5.1 Extraocular muscle assessment, versions & ductions

Extraocular Muscle Assessment

Each eye is moved by six extraocular muscles (EOMs): four rectus muscles — superior rectus (SR), inferior rectus (IR), medial rectus (MR), and lateral rectus (LR) — and two oblique muscles — superior oblique (SO) and inferior oblique (IO). Five of the six originate from the annulus of Zinn at the orbital apex; the inferior oblique is the exception, arising from the floor of the orbit near the lacrimal fossa. The recti insert anterior to the equator of the globe, while the obliques insert posterior to the equator, which is why the obliques generate torsional and vertical actions rather than simple horizontal movement. The COT must recall each muscle's primary action and cranial-nerve supply without hesitation.

The six muscles, their actions, and innervation

A reliable memory aid is LR6-SO4-rest3: the lateral rectus is supplied by cranial nerve VI (abducens), the superior oblique by cranial nerve IV (trochlear), and the remaining four muscles (SR, IR, MR, IO) by cranial nerve III (oculomotor).

The horizontal recti (MR, LR) have a single, pure action because their line of pull runs straight along the horizontal meridian. The vertical recti and obliques carry secondary and tertiary actions that shift with eye position — SR and IR elevate or depress most purely when the eye is abducted about 23°, whereas the obliques exert their vertical action best when the eye is adducted. This anatomy is the reason testing must be done in specific gaze positions rather than in primary gaze alone.

The nine diagnostic positions of gaze

Ocular motility is evaluated in nine positions: primary gaze (straight ahead), four secondary positions (right, left, up, down), and four tertiary positions (up-right, up-left, down-right, down-left). Each of the six cardinal fields isolates one muscle as the prime mover — for example, the right SR is isolated in up-and-right gaze and the right IO in up-and-left gaze. The technician moves a fixation target in a broad "H" pattern and observes whether both eyes track fully, smoothly, and together.

Versions versus ductions

• Ductions are monocular movements — movement of ONE eye while the fellow eye is covered: adduction (nasal), abduction (temporal), supraduction (up), infraduction (down), incycloduction and excycloduction (torsion).
• Versions are binocular, conjugate movements — BOTH eyes moving together in the same direction: dextroversion (both right), levoversion (both left), supraversion (both up), infraversion (both down).
• Vergences are binocular, disconjugate movements — the eyes move in opposite directions (convergence, divergence).

When a version appears limited, cover the fellow eye and re-test the affected eye alone as a duction. A full duction with a limited version points to a binocular coordination problem rather than a mechanically restricted or paretic muscle.

Yoke muscles and Hering's law

Yoke muscles are the pair — one per eye — that contract simultaneously to drive the eyes into a cardinal position. In dextroversion the yoke pair is the right LR and the left MR. Hering's law of equal innervation states that yoke muscles receive equal, simultaneous neural input. This explains why the secondary deviation exceeds the primary deviation in a paretic muscle: when the patient fixates with the paretic eye, the extra innervation it demands also floods its normal yoke, overdriving the sound eye.

Sherrington's law and grading under/overaction

Sherrington's law of reciprocal innervation operates within a SINGLE eye: as the agonist contracts, its ipsilateral antagonist relaxes — as the MR contracts to adduct, the LR of the same eye relaxes. Under- and overactions are documented on a scale centered on 0 (normal), using minus signs for underaction (-1 to -4) and plus signs for overaction (+1 to +4). Underaction means the eye fails to reach the expected endpoint in a gaze field; overaction means it travels beyond normal. Recording these grades in each diagnostic position produces the motility chart that localizes a palsy or restriction and separates a neurogenic cause from a mechanical one.

Performing the motility exam and clinical relevance

In practice, seat the patient at eye level and hold an accommodative fixation target — a small letter or picture rather than a bare light for children, so accommodation stays engaged. Ask the patient to keep the head still (steady the chin if needed) and follow the target with the eyes only, moving it smoothly through the H pattern at a comfortable speed. Watch for lag, jerkiness, nystagmus, or an eye that stops short of the endpoint. The field of action of a muscle is the gaze direction in which it is the strongest mover; a limitation seen only in one muscle's field of action helps localize the affected muscle. If a limitation is symmetric and worsens equally in both eyes, suspect a supranuclear (gaze) problem rather than a single muscle. Documenting the direction, degree (-4 to +4), and whether ductions differ from versions turns a set of observations into a chart the ophthalmologist can use to diagnose paresis, restriction, or a gaze palsy.