Introduction to Refractive Error and Prescription Writing

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Introduction to Refractive Error and Prescription
Writing

• Walter Huang, OD
• Yuanpei University
• Department of Optometry

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Vision

• Optics
• Cornea
• Aqueous humor
• Lens
• Vitreous humor
• Retina

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Refractive Error

• The result of a mismatch between optics and the
  growth of the eye
• It is due to a combination of genetic and
  environmental influences
• It is NOT considered an eye disease
• Treatment includes spectacles, contact lenses,
  and refractive surgery

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Types of Refractive Error

• Emmetropia
• Myopia
• Hyperopia
• Astigmatism
• Presbyopia

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Emmetropia

• The average emmetrope has a VA of 20/20 or better

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Myopia

• When parallel rays of light enter the eye (with
  accommodation relaxed) and come to a single point
  focus in front of the retina

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Myopia

• Blurry vision at distance
• Clear vision at near

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Myopia

• It is corrected by divergent or minus lenses
• Power of corrective lens needed can be estimated
  by finding the far point where the patient can
  achieve clear vision
• Example
• Far point is at 20cm
• Focal length of corrective lens needed 20cm
• Power of corrective lens needed 1/f 1/0.2m
  -5.00D
• Unit for Power Diopter (D)

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Hyperopia

• When parallel rays of light enter the eye (with
  accommodation relaxed) and come to a single point
  focus behind the retina

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Hyperopia

• Blurry vision at distance and near
• Intermittent blurring of vision

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Hyperopia

• It is corrected by convergent or plus lenses
• A young patient with low hyperopia can
  accommodate to focus the distant image on the
  retina
• Since accommodation decreases with age, a low
  hyperopic patient tends to wear corrective lenses
  for near work at an earlier age

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Astigmatism

• When parallel rays of light enter the eye (with
  accommodation relaxed) and do not come to a
  single point focus on or near the retina

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Astigmatism

• It is due to a distortion of the cornea and/or
  lens
• The refracting power is not uniform in all
  meridians
• The principal meridians are the meridians of
  greatest and least refracting powers
• The amount of astigmatism is equal to the
  difference in refracting power of the two
  principal meridians

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Astigmatism

• Distorted vision
• Letter confusion
• P versus F
• A versus R
• H versus N

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Astigmatism

• It is corrected by cylindrical or
  spherocylindrical lenses

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Presbyopia

• Presbyopia old mans eye (Latin)

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Presbyopia

• Decrease in the amplitude of accommodation or
  loss of accommodative ability with age

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Presbyopia

• It is a natural part of the aging process
• The onset of presbyopia is at approximately 40
  years of age and over though it may be earlier in
  low hyperopes

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Presbyopia

• Blurry vision at near
• Difficult or impossible to accommodate
  sufficiently for near work

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Presbyopia

• It is corrected by convergent or plus lenses for
  near work only (near Add)

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Types of Lens

• Spherical lens
• Cylindrical lens
• Spherocylindrical lens

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Spherical Lens

• A plus or minus lens where the refracting power
  is equal in all meridians
• Diopter Sphere (DS) is the measuring unit used to
  differentiate the spherical lens from lenses with
  cylindrical component
• Power cross and prescription writing for a
  spherical lens require the specification of the
  spherical power component only

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Spherical Lens

• Power cross
• Prescription form 2.50DS

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Cylindrical Lens

• A flat or plano (pl) axis meridian perpendicular
  to a power meridian

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Cylindrical Lens

• Diopter Cylinder (DC) is the measuring unit used
  to differentiate the cylindrical lens from lenses
  with spherical component
• Power cross and prescription writing for a
  cylindrical lens require the specification of
  both the cylindrical power and axis components

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Cylindrical Lens

• Power cross
• Prescription form -4.00 x 180

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Spherocylindrical Lens

• A toric lens consists of two perpendicular
  principal meridians
• Power cross and prescription writing for a
  spherocylindrical lens require the specification
  of the spherical power, cylindrical power, and
  axis components

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Spherocylindrical Lens

• Power cross
• Prescription form
• 3.00 -1.00 x 180
• 3.00/-1.00 x 180

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Prescription Writing

• Example 1
• Power cross
• Prescription form 1.00 -0.50 x 120

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Prescription Writing

• Example 2
• Power cross
• Prescription form -3.00 -0.50 x 084

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Prescription Writing

• Example 3
• Power cross
• Prescription form 1.25 -2.50 x 005

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Rules for Power Cross

• Specify both power and axis
• Power is always represented by plus or minus sign
  in front and contains two digits after the
  decimal point
• Power is presented in 0.25D steps

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Rules for Power Cross

• Axis meridian starts counter-clockwise from 0 to
  180

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Rules for Power Cross

• The cross orientation is drawn to the actual
  meridian
• When axis is at the 0 to 180 horizontal, use 180
  instead of 0
• Degree notation may or may not be used for axis
• If degree notation is NOT used for axis, three
  digits must be used for axis, except in the case
  of 0

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Rules for Prescription Writing

• Always include power, cylinder, and axis, except
  for spherical lenses (specified as DS)
• Degree notation is NOT used for axis

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Minus versus Plus Cylinder

• In Optometry, prescription writing is in minus
  cylinder (-cyl) form
• Sphere and axis specified is the most plus
  principal meridian

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Minus versus Plus Cylinder

• In Ophthalmology, prescription writing is usually
  in plus cylinder (cyl) form
• Sphere and axis specified is the most minus
  principal meridian

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Conversion between Minus and Plus Cylinder

• Be sure to know how to convert between minus and
  plus cylinder form and back

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Conversion between Minus and Plus Cylinder

• Example
• Minus cylinder form 1.00 -3.00 x 180
• Plus cylinder form -2.00 3.00 x 090