Ametropia = any refractive error where the eye can’t focus light directly on the retina without help.
Main types
- Myopia (nearsightedness)
- Eye is too long or cornea too curved → light focuses in front of retina.
- Close objects are clear, distant ones are blurry.
- Hyperopia (farsightedness)
- Eye is too short or cornea too flat → light focuses behind retina.
- Near vision is harder; distance vision can be okay (but may be strained).
- Astigmatism
- Cornea (or lens) is irregularly shaped (more curved in one direction) → light focuses on multiple points.
- Vision is blurry at all distances, often with a “stretched” look.
- Presbyopia
- Age-related loss of lens flexibility (usually after 40).
- Near focus gets tough, even if distance vision is fine.
How it’s measured
- Spherical equivalent (SE): Combines sphere (myopia/hyperopia) + half of cylinder (astigmatism) to give one number.
- Diopter (D): Unit of lens power. Negative D = myopia, positive D = hyperopia.
Causes & risk factors
- Genetics: Family history strongly influences myopia/hyperopia risk.
- Environment: Too much near work (reading, screens) + little outdoor time → higher myopia risk.
- Age: Lens stiffens → presbyopia.
- Eye injuries or surgeries can induce irregular astigmatism.
Correction options
- Glasses: Simple, non-invasive.
- Contact lenses: Directly on eye; options like toric for astigmatism, multifocal for presbyopia.
- Refractive surgery: LASIK, PRK, SMILE (laser reshapes cornea). Lenses or implants for high prescriptions.
- Orthokeratology (ortho-k): Wear special rigid lenses overnight to temporarily reshape cornea (often for myopia control).
Management (especially for myopia control)
- Outdoor time: 2+ hours daily linked to slower myopia progression in kids.
- Low-dose atropine drops: Can slow myopia worsening.
- Multifocal/Defocus lenses: Some contact lenses or glasses designed to reduce progression.