Astigmatism – Written by Purrven Bajjaj, B Optom, MSc Clinical Optom.
Astigmatism is a type of refractive error that occurs when the cornea (transparent thin layer that covers the front portion of the eye) or the lens (responsible for the focusing mechanism of the eye, allowing clear vision) of the eye is not perfectly smooth and round as it should be.
This causes light entering the eye to scatter and get focused at multiple points on the retina (light sensitive tissue at the back part of the eye) instead of just at one point directly on the retina (Figure 1).
The following sections will briefly cover the public health implications of astigmatism, the causes and different types of astigmatism, its signs and symptoms, and some insights into its management and treatment.
Astigmatism affects people of all age groups, gender, and ethnicity. Globally, the condition affects approximately 1 in 10 children and teenagers (aged 20 years and below) and 4 in 10 adults (aged 30 years and above) .
A recent (2018) study investigated the number of people with astigmatism in different parts of the world. The study found that among those aged 20 years and below, astigmatism was most common in the United States (US), with 3 in 10 American children and teenagers affected . The same study also investigated the number of adults aged 30 years and above with the condition and found that the condition was most common among adults in the US and Southeast Asian countries, with close to 5 in 10 having the condition .
A study published in 2018 that aimed to understand the economic burden of refractive errors in Iran reported that astigmatism incurred the highest burden to the Iranian economy at US$ 5.5 billion each year . Global estimates of the direct and indirect costs of astigmatism are still emerging, however uncorrected refractive errors (myopia, hypermetropia, astigmatism, presbyopia) can result in substantial economic costs due to loss of productivity. The global potential loss of productivity due to uncorrected refractive errors has been estimated to be US$ 244 billion per year .
The two main types of astigmatism are corneal (when the cornea is irregularly shaped) and lenticular (when the lens is irregularly shaped).
The cornea has 2 axes. In astigmatism, one axis is steeper (higher degree) and the other axis is flatter (lower degree). Astigmatism of the eye is calculated as the difference between the steeper and flatter axis.
Corneal astigmatism is further classified into regular and irregular astigmatism. With regular astigmatism, the cornea is more curved in one particular direction than the other. The axes of the eye are 90 degrees apart or perpendicular to each other. Irregular astigmatism is when the curvature of the eye isn’t an even curve across the cornea.
In ATR astigmatism, the horizontal meridian (as seen in Figure 2) is steeper than the vertical meridian. Vision will be blurry when looking at horizontal lines.
In WTR astigmatism, the vertical meridian (as seen in Figure 2) is steeper than the horizontal meridian. Vision will be blurry when looking at vertical lines. WTR astigmatism, commonly found in children between 7 to 16 years old, is also the most common type of astigmatism.
Oblique astigmatism occurs when the steepest axis of the cornea is neither the vertical nor horizontal meridian.
Dioptres (D) is the unit used to measure the degree of the corrective lens required to achieve clear vision.
The following table describes the different severities of astigmatism :
|Mild astigmatism||Less than 2.00D|
|Moderate astigmatism||2.00D to 4.00D|
|High astigmatism||Greater than 4.00D|
Astigmatism, is a complex eye disease that can develop due to genetics and other factors such as an irregular shape of the cornea or lens or due to corneal diseases.
Astigmatism occurs when light is bent differently depending on where it hits the cornea and then through the lens. In an eye with no astigmatism, the cornea or lens has equal roundness in all areas, like a soccer ball. In an eye with astigmatism, the cornea or lens is curved like a rugby ball, with some areas steeper or more rounded than others.
Studies have identified 3 major genes that are collectively responsible for the development of astigmatism, but the exact gene has not been identified yet . In a study analyzing astigmatism in families, it was reported that children who have one parent or a sibling with astigmatism have a close to 50% increased risk of developing the condition themselves .
A study analyzing refractive errors (myopia, hypermetropia, astigmatism) in premature infants (born earlier than 32 weeks of pregnancy) reported that 2 in 10 premature infants had astigmatism . This may be because the cornea did not have enough time to develop properly in the womb. Structural issues that affect the development of the cornea may also cause astigmatism.
