Binocular Vision and Ocular Motility
Core framework
Start with the cover test and versions/ductions to localize a deviation. Then classify: phoria vs tropia, comitant vs incomitant, and eso/exo/hyper. Understand accommodation-convergence linkage (AC/A ratio) and the near triad.
Concepts to know cold
- Cover test: uncover test reveals phoria; alternate cover breaks fusion to measure total deviation.
- AC/A ratio: high AC/A causes esotropia at near; low AC/A causes exotropia at near. This guides whether to add plus at near or use prism.
- Amblyopia and suppression: strabismic amblyopia arises from constant unilateral deviation; treat with patching or atropine penalization.
- Motility defects: cranial nerve palsies produce incomitant deviations. CN III palsy gives down-and-out eye with ptosis and mydriasis; CN VI gives abduction deficit; CN IV gives vertical diplopia worst on contralateral gaze and head tilt.
Traps
Distinguish comitant (equal in all gazes, often refractive/accommodative) from incomitant (suggests palsy or restriction). Do not confuse a phoria (latent, controlled) with a tropia (manifest).
Rule of thumb
If the deviation changes with gaze direction, think palsy or restriction; if it is equal everywhere, think refractive or accommodative.
Worked example
A child has esotropia greater at near than distance with high AC/A. Adding plus lenses at near reduces accommodative demand and convergence, improving alignment. This is the basis for near plus add treatment.