23.2 Core Workflows and Decision Points
Key Takeaways
- The nine diagnostic positions of gaze isolate yoke-muscle pairs; six are 'cardinal' positions.
- Cover-uncover test detects a tropia (manifest deviation); alternating cover test reveals total deviation including phorias.
- The Worth 4-Dot test screens fusion and suppression using red/green glasses and a 4-light target.
- Prism cover testing quantifies a deviation in prism diopters; base direction is opposite the deviation.
23.2 Core Workflows and Decision Points
Motility testing follows a predictable sequence: assess gross movements (versions), localize a weak muscle (diagnostic positions), then determine whether the eyes are straight (cover testing), whether the patient fuses (Worth 4-Dot, stereopsis), and how large any deviation is (prism cover test). Each step has a control that protects the result.
Nine positions of gaze
Versions are tested by having the patient follow a target (penlight or fixation stick) at about 14 inches through the nine positions of gaze: primary, up, down, right, left, and the four oblique positions. The six cardinal positions (up-right, up-left, right, left, down-right, down-left) each isolate one yoke-muscle pair, so an underaction in a cardinal position localizes the weak muscle.
| Direction of gaze | Right-eye muscle | Left-eye yoke muscle |
|---|---|---|
| Up & right | Right SR | Left IO |
| Right | Right LR | Left MR |
| Down & right | Right IR | Left SO |
| Up & left | Right IO | Left SR |
| Left | Right MR | Left LR |
| Down & left | Right SO | Left IR |
The rule to recite: the superior oblique depresses the eye in adduction (looking down and in), so SO weakness shows up best in the down-and-in position — the basis of the Park's three-step test for a CN IV palsy.
Cover testing
- Cover-uncover test detects a tropia (a manifest misalignment present with both eyes open). Cover one eye and watch the fellow eye for refixation movement. Inward movement = exotropia; outward = esotropia.
- Alternating (cross) cover test dissociates the eyes and reveals the total deviation, including any latent phoria. It cannot distinguish a phoria from a tropia by itself, so always perform cover-uncover first.
- A control: the patient must keep accommodative fixation on a clear, accommodative target (a letter, not a bare light) so an accommodative esotropia is not missed.
Sensory fusion tests
The Worth 4-Dot test uses red/green glasses (red over the right eye by convention) and a target of four lights: one red, two green, one white. The patient's report localizes the problem:
| Patient sees | Interpretation |
|---|---|
| 4 lights | Normal fusion |
| 2 red lights | Left-eye suppression |
| 3 green lights | Right-eye suppression |
| 5 lights | Diplopia (no fusion); red/green positions indicate eso vs exo |
Quantifying the deviation
The prism and alternate cover test (PACT) measures the deviation in prism diopters (PD). Place increasing prism in front of one eye until the alternating cover produces no refixation movement; that prism power is the deviation. Remember the base rule: for an esotropia use base-out prism, and for an exotropia use base-in prism (base goes opposite the direction the eye is turned). Stereopsis (e.g., Titmus or Randot) is recorded in seconds of arc, with lower numbers (e.g., 40 arcsec) indicating better depth perception.
Confrontation versus instrument testing
Gross versions with a penlight are a confrontation screen the COA performs at the chair; they are quick and need no equipment beyond a fixation target. When the screen is abnormal, the workup escalates to cover testing for alignment, prism cover testing for magnitude, and sensory tests (Worth 4-Dot, stereopsis) for fusion. Knowing the order prevents the trap of measuring with prisms before you have even confirmed there is a manifest deviation.
Near point of convergence (NPC)
A frequently tested bedside vergence measure is the near point of convergence (NPC): slowly bring an accommodative target toward the patient's nose along the midline and note where one eye breaks outward (the break point) and where it recovers fusion (the recovery point). A normal NPC break is roughly 5–10 cm from the nose; a remote NPC (e.g., beyond 10 cm) suggests convergence insufficiency, a common cause of near blur and asthenopia. Record the actual centimeters, not just "normal."
Controls that keep results valid
| Test | Critical control | Failure if ignored |
|---|---|---|
| Versions | Hold target ~14 in, keep head still | Misreads head turn as gaze limitation |
| Cover-uncover | Watch the fellow (uncovered) eye | Misses the manifest deviation |
| Worth 4-Dot | Red lens over the right eye | Reverses suppression interpretation |
| Prism cover | Use an accommodative target | Misses accommodative esotropia |
| NPC | Move slowly along midline | Overestimates convergence ability |
Every one of these controls is a defensible-answer cue: when a stem describes a sloppy technique, the correct answer usually restores the missing control before reporting a result.
Sequencing the full motility workup
Think of the workup as a funnel. Begin with a history of when, where, and in what direction the diplopia occurs, because the direction predicts the muscle. Next perform versions in the nine positions to find the lagging field. Then run cover-uncover to confirm a manifest tropia, alternating cover to expose the total deviation, and prism cover to measure it in prism diopters at distance and near. Add sensory tests (Worth 4-Dot, stereopsis) to judge whether the patient still fuses, and an NPC when near complaints dominate. Each output feeds the next decision, and skipping a step is the usual wrong answer.
On the cover-uncover test, you cover the right eye and observe the left eye move outward (temporally) to take up fixation. What does this finding indicate?