Keratoconus is an eye disease where the cornea gradually thins and changes to a cone shape. This can lead to irregular astigmatism. Keratoconus affects 1 in 2000 Americans. Keratoconus can be acquired, or it can also be inherited through genes. It was found that 1 in 10 people affected with Keratoconus has a parent with the disease .
Rubbing the eyes too hard or too often might damage the cornea or lens of the eye. It can affect vision or lead to an infection that needs medical attention. Some people who have allergies experience itchy eyes, causing them to rub their eyes. Eye rubbing might also be a reflex or a habit. Rubbing the eyes hard enough to make them lose the roundness of its shape can damage the eyes over time and has been associated with the development of astigmatism in some cases.
Adults with mild astigmatism (less than 2.00D) may realize their vision deteriorating. Children with symptoms of astigmatism may not be aware that they have the condition and are unlikely to complain. Symptoms vary between individuals but in mild astigmatism (less than 2.00D), symptoms may not be noticeable and spectacles or contact lenses might not be needed. It is best to seek advice from a trained eye health professional by going for regular eye checks.
If you experience any of these signs and symptoms, schedule an appointment with an eye health professional to get your eyes checked. It is also important to note that the development of eye conditions may even start before symptoms appear, which makes going for regular and timely eye checks that much more essential.
Astigmatism is usually diagnosed by an eye health professional based on the results of a comprehensive eye check. Vision will be tested, where the eye health professional will have you read letters or numbers off a visual acuity chart. In some cases, if astigmatism was not previously present, an additional test called retinoscopy is conducted. The eye health professional will use a retinoscope to shine light into the eye and check how light bends in your eye. The degree or amount of astigmatism is then checked by a test called refraction, where the eye health professional will ask the patient to compare various lenses and decide which lens makes the patient’s vision clear.
Specialized equipment like a keratometer is also used to check the curvature of the cornea. This equipment allows the exact curvature of a specific area of the cornea to be measured. Another sophisticated device called a topographer is also used and it generates a map of the cornea providing more detail of its shape.
Astigmatism cannot be cured. When blurring of vision due to high astigmatism (more than 4.00D) in children is left undetected or uncorrected, the eye may get ignored by the brain due to the lack of reception, leading to amblyopia or lazy eye.
Astigmatic vision is corrected with spectacles, contact lenses, or vision correction surgery. An eye health professional will recommend the most suitable option based on age, degree of astigmatism, the health of the eye, and vision requirements.
Spectacles can be used to help improve the vision of people with astigmatism. Spectacles might be required all the time or only when performing visual tasks that require clear vision, like driving or reading. Prescription spectacles that help to correct the condition contain a special cylindrical lens prescription that provides the amount of lens power needed to correct the condition.
Contact lenses are divided into 2 main categories, soft and hard. Soft contact lenses, which is the more common choice among contact lens wearers, are flexible and is moulded to the shape of the eye. In eyes with irregular astigmatism, the eye health professional may recommend hard contact lenses since the hard nature of the lens provides regularity on the surface of the eye, therefore improving the way light enters and interacts with the eye and providing clear vision.
The US Food and Drug Administration (FDA) regulates contact lenses as a medical device. It is important for contact lens users to maintain good contact lens care and hygiene. Routine eye checks with an eye health professional are also essential. Seek prompt medical attention when experiencing discomfort, soreness, itching and redness of the eye from wearing contact lenses.
Refractive surgery is when a surgeon applies laser to the cornea (transparent layer protecting the surface of the eye) to change the way light enters your eye, so that it is focused on the retina (back portion of the eye) to produce clear vision. Common refractive surgeries include laser-assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).
LASIK is the most common refractive surgery and involves creating a thin flap on the surface of the cornea. A laser then removes a precise amount of tissue from the inner layers of the cornea needed to change its shape for clear vision. The flap is then placed back into its original position to heal. In PRK, lasers remove a thin layer of tissue from the surface of the cornea instead in order to flatten its shape, which helps to refocus incoming light onto the retina.
As per the FDA, refractive surgeries should only be performed in those aged 18 years or older. The eligibility for surgery and type of surgery is dependent on the general health of the person and the results of a comprehensive eye check that includes additional tests not done routinely for an accurate assessment of the cornea (transparent layer protecting the surface of the eye).
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