Astigmatism(Theory,Pathophysiology)

Astigmatism is a common vision condition that causes blurred vision. The curvature of the cornea and lens bends the light entering the eye in order to focus it precisely on the retina at the back of the eye. In astigmatism, the surface of the cornea or lens has a somewhat different curvature. The surface of the cornea is shaped more like a football instead of round like a basketball, the eye is unable to focus light rays to a single point. Vision becomes out of focus at any distance. In addition, the curvature of the lens inside the eye can change, resulting in an increase or decrease in astigmatism. This change frequently occurs in adulthood and can precede the development of naturally occurring cataracts.

History

As a student, Thomas Young discovered that he had problems with one eye in 1793. In the following years he did research on his vision problems. He presented his findings in a Bakerian Lecture in 1801. Independent from Young, George Biddell Airy discovered the phenomenon of astigmatism on his own eye. Airy presented his observations on his own eye in February 1825 at the Cambridge Philosophical Society. Airy produced lenses to correct his vision problems by 1825, while other sources put this into 1827 when Airy obtained cylindrical lenses from an optician from Ipswich. The name for the condition was not given by Airy, but from William Whewell. By the 1860s astigmatism was a well established concept in ophthalmology.

Focus of the principal meridian

With accommodation relaxed:

1. 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 in front of the retina, while the second is on the retina.

2. 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.

3. Mixed astigmatism – focal lines are on both sides of the retina (straddling the retina)