Binocular Vision and Accommodation
What this domain tests
This domain tests cover testing, prism measurement, accommodative amplitude and facility, convergence, suppression and stereopsis. Expect questions that require you to distinguish a refractive from a non-refractive deviation and to calculate AC/A ratio.
Core rules and concepts
- Cover testing detects manifest and latent deviation; alternate cover testing reveals the total deviation by dissociating fusion.
- Prism bars measure deviations in prism dioptres; the prism base is placed opposite the direction of deviation.
- AC/A ratio = change in deviation in prism dioptres per dioptre of accommodative demand.
- Accommodative amplitude declines with age; insufficiency produces near blur and asthenopia.
- Convergence excess and convergence insufficiency differ by deviation at near versus distance.
Worked example
A patient is orthophoric at 6 m but shows 10 prism dioptres of esotropia at 40 cm. The accommodative demand at 40 cm is 2.50 D. AC/A = 10 ÷ 2.50 = 4 prism dioptres per dioptre. Reasoning: the near deviation is driven by accommodation, so the ratio is high-normal to high; a near add or bifocal may be considered after full refractive correction and binocular assessment.
Traps that cost marks
Measuring deviation without correcting refractive error; confusing esophoria with esotropia; calculating AC/A using centimetres instead of dioptres; missing suppression in a patient with an apparently normal cover test.
Key points
- Alternate cover testing breaks fusion and reveals total deviation.
- AC/A uses prism dioptres per dioptre of accommodation.
- Correct refractive error before interpreting binocular findings.
- Convergence excess is greater at near; insufficiency is greater at near but with low AC/A.
- Suppression can mask a strabismus on basic cover testing.