Ocular Motility Testing

Ocular Motility Testing is 4% of the exam. Its measurements show the physician whether the eyes are aligned, how far they deviate, and whether double vision is urgent.

Six extraocular muscles move each eye; remember innervation as LR6-SO4-3: lateral rectus (CN VI), superior oblique (CN IV), everything else (CN III).

Why the obliques are tested looking in — with the eye turned toward the nose, the superior oblique mainly lowers it and the inferior oblique mainly raises it, so a weak superior oblique shows up in reading and on stairs.

Yoke muscles — the pair, one in each eye, that moves both eyes the same way; they receive equal nerve signals (Hering's law).

Cardinal positions — six positions of gaze isolate each yoked pair (right gaze: right LR + left MR); record over- and under-actions systematically. Each position tests one muscle of each eye in its main field of action. Directions are always the patient's.

Ductions vs versions — one eye moving (fellow covered) vs both together; a movement missing on both tests tells you the muscle CAN'T, not just won't. Vergences move the eyes in opposite directions, converging for near.

Running versions — with the head still, the patient follows a small target through the cardinal positions, often traced as a large H. Note any head turn or tilt the patient adopts.

Deviation vocabulary — eso (in), exo (out), hyper (up), hypo (down); -tropia = manifest, -phoria = latent (held in check by fusion). A tropia is constant or intermittent (showing with fatigue, illness, or daydreaming), and alternating (either eye fixates while the other turns) or unilateral (the same eye always turns). Distance and near deviations can differ, so each is measured and charted.

Chart shorthand — ortho for no deviation; E and X for esophoria and exophoria; ET and XT for esotropia and exotropia; E(T) and X(T) for the intermittent tropias; RHT or LHT for a right or left hypertropia (vertical deviations are usually named for the higher eye); a prime mark for near (ET′); cc and sc for with and without correction.

Set up for an honest number — the patient fixes an accommodative target, such as a small letter or picture, not a bare light, so a turn driven by focusing effort shows in full. Test at distance (6 m, about 20 feet) and near (about 33 cm), usually in the patient's glasses, and chart the correction worn.

Cover-uncover test — detects manifest deviations (tropias): cover the fixating eye and watch the fellow eye for a movement to pick up fixation. An eye that moves OUT to fixate was turned in (esotropia); one that moves IN was turned out (exotropia); one that moves DOWN was turned up (hypertropia). Repeat on the other eye. If neither cover moves the other eye, no tropia shows at that distance, provided each eye sees well enough to fixate. Only when no tropia is found, watch each covered eye as the cover comes off: one that moves back into line had drifted behind the cover, a phoria.

Alternate cover test — never allows fusion, so it reveals the TOTAL deviation (tropia + phoria). Move the occluder straight across, about 2 to 3 seconds per eye, so the eyes never see together. Alone, it cannot separate tropia from phoria, so cover-uncover comes first.

Prism and alternate cover test — the alternate cover test with a prism held before one eye, apex toward the deviation; base-out for eso, base-in for exo. Increase the power until refixation movement neutralizes: that's the measurement. For a vertical deviation, base-down goes over the higher eye. Never stack two prisms in the same direction over one eye: their labeled powers do not simply add. Record the prism diopters with the direction (eso, exo, hyper), distance, correction, and test.

Hirschberg — with the patient looking at a penlight, compare corneal light reflexes: displacement TEMPORAL to pupil center means the eye is turned IN (esotropia); each millimeter ≈ 7° (~15 prism diopters). The reflex sits opposite the turn: nasal in exotropia, below center in hypertropia. In straight eyes the two reflexes match, usually centered or slightly nasal.

Krimsky — prism placed before an eye and increased until the reflexes look symmetric: the measurement rescue for infants and non-fixing eyes. Both reflex tests are estimates; cover testing is preferred whenever each eye can fixate.

Near point of convergence — bring a target to the nose until an eye drifts or doubling starts; a receded NPC (~beyond 10 cm) with reading strain suggests convergence insufficiency. Record it in centimeters.

Stereopsis — good random-dot or Titmus stereo implies functional alignment; it's the pediatric screening ally of cover testing. Test it before the cover tests, because breaking fusion first can lower the result. The Supplemental Testing guide covers the stereo tests and the Worth 4-dot.

Amblyopia — reduced best-corrected vision, usually in one eye, because that eye did not give the brain a clear, aligned image in early childhood. Classic causes: a constant eye turn, a large difference in prescription between the eyes (anisometropia), or something blocking the view, such as a congenital cataract.

A constant eye turn in a young child is never 'wait and see': constant deviation invites strabismic amblyopia while the visual system is still wiring. The brain suppresses the turned eye's image, and that eye's sight stops developing. Treatment, set by the physician (glasses, patching), works best early, so make sure the physician knows about the turn at that visit.

Pseudostrabismus — epicanthal folds (wide skin folds at the inner corners) or a broad nasal bridge can make a baby's straight eyes look crossed. The physician makes that call, after matching reflexes and a normal cover test.

Testing acuity — use full lines or crowding bars, since amblyopic eyes read single letters better (see the Visual Assessment guide).

First question at the chair — does it survive covering one eye? Binocular diplopia disappears when either eye is covered and means the eyes are out of line. Monocular diplopia (persists one-eyed) is optical — astigmatism, cataract — not neurologic, and often clears through a pinhole.

Describe it — side by side or one above the other; distance or near; which gaze; constant or varying. Check pupils before any dilating drop.

New binocular diplopia in an adult goes to the physician promptly (nerve palsies, thyroid disease, neurologic causes), never into a routine booking. With pain, a droopy lid, or a large pupil, it goes immediately.

The third-nerve eye rests down and out because the lateral rectus (VI) and superior oblique (IV) still pull. Thyroid eye disease and myasthenia gravis also cause double vision (see the General Medical Knowledge guide). Pupil findings are in the Pupil Assessment guide.

Be ready to name each muscle's action and nerve, pair the yoke muscles in each cardinal position, separate tropia from phoria and ductions from versions, choose and read the cover tests, orient a prism, read a Hirschberg reflex, say when Krimsky is used, give the NPC cutoff, explain why a constant turn in a child threatens vision, and sort double vision by urgency.