Astigmatism is perhaps one of the most common conditions of the eye, yet one of the most misunderstood.
The transparent layer of tissue at the front of the eye is called the Cornea. When the cornea or the lens is stretched to an imperfect curve, it leads to a degree of astigmatism.
Astigmatism falls under a group of related eye conditions known as refractive errors. Contrary to popular belief, astigmatism is not an eye disease; it can be boiled down to simply to how the eye interprets light falling on it.
In an eye with astigmatism, the light fails to focus on a singular point on the retina to produce clear vision. Instead, there will be multiple focus points, either in front of the retina or behind it (or both), resulting in blurred or distorted vision.
Since astigmatism is an anomaly in the cornea, the most common cause is heredity or family history.
Other causes include:
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Determining the cause of the condition (mentioned above) helps determine the type of astigmatism and this helps pin down the appropriate treatment for the condition.
To understand the different types of astigmatism, we should first understand the term ‘meridian’ used in relation to the eye.
Meridians are the imaginary perpendicular lines along the center of the eye (much like the lines connecting 12, 3, 6 and 9 on the face of a clock)
In cases of astigmatism, the steepest and the flattest meridians passing through the eye are called the principal meridians.
Corneal astigmatism: When the cornea has an anomalous shape, it is called corneal astigmatism. There are two kinds of corneal astigmatism.
1. Regular astigmatism: In this condition, the cornea is curved more in one direction than the other. It is often described as an oblong or “football” shape of the cornea.
2. Irregular astigmatism: In this condition, the curvature of the cornea is not even across the surface of the eye. The curvature is in multiple directions in varying degrees. Irregular astigmatism occurs more as a result of eye injuries that leads to scar formation on the cornea. These can occur due to an infection of the eye, changes to the cornea after surgery or even an abrasion of the cornea. It can also result from the gradual thinning of the cornea, known as keratoconus .
Lenticular astigmatism: Astigmatism resulting from the distortion of the lens in the eye is called lenticular astigmatism.
Astigmatism can also occur in combination with other refractive disorders.
Myopic astigmatism: One or both principal meridians of the eye are nearsighted. Myopia can occur in differing degrees if both meridians are nearsighted.
Hyperopic astigmatism: One or both principal meridians of the eye are farsighted. When both meridians are farsighted, they cause hyperopic astigmatism in differing degrees.
Mixed astigmatism: This condition occurs when one principal meridian is nearsighted, and the other is farsighted.
The most common form of astigmatism is regular corneal astigmatism.
Sometimes, the symptoms of astigmatism are very mild and need no treatment for vision correction. However, in cases where vision is significantly affected by this condition, leaving it untreated can lead to the following symptoms.
These symptoms significantly impact the quality of life and productivity for the patient, especially in cases where the patient’s work involves long hours in front of a screen or requires reading or writing. Astigmatism is non-contagious.
While it is relatively easier to identify the condition in an adult, astigmatism in a child is harder to identify because the child does not have a frame of reference for perfect vision and does not even know that the condition needs attention.
It becomes the onus of the caretakers to look for symptoms the child may exhibit. Here are some things that signal that attention is required.
It becomes paramount to schedule regular eye screening for children which can lead to an early diagnosis for the growing child. The recommended ages for screening are as follows:
One or more of these tests are commonly used to diagnose astigmatism.
The Vision Test: In a typical Vision test , the doctor will have you read some letters on a standard eye chart from nearly 20 feet away. Your distance to vision ratio is measured against that of a normal eye.
For example: For an eye with 20/20 vision, the patient can see from 20 feet what a normal eye can see from 20 feet. If the vision is 20/60, this eye requires a distance of 20 feet to read what a normal eye can read from 60 feet.
Refraction Test : The doctor uses a big lens machine called a phoropter to change the lens power as you look through it, until you can read the vision chart clearly and arrive at the right prescription for you.
Sometimes, a handheld device called a retinoscope is also used to diagnose the condition.
Keratometry: This machine is designed to measure the bend center of your cornea and the steepest and flattest curves (the principal meridians). This will allow the doctor to determine the shape of your cornea and how well it can focus.
Corneal topography: This is an advanced machine equipped to give the most details about your cornea’s shape. The device collects thousands of tiny measurements of the cornea to determine this shape. A color map is built by the computer as a display of the patient’s corneal shape. Corneal topography is utilized to diagnose keratoconus , a condition which leads to progressive thinning of the cornea and leading to irregular astigmatism.
Based on the results of the diagnoses from one or a combination of the methods mentioned above, the doctor recommends a treatment method for astigmatism.
