Binocular Vision, Strabismus and Paediatric Optometry
Concepts
Master the cover test (uncover, cover-uncover, alternate), the Hirschberg and Krimsky tests, and the difference between comitant and incomitant strabismus. Understand AC/A ratio (normal 4–6 Δ/D), the near point of convergence, and fusional reserves. Know the types of amblyopia (strabismic, refractive, anisometropic, deprivation), the critical period, and the principles of occlusion therapy. Understand accommodative esotropia and its management with plus lenses.
Traps
Questions often confuse esotropia (inward) with exotropia (outward), and comitant (equal in all gazes) with incomitant (varies with gaze, suggests palsy). Watch for the AC/A ratio calculation: AC/A = (deviation at near − deviation at distance)/accommodative demand. Also know that amblyopia treatment is most effective before age 7–8.
Worked example
A child has 10 Δ esotropia at distance and 20 Δ at near. AC/A = (20 − 10)/2.5 D (at 40 cm) = 4 Δ/D, which is normal. If the near deviation were 30 Δ, AC/A = (30 − 10)/2.5 = 8 Δ/D, indicating a high AC/A ratio - consider plus lenses.