Astigmatism

Overview of Astigmatism

The World Health Organisation (WHO) has estimated that around 39 million people all over the world are blind and 246 million have poor vision. The statistics of WHO have also shown that India has the highest blind population in the world. A recent report published in the Times of India has shown that India loses around 37 billion dollars due to eye-sight problems.

Many of these vision problems arise due to refractive errors of the eye lenses. Astigmatism is one such error that gives rise to both near-sight and far-sight problems. Although it can be corrected by proper lenses, a delayed diagnosis and treatment can even lead to a permanent loss of vision.

What is Astigmatism?

Astigmatism is a refractive error in which an even distribution of light does not occur over the retina, the light-sensitive tissue at the back of the eye. It arises due to the imperfect curvature of the cornea. The disease is often attributed to genetic factors. It is characterized by an extreme blurry vision. An abnormal curvature of the cornea occurs in this disease. Under normal condition, the cornea has a spherical shape. But an astigmatic cornea mostly has two curves. Astigmatism may be characterized by either nearsightedness or farsightedness.

Astigmatism takes place when the light is bent differently after it strikes the cornea and passes through the eyeball. A normal human eye has a cornea that is round and even. An astigmatic cornea, on the other hand, has some areas steeper or more rounded than the others. This causes the images to appear blurred or abnormally stretched.

Who form the vulnerable groups?

Astigmatism can affect children and adults alike. People having a very mild degree of astigmatism may not be aware of the problem. For small children, especially one with an ancestral history of vision problems should be taken for regular check-ups in order to detect astigmatic problems if any at a very early stage. It is often said that the glare from certain objects such as a whiteboard or some types of bulbs may result in Astigmatism. But this fact has not been scientifically established.

What causes Astigmatism?

Astigmatism usually occurs in people with one or both parents having some vision-related difficulties. So in most of the cases, it is an inherited trait. It may also result from certain environmental factors. People spending long hours in a dimly lit environment, working or studying under similar conditions have a greater likelihood of developing Astigmatism. Astigmatism, whether regular or irregular is caused by a combination of external (corneal surface) and internal (posterior corneal surface, fluids, retina and the eye-brain interface) optical properties.

What are the main signs and symptoms of Astigmatism?

Astigmatism may present with either nearsightedness or farsightedness or in some cases both. It is easy to tell if one has Astigmatism if the following symptoms are observed-

• Blurred and distorted vision
• Nearsightedness and farsightedness
• Tendency to squint
• Eye twitching and eye fatigue
• Eye-strain while concentrating on a subject
• Pain in the eyes after long hours of performing visual tasks
• Problems viewing the blackboard
• The images may appear extremely blurred and stretched out
• Problems driving at night
• Lines running across the page appear less distinct than
• Migraine headaches (occurs in the extreme stage)

What are the different types of Astigmatism?

Astigmatism has a number of subtypes based on the axis of the principal meridian, the focus of the principal meridian and the distribution of incoming light over the retina. According to the axis of the principal meridian, Astigmatism is of two types-

• Regular Astigmatism: In this case, the principal meridians are perpendicular. It is of three types-
• With-the-rule astigmatism: Here the vertical meridian is the steepest. To correct this type of astigmatism, a minus cylinder can be placed in the horizontal axis. This makes the horizontal axis steeper and the vertical axis less steep. In a word, both the axes are made equally steep in order to avoid refractive errors.
• Against-the-rule astigmatism: Here the horizontal meridian is the steepest. It can be fixed by placing a minus cylinder in the vertical axis or a plus cylinder in the horizontal axis.
• Oblique astigmatism: In this type, the steepest curve lies either between 120 and 150 degrees or between 30 and 60 degrees.
• Irregular astigmatism: Here the principal meridians are not perpendicular.

Based on the focus of the principal meridian, Astigmatism can be categorized into three types-

• Simple astigmatism: It has two subtypes-
• Simple hyperopic astigmatism: Here the first focal line is on the retina, while the second focus line is behind the retina.
• Simple myopic astigmatism: Here the first focal line is in front of the retina, while the second focal line is on the retina.
• Compound astigmatism: It has two subtypes-
• Compound hyperopic astigmatism: Here, both the focal lines are located behind the retina.
• Compound myopic astigmatism: Both the focal lines are located in front of the retina in this case.
• Mixed astigmatism: Here the focal lines are on both sides of the retina, straddling the retina.

Diagnosis and treatment

The ophthalmologist conducts a series of investigations in order to formulate a treatment plan.

Some of the options are as follows:

Corrective lenses: The corrective lenses such as eyeglasses and contact lenses can be used to rectify the refractive errors of the eye.

Contact lenses are of several types– extended wear, disposable soft, gas permeable, rigid and bifocal.

Orthokeratology (ortho-k) is a treatment method in which a series of a customized pair of rigid lens is worn for long durations in a day till the cornea becomes even and uniform. In case the treatment is discontinued, the eye returns to its original shape. Wearing lenses for a long duration can lead to increased risk of infection.

Surgical intervention: The refractive surgeries can improve the vision of an individual without the use of eyeglasses or contact lenses. The various types of refractive surgeries are Laser-assisted in-situ keratomileusis (LASIK), Laser-assisted subepithelial keratectomy (LASEK), Photorefractive keratectomy (PRK) and Epi-LASIK.

Refractive surgeries might have side effects such as infection, dry eye, corneal scarring, under correction or overcorrection, the appearance of halo or starburst appearance around the lights and many more. Since the exact cause of the condition is unknown, and it might be inherited, the prevention of the condition is not possible.

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