Refractive Error

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

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II. 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. Convex lens (e.g. human eye) converges light rays
        1. Concave lens moves focal point closer to lens
        2. Distant objects have a closer focal point than near objects
      4. Concave lens (e.g. glasses correction for Myopia) diverges light
        1. Concave lens moves focal point further from lens
    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
    2. Near objects are seen best
      1. Light is focused in front of the Retina
  4. Hyperopia (Farsightedness)
    1. Far objects are seen best
      1. Light is focused behind the Retina
    2. Normal in infants (+0.50 to +2.50 Diopters)
      1. Vision normalizes by age 5 to 8 years old
    3. Light rays focus behind Cornea
      1. Cornea too flat or lens too weak for globe
      2. Near objects not seen clearly
    4. Correction: Convex lenses (convergent, plus power)
    5. Causes
      1. May be congenital with early onset
      2. Presbyopia occurs with normal aging (age >45 years)
  5. Astigmatism
    1. Non-spherical Corneal surface
      1. Represents a cylindrical Refractive Error
      2. Contrast with Myopia and Hyperopia which are spherical Refractive Errors
    2. Parts of surface (Meridians) are steeper than others
      1. Objects appear blurry at any distance
    3. Corrected with cylindrical lens oriented to appropriate axis
      1. Typically used in combination with spherical lens to correct Myopia or Hyperopia

III. Assessment

  1. See Visual Acuity

IV. Management: Refractive Error Correction

  1. Corrective lens prescriptions
    1. Example
      1. OD: -1.5 + 0.25 x180
      2. OS: -2.0 + 0.50 x180
    2. Interpretation
      1. Side (OD, OS)
      2. Spherical Lens
        1. Minus (convex) or Plus (concave)
        2. Diopter
      3. Cylindrical Lens correction
        1. Diopter
        2. Axis in Degrees
      4. Additional bifocal prescription
        1. Denoted with, for example, "Add +1.25 OU"
  2. Non-Surgical Options
    1. Eye Glasses
    2. Contact Lenses
  3. Refractive Surgery
    1. Laser In-Situ Keratomileusis (LASIK)
    2. Intrastromal corneal ring (ICR or Intacs)
    3. Phakic Intraocular Lenses

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