SAS 7 | ASTIGMATISM

ASTIGMATISM

A refractive condition parallel rays of light from infinity with accommodation relax would form two or more focal points in relation to the retina.

- Is an imperfection in the curvature of your eye’s cornea or lens.

Normally, the cornea and lens are smooth and curved equally in all directions. This helps to focus light rays sharply onto the retina at the back of your eye.

Can occur with myopia or hyperopia and can be easily diagnosed with a simple eye exam.

Derived from the Greek word stigma (point) and literally means lack of a focal point.

In normal astigmatism the clearest and blurrest orientations are 90 degrees apart.

The result of astigmatism is constant blur at both distance and near which accommodation is unable to correct.

Donders(1860)

Reported that the first possible clue of a defect of vision could be remedied by cylindrical lenses.

Reported that only eleven cases of astigmatism had been reported in a period of nearly seventy years.

Pare(1575)

Produced stenopaic spectacles.

Thomas Young(1800)

Made the first accurate description of astigmatism.

George Biddle Airy(1825)

Described the construction of the first sphero-cylindrical lens.

1828

The first sphero-cylindrical American lenses were made.

John Green(1866)

Produced the first distance test chart for astigmatism including the clock dial.

Barnett(1877)

Cylindrical trial cases came into use .

PRINCIPAL 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 principal meridians

Power meridian

Meridian of maximum curvature or greatest power.

Power in the horizontal plane projects a vertical focal line .

Power in the vertical plane projects a horizontal focal line image.

Axis meridian

Meridian of minimum curvature or least power.

Astigmatic Chart

Consists of a series of radiating lines known as clock dial

Corneal

Irregular curvature of the anterior cornea

Meridional differences in refractive index

Lenticular

Irregular curvature of the crystalline lens or at it’s layers

Tilted crystalline lens

Varying refractive index

Others

Posterior retina may be

Asymmetric, tilted or decentered

Eyelid mass lesion

Changes in the vitreous gel with Variations in index

Any healed aberration or injury or scarring of the cornea which can be due to any infection or trauma.

The weight of the upper eyelid resting on the eyeball, traction of the external ocular muscles upon the eyeball may force the sclera into a toroidal shape without altering the cornea.

Total astigmatism

The sum resultant of all factors of the ref. system due to normal variations in the surfaces of the different media.

Corneal astigmatism

The cornea exhibits a variation of curvature throughout different meridians.

A. Regular Astigmatism

The type of astigmatism where the principal meridians are at angle of 90 degrees apart (perpendicular to each other)

With-the-rule or Direct

When the meridian with the greater refractive power is vertical falling within the area between the 60 & 120 meridian.

Where the vertical meridian is the steepest and it stays close to 90 ̊. - common in children.

Range: 60-120

Against-the-rule (Inverse or Perverse)

The horizontal meridian stays close to 180 ̊.

Horizontal meridian is more steeper than the vertical meridian. - common in the elderly.

Range: 0-30 ; 150-180

Oblique astigmatism

If the principal meridians are not at 90 ̊ and 180 ̊, it is categorized as oblique astigmatism.

Here, the shape of the eyeball resembles a tilted American football with its prime meridians lying

Between 30 ̊–60 ̊ and 120 ̊–150 ̊. (31-59 & 121-179)