Source: The Conversation (Au and NZ) – By Flora Hui, Research Fellow, Centre for Eye Research Australia and Honorary Fellow, Department of Surgery (Ophthalmology), The University of Melbourne
Have you ever gone to the optometrist for an eye test and were told your eye was shaped like a football?
Or perhaps you’ve noticed your vision is becoming increasingly blurry or hard to focus?
You might be among the 40% of people in the world who live with astigmatism.
What causes astigmatism?
The eye acts like a camera, capturing light through the front surface (the cornea) and focusing it onto the “film” at the back of the eye (retina).
To get a clear picture, the eyeball and all of its surfaces (cornea, lens and retina) have to meet certain specifications of size and shape.
Otherwise, vision can appear blurred and out-of-focus, known as “refractive error”.
Astigmatism (uh-STIG-muh-tiz-um) is a type of refractive error where one or more of the eye’s surfaces are not smooth and/or round. It is broadly classified into two types: regular and irregular.
Regular astigmatism is the most common. It typically comes from changes in the shape of the cornea. Instead of being round, it is more oval, like a football or an egg. We don’t fully understand why some people develop regular astigmatism, but it’s partly due to genetics.
Irregular astigmatism is rarer. It occurs when a part of the cornea is no longer smooth (from scarring or growths on the cornea), or its shape has changed in an uneven or asymmetrical way.
Eye conditions such as keratoconus – where the cornea weakens over time and becomes cone-like in shape – causes irregular astigmatism.
If the cornea is no longer round or smooth, light entering the eye is scattered across the retina. This can cause blurry or distorted vision, reduced sensitivity to contrast, shadows or double vision and increased sensitivity to bright lights.
Is astigmatism a new condition?
In 1727, Sir Isaac Newton was the first to describe the physics of how an irregular surface might affect the focus of light passing through it.
This was followed in 1800 by Thomas Young, a scientist who had astigmatism and described how it affected his vision in a lecture.
In 1825, Sir George Airy, an astronomer who also had astigmatism, discovered he could see more clearly when he tilted his glasses on an angle. He became the first person to suggest using cylindrical lenses to correct for astigmatism. These are still used today.
The name “astigmatism” came last, coined by William Whewell in 1846. The name was derived from Greek: “a-” (“without”), and “stigma” (“a mark/spot”), literally translating as “without a point”, referring to the lack of a single, clear focal point of vision.
How is astigmatism measured?
Optometrists usually detect and measure regular astigmatism during refraction, when they place different lenses in front of the eye to determine a spectacle prescription.
As irregular astigmatism can involve very small rough patches or bumps, it is best seen with specialised imaging such as corneal topography. This creates a 3-dimensional map to show local bumps and irregularities on the cornea.
I’ve got astigmatism, what do I need to know?
Astigmatism can present at any age but becomes more common as we get older.
You can develop astigmatism over time, and the level of astigmatism can change as well.
With mild astigmatism, you may not notice any problems with your vision. With increasing levels of astigmatism, your vision becomes less crisp. This can lead to reduced vision, eye strain, or fatigue.
You may need astigmatism correction to see clearly and effortlessly. Correcting astigmatism aims to compensate for the differing curvatures of the cornea, to ensure that light entering the eye focuses correctly on the retina.
To correct regular astigmatism, cylindrical lenses compensate for each curvature in the “football”. Cylindrical lenses are prescribed as either glasses, contact lenses.
Astigmatism can also be corrected with laser eye surgery.
Orthokeratology (ortho-k) can also be used. This involves wearing specialised hard contact lenses overnight. These hard contact lenses temporarily reshape the cornea, allowing the wearer to be glasses-free during the day.
To manage irregular astigmatism, it is important to treat the underlying condition causing astigmatism as well. But often, hard contact lenses are needed for clear vision during the day, as they can sit on the surface of the eye to compensate for local uneven patches in a way that glasses or soft contact lenses cannot.
Surgery, such as corneal transplants, is also sometimes needed as a last resort to replace a damaged, misshapen cornea and manage the irregular astigmatism.
Do I need to worry about astigmatism in my children?
In children, if there is enough astigmatism present to cause blurred or distorted vision, it can impact their learning and development both in the classroom and during sporting activities.
Untreated astigmatism is not dangerous, but high levels of astigmatism in young children can cause other vision problems such as “eye turns” or “lazy eye” (amblyopia).
But don’t worry, regular eye checks with the optometrist for children (and adults as well) allows for early detection and management, when needed.
Flora Hui works part-time in private practice as an optometrist.
Angelina Duan works in private practice as an optometrist.
– ref. What is astigmatism? Why does it make my vision blurry? And how did I get it? – https://theconversation.com/what-is-astigmatism-why-does-it-make-my-vision-blurry-and-how-did-i-get-it-256235
