ASTIGMATISM



ASTIGMATIC EYE

What is it?

 This is an error with the way light is focused on the retina.  Similar to nearsightedness or farsightedness, astigmatism is a refractive error.  In a normal eye, light rays come to a single point focus on the retina.   In an astigmatic eye, however, light rays do not come to a single point focus and thus a clear image is not produced.  There are many focus points in an astigmatic eye—some in front or behind the retina, or both.

The cornea of an eye is normally shaped like a ball, or sphere.  However, this need not always be the case.  An astigmatic eye is shaped like a football with one meridian being more curved than the one 90 degrees away from it.   To understand meridians, think of the directional North South East West cross sign.  The North-South is the 90 degree meridian and the East West is the 180 degree meridian. 

The meridians are measured in terms of their ‘steepness.’  One meridian is usually flatter than the other.

Astigmatism may also be caused by the shape of the lens within the eye.  This is referred to as lenticular astigmatism as opposed to corneal astigmatism. 

SYMPTOMS OF ASTIGMATISM

Vision appears to be blurry and distorted at various distances for a person experiencing uncorrected astigmatism.   Generally, after a period of time, a person may experience eye strain, headaches and will often find the need to be squinting.  Astigmatism may be classified as regular and irregular.

REGULAR ASTIGMATISM

Regular astigmatism can be broken down into five categories:

1.) SIMPLE MYOPIC ASTIGMATISM

 -One meridian is emmetropic—light rays focus on the retina(no correction is required—Rx Plano)

Example:  Plano -2.00 x 090

-Opposite meridian is myopic—light rays come to a point focus in front of the retina.  A minus lens is required for correction

2.) COMPOUND MYOPIC ASTIGMATISM

-Light rays in both meridians fall short of the retina

Example:  -1.00 -2.00 x 090

The first number (-1.00) is the sphere power (in diopters) for the correction of myopia in the flatter (less nearsighted) principal meridian of the eye.

          The second number (-2.00) is the cylinder power for the additional  myopia       correction in the more curved principal meridian.

The third number (90) is called the axis of astigmatism. This is the location (in degrees) of the flatter principal meridian, on a 180-degree rotary scale (in which 90 degrees designates the vertical meridian of the eye, and 180 degrees designates the horizontal meridian).

3.) SIMPLE HYPEROPIC ASTIGMATISM

-One meridian is emmetropic (light rays focus on the retina)

                     -One meridian is hyperopic (light rays focus behind the retina and a plus                lens is required for correction)
                     Example:  Plano +2.00 x 090

4.)  COMPOUND HYPEROPIC ASTIGMATISM 

-Both meridians are hyperopic

-Light rays in both meridians focus behind the retina.

Example:  +1.00+2.00 x 090

     5.)  MIXED COMPOUND ASTIGMATISM

                   -One meridian is farsighted and the other is       nearsighted

  IRREGULAR ASTIGMATISM

Irregular astigmatism results when the principal meridians are not 90 degrees apart.  This ends up happening as a result of an eye injury which may scar the cornea.  Eye surgery and a condition called Keratoconus (cone shaped cornea as a result of thinning) as well results in the principal meridians not being perpendicular to one another. 

Astigmatism is corrected by using contact lenses, glasses or by having refractive surgery.