How to measure astigmatism

What Is Astigmatism?

What is Astigmatism?

Astigmatism usually occurs when the front surface of your eye, the cornea, has an irregular curvature. Astigmatism is one of a group of eye conditions known as refractive errors and these errors cause a disturbance in the way that light rays are focused within your eye. Astigmatism often occurs with nearsightedness and farsightedness, conditions also resulting from refractive errors. Astigmatism is not a disease nor does it mean that you have “bad eyes.” It simply means that you have a variation or disturbance in the shape of your cornea.

Symptoms

  • Distortion or blurring of images at all distances
  • Headache and fatigue
  • Squinting and eye discomfort or irritation

The symptoms described above may not necessarily mean that you have astigmatism. However, if you experience one or more of these symptoms, contact your ophthalmologist for a complete exam.

Causes

Normally, the cornea is smooth and equally curved in all directions and light entering the cornea is focused equally on all planes or in all directions. In astigmatism, the front surface of the cornea is curved more in one direction than in the other. This abnormality may result in vision that is much like looking into a distorted, wavy mirror. The distortion results because of an inability of the eye to focus light rays to a single point.

If the corneal surface has a high degree of variation in its curvature, light refraction may be impaired to the degree that corrective lenses are needed to help focus light rays better. At any time, only a small proportion of the rays are focused and the remainder are not, so that the image formed is always blurred. Usually, astigmatism causes blurred vision at all distances.

Risk Factors

A small amount of astigmatism is very common and the tendency to develop astigmatism is inherited. A larger amount of astigmatism can be associated with diseases such as keratoconus.

Tests and Diagnosis

The amount of astigmatism in the eye can be measured in various ways. The autorefraction or the subjective refraction—based on the patient’s response—that are done at the beginning of an eye exam is one way to measure astigmatism. The amount of astigmatism caused by the cornea is measured in the clinic by a diagnostic instrument called a keratometer.

Treatment and Drugs

If the degree of astigmatism is slight and there are no other problems of refraction, such as nearsightedness or farsightedness, corrective lenses may not be needed. If the degree of astigmatism is great enough to cause eye strain, headache, or distortion of vision, corrective lenses will be needed for clear and comfortable vision.

The corrective lenses needed for astigmatism are called toric lenses and they have an additional power element called a cylinder. They have greater light-bending power in one axis than in others. Your ophthalmologist will perform precise tests during your exam to determine the ideal lens prescription. Refractive surgery also may be an option for correcting some forms of astigmatism.

Astigmatism may increase slowly. Regular eye care can help to insure that proper vision is maintained. You may have to adjust to wearing eyeglasses or contact lenses if you do not wear them now. Other than that, astigmatism probably will not significantly affect your lifestyle.

This article was co-authored by Rajesh Khanna, MD. Dr. Rajesh Khanna is a board certified Ophthalmologist and the Founder of Khanna Vision Institute in Los Angeles, California. Dr. Khanna specializes in Lasik, cataract, and refractive eye surgery as well as treatments for presbyopia and keratoconus. Dr. Khanna completed his first Ophthalmology Residency in Mumbai and his second Ophthalmology Residency at SUNY Downstate in New York City. He went on to complete a fellowship training in corneal and refractive surgery from the University of Cincinnati in Ohio and a Neurophthalmology fellowship from Kingsbrook Jewish Hospital in New York City. Dr. Khanna is also a voluntary member of the UCLA faculty and is an Internationally recognized top Lasik, presbyopic implants in the eye (PIE), and Refractive vision care specialist. He is board certified by the American Board of Ophthalmology and is a certified Master of Surgery by the University of Bombay.

There are 8 references cited in this article, which can be found at the bottom of the page.

This article has been viewed 106,019 times.

Astigmatism is a common eye condition that causes blurry vision. It occurs when your cornea is more oblong, or football-shaped, than round. Astigmatism can be diagnosed by your optometrist or ophthalmologist with a comprehensive eye exam. It can be corrected with glasses, contacts, or sometimes surgery – but measuring the shape of your cornea is important for getting the right lens prescription. [1] X Trustworthy Source American Optometric Association Professional medical organization dedicated to supporting optometrists and improving public eye and vision health Go to source Astigmatism is usually measured with a series of eye exams, some that are standard practice and others that may apply only to certain circumstances.

