviation means either a depressor of the right eye is weak (RIR or RSO) or an elevator of the left eye is weak (LIO or LSR). Four candidates remain.

Step 2 narrows by gaze. Each candidate muscle acts vertically in a specific gaze direction. RSO depresses most strongly when the right eye is adducted, so an RSO palsy shows greatest hyperdeviation on left gaze. RIR depresses most strongly in abduction (right gaze). After step 2, two candidates remain — typically one rectus and one oblique on opposite eyes.

Step 3, the Bielschowsky head-tilt test, separates the final two by exploiting torsion. Tilting the head to the right intorts the right eye and extorts the left. Intorsion is performed primarily by SR and SO; extorsion by IR and IO. SR and SO are antagonistic in vertical action — SR elevates while SO depresses — but they share the intorsion duty. With an RSO palsy, tilting right requires the right eye to intort, but the paretic SO can't contribute, so the SR is recruited more strongly to do the work. Since the SR's primary action is elevation, that recruitment produces extra elevation and exaggerates the hyperdeviation. The Bielschowsky maneuver pulls the localization out of the noise.

That logic produces the canonical CN IV palsy result: right hyperdeviation, worse on left gaze, worse on right tilt — which uniquely localizes to the right superior oblique. It is the three-step pattern every clinician memorizes.

When Parks 3-Step doesn't apply

Parks assumes the deviation arises from a single paretic muscle and is comitant in the same way across all gazes. Real-world vertical strabismus violates this assumption often, and recognizing those situations is more clinically important than running the test mechanically.

Skew deviation is a supranuclear vertical misalignment from brainstem or cerebellar pathology. It can mimic a peripheral palsy on Parks but doesn't follow expected anatomical logic. The ocular tilt reaction (skew + head tilt + conjugate ocular torsion) and the upright-supine test help identify it. Urgent neuroimaging is indicated.
Restrictive strabismus from thyroid eye disease, orbital floor fracture, or post-surgical scarring produces gaze-incomitant patterns that don't reflect a paretic muscle. Forced duction testing distinguishes restrictive from neurogenic causes.
Long-standing dissociated vertical deviation (DVD) varies with attention and may switch eyes — it doesn't follow innervational anatomy.
Prior strabismus surgery mechanically alters muscle action vectors, invalidating the standard Parks logic.
Multiple-muscle palsies — myasthenia gravis, partial CN III palsies affecting more than one muscle, complex orbital trauma — produce inconsistent results across repeated testing. Ice test, fatigability assessment, and AChR antibodies help confirm MG.

When Parks gives an unexpected answer or the localization doesn't fit the rest of the clinical picture, treat that as a signal to consider these alternatives rather than forcing a single-muscle diagnosis.

Clinical pearls for ocular motility evaluation

Vertical fusional amplitude is a chronicity clue. A normal patient can fuse only 2–4 prism diopters of vertical deviation. Patients with long-standing vertical strabismus develop unusually large amplitudes (often 10+ prism diopters), a finding that supports congenital decompensation over acute palsy.
Double Maddox rod measures cyclotorsion. SO palsy classically shows extorsion of the affected eye; intorsion of the higher eye is more typical of skew deviation. Objective extorsion plus a positive Parks result strengthens the SO localization.
Imaging is warranted in any acquired CN IV palsy without a clear traumatic history, in any pupil-involving CN III palsy (concern for posterior communicating artery aneurysm), and in any vertical deviation with new neurologic symptoms. A pupil-sparing CN III palsy in an adult with vascular risk factors is more often microvascular and usually does not need urgent imaging.
CN VII palsy doesn't cause motility issues but produces a clinically critical eye finding: incomplete lid closure (lagophthalmos) and reduced blink, leading to exposure keratopathy. Bell's palsy management routinely involves the optometrist's office for ocular surface protection — daytime lubrication, ointment and tape at night, and often a moisture chamber.
Forced duction testing is the single most useful chair-side maneuver for separating restrictive from neurogenic disease. It is positive in TED and orbital fracture, negative in true paresis.

Cranial Nerves of the Eye

Nerve	Origination	Purpose	Innervation	Palsy Signs
II - Optic	Cerebrum	VisionAfferent pupillary responseVisual fields	Sensory	APDDecreased VAColor desaturationVisual field defect
III - Oculomotor	Midbrain	EOMs (MR, IR, SR, IO)LevatorPupil constrictionAccommodation	Motor	Ptosis"Down and out" eyeMydriasis if pupil-involving
IV - Trochlear	Midbrain	Superior oblique onlyIntorsionDepression in adduction	Motor	Vertical diplopia worse on downgazeHead tilt away from affected side
V - Trigeminal	Pons	Corneal and periocular sensation (V1)Midface sensation (V2)	Sensory & Motor	Decreased corneal sensation (V1)Risk of neurotrophic keratopathy
VI - Abducens	Pontomedullary Region	Lateral rectus onlyAbduction	Motor	EsotropiaAbduction deficitMay signal raised intracranial pressure
VII - Facial	Pontomedullary Region	Orbicularis oculi (lid closure)Lacrimal glandTaste anterior 2/3 tongue	Sensory & Motor	LagophthalmosDecreased blinkExposure keratopathy

Extraocular Muscles and Their Actions

Muscle	Primary Action	Secondary Action	Tertiary Action	Nerve
Superior Rectus	Elevation	Intorsion	Adduction	CN III
Superior Oblique	Intorsion	Depression	Abduction	CN IV
Medial Rectus	Adduction	—	—	CN III
Inferior Rectus	Depression	Extorsion	Adduction	CN III
Inferior Oblique	Extorsion	Elevation	Abduction	CN III
Lateral Rectus	Abduction	—	—	CN VI

Parks 3-Step Test

Findings

EOMs and Cranial Nerves FAQs

What is the Parks 3-Step test used for?

