Vision

Refractive Error

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Refractive Error, Refraction, Emmetropia, Hyperopia, Farsighted, Astigmatism

  • Definitions
  1. Refraction
    1. Measured in Diopters
    2. Bending of light rays
      1. Junction of two transparent media
      2. Media must have different densities to bend light
    3. Human eye refracts light at two locations
      1. Cornea (66% of eye's focusing power)
        1. Fixed focusing power
      2. Crystalline lens (33% of eye's focusing power)
        1. Accommodation changes lens shape to focus objects
    4. Refractive power of lens
      1. Reciprocal of focal length measured in meters
      2. Examples
        1. One diopter lens has focal point of 1 meter
        2. Two diopter lens has focal point of 0.5 meters
  2. Emmetropia (normal vision)
    1. Eye focusing power perfectly matched to globe length
    2. Image focused precisely on Retina
    3. Normal vision confers focal length of infinity
  3. Myopia (Nearsightedness)
    1. See Myopia
  4. Hyperopia (Farsightedness)
    1. Normal in infants (+0.50 to +2.50 Diopters)
      1. Vision normalizes by age 5 to 8 years old
    2. Light rays focus behind Cornea
      1. Cornea too flat or lens too weak for globe
      2. Near objects not seen clearly
    3. Correction: Convex lenses (convergent, plus power)
  5. Astigmatism
    1. Non-spherical Corneal surface
    2. Parts of surface (Meridians) are steeper than others
    3. Objects blurry at any distance
  • Assessment
  1. See Visual Acuity
  • Management
  • Refractive Error Correction
  1. Non-Surgical Options
    1. Eye Glasses
    2. Contact Lenses
  2. Refractive Surgery
    1. Laser In-Situ Keratomileusis (LASIK)
    2. Intrastromal corneal ring (ICR or Intacs)
    3. Phakic Intraocular Lenses