You may wonder if Jake is just one odd guy, refuting all this multi-thousand dollar equipment, and tens of thousands of optic shops practices. And really, considering this platform (internet), you should maybe question the veracity of my statements. And you can, most easily. Here is how:

Just go to one optic shop, get your vision measured. Get your astigmatism checked. Take the results with you.

Go to another optic shop. Repeat the process.

Now, look at those results. Are they identical?

Do it again. Go to a third optic shop. If you repeat this ten times, at varying times in the day, using whatever equipment they have, how many of the results, would you say, will be identical? Especially on the astigmatism side, you may not even get two identical prescriptions, out of ten.

Ironically enough, you can quite accurately determine your own refractive error, and prescription needs, at home (more so than most shops will).

You can also order the correct prescriptions, thanks to the Internet.

So, the astigmatism measurement is flawed. You want to reduce this kind of prescription error, in front of your eyes all day every day, as much as possible. Look for the upcoming installments in this series, to cover the remaining topics:

3. Should you actually use astigmatism correction in your lenses?

4. How to cure your astigmatism

5. Astigmatism and the problem with close-up focus

6. Astigmatism in children

How to measure astigmatism

How to measure astigmatism

Overview: What is Astigmatism?

Astigmatism refers to a common eye condition that causes your vision to be blurry at all distances—both near and far—because the curvature of your cornea (the clear front cover of your eye) or natural lens is irregularly shaped. This refractive error occurs because your eye cannot focus light properly, directly onto your retina, the light-sensitive surface that lines the back of your eye.

How to measure astigmatism

If your cornea is shaped more like a football than a soccer ball, your eye won’t be able to focus light rays at a single point. If the eye’s lens curvature changes, this can worsen your astigmatism. This can happen in adulthood and with naturally occurring cataracts.

There are three different types of refractive errors, including astigmatism:

  • Myopia (Nearsightedness) — This occurs when your eye focuses light in front of the retina, so you have trouble seeing objects that are far away.
  • Hyperopia (Farsightedness) — This occurs when your eye focuses light behind the retina, so you have trouble seeing objects that are nearby.
  • Astigmatism — This occurs when your eye focuses light at different points both in front of and behind the retina, so you have trouble seeing objects both far away and close up.

How to measure astigmatism

You can inherit corneal errors like astigmatism through your genetics or develop it after an eye injury, eye surgery, or eye disease like keratoconus, which causes your cornea to form into a conical shape. Keratoconus is an eye disease that causes irregular and often high amounts of astigmatism.

Effects of Astigmatism on Visual Impairment

Whether you’re born with it or it develops later in life, the symptoms of astigmatism generally include the following:

  • Blurry vision
  • Trouble seeing objects both close up and far away
  • Fluctuating vision
  • Eye strain
  • Squinting
  • Headaches
  • Difficulty seeing at night
  • Seeing glare and halos at night
  • Seeing shadowy images

If you only have a mild case of astigmatism, you may not notice any of the above symptoms or require correction. However, if you do need treatment, options are available, including corrective eyeglasses, contact lenses, and refractive surgery.

If you leave it alone, astigmatism can get better or worse over time. If you have vision problems, you should see your eye doctor to take an eye exam and discuss your options to help you achieve clear vision.

New colour LED technology provides increased repeatability and accuracy

How to measure astigmatism

Posted: Tuesday, April 1, 2014

Accurate determination of the axis of astigmatism is critical for optimising visual outcomes with toric IOLs. Therefore, in seeking precision, surgeons often obtain multiple keratometric readings. However, results of a comparative study presented at the XXXI Congress of the ESCRS in Amsterdam suggest that the need for repeat astigmatism measurements and the extra time and effort involved can be avoided by using a new topography platform that uses colour LED triangulation technology and a unique ray tracing principle to analyse the corneal surface (Cassini, i-Optics).

