A 60-year-old patient complains of decreased distant vision but now he does not use spectacles for near vision. The image of his ocular examination is given below. What is this type of refractive error called?

Explanation

## Correct Answer: B. Index myopia Index myopia occurs when there is an increase in the **refractive index of the lens**, causing excessive convergence of light rays without any change in axial length or corneal curvature. In this 60-year-old patient, the clinical presentation is pathognomonic: decreased distant vision (myopia) but paradoxically improved near vision without spectacles. This occurs because the increased lens density (nuclear sclerosis) in age-related cataracts increases the refractive power of the lens, creating a myopic shift. The patient no longer needs reading glasses because the lens's enhanced refractive index provides additional convergence for near objects—a phenomenon called **"second sight"** in elderly patients. This is distinct from true myopia where axial length is increased. The increased refractive index of the lens nucleus is the sole pathophysiological mechanism, making this index myopia. In Indian clinical practice, this is commonly seen in patients with early nuclear cataracts (age-related) and is a classic presentation in geriatric ophthalmology clinics. The absence of any structural change in eye length or corneal radius of curvature confirms the refractive error is purely due to lens optics. ## Why the other options are wrong **A. Curvatural myopia** — Curvatural myopia results from increased curvature of the cornea or lens surface, increasing the refractive power of the cornea. However, this patient's myopia is not due to corneal steepening or lens surface curvature changes—it is due to increased lens density. The clinical clue of improved near vision without spectacles rules out corneal refractive changes, which would not selectively improve accommodation. **C. Axial myopia** — Axial myopia occurs when the **axial length of the eye is increased** beyond the normal 24 mm, causing light to focus anterior to the retina. This is the most common form of myopia in younger populations and is structural. The question stem does not suggest increased eye length, and the paradoxical improvement in near vision is incompatible with axial myopia, which would not improve accommodation. **D. Positional myopia** — Positional myopia is a rare condition where the lens is displaced anteriorly (subluxation), increasing its refractive power. While anterior lens displacement could theoretically cause myopia, it is not the primary mechanism here. The clinical presentation of 'second sight' in an elderly patient with no history of trauma or connective tissue disorder strongly suggests age-related nuclear sclerosis (index myopia), not lens subluxation. ## High-Yield Facts - **Index myopia** is caused by increased refractive index of the lens nucleus, not by changes in axial length or corneal curvature. - **'Second sight'** phenomenon: elderly patients with nuclear cataracts develop myopia but paradoxically improve near vision without reading glasses due to increased lens refractive power. - **Nuclear sclerosis** (age-related cataract) is the most common cause of index myopia in Indian geriatric populations. - **Axial myopia** (increased eye length) is the most common form of myopia overall, but does NOT produce improved near vision. - **Curvatural myopia** involves increased corneal or lens surface curvature; ruled out here because corneal changes would not selectively improve near accommodation. ## Mnemonics **INDEX vs AXIAL Myopia** **INDEX** = Lens density ↑ (refractive index ↑) → 'Second sight' (near vision improves). **AXIAL** = Eye length ↑ → No accommodation benefit. Remember: INDEX is about the lens material itself; AXIAL is about the eye's physical size. **Second Sight = Index Myopia** When an elderly patient says 'I can read without glasses now' despite new myopia → think **nuclear sclerosis** → **index myopia**. The increased lens density acts like a stronger lens, helping near vision. ## NBE Trap NBE pairs "myopia + improved near vision" to trap students into choosing axial myopia (the most common form overall). However, axial myopia never improves near vision; only index myopia's increased lens refractive power can paradoxically enhance accommodation in elderly patients. ## Clinical Pearl In Indian geriatric clinics, a 60+ year-old patient presenting with "I can suddenly read without my reading glasses but can't see the TV clearly" is index myopia until proven otherwise. This 'second sight' is a harbinger of nuclear cataract progression and warrants early ophthalmology referral to prevent vision loss. _Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 29 (Eye); Harrison's Principles of Internal Medicine, Ch. 28 (Disorders of Vision); Duke-Elder's Practice of Refraction, Ch. 3 (Classification of Refractive Errors)_