What is lazy eye (amblyopia)? How it differs from nearsightedness

Amblyopia, commonly called lazy eye, means the eye itself has no structural disease, but it did not receive clear enough visual input during the critical years of visual development -- so even with properly fitted glasses, its corrected vision stays below the normal level for that age.

It is not the same thing as nearsightedness. Myopia is the eye's lens failing to focus, and the right glasses usually fix the picture; amblyopia lives in how the brain processes the visual signal -- glasses answer whether the image is sharp, not whether the brain uses it.

Common causes include a large difference in prescription between the two eyes, significant farsightedness or astigmatism, strabismus, and anything that blocks vision early, such as congenital cataract or a drooping eyelid. The brain gradually favours the easier eye, and the other one participates less and less.

Visual development has a critical period, and plasticity is greatest at young ages -- which is why paediatric guidelines everywhere stress regular vision screening: find it early, act early.

Beyond single-eye acuity, whether the two eyes work together -- fusion and stereopsis -- matters just as much. That cooperation is exactly what binocular vision training is about.