In the study, the colour LED topographer, an optical biometer (Lenstar, Haag- Streit), a Scheimpflug device (Pentacam, Oculus), and a Placido ring topographer (Keratron, Optikon) were used to obtain anterior corneal measurements (flat K, steep K and flat axis) in 60 eyes of cataract surgery patients. Three measurements were obtained in each eye with each device and the standard deviation of the three within subject values was analysed to evaluate repeatability for each system.

Although the four instruments had similar repeatability for determining the magnitude of astigmatism, the colour LED system had significantly better repeatability than each of the other devices in measuring astigmatism axis, reported Victor Arni Sicam PhD, inventor of the Cassini.

Mean standard deviation for the axis measurements was 4.8o using the colour LED topographer, 6.5o with the optical biometer, 9.4o using the Placido topographer and 14.9o using the Scheimpflug device. Further analyses showed the repeatability of measuring astigmatism axis was improved for each of the instruments when only data from eyes with cylinder greater than 0.5 D was used. Again, however, the mean standard deviation values showed the colour LED topographer had significantly better repeatability than the optical biometer, Placido topographer and Scheimpflug device- 4.1o vs. 6.0o, 7.3o and 12.2o respectively.

“Results from previously published studies using various keratometric instruments highlight there is only moderate repeatability of multiple axis measurements. The current study shows that with the colour LED system, only one measurement is needed to obtain accurate astigmatism information for toric IOL implantation. The first time will be right,” Dr Sicam told EuroTimes. He explained that the nature of its measurement technology accounts for the superior repeatability of the colour LED topographer.

“The measurements are taken instantaneously and the acquired information is simple points rather than rings so that image and data processing are easier, this study used a Cassini clinical prototype and it is expected that the commercial version will perform much better,” Dr Sicam said.

The study also investigated agreement of astigmatism magnitude measurements between the colour LED topographer and the other instruments based on Bland- Altman plots of the median data point for each eye. A systematic difference was noted between the colour LED topographer and the optical biometer and was even stronger comparing the colour LED topographer and the Placido ring topographer. In both cases, there was a trend for the inter-instrument difference to increase as the magnitude of astigmatism increased.

“The Bland-Altman comparison can only establish differences between technologies but cannot identify which device is more accurate. For that reason we used the instruments to measure an artificial toric surface,” Dr Sicam said. For the latter measurements, the colour LED topographer and the optical biometer performed similarly and provided data within the confidence limits for accuracy. The Placido ring system measurements had an error of four to five per cent.

Victor A Sicam: [email protected]

    30 Jun Supplement: Keys to success with today’s presbyopia and toric lens technologies 17 Aug Presbyopia & Astigmatism correcting IOLs: Key clinical opinions & practice patterns 10 Dec Supplement: Presbyopia & Toric IOL Correction: Keys to Success with Multifocal & EDOF IOLs 29 Jan Posterior corneal astigmatism 29 Jan Toric IOL realignment 29 Jan Improving toric iol calculations – 3D quantitative OCT and ray tracing show promise for achieving better outcomes 29 Jan Outcomes in toric iols – Improved outcomes with intraoperative aberrometry in difficult cases 29 Jan Best Timing for Biometry – Improved outcomes with intraoperative aberrometry in difficult cases 29 Jan Optimising Outcomes – Data from devices that measure the posterior cornea improve power calculations 11 Jan Improving accuracy in predicting IOL power in cataract surgery patients – New biometric devices and data analysis techniques combine in the quest for optimum outcomes in cataract surgery

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Study shows this imaging technique achieved higher precision than topography and biometry in post-refractive patients.

Measuring corneal astigmatism is important for patients undergoing cataract surgery, and there are several different imaging techniques, but which is the most accurate? In this study, researchers assessed the repeatability and accuracy of corneal astigmatism measurement with a spectral-domain OCT system and compared them with Scheimpflug-based topography and swept-source optical biometry.

Sixty pseudophakic eyes with monofocal non-toric intraocular lens that previously had refractive surgery were analyzed. To assess accuracy, simulated keratometric (SimK) and net corneal astigmatism obtained from each device were compared with subjective manifest refraction astigmatism.

