Ocular Motility Made Easy: Muscle Actions, Palsies, Three-Step
Last updated: October 2026
Muscle actions, yoke pairs and the three-step test feel like separate lists, but they come from one idea: where each muscle pulls from compared with where the eye is pointing. Get that, and palsies, the Hess chart and head tilts fall into place.
Four ideas behind every motility question
1. Recti pull from behind; obliques pull from the frontThe recti start at the orbital apex and pass backwards and medially at about 23° to the visual axis. The obliques work from the front of the orbit (the trochlea and the anterior orbital floor) and insert behind the equator, so they move the back of the eye and the cornea goes the opposite way: the superior oblique depresses.
2. Two letter-rules give every secondary actionSIN: the Superior muscles (SR, SO) INtort; the inferiors extort. RAD: the vertical Recti ADduct; so the obliques abduct.
3. Test each vertical muscle where it works aloneVertical recti line up with the visual axis in abduction; obliques in adduction. So test SR and IR looking out, SO and IO looking in.
4. Both eyes get the same commandHering's law: yoke muscles receive equal innervation. Sherrington's law: when a muscle contracts its antagonist relaxes. Fixing with a paretic eye needs extra drive, so the secondary deviation is bigger than the primary.
Primary, secondary and tertiary actions
| Muscle | Primary | Secondary | Tertiary | Nerve |
|---|---|---|---|---|
| Medial rectus | Adduction | — | — | III (inferior division) |
| Lateral rectus | Abduction | — | — | VI |
| Superior rectus | Elevation | Intorsion | Adduction | III (superior division) |
| Inferior rectus | Depression | Extorsion | Adduction | III (inferior division) |
| Superior oblique | Intorsion | Depression | Abduction | IV |
| Inferior oblique | Extorsion | Elevation | Abduction | III (inferior division) |
Memory hook: LR6 SO4, the rest are III. SIN: superiors intort. RAD: recti adduct.
The Parks–Bielschowsky three-step test
Use it for a vertical deviation from a single paretic cyclovertical muscle.
| Step | Question | What it leaves |
|---|---|---|
| 1 | Which eye is higher in primary position? | Right hyper: RIR, RSO (right depressors weak) or LSR, LIO (left elevators weak) |
| 2 | Is the hyper worse looking left or right? | Worse looking left: RSO or LSR (the muscles that act in left gaze) |
| 3 | Is it worse tilting the head right or left? | Worse on right tilt: RSO (the right eye intorts on right tilt; the weak SO cannot, so the SR intorts it and elevates it) |
Memory hook: A superior oblique palsy is worse looking away from the bad side and tilting towards it. The patient tilts the head away to fuse.
Each cranial nerve in one box
Third nerve (III)Eye down and out, ptosis, ± dilated pupil. The SR subnucleus supplies the opposite eye and one central nucleus serves both levators, so a nuclear III palsy gives bilateral ptosis and contralateral elevation weakness. Superior division: SR + levator. Inferior: MR, IR, IO and the pupil.
Fourth nerve (IV)The thinnest nerve with the longest intracranial course, the only one that leaves the back of the brainstem, and its fibres cross (the nucleus is on the opposite side). Most common cause of an acquired palsy: head trauma. Big vertical fusional amplitudes (>3–4 PD) and an old head tilt in photos suggest a congenital palsy.
Sixth nerve (VI)Esotropia worse in the distance and looking towards the weak side. Runs up the clivus and over the petrous apex (Dorello's canal), so raised intracranial pressure can stretch it: a false localising sign. VI + facial pain + ear infection = Gradenigo syndrome.
The VI nucleus is a gaze centreIt holds interneurons that cross to the opposite MR subnucleus through the MLF. A lesion of the nucleus therefore causes a horizontal gaze palsy to that side, not just an abduction weakness.
Brainstem syndromes worth knowing
| Syndrome | Lesion | Signs |
|---|---|---|
| Weber | Midbrain, cerebral peduncle | III palsy + contralateral hemiparesis |
| Benedikt | Midbrain, red nucleus | III palsy + contralateral tremor or chorea |
| Claude | Midbrain, superior cerebellar peduncle | III palsy + contralateral ataxia |
| Internuclear ophthalmoplegia (INO) | MLF | Weak adduction on the side of the lesion + abducting nystagmus of the other eye; convergence usually preserved. Bilateral in a young adult: MS; unilateral in an older adult: stroke |
| One-and-a-half | VI nucleus (or PPRF) + MLF, same side | No horizontal movement of the ipsilateral eye; the other eye can only abduct |
| Millard–Gubler | Ventral pons | VI and VII palsy + contralateral hemiparesis |
| Möbius | Congenital VI and VII nuclei | Bilateral abduction and facial weakness from birth |
Memory hook: In INO the name follows the eye that cannot adduct: a right INO means the right eye fails to adduct, from a right MLF lesion.
Paralysis or restriction?
| Test | Paretic muscle | Restriction (TED, blowout, fibrosis) |
|---|---|---|
| Forced duction | Free | Restricted |
| Saccade speed into the field | Slow (floaty) | Normal speed, sudden stop |
| IOP looking into the field | Unchanged | Rises |
| Hess chart | Underaction of the paretic muscle; overaction of its yoke | Compressed field, often both up and down |
- Thyroid eye disease muscles in order of involvement: I'M SLOw: inferior rectus, medial, superior, lateral, obliques. A tight IR limits elevation and mimics an SR palsy.
- Orbital floor blowout: trapped IR or its septa limit up- and down-gaze; with a positive forced duction and a white-eyed blowout in a child, operate urgently (oculocardiac reflex).
- Hess chart: the smaller field is the affected eye; find the muscle with the greatest underaction, then look for overaction of its yoke and, later, the contracture of its antagonist ("spread of comitance").
III palsy: when to worry
- Pupil involved (dilated, poorly reactive): compressive until proven otherwise; same-day CTA or MRA for a posterior communicating artery aneurysm.
- Complete motor palsy with a normal pupil in a vasculopath older than 50: usually ischaemic, recovering in 3 months. Still image if it does not recover, if other nerves join in, or if the patient is young.
- Aberrant regeneration (lid rises on down-gaze or adduction, pupil constricts on adduction) never follows ischaemia: it means trauma or compression. Primary aberrant regeneration without a preceding palsy suggests a slow-growing cavernous sinus mass (meningioma, aneurysm).
Where residents slip
- Mixing up which way the obliques move the eye: they pull from the front, so the superior oblique depresses (most in adduction).
- Testing SO in abduction: test the obliques with the eye adducted, the vertical recti with it abducted.
- Forgetting the head tilt points away from a SO palsy: the patient tilts to the opposite shoulder.
- Calling a nuclear VI lesion an abduction palsy: it is a gaze palsy because the nucleus drives the opposite MR too.
- Missing bilateral IV palsy: look for a V-pattern, more than 10° of excyclotorsion and a hypertropia that switches sides on side gaze.
Test yourself: flip cards
Quick quiz
Related: Strabismus hub, Neuro-ophthalmology hub, Orbital anatomy made easy, Pupils made easy, Nystagmus.
Key sources
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 6: Pediatric Ophthalmology and Strabismus; Section 5: Neuro-Ophthalmology (current editions)
- von Noorden GK, Campos EC. Binocular Vision and Ocular Motility, 6th ed. Mosby; 2002
- Parks MM. Isolated cyclovertical muscle palsy. Arch Ophthalmol. 1958;60:1027-1035
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