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ASTIGMATISM (PowerPoint)

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					   Astigmatism is a type of refractive error
    wherein the refraction varies in the
    different meridian.

 Astigmatism is a defect in which light
entering the eye is unable to be brought to
a single focus on retina resulting in blurred
vision at all distances.
 It simply means that there is a variation
and disturbance in the shape of the
cornea or lens. It is usually due to an –

*Irregularly shaped cornea
 * Scarring of the cornea
 *Abnormities in the crystalline lens
**Some eyes conditions are seen along with
  astigmatism as
 Nystagmus
 Cataract
 Ptosis
 Retinitis pigmentosa
 Albinism
 Inflammation
 degeneration
Symptoms:-

    Eye strain
 discomfort
 headache
 dry eye
 sensitivity to light
 Blurred vision
                     Classification of astigmatism

             Physiology                                 Pathological

                                 regular
                                                                             Irregular


   simple                           compound                                   mixed
                                       myopic             hypermetropic
myopic         Hypermetropic                                                           With the rule

                                            With the rule
                            With the rule                          With the rule
    With the rule                                                                     Against the rule
                       Against the rule         Against the rule
                                                                   Against the rule
         Against the rule
    Type of astigmatism :-
     there are two type of astigmatism:-

1.   Regular astigmatism
2.   Irregular astigmatism
          Regular astigmatism
   The astigmatism is regular when the
    refractive power changes uniformly from
    one meridian to another
    A etiology :-

1.   Corneal astigmatism
2.   Lenticular astigmatism
3.   Retinal astigmatism
    corneal astigmatism

    it is the result of abnormality of
  curvature of cornea
 It is usually congenital
 acquired corneal astigmatism is often
  irregular
                Causes
 Congenital curvature astigmatism
 Keratoconous
 Trauma to the cornea or lens
 After cataract operation
         Lenticular astigmatism
   Mainly due to

*congenital abnormality of lens
*refractive index of lens
*congenital or traumatic subluxation of the
  lens
         Retinal astigmatism
*Due to oblique placement of macula

* It can be corrected by visual aid
   type of regular astigmatism
    It is subdivided into four type ;–
     With-the-rule astigmatism
     Against-the-rule astigmatism
     Oblique astigmatism
     bi-oblique astigmatism
    With-the-rule astigmatism:


   Refractive power of the vertical meridian
    is greatest
   .The vertical meridian is steepest than
    horizontal meridian.
    Eyes see vertical lines more sharply
    than horizontal lines.
    Requires concave cylinders at 180+ /-20
   Or convex cylinders at 90+/-20 degrees
     Against-the-rule astigmatism:
   the refractive power of horizontal meridian is
    greatest than vertical meridian.
    Horizontal meridian is steepest than vertical
    meridian.
    Eyes see horizontal lines more sharply than
    vertical lines.
   Requires concave cylinders at 90 +/-20degrees
   Or convex cylinders at 180+/-20 degrees.
        Oblique astigmatism:

   In which the two principle meridians lies
    somewhere between the axis defining
    either with-the-rule or against-the-rule
    astigmatism.
   Complementary (45 in one meridian,135
    in other meridian)
      Bi –oblique astigmatism
 Principle meridian are not at right angle
 One at 30 , other at 100
optics of regular astigmatism
 Light entering the eye cannot converge to
  a point focus but form focal lines
 Configuration of rays refracted from the
  astigmatic surface –( sturms conoid) .
 Lens with cylinder produces an astigmatic
  focus .this astigmatic focus contains
  vertical line ,corresponding to the focus of
  the horizontal principal meridian .and
  horizontal focal line corresponding to the
  focus of the vertical principal meridian .
 The region between these two lines is
  known as the conoid of sturm .
 At the dioptric mid –point between these
  two focal line , the astigmatic focus forms
  a circular patch known as circle of least
  confusion
a. Simple hypermetropia astigmatism
    When refracted rays from one meridian
    takes focus on the retina and refracted
    rays from other meridian takes focus
    behind the retina.
b)Simple myopic astigmatism
   When refracted rays from one meridian
   takes focus on the retina and refracted
   rays from other meridian takes focus in
   front of retina.
c) Compound hypermetropic

astigmatism:

In this condition of astigmatism refracted
rays from both meridians take focus behind
the retina.
d) Compound myopic astigmatism:
  In this condition of astigmatism refracted
  rays from both meridians take focus in
  front of retina.

e) Mixed astigmatism: When refracted
  rays from one meridian take focus in front
  of retina and refracted rays from other
  meridian take focus behind the retina.
   b) Irregular astigmatism:
  Irregular differences in curvature or
  refractive index of the optic components
  or misalignment of any optical component
  may lead to irregularity.
 It is often the result of trauma,
 pterygium,
 corneal scars,
 and complication of refractive surgery.
 Another not uncommon condition is
  conical cornea (keratoconous)
              Diagnostic
Objectively:
      With the help of retinoscopy
 Autorefractometer
 Keratometer
 Corneal topography
Subjectively:
 By Jackson’s cross cylinder
 By astigmatic fan
 By stenopaeic slit
               management
   CONSERVATIVE MANAGEMENT:
     Prescription eye wear
     Spherical soft contact lenses
     Spherical RGP contact lenses
     Toric soft contact lenses
      SURGICAL MANAGEMENT
1.   Conventional LASIK
2.   Astigmatic Keratotomy
3.   Photorefractive Keratectomy
4.   PAI LASIK
5.   LASEK
6.   Orthokeratology
7.   Phakic Intraocular Lens(PIOL)
       Physiological astigmatism

 Almost all have minor degree of
  astigmatism {physiological }
 Diameter of cornea in vertical axis is
  11.5mm and in horizontal meridian is
  12mm.
 This means horizontal meridian is flatter
  than vertical.
 Because diameter is inversely proportional
  to curvature.
 Occur as lid press on anterior corneal
  surface.
 Make vertical meridian steeper
 Only small astigmatic error
 Usually 0.12D
 NO TREATMENT REQUIRED,
  life is an echo ,all comes back ,the good
,the bad, the false and the true ,
 so give the world the best u have and the
best will come back to u .
For Further Queries Contact :
Ms. Priyanka Singh
Head – Optometry Service
Email – optometry@venueyeinstitute.org

				
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posted:3/24/2012
language:English
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