OCT net astigmatism showed the highest accuracy for manifest astigmatism prediction. OCT net corneal astigmatism measurement was more accurate than that of Pentacam HR in both post-LASIK/PRK eyes and post-RK eyes. The authors noted this showed the advantage of using OCT in both mildly and highly irregular corneas. OCT was also more accurate than biometry overall, though the difference was not significant in the post-LASIK/PRK subgroup.

“Astigmatism derived from OCT net readings was shown to have good test-retest repeatability with a coefficient of repeatability similar to that reported for manifest refraction, and significantly lower than that found for [topography],” the authors concluded in their study. “These results are in agreement with previous studies, where OCT astigmatism showed better repeatability than Scheimpflug imaging.”

One reason the OCT had better accuracy could be that the smaller number of meridians in the other technology could limit the precision of astigmatism measurement, the authors noted. Another could be the size of the analytical zone: the study used a 4mm zone for both OCT and topography but 2.5mm for biometry, the authors explained in their study. “The smaller measurement zone may be less precise,” they suggested, “because our post-refractive surgery study population have less uniform corneal curvatures.”

Llorens-Quintana C, Pavlatos E, Thaware O, et al. Accuracy of OCT-derived net corneal astigmatism measurement. Journal of Cataract and Refractive Surgery. [Epub ahead of print].

Articles On Astigmatism

  • What Is Astigmatism?
  • Symptoms
  • Diagnosis

What Is Astigmatism?

Astigmatism is a condition in which your eye isn’t completely round.

Ideally, an eyeball is shaped like a perfectly round ball. Light comes into it and bends evenly, which gives you a clear view. But if your eye is shaped more like a football, light gets bent more in one direction than another. That means only part of an object is in focus. Things at a distance may look blurry and wavy.

It’s common to have astigmatism along with nearsightedness (myopia) or farsightedness (hyperopia). These three conditions are called refractive errors because they involve how your eyes bend (refract) light. Learn more: Does astigmatism get worse with age?

Astigmatism Symptoms

  • Blurry or distorted vision
  • Eyestrain
  • Headaches
  • Trouble seeing at night

Astigmatism Causes

Most people are born with it, but experts don’t know why. You can also get it after an eye injury, an eye disease, or surgery.

Rarely, a condition called keratoconus can cause astigmatism by making the clear front part of your eye (your cornea) thinner and more cone-shaped. You’ll probably need contacts (but not glasses) to see clearly.

You can’t get astigmatism from reading in low light or sitting too close to the TV.

Astigmatism Diagnosis

Astigmatism symptoms come on slowly. Go to an eye doctor if you notice changes in your vision. You’ll need a complete eye exam. Your doctor will test the sharpness of your eyesight by asking you to read an eye chart. They’ll also use tools to measure your vision, including:

  • Phoropter. You look through a series of lenses to find the ones that give you the clearest vision.
  • Keratometer/topographer. This machine uses a circle of light to measure the curve of your cornea.
  • Autorefractor. This device shines light into your eye and measures how it changes as it bounces off the back. This gives your doctor an idea of which lenses you need.

Astigmatism Prescription Measurements

Your prescription will have some letters and numbers. OD means oculus dexter, your right eye, and OS is oculus sinister, your left eye. OU means oculus uterque, or both eyes.

The numbers are measurements called diopters.

  • The first number is for something called spherical correction. If it has a minus sign, you’re nearsighted. If there’s a plus sign, you’re farsighted. A higher number means blurrier vision.
  • The second numbers are your cylindrical correction. This is how strong your astigmatism is.
  • The third one is the axis, the location of the astigmatism on your cornea.

For example, a prescription of “OD -1.00 x -2.00 x 155” means your right eye has 1 diopter of nearsightedness and 2 diopters of astigmatism at 155 degrees on your cornea.

Astigmatism Treatment

Glasses or contacts can correct almost all cases of astigmatism. But if you have only a slight astigmatism and no other vision problems, you may not need them.В Learn more: Do eye exercises work for astigmatism?

