Unlocking the Mysteries of Astigmatism

Eye care
Written By Lucinda Howard

Astigmatism is a common ocular defect that occurs when the curvature of the cornea or lens is not symmetrical or uniform. It is usually a minor eye condition that many people will have to some extent, especially those who wear glasses. However, symptoms can be mild and vision correction is not always needed.

Astigmatism is often grouped with two related conditions - short-sightedness (myopia) and long-sightedness (hypermetropia) - all defined as refractive errors. Those with astigmatism are also likely to be either long or short-sighted. Astigmatism is most commonly present from birth, but can also develop following eye injuries or as a complication from eye surgery.

TYPES OF ASTIGMATISM
There are two types:

Regular astigmatism – the cornea is curved more in one direction. It is the more common of the two types and can be treated with specialist glasses or contact lenses.

Irregular astigmatism –the cornea may be uneven and curve in a number of directions or be curved steeper on one axis. This type of astigmatism may be a result of an eye injury leading to a scar on the cornea. Irregular astigmatism can be corrected with contact lenses but not glasses.

Symptoms of astigmatism can include: 

  • Blurred or distorted near & far vision

  • Unclear peripheral vision

  • Difficulty distinguishing certain shapes & details

  • Inability to see clearly between horizontal & vertical lines

If untreated, astigmatism can cause:

  • Headaches

  • Eyestrain

  • Reading difficulties

  • Fatigue, especially when doing tasks such as computer work & reading for long periods

WHAT CAUSES ASTIGMATISM?

Astigmatism is usually caused when the cornea or lens is not quite the right shape. Most people who have astigmatism are born with it, however, it is not known why it happens. To understand astigmatism and its effects, it is important to understand how the eye works.

HOW YOUR EYE WORKS

Anatomy of an eye
Astigmatism

There are four main components that help the eye function. 

Cornea & Lens – these sit at the front of the eye to help focus the light coming in, like a camera lens. This allows you to form an image on the retina. 

Retina – this sits at the back of the eye and is a layer of tissue that senses light and colour. It converts light into nerve signals and differentiates between wavelengths to interpret colour.
Optic Nerve – this sends the electrical signals from the retina to the brain where they are decoded and understood.

ASTIGMATISM & THE CORNEA & LENS

The cornea is at the front of the eye and is a thin layer of tissue. It helps to protect the eye, and working with the lens, focusses incoming light onto the retina to sharpen images. It does this by refracting or bending the light evenly to give a clear view of the object.

To work correctly, the cornea should be a regular curve, like a sphere. When you have astigmatism, it has an irregular curve, like the shape of a rugby ball or the back of a spoon.

What does this mean? Light rays hitting the irregular-curved cornea are refracted more in one direction than another, preventing them from focusing on the right part of the retina, and resulting in a blurred or distorted image. This is known as corneal astigmatism.

If the lens is an irregular shape, the light also bends unevenly, causing the same problem as corneal astigmatism. This is called lenticular astigmatism.

OTHER CAUSES

Astigmatism may also be caused by:

  • Corneal Injuries

  • Eye Surgery

  • Keratoconus or Keratoglobus - The cornea can bulge, get thinner or change shape on account of these disorders.

HOW ASTIGMATISM IS DIAGNOSED

Astigmatism is usually diagnosed during routine eye tests. Those that have astigmatism are normally born with the defect, but the condition can go undiagnosed for a long time if regular eye tests are not conducted. 

It is especially important for children to get routine eye tests as they may not realise there is a problem with their eyesight. If astigmatism is left untreated, it can end up causing a lazy eye and may also affect your child’s ability to read and focus in school. 

ASTIGMATISM TESTS

There are a few tests that can be done to check for astigmatism, but the two most common ones are: 

  • Visual Acuity Test – This tests a person’s ability to focus on objects at different distances. Most commonly, this includes reading letters on a ‘Snellen chart’ – with each descending line of letters smaller than the last.

  • Keratometer Test – the keratometer device measures the extent of corneal astigmatism. It is able to measure how the cornea is focusing the light, as well as to detect curve irregularities.

HOW DO YOU TREAT ASTIGMATISM? 

The symptoms of astigmatism can be mild and sometimes no treatment is needed to correct your eyesight.

In cases where the vision is being significantly affected by astigmatism, corrective lenses (glasses or contact lenses) or laser eye surgery can be used to treat it - depending on the type you have (regular or irregular).

CORRECTIVE LENSES

Corrective lenses (spectacles or contact lenses) compensate for the cornea’s irregular curve. The light rays pass through the lens and focus onto the retina in the correct place to provide a sharp, in-focus image. 

Both glasses and contact lenses can be effective at treating astigmatism, and the type of corrective lens you choose very much depends on both your personal preference and on the advice of your optometrist. 

LASER EYE SURGERY

Laser eye surgery or laser refractive surgery can also be used to permanently correct astigmatism. The surgery starts by removing the cornea’s outer layer of cells, then using a laser, removes tissue to physically and permanently change the curve of the cornea.

I hope this blog post has been of interest and I look forward to welcoming you in practice again soon

Lucinda

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