Ocular Motility Testing

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

Deviation vocabulary — eso (in), exo (out), hyper (up), hypo (down); -tropia = manifest, -phoria = latent (held in check by fusion).

Cover-uncover test — detects manifest deviations (tropias): cover the fixating eye and watch the fellow eye for a movement to pick up fixation.

Alternate cover test — never allows fusion, so it reveals the TOTAL deviation (tropia + phoria). Add prisms — apex toward the deviation; base-out for eso, base-in for exo — until refixation movement neutralizes: that's the measurement.

Ductions vs versions — one eye moving (fellow covered) vs both together; a movement missing in both tells you the muscle CAN'T, not just won't.

Cardinal positions — six positions of gaze isolate each yoked pair (right gaze: right LR + left MR); record over- and under-actions systematically.

Hirschberg — compare corneal light reflexes: displacement TEMPORAL to pupil center means the eye is turned IN (esotropia); each millimeter ≈ 7° (~15 prism diopters). Krimsky stacks prism until the reflexes symmetrize — the measurement rescue for infants and non-fixing eyes.

Stereopsis — good random-dot or Titmus stereo implies functional alignment; it's the pediatric screening ally of cover testing.

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.

The two red flags —

New binocular diplopia in an adult goes to the physician promptly (nerve palsies, thyroid disease, neurologic causes). First question at the chair: does it survive covering one eye? Monocular diplopia (persists one-eyed) is optical — astigmatism, cataract — not neurologic.

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. Classic story to recognize: CN IV palsy — vertical diplopia worst reading or on stairs, with a head tilt away from the palsied side.