According to standard definitions used in Indian postgraduate examinations, amblyopia is diagnosed when there is reduced best-corrected visual acuity in an eye without any demonstrable structural pathology. What minimum interocular difference defines unilateral amblyopia?
- A A difference of at least one Snellen line between the eyes
- B A difference of at least two Snellen lines between the eyes ✓
- C A difference of at least three Snellen lines between the eyes
- D Any acuity worse than 6/60 in the affected eye
Correct answer: B. A difference of at least two Snellen lines between the eyes
Explanation
Unilateral amblyopia is conventionally defined as a two-line (0.2 logMAR) interocular difference in best-corrected acuity without structural cause, corresponding roughly to 6/6 versus 6/18. A one-line difference falls within normal interocular variation and test-retest variability, which makes option A the tempting distractor. Bilateral amblyopia is defined separately, usually as acuity worse than 6/12 in both eyes. Acuity worse than 6/60 merely indicates dense amblyopia and is not part of the diagnostic threshold.
Reference: Parsons' Diseases of the Eye, 23rd ed.
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