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Accommodation and optics
Anatomy & Optics
Key Points
- •Helmholtz theory: ciliary muscle contraction → zonular relaxation → lens becomes more convex (anterior surface curvature increases) → increased refractive power
- •Amplitude of accommodation decreases with age (Donders): 14 D at age 10 → 1 D by age 60; presbyopia manifests at 40–45 years when amplitude drops below ~4 D
- •AC/A ratio (normal 3–5Δ/D): high AC/A → convergence excess esotropia (near >> distance); low AC/A → convergence insufficiency (exophoria at near)
- •Near triad: accommodation + convergence + miosis - these three responses are linked and occur simultaneously during near fixation
- •Presbyopic near add increases by +0.50 D every 5 years from age 40; maximum +2.50 to +3.00 D by age 60
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