There are two treatments for the common levels of astigmatism:

  • Corrective lenses. That means glasses or contacts. If you have astigmatism, your doctor will probably prescribe a special type of soft contact lenses called toric lenses. They can bend light more in one direction than the other. If your case is more severe, you might get gas-permeable rigid contact lenses for a procedure called orthokeratology. You wear the lenses while you sleep, and they reshape your cornea. You’ll need to keep wearing the lenses to hold this new shape, but you won’t have to wear them as often.
  • Refractive surgery. Laser surgery also changes the shape of your cornea. Types of refractive surgery include LASIK and PRK. You’ll need to have otherwise healthy eyes with no retina problems or corneal scars.В Get more information about LASIK surgery for astigmatism.

Irregular astigmatism is far less common and is linked to problems with your cornea, the front part of the eye. Keratoconus is one example.

Astigmatism in Children

Many infants are born with astigmatism, and it often goes away before their first birthday.В A child with an untreated vision condition might have a hard time at school. This could lead to an incorrect diagnosis of a learning disorder.

Because children usually can’t tell that there’s a problem with their vision, they need regular eye exams starting about 6 months of age.

Sources

Yanoff, M. Ophthalmology, Mosby, 2008.

MedlinePlus: “Vision Problems.”

National Eye Institute: “Facts About Refractive Errors,” “Astigmatism.”

American Academy of Ophthalmology: “What Causes Astigmatism?” “Astigmatism Diagnosis and Treatment,” “What Do Astigmatism Measurements Mean?”

American Optometric Association: “Astigmatism.”

Mayo Clinic: “Astigmatism.”

Canadian Association of Optometrists: “How to Read Your Eyeglass Prescription.”

Boston Children’s Hospital: “Astigmatism.”

When you have an eye exam, your eye doctor will test you for astigmatism, a condition that may lead to blurred or double vision.

Astigmatism usually is an imperfection in the curvature of your cornea — the clear, round dome covering the eye’s front surface. If you wear a contact lens, the lens sits on your cornea.

Sometimes, more rarely, astigmatism is an irregularity in the shape of the lens of your eye, which sits behind the pupil.

An eye without any astigmatism is basketball-shaped (round), while an eye with severe astigmatism is football-shaped (oblong).

-1.0 Astigmatism, -2 Astigmatism Prescription: What Do These Numbers Mean?

Astigmatism is measured in diopters. A perfect eye with no astigmatism has 0 diopters. Most people have between 0.5 to 0.75 diopters of astigmatism. People with a measurement of 1.5 or more typically need contacts or eyeglasses to correct their astigmatism in order to maintain clear vision.

Glasses or contacts correct astigmatism by counteracting uneven curvatures of your cornea and lens. Treatment such as LASIK can treat astigmatism as well.

A person with the eye condition known as kerataconus, in which the cornea becomes thin and develops a cone-like bulge, can have astigmatism of up to 20 diopters. In such extreme conditions, a corneal transplant may be necessary to restore the spherical shape of the cornea. Ophthalmologists and optometrists sometimes use different measurements for astigmatism. Optical shops — who actually make the glasses or contacts to correct vision — have no problem understanding the prescription, no matter who is providing it.

Of the three numbers on your contacts or glasses prescription, the last two refer to astigmatism:

  • Spherical” indicates whether you are nearsighted or farsighted. A plus sign indicates you are farsighted, a minus sign indicates you are nearsighted. The higher the number, the stronger your prescription.
  • Cylinder” measures what degree of astigmatism you have, or how flat or irregular your cornea is shaped.
  • Axis” is measured in degrees, and refers to where on the cornea the astigmatism is located. Axis numbers go from 0 to 180.

Astigmatism Prescription Measurement Machines

Your eye doctor may use several machines to measure you for astigmatism:

  • An auto refractor shines light into the eye and then measures how it changes as it bounces off the back of the eye.
  • A topography machine (also called a keratometer) focuses a circle of light on the cornea and measures its reflection. This determines the curvature of the cornea and helps the doctor properly fit your contact lenses.
  • A phoropter measures how a lens must be shaped and curved to correct your vision to a normal state. The doctor has you look through a series of lenses in front of your eyes, and asks which ones make your vision better. Based on your answers, the doctor determines the lenses that provide the clearest vision.