A 68-year-old man reports that his distance vision has worsened over two years and he now reads newspaper print without his old glasses. Slit lamp examination shows dense nuclear sclerosis. The refractive shift responsible for his improved near vision is best explained by:

• A Increase in the refractive index of the lens nucleus causing index myopia ✓
• B Cortical cataract producing irregular astigmatism
• C Posterior subcapsular cataract reducing accommodative demand
• D Axial shortening of the globe with ageing

Explanation

Nuclear sclerosis raises the refractive index of the lens nucleus, increasing its converging power and producing a myopic shift called index myopia, which gives the classic 'second sight' phenomenon where the elderly patient reads again without glasses. Cortical cataract causes irregular astigmatism and glare rather than a uniform myopic shift, killing option B. Posterior subcapsular cataract affects near vision disproportionately due to pupillary constriction. The axial length does not shorten with cataract formation.

Reference: Parsons' Diseases of the Eye, 23rd ed.

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Written and medically reviewed by the StethoPrep medical team.