23.4 Common Traps in Ocular Motility Testing

23.4 Common Traps in Ocular Motility Testing

The motility section of the COA exam rewards precision with directional terms and test mechanics. Most missed questions come from a small set of recurring traps.

Trap 1 — reversing eso/exo and prism base

Eso means turned in (toward the nose); exo means turned out (toward the temple); hyper = up, hypo = down. The neutralizing prism base goes opposite the deviation: esotropia → base-out, exotropia → base-in, hypertropia → base-down over the higher eye. Students routinely match base to the same direction as the turn; that is wrong. Recite "base points where the eye should go," i.e., toward the apex of the deviation.

Trap 2 — watching the wrong eye on cover-uncover

On the cover-uncover test for a tropia, you cover one eye and watch the uncovered (fellow) eye for a refixation shift. The novice mistake is to stare at the eye under the cover. The eye you cover is hidden; only the fellow eye reveals a manifest deviation.

Trap 3 — confusing phoria and tropia

The alternating cover test alone cannot tell a phoria from a tropia, because it breaks fusion and shows the total deviation. You must do cover-uncover first. A frequent distractor claims the alternating cover test diagnoses a tropia by itself.

Trap 4 — treating monocular diplopia as a muscle problem

If double vision persists with the fellow eye covered, it is monocular and optical (astigmatism, cataract, dry tear film, lens dislocation), not an EOM or nerve issue. The pinhole test often collapses monocular diplopia to a single image. Ordering a motility workup here is the trap.

Trap 5 — testing without correction or at one distance only

• Always test with the patient's habitual correction unless instructed otherwise; an uncorrected accommodative esotropia will be misjudged.
• Always test distance and near; a deviation can differ markedly between the two (intermittent exotropia is often larger at distance, accommodative esotropia larger at near).
• Use an accommodative target (a letter), not a bare penlight, when alignment depends on accommodation.

Trap 6 — vague documentation

Writing "EOMs full" when a -1 underaction exists, or "diplopia" with no direction or test, will be marked incomplete. The defensible entry names the test, eye, gaze position, and magnitude (grade or prism diopters).

Trap 7 — mixing up primary and secondary deviation

Because of Hering's law, the deviation measured when the patient fixates with the non-paretic eye (the primary deviation) is smaller than the deviation measured when fixating with the paretic eye (the secondary deviation). The exam may ask which is larger; the secondary deviation is larger because extra innervation is sent to the weak muscle and equally to its yoke partner. Students who do not know this miss the comparison item.

Trap 8 — assuming a palsy when the problem is restrictive

Limited movement does not always mean a weak muscle. Restriction (entrapment from a blowout fracture, fibrosis in thyroid eye disease, or scarring) mechanically blocks movement while the nerve and muscle are intact. The physician's forced duction test separates the two: free movement on passive rotation points to a palsy, resistance points to restriction. Treating every limitation as a nerve palsy is a classic distractor.

Trap 9 — letting a head turn fake a gaze limitation

Patients with diplopia adopt a compensatory head posture (a turn or tilt) to avoid the field where they see double. If the COA does not straighten and stabilize the head, the eyes appear to move fully when they do not. Always control head position before grading versions.

Quick self-check before answering

Running this five-line check defuses the majority of motility distractors before you commit to an answer.

Trap 10 — forgetting that secondary actions flip the simple picture

The vertical recti and obliques have secondary actions that depend on eye position, and ignoring them produces wrong localizations. The superior rectus elevates best when the eye is abducted, while the superior oblique depresses best when the eye is adducted; the inferior muscles mirror this. That is why the diagnostic positions test each vertical muscle in a specific gaze, not in primary position. A distractor that tests a vertical muscle in straight-ahead gaze, where its action overlaps with another muscle, is set up to mislead.

When a vertical-diplopia stem omits the gaze direction, treat the item as incomplete and look for the position cue.