Astigmatism

  Astigmatism is a refractive condition, not pathological in nature in which the optical system of the eye is incapable of producing a point image on the retina for a point object. This is due to varying refractive power from one meridian to the other of the eye’s optical system.
In an eye with astigmatism light fails to fall on a single focus on the retina to produce clear image. Instead, multiple focuses occur, either in front of the retina or behind it (or both).
SIGNS AND SYMPTOMS
Astigmatism usually causes vision to be blurred or distorted to ssome degree at all distances.
Symptoms of uncorrected astigmatism ar eye strain,fatigue and headaches, especially after reading or other prolonged visual tasks.
Squinting is also a very common symptom.
Types
Axis of the principal meridian
Regular astigmatism. Principal meridians are perpendicular(meridian of greatest refraction and meridian of least refraction)
 The cornea is usually the cause of clinically significant astigmatism, although the crystalline lens tends to cause astigmatism of small amount. Most corneas are more steeply curved in the vertical meridian than in the horizontal meridian ,causing the vergence of light to be greatest in the vertical meridian than in the horizontal meridian.

Wit- the-rule astigmatism- the vertical meridian is steepest(the vertical meridian is the meridian of greatest refraction)
Against-the-rule astigmatism- the horizontal meridian is steepest(the horizontal meridian is the meridian of greatest refraction )
Oblique astigmatism- the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees
Irregular astigmatism- principal meridians are not perpendicular.
In with-the-rule astigmatism, a minus cylinder is placed in the horizontal axis to correct the refractive error(or a plus cylinder in the vertical axis). Adding a minus cylinder in the horizontal asix makes the horizontal axis “steeper” (or better: makes the vertical axis “less steep”) which makes both axis equally “steep”. In against-the-rule astigmatism, a plus cyclinder in the vertical axis).

Focus of the principal meridian
With accommodation relaxed:
Simple astigmatism
Simple hyperopic astigmatism- first focal line is on the retina, while the second is located behind the retina
Simple myopic astigmatism- first focal line is on the retina, while the second is located in front of the retina
Compound astigmatism
Compound hyperopic astigmatism- both focal lines are located behind the retina.
Compound myopic astigmatism- both focal lines are located in front of the retina
Mixed astigmatism- focal lines are on both sides of the retina
Corneal, Internal and Refractive Astigmatism
The cornea, as already noted, is usually the source of clinically significant astigmatism. The amount of corneal astigmatism, along with location of the meridian of least and greatest refraction , can easily be determined with a keratometer.
Internal astigmatism is relatively small in amount, as compared with corneal astigmatism, tends to vary little from one person to another, and is almost always against-the-rule. The main causes of internal astigmatism are the toricity of the back surface of the cornea and tilting of the crystalline lens.
Refractive astigmatism (also called total astigmatism) is the astigmatism of the eye as determined by objective refraction (retinoscopy) or by subjective refraction. Thus, if both corneal and refractive astigmatism are known for a given eye, the internal astigmatism can by means of the following formula: internal astigmatism= refractive astigmatism – corneal astigmatism.
Javal proposed a rule for predicting the subjective astigmatism on the basis of the corneal astigmatism as determined by a keratometry.
At= p(Ac) + K
Where
   At= the total (refractive) astigmatism of the eye
   Ac= the corneal astigmatism
   P= approximately 1.25
   K= approximately 0.50D against-the-rule
Javal made it clear that the rule was strictly empirical and that the constants p and k were only approximations.