Skip to content

Ametropia

Table of Contents

  • Introduction
  • Myopia
  • Hypermetropia
  • Astigmatism
  • Presbyopia
  • References

Introduction

  • Ametropia: umbrella term for visual disorders caused by abnormal refractive power of the eye, resulting in blurred vision.
  • Main components of refraction:
  • Cornea (responsible for 2/3 of refraction)
  • Lens (responsible for 1/3 of refraction)
  • Axial length (eye’s anterior-posterior length)
  • Emmetropia: normal refraction where light focuses directly on the retina.
  • Four main types of ametropia: myopia, hypermetropia, astigmatism, presbyopia.
  • Axial length abnormalities are most common causes of myopia and hypermetropia.

Myopia

  • Also called near-sightedness.
  • Occurs when the eye is too long or the refractive power is too strong, focusing light in front of the retina.
  • Results in blurring of distant objects; near objects seen clearly (due to accommodation).
  • Prevalence varies globally: 20-50% in US, 80-90% in parts of Asia.
  • Risk factors: genetics, excessive near work, urbanization, ambient lighting, nutrition.

Management

  • Corrected with concave (minus) lenses to move focal point back onto retina.
  • Contact lenses (soft and hard) also used.
  • LASIK surgery: reshapes cornea to reduce myopia; suitable only for stable myopia in adults.

Hypermetropia

  • Also called farsightedness.
  • Eye too short or lens too weak, causing light to focus behind the retina.
  • Distant objects clearer than near objects.
  • Prevalence around 10% in the US.

Management

  • Corrected with convex (plus) lenses to move focal point forward onto retina.
  • Contact lenses also used.

Astigmatism

  • Cornea is oval-shaped rather than spherical.
  • Causes uneven refraction in horizontal and vertical axes leading to distorted vision.

Management

  • Corrected with cylindrical lenses, incorporated in glasses or contact lenses.
  • LASIK surgery can also correct astigmatism.

Presbyopia

  • Age-related loss of accommodation (focusing ability).
  • Lens becomes stiffer with age, reducing ability to focus on near objects.
  • Usually starts from fourth decade of life and progresses.

Management

  • Corrected with reading glasses for near vision.
  • Bifocals or trifocals can correct presbyopia plus other ametropias simultaneously.

References

  • Root T. Chapter 9: Introduction to Optics.
  • American Academy of Ophthalmology. Myopia, Hyperopia, Presbyopia.
  • Oxford Handbook of Ophthalmology, 2018.
  • Eyedisorder.com and other licensed image sources.
  • Acute Angle-Closure Glaucoma
  • Age-related Macular Degeneration (ARMD)
  • Amblyopia
  • Anisocoria
  • Anterior Uveitis (Iritis)

```