Sometimes, diagnoses reveal astigmatism in only one eye, which is then treated by appropriate corrective lenses or surgery for only the affected eye.
Some common treatment recommendations are:
1. Glasses
For regular astigmatism, the most common correction method is the use of glasses.
2. Contact lenses
Irregular astigmatism cannot be corrected with glasses and requires contact lenses to correction. They work by acting as the first refractive surface for light rays entering the eye and helping to focus the refraction to a single point. Contact lenses also provide clearer vision, a wider field of vision, and greater comfort. When fitted properly, they are a safe and efficient option.
1. Toric contact lenses
Toric contact lenses are specially designed soft contact lenses, made from conventional hydrogel or highly breathable silicone hydrogel material. Toric contact lenses work in two ways:
Toric lenses adapt to the different meridians of the cornea and come fitted with appropriate powers to correct the degree of nearsightedness or farsightedness along the different meridians that characterize astigmatism.
Toric lenses are equipped with a design feature that allows the lens to rotate to the proper orientation on the cornea
and aligns the power meridians of the contact lens with the appropriate meridians of the eye for clear vision.
2. Gas Permeable contact lenses
Popularly known as “hard” lenses, these are rigid contact lenses are made of durable plastic that transmits oxygen. The rigid nature of these lenses helps maintain the spherical shape of the cornea to help correct the vision. Due to this feature, they are also called rigid gas permeable (RGP) lenses or oxygen permeable lenses. Most gas permeable lenses are made of silicone, a material that allows the passage of oxygen to the cornea to keep it healthy, without having to rely on the supply of oxygen contained in tears that wash the cornea with each blink.
RGP lenses are harder to adapt to because of the rigid and thick structure of the lenses, but they are known to provide noticeably sharper vision than toric lenses.
3. Hybrid contact lenses
Hybrid contact lenses combine the best features of the soft toric lenses and RGP lenses. The central zone of these lenses is made of a RGP material and surrounded by a fitting area of hydrogel or silicone hydrogel material.
Hybrid contact lenses provide the sharp vision of RGP lenses combined with the wearing comfort of soft lenses.
Since hybrid contact lenses are about the same size as toric soft lenses and have thinner edges than RGP lenses, there is a lower risk of these lenses dislodging from the eyes while engaging in sports or other activities.
3. Refractive Surgery
Refractive surgery works by changing the shape of the cornea permanently. The change in the shape of the cornea helps enhance or even restore the ability of the cornea to focus light rays on a precise singular point, thus improving vision. Eye doctors recommend refractive surgery after a thorough examination, that includes keratometry and corneal topography (described above).
There are many types of refractive surgeries. Your eye care professional can help you decide which surgery is the right option for you.
The most common surgery for astigmatism is LASIK Surgery.
LASIK surgery uses ultra-precise laser technology to reshape the cornea to correct the way light enters the eye. The doctor creates a flap in the outer layer of the cornea so he or she can access the tissue that lies beneath. This flap was traditionally created with a blade called the microkeratome; however, with the latest advancements, laser technology has essentially replaced this blade. The laser technology, known as a femtosecond laser, uses precise laser beams to create a sophisticated and uniform flap. The flap is then lifted and the underlying corneal tissue bed is then ablated or removed to reshape the patient’s cornea. Using computer imaging technology called wave-front technology, the doctor creates a detailed image of the cornea as a guide for either optimized or customized ablation treatment to shape the patient’s cornea. After the completion of this ablation, the flap is placed back on the patient’s residual bed. LASIK is seen as a permanent solution to astigmatism as opposed to using glasses or contact lenses.
2. Photorefractive keratectomy (PRK)
Unlike LASIK, PRK does not involve the creation of a flap. Instead the surgeon first simply removes the skin cells of the cornea, known as epithelium. Using the same excimer laser that is used for LASIK, the surgeon then ablates the underlying corneal issue to rehape the patient’s cornea. The epithelium cells are then allowed to naturally heal in over the span of 3-5 days.
3. Refractive lens exchange (RLE)
Also known as clear lens exchange, clear lens extraction, and refractive lens replacement, it resembles cataract surgery whereby the doctor removes the natural lens and replaces it with an artificial implant through a small incision made into the cornea. Astigmatism can be corrected with the artificial implant placed in the eye. See article on Refractive Lense Exchange.
Conclusion
Astigmatism is a readily treatable condition, thanks to the many current advances in ophthalmology. All it requires is attention and timely care so that you can use your eyes to their maximal potential.
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