Astigmatism 1

• 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

  
• Etiology of 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
• Classification of astigmatism

  • Based on etiology
  • Based on relation between principal meridians
  • Based on orientation of meridian or axis
  • Based on focal points relative to the retina
  • Based on relative locations of principal meridians or axes when comparing the two eyes

  
• Corneal astigmatism


  When the cornea has unequal curvature on the anterior surface
• Lenticular astigmatism


  When the crystalline lens has an unequal curvature on the surface or in its layers

  
• Total astigmatism


  The sum of corneal astigmatism and residual astigmatism
• Residual astigmatism


  • Posterior corneal surface
  • Lenticular surface
  • Lenticular zonule
• Regular astigmatism


  When the two principal meridians are perpendicular to each other
• Most cases of astigmatism are regular astigmatism
• With-the-rule (WTR) astigmatism


  • When the greatest refractive power is within 030 of the horizontal meridian (i.e., between 0 and 30, 150 and 180 meridians)
  • Minus cylinder axis around horizontal meridian
  • The most common type of astigmatism based on the orientation of meridians
• Against-the-rule (ATR) astigmatism


  • When the greatest refractive power is within 030 of the vertical meridian (i.e., between 060 to 120 meridians)
  • Minus cylinder axis around vertical meridian
• Oblique (OBL) astigmatism


  When the greatest refractive power is within 030 of the oblique meridians (i.e., between 030 and 060 or 120 and 150)

  
• Irregular astigmatism


  • When the two principal meridians are not perpendicular to each other
  • Curvature of any one meridian is not uniform
  • Associated with trauma, disease, or degeneration
  • VA is often not correctable to 20/20
• Simple astigmatism


  When one of the principal meridians is focused on the retina and the other is not focused on the retina (with accommodation relaxed)
• Simple myopic astigmatism


  When one of the principal meridians is focused in front of the retina and the other is focused on the retina (with accommodation relaxed)

  
• Simple hyperopic astigmatism


  When one of the principal meridians is focused behind the retina and the other is focused on the retina (with accommodation relaxed)

  
• Compound astigmatism


  When both principal meridians are focused either in front or behind the retina (with accommodation relaxed)
• Compound myopic astigmatism


  When both principal meridians are focused in front of the retina (with accommodation relaxed)

  
• Compound hyperopic astigmatism


  When both principal meridians are focused behind the retina (with accommodation relaxed)
• Mixed astigmatism


  When one of the principal meridians is focused in front of the retina and the other is focused behind the retina (with accommodation relaxed)
• Symmetrical astigmatism


  • The principal meridians or axes of the two eyes are symmetrical (e.g., both eyes are WTR or ATR)
  • The sum of the two axes of the two eyes equals approximately 180
• Symmetrical astigmatism


  • OD: pl -1.00 x 175
  • OS: pl -1.00 x 005
• Asymmetrical astigmatism


  • The principal meridians or axes of the two eyes are not symmetrical (e.g., one eye is WTR while the other eye is ATR)
  • The sum of the two axes of the two eyes does not equal approximately 180

  
• Asymmetrical astigmatism


  • OD: pl -1.00 x 180
  • OS: pl -1.00 x 090
• Prevalence of astigmatism by age


  • Infants are born with ATR astigmatism, where the cornea is the source of the astigmatism
  • Preschool children have little or no astigmatism
  • Teenage children demonstrate a shift towards WTR astigmatism
  • Older adults show a shift towards ATR astigmatism
• Prevalence of astigmatism by gender


  • In general, there are no significant differences between males and females
• Prevalence of astigmatism by ethnicity


  • Higher prevalence in North Americans, Latinos
  • Asian infants tend to be WTR astigmatism
  • Caucasian infants tend to be ATR astigmatism
• For older adults, the average rate of change towards ATR astigmatism is less than or equal to 0.25D every 10 years
• Visual acuity of uncorrected astigmatism


  • Theoretically, at NO distance does an uncorrected astigmat have a sharp retinal image
  • Clinically, if astigmatism is small (less than 0.50DC), the patient may not notice blur

  
• Visual acuity of simple or compound myopic astigmatism


  Accommodation may make the retinal image even more blurry

  
• Visual acuity of simple or compound hyperopic astigmatism

  Accommodation may improve VA to some extent
• Visual acuity of mixed astigmatism


  • VA is relatively good
  • May not need much accommodation
• Uncorrected visual acuity and spherical/astigmatic ametropia

  • 20/30 with 0.50D spherical and 1.00D astigmatism
  • 20/40 with 0.75D spherical and 1.50D astigmatism
  • 20/60 with 1.00D spherical and 2.00D astigmatism
  • 20/80 with 1.50D spherical and 3.00D astigmatism
  • 20/120 with 2.00D spherical and 4.00D astigmatism
  • 20/200 with 2.50D spherical and >4.00D astigmatism

  
• Relationship between spherical refractive error and astigmatism

  Spherical refractive error (D)

  Myopia or absolute hyperopia

  When multiplied by a factor of two, it equals astigmatism (D)
• Symptoms of astigmatism


  • Distorted vision at distance and near
  • Letter confusion
  • Asthenopia or ocular fatigue
  • Headaches
  • Squinting

  
• Signs of astigmatism


  • Decreased visual acuities at distance and near

  
• Clinical tests for astigmatism


  • Visual acuity tests – distance and near
  • Autorefraction
  • Keratometry
  • Retinoscopy (most reliable source of information for cylinder power and axis)
  • Monocular subjective refraction, including Jackson cross cylinder
• Management of astigmatism


  • Cylindrical lenses and spherocylindrical lenses in spectacles and contact lenses for simple astigmatism and compound astigmatism, respectively
  • Refractive surgery

  
• Spectacle management

  Single vision glasses with cylinder
• Contact lens management


  • Toric soft contact lenses
  • Toric rigid gas permeable contact lenses