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Prisms and optics
Anatomy & Optics
Key Points
- •A prism deviates light towards its base while the image shifts towards the apex; 1Δ = 1 cm deviation at 1 m
- •Fresnel prisms are thin, adjustable press-on prisms ideal for acute/recovering palsies and trial use; they reduce VA by 1–2 lines due to diffraction
- •Prism adaptation occurs when the deviation increases after wearing prisms - the prism adaptation test (PAT) reveals the full surgical target in esotropia
- •Base-out prism for eso-deviations, base-in for exo-deviations; place prism base towards the deviation on the deviating eye
- •The 4Δ base-out prism test is specific for detecting microtropia - absence of refixation = microtropia present
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- Definition
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