The Parks 3-Step test (also called the Bielschowsky 3-step test) localizes the paretic extraocular muscle in patients with a vertical strabismus thought to result from a single weak muscle. The three steps are: (1) which eye is hyperdeviated, (2) is the deviation worse on right or left gaze, and (3) is the deviation worse with right or left head tilt. The combination uniquely identifies one of the eight depressors or elevators of the right or left eye and most commonly localizes a CN IV (trochlear) palsy of the superior oblique.

Which cranial nerve controls the superior oblique?

The superior oblique is innervated solely by cranial nerve IV (the trochlear nerve). It's the only EOM with this innervation. CN IV is also the only cranial nerve to exit the brainstem dorsally and the only one to decussate before reaching its target — features that make it the longest CN intracranially and especially vulnerable to closed head trauma. The mnemonic LR6 SO4 R3 captures the rule: lateral rectus by CN VI, superior oblique by CN IV, all the rest by CN III.

Why is the trochlear nerve (CN IV) so commonly affected?

CN IV is the longest cranial nerve intracranially and the only one to exit the brainstem dorsally. That long, exposed dorsal course makes it uniquely vulnerable to closed head trauma — even minor injuries can produce unilateral or bilateral palsy. Many adult-onset SO palsies are actually congenital with late decompensation, catching patients off guard when their compensatory head tilt is challenged by tasks like sustained reading or downgaze. Microvascular ischemia in HTN or diabetes and demyelinating disease account for most of the remaining acquired cases.

What does the LR6 SO4 R3 mnemonic mean?

LR6 SO4 R3 is the standard mnemonic for cranial nerve innervation of the EOMs:

LR6 — lateral rectus is innervated by CN VI (abducens).
SO4 — superior oblique is innervated by CN IV (trochlear).
R3 — all the remaining muscles (medial rectus, inferior rectus, superior rectus, inferior oblique) are innervated by CN III (oculomotor).

CN III also supplies the levator palpebrae superioris, the iris sphincter, and the ciliary muscle. A complete CN III palsy therefore presents with ptosis, a “down and out” eye, and a fixed dilated pupil.

When does the Parks 3-Step test fail or give a wrong answer?

Parks assumes a single paretic muscle and a comitant pattern that follows normal anatomy. It can mislead in skew deviation (a supranuclear cause that mimics a peripheral palsy), restrictive strabismus (thyroid eye disease, orbital floor fracture, post-surgical scarring), long-standing dissociated vertical deviation, prior strabismus surgery, and multiple-muscle palsies like myasthenia gravis or a partial CN III palsy. When Parks gives an unexpected answer or the clinical picture doesn't fit, that's a cue to consider these alternatives — forced duction testing, the upright-supine test for skew, and ice or fatigability testing for MG help differentiate.

What is the difference between a CN IV palsy and a skew deviation?

A CN IV palsy is a peripheral lesion of the trochlear nerve causing weakness of the contralateral SO. The deviation follows Parks 3-Step logic, the affected eye shows extorsion on double Maddox rod, and the patient typically tilts the head away from the affected side to compensate. A skew deviation is a supranuclear (brainstem or cerebellar) misalignment that can mimic Parks results but follows different rules. Skew often shows intorsion of the higher eye on double Maddox rod (the opposite of CN IV), is part of the ocular tilt reaction triad with head tilt and conjugate torsion, and typically improves supine (the upright-supine test). Skew warrants urgent neuroimaging because it points to brainstem pathology.

Which extraocular muscle is most commonly affected in thyroid eye disease?

The inferior rectus is the muscle most commonly affected in thyroid eye disease, followed by the medial rectus. TED produces fibrosis and restriction rather than paresis — the affected muscle becomes mechanically tethered, restricting movement away from its primary action (so an enlarged, fibrotic IR restricts upgaze rather than weakening downgaze). Forced duction testing is positive in restrictive TED and negative in true paresis. Because TED restricts the IR, it produces a vertical deviation that can mimic a contralateral SR palsy on Parks 3-Step — a classic restrictive-mimics-neurogenic pitfall.

What does it mean if a CN III palsy involves the pupil?

A pupil-involving CN III palsy — fixed and dilated pupil with reduced reactivity to light, alongside the typical ptosis and “down and out” eye — is a neurosurgical emergency until proven otherwise. The classic concern is a posterior communicating artery aneurysm compressing the nerve from outside, since the pupillary parasympathetic fibers run on the surface of CN III and are affected first by external compression. Urgent CTA or MRA is indicated. By contrast, a pupil-sparing CN III palsy in an older adult with vascular risk factors usually reflects microvascular ischemia of the central nerve fibers, which spares the surface pupillary fibers; these typically resolve spontaneously over 8–12 weeks. Any progression of a pupil-sparing presentation, or onset in a younger patient without vascular risk, warrants imaging.