Astigmatism 2

• Astigmatism


  Refractive condition in which a variation of power exists in different meridians of the eye

  
• Astigmatism


  • Generally one meridian exhibits the greatest and one the least power and these are known as the principal meridians
• Thomas Young provided the first accurate description of astigmatism


  1800
• 11 cases of astigmatism were reported in this period


  Nearly seventy years
• John Green was the first to produce a distance test chart for astigmatism


  Unknown
• Charts for astigmatism


  • Clock dial
  • Fan dial
• Corneal astigmatism


  The major source of astigmatism is the anterior surface of the cornea, which usually exhibits direct or with-the-rule astigmatism

  
• With-the-rule astigmatism


  A common form of astigmatism that causes the cornea to be oblong shaped with high refractive power in the vertical meridian

  
• Lens astigmatism


  The major astigmatic effect of the lens is not attributed to unequal curvature of the lens, but to the potentiality of the lens to be tilted in relation to the visual axis or shifted in position within the eye

  
• Other causes of astigmatism


  • Traction of the external ocular muscles upon the eyeball
  • Eccentric position of the fovea in relation to the visual axis
  • Changes in the vitreous gel
  • Irregularity of the fundus surface

  
• Methods to diagnose astigmatism


  • Visual acuity
  • Keratometry
  • Refraction
  • Retinoscopy

  
• Keratometer


  An instrument used to measure the curvature of the cornea
• Types of astigmatism


  • Total astigmatism
  • Corneal astigmatism
  • Lenticular astigmatism
  • According to the resultant astigmatism in relation to the total refractive system

  
• Total astigmatism


  The sum resultant astigmatism of all factors of the refractive system, due to normal variations in the surfaces of the different media

  
• Components of total astigmatism


  • Anterior corneal surface
  • Residual (posterior corneal surface, lenticular surface, lenticular zonule)

  
• Types of corneal astigmatism


  • Regular
  • Irregular

  
• Regular corneal astigmatism


  Two principal meridians exist at right angles to each other, one of the greatest and one of the least curvature
• Irregular corneal astigmatism


  Either the two principal axis are not at right angles to each other or the curvature of any one meridian is not uniform

  
• Types of irregular corneal astigmatism

  • With-the-rule (direct)
  • Against-the-rule (inverse or perverse)
  • Oblique

  
• Corneal astigmatism based on both eyes


  • Symmetric
  • Asymmetric

  
• Symmetric corneal astigmatism


  The total of the degrees representing the two weakest (or two strongest) meridians for both eyes equals 180°

  
• Symmetric corneal astigmatism types


  • Homologous (if with-the-rule)
  • Heterologous (if against-the-rule)

  
• Asymmetric corneal astigmatism


  When the two weakest or the two strongest meridians of both eyes do not total 180° upon addition of their meridional locations

  
• Asymmetric corneal astigmatism types


  • Homonymous (both eyes are with-the-rule or both are against-the-rule)
  • Heteronymous (one eye is with-the-rule and the other is against-the-rule)
• Lenticular astigmatism


  Astigmatism due to unequal curvature of the lens surfaces or layers

  
• Types of astigmatism by resultant in relation to total refractive system


  • Simple hyperopic astigmatism
  • Compound hyperopic astigmatism
  • Simple myopic astigmatism
  • Compound myopic astigmatism
  • Mixed astigmatism

  
• Simple hyperopic astigmatism


  With accommodation relaxed, one meridian would focus on the retina while the other would tend to focus behind the retina

  
• Compound hyperopic astigmatism


  With the accommodation completely relaxed, the posterior principal of foci of both meridians would tend to fall beyond the retina
• Simple myopic astigmatism


  With accommodation relaxed, one meridian would focus on the retina and the other would focus in front of the retina

  
• Compound myopic astigmatism


  With accommodation relaxed, both principal meridians focus in front of the retina

  
• Mixed astigmatism


  With accommodation relaxed, one meridian tends to focus behind the retina, while the other focuses in front of the retina

  
• Variation in astigmatism


  Something different from another of the same type

  
• Variations in astigmatism


  • Change in amount at near
  • Change in axis
• Change in amount of astigmatism at near


  The amount of astigmatism appeared to vary when attention was fixed at a near point from that amount exhibited at the time of far point observation, usually towards an increase in with-the-rule astigmatism

  
• Change in axis of astigmatism


  Shift in the axis of the correcting cylinder, believed to be due to lenticular changes and torsional effect of the eye when converging at near point

  
• Symptomatology


  The medical science of symptoms, the combined symptoms of a disease
• Types of symptoms


  • Subjective (observed only by the patient)
  • Objective (evident to the observer)
• Asthenopia


  Symptoms of astigmatism including frontal and temporal headaches, tearing, smarting and eye strain

  
• Basic correction for astigmatism


  Consists of the cylindrical power which equalizes the discrepancy between the two principal meridians
• Considerations in basic correction


  • Effect upon image
  • Previous adaptation
  • Different amount of astigmatism at near point than at far point