What is astigmatism?
Astigmatism is a structural abnormality of the eye that bends light as it passes through the eye in such a way that makes it difficult to focus properly, causing blurred vision. It is due to an irregularity in the shape of the cornea, which in mostly spherical in a normal eye, but more egg shaped in an eye with astigmatism.
The causes of astigmatism
There is some evidence that suggests that regular astigmatism may be hereditary in nature, passing from parents to their children, but there's not enough information available to know for certain. Irregular astigmatism is often caused by damage to the eye, resulting in corneal scarring, or scattering within the crystalline lens inside the eye.
In some cases, astigmatism can be present at the time of birth, although detecting vision problems in very young children is very difficult. In other cases, astigmatism can develop later in life. It's also possible for the condition to either improve or worsen as time passes. Everyone's own experience will be different.
Symptoms of Astigmatism
For people with very minor levels of astigmatism, there may not be any obvious symptoms at all, aside from requiring a slight change in prescription to match the shape of the eye. Otherwise, there may not be any noticeable issues. For others, a severe case of astigmatism may cause blurred vision and squinting, which can then also lead to headache and eye fatigue.
If you're experiencing any of these symptoms, it's recommended that you see you eye doctor as soon as possible.
Types of Astigmatism
The measurements of the shape and curvature of the eye, using an instrument called a Keratometer, are used to determine the type of and severity of the astigmatism. In addition to regular astigmatism, which develops naturally, and irregular astigmatism, which is the result of damage to the eye, further categorization of astigmatism includes:
- Simple Myopic Astigmatism – The shape of the eye causes light to refract onto two separate focal points, one on the retina, and the other just in front of it.
- Simple Hyperopic Astigmatism – The inverse of simple myopic astigmatism. Also causing two focal points of light, with one being on the retina, but the other behind it.
- Compound Myopic Astigmatism – Similar to simple myopic astigmatism, but with both focal points in front of the retina, but distinct from one another.
- Compound Hyperopic Astigmatism – Following the same pattern, both focal points are found behind the retina, separate from one another.
- Mixed Astigmatism – A hybrid of the other forms of astigmatism, where one focal point exists in front of the retina, and the other behind it.
How is Astigmatism Diagnosed?
A standard comprehensive eye exam is going to be enough to detect astigmatism. The eye exam will consist of several different tests, each designed to measure a different aspect of vision and eye health. The primary tests performed during the exam are:
- The Visual Acuity Test – Probably the most recognizable eye test in the world, where lines of random letters that decrease in size are viewed from a fixed distance of 20 feet (or 6 metres). A person with normal “20/20” vision (or 6/6 in the UK) can see exactly what their expected to see from 20 feet away. A person with 20/40 vision (6/12 in the UK), for example, can see letters clearly at 20 feet that a person with normal vision could see from 40 feet away.
- The Refraction Test – A vision testing device called a phoropter is used to determine the exact numerical value of someone’s prescription. The phoropter, which looks like a large mask with many dials and lenses, is put in front of the patient's face. While looking at the same lettered chart, different lenses are changed out with one another to find which one provides a clearer image. Through a process of elimination, the best lenses are determined, and the results used to write a prescription for eyeglass lenses.
- The Keratometry Test – The tool most relevant to diagnosing the cause of astigmatism in an individual, is a Keratometer. This is a device that shines a light across the surface of the cornea, to measure the exact curvature of the surface of the eye. This is used not only to detect astigmatism, but also for the proper fitting of contact lenses. In some cases, a more complex variation of this test called corneal topography may be used for a more detailed mapping of the cornea. The keratometer can give an insight as to the location of the astigmatism - often it is corneal in nature, but sometimes it can be caused by the lens inside the eye (lenticular astigmatism). This is very important information to determine the best means of correcting it with contact lenses.
How to treat astigmatism
The recommended treatment will depend largely on the type and severity of astigmatism in question.
Regular astigmatism can be corrected with any sort of standard corrective lens - contacts or spectacles - while irregular astigmatism can only be rectified with contact lenses. Laser refractive surgery, such as LASIK or PRK, can be beneficial, but can cost quite a bit more than the alternatives, and require a complicated surgical process.
The type of contact lenses used to correct astigmatism, known as toric lenses, can come in both soft and rigid gas permeable varieties. Which lens is the right one for you will depend on both the strength of the prescription, the nature of the astigmatism, as well as personal preference.
Small amounts of astigmatism (< 0.75D) can often be ignored with soft lenses - most people will experience vision only slightly less clear than their glasses.
However if you want any astigmatism corrected up to 3.00D there are now soft lenses capable of that - even daily ones. Focus Dailies Aquacomfort Plus Toric can correct up to 2.00D of astigmatism easily and 1 Day Acuvue Moist for Astigmatrism can correct up to 3.00D and may be just what you need to sharpen up your vision.
Correct your astigmatism with contact lenses
If you are concerned about problems with your vision, the best thing to do is get an eye examination. Your eye care professional will carry out various tests to determine the nature and severity of your condition. They will ask you questions about your lifestyle and working habits to determine the best way to get you seeing clearly again.
If you have been diagnosed with astigmatism, a common solution is the use of corrective contact lenses. At Contactlenses.co.uk, we believe that visual problems should not get in the way of your normal lifestyle. That's why we've made it simple, fast and affordable to get contact lenses to correct your vision.
Author: John Dreyer Optometrist Bsc(Hons), MCOPTOM, DipCLP
Created: 24 Apr 2015, Last modified: 17 Jul 2019