Astigmatism is a type of refractive error wherein the refraction varies in different meridian. As a result, the light rays entering the eye cannot converge to a point focus but form focal lines.
As described by Bannon and Walsh (1945), astigmatism was first mentioned in 1727 by Sir Issac Newton. But Thomas Young in 1800 published first description of astigmatism.
Refractive power of the eye is determined predominantly by variables like power of the cornea, power of the lens, and axial length of the eyeball. In emmetropia, these three components of refractive power combine to produce normal refraction to the eye.
Emmetropia is the condition where the eye has no refractive error and requires no correction for distance vision. In an emmetropic eye, rays of light parallel to the optical axis focuses on the retina. The far point in emmetropia (point conjugate to retina in non- accommodating state) is optical infinity, which is 6 meters. Ametropia (refractive error) results when cornea and lens inadequately focus the light rays. The measuring unit for refractive error is dioptre (D), which is defined as the reciprocal of the focal length in meters.
The term ametropia (refractive error) describes any condition where light is poorly focused on light sensitive layer of eye, resulting in blurred vision. This is a common eye problem and includes conditions such as myopia (near- sightedness), hypermetropia (far- sightedness), astigmatism, and presbyopia (age- related diminution of vision). A person who is able to see without the aid of spectacles or contact lenses is emmetropic.
Prevalence and distribution of ametropia vary greatly with age. Majority of children in early infancy are found to be somewhat hypermetropic. During the school years, children begin to become myopic in increasing numbers. Astigmatism change relatively little with age. The majority of children and young adults have a small amount of with-the-rule astigmatism, but in later adult years, there is a tendency for with-the-rule astigmatism to decrease in amount and for against-the-rule astigmatism to increase.
Astigmatism is divided as
· Regular astigmatism: It is correctable with cylindrical or sphero-cylindrical lenses. It may be
- With-the-rule astigmatism: When the steepest corneal meridian is close to 90?.
- Against-the-rule astigmatism: When the steepest meridian is close to 180?.
- Oblique astigmatism: Principal meridians do not lie close to 90? or 180?.
- Bi- oblique astigmatism: The two principal meridians are not at right angle to each other.
· Irregular astigmatism: It is not correctable with cylindrical or sphero-cylindrical lenses. Patients with irregular astigmatism typically suffer from reduced visual acuity and poor quality of vision. Irregular astigmatism may result from corneal diseases, such askeratoconus, trauma or scarring following herpes infection. It may also be produced due to cataract surgery, penetrating keratoplasty or elective kerato-refractive surgery.
Depending on spherical ametropia of a particular eye, astigmatism may be classified as simple or compound based on whether one or both meridians, respectively, are focused outside the retina. If one meridian focuses in front of the retina and the other meridian focuses behind it, the astigmatism is called mixed astigmatism.
In astigmatism, the eye has different refractive powers along different meridians. Light entering in vertical direction gets focused differently than light in the horizontal direction. The meridian of steeper curvature has greater refractive power. The astigmatic eye produces a blurred image because two focal lines of images are being produced. This requires different corrections along each of these meridians to produce a single focused image on the retina.
Agarwal Sunita, Agarwal Athiya, Apple David J, Buratto Lucio, Ali? Jorge L, Pandey Suresh K, Agarwal Amar. Textbook of Ophthalmology Vol 1. Jaypee Brothers Medical Publishers (P) Ltd 2002. P 174- 177.
Selser Richard E Jr. Proceedings of the 42nd Annual Symposium on Medical Cornea- Corneal and Refractive Surgery, New Orleans, LA, USA, February 26- 28, 1993 organised by New Orleans Academy of Ophthalmology. Kugler Publications 1994. P 87- 99.
Bannon RE, Walsh R. On Astigmatism, Parts 1 and 2. Amer. J. Optom., 1945; Vol. 22: 101-111, 162- 179.