The vision problems caused by astigmatism are a result of defects within the eye.

These defects are usually present from birth.

How the eye works

The eye is made up of three functional components. They are the:

• optical system (cornea and lens) – the cornea and lens are positioned at the front of the eye and act like the lenses of a camera, focusing the light coming into the eye to form an image on the retina
• retina – the light-sensitive layer of tissue at the back of the eye that senses light and colour and converts them into electrical signals
• optic nerve – the "cable" that transmits the electrical signals from the retina to the brain, where they're interpreted and understood

Astigmatism usually occurs as a result of problems with the cornea or lens.

The cornea and lens

The cornea is a transparent layer of tissue that covers the front of the eye and helps protect the eye from damage. The cornea and lens are also responsible for focusing incoming light on to the retina to create a clear image.

To work properly, the cornea needs to be perfectly curved, like the surface of a football. However, in astigmatism the cornea is shaped irregularly, like the surface of a rugby ball.

When light hits an irregularly curved cornea, it's not focused correctly on to the retina. The incorrect focus blurs the image, resulting in blurred vision.

Astigmatism caused by problems with the cornea is known as corneal astigmatism.

The same problem can also sometimes be caused by an irregular-shaped lens that bends light unevenly in the eye. This is known as lenticular astigmatism.

It's not known why some people are born with astigmatism, but there may be a hereditary element.

Other causes

Other possible causes of astigmatism include:

• injuries to the cornea, such as an infection that scars the cornea 
• changes to the cornea caused by eye surgery
• keratoconus and keratoglobus – eye conditions that cause the cornea to bulge, get thinner and change shape
• some conditions that affect the eyelids and distort the cornea 
• other conditions that affect the cornea or lens