Refractive error development

• Refractive error


  Occurs when there is a failure of or disruption to the emmetropisation process
• Components of the eye that can influence refractive status


  • Cornea (2/3 optical power)
  • Crystalline lens (1/3 optical power)
  • Axial (overall) length
• Ocular biometry


  Measurement of the components of the eye
• Emmetropia


  The power of the eye and the axial length are balanced - the image forms on the retina
• Myopia


  The eye has too much power for the axial length/ the eye is too long - the image forms in front of the retina
• Hypermetropia


  The eye is not powerful enough/ the axial length is too short - the image forms 'behind' the retina
• Refractive error development


  1. Eyes are still developing at birth

  2. Vision and acuity development can continue until 8-10 years old (the 'plastic period')

  3. Components of the refractive system can vary with individuals

  4. Some characteristics are hereditary (genetic element)

  5. Environmental factors can also affect development

  6. Sunlight has been shown to be a big factor
• Cornea provides 2/3 of the refractive power, crystalline lens provides 1/3 of the refractive power
• Axial length is the distance from the cornea to the macula
• Emmetropisation


  An active, finely regulated process whereby the components of the eye grow in a co-ordinated manner rather than independently, stimulated by blurred image
• There is a higher prevalence of emmetropia than would be predicted on the basis of chance
• Much of the developed world is becoming increasingly myopic
• Influence of visual environment on refractive error

  1. Refractive blur is induced with spectacle or contact lenses and refractive error and axial length are monitored over time

  2. When no lenses are worn, hypermetropic eyes grow towards a state of low hypermetropia or emmetropia

  3. When minus lenses are worn, a high level of hypermetropic blur is induced and extreme axial eye growth occurs - when these lenses are removed a high level of myopia is present

  4. When plus lenses are worn, normal hypermetropic blur is cancelled and there is no stimulus for eye growth
• Component ametropia


  Occurs when one or more of the components of the eye is abnormal, i.e. falls outside the range of values where emmetropia would be possible
• Correlation ametropia


  Occurs when all the components of the eye fall within the normal range but there is an abnormal correlation between them
• Reasons to determine refractive error

  • Provide correction
  • Assess ocular disease risk
  • Assess suitability for occupations
  • Monitor for changes
• Fluctuations in refractive error

  • May indicate diabetes
  • Increase in astigmatism
  • Increase in hypermetropia can indicate mechanical changes in the retina (bleeds, swelling or malignancies)
  • Increases in myopia due to cataracts
• Myopic Macular Degeneration


  • Progression increases the axial length of the eye
  • The sclera can stretch and grow but the retina can only thin as it is stretched
  • The retinal integrity is compromised as it thins and leads to an increased risk of degeneration: cells break down, cell death, new blood vessels form and leak
• An eye with a long axial length will be myopic AND have a compromised retina
• A short eye will be hypermetropia AND compressed ocular structures which can affect ocular pressure