Astigmatism is the condition of refraction when the eye does not have the same power of refraction in all meridians (i.e., unequal refraction of light). Parallel light rays entering the eye will not come to a focus image on one point on the retina.
Structure of the Astigmatic Eye:
In normal eye, the cornea is spherical in shape. Light rays passing through all surfaces of the cornea undergo the same degree of refraction (i.e., refracted equally). As a result, these rays come to be focused exactly on the retina at the back of the eye, creating a sharp image.
In astigmatism, the cornea is oval like a football instead of spherical like a basketball. Most astigmatic corneas have two curves; a steeper curve (has too much curvature) and a flatter one. Light rays passing through different parts of the cornea are refracted by variable amount. This causes light to focus on more than one point at the back of the eye, resulting in blurred vision at distance or near.
Symptoms and Signs of Astigmatism:
In addition to blurring of vision, astigmatism may cause headache, pain in and around the eye, redness of the eyelids margin and tearing. Persons with astigmatism may also try to improve vision by squinting (cross-eyed). In children, it may cause delay in learning and poor schoolwork.
Causes and Types of Astigmatism:
The specific cause of astigmatism is unknown. It can be hereditary and may be present at birth. It can change as with age and may decrease or worsen over time. It can range from mild to severe and often occurs along with other refractive errors as myopia or hyperopia. There are two types of astigmatism:
- Regular Astigmatism: here the meridian of maximal and minimal powers are at right angel to each other and the change from the meridian of highest power to that of lowest power is gradual. A toric lens can correct regular astigmatism arising from either the cornea or crystalline lens.
- Irregular Astigmatism: Here the meridian of maximal and minimal powers are not at right angel to each other and the change from the meridian of highest power to that of lowest power is not gradual. Irregular Astigmatism can be corrected by contact lenses but cannot be corrected by standard spectacle lenses.
Diagnosis and Treatment of Astigmatism:
Most people have some degree of astigmatism. Small amounts of astigmatism usually do not affect vision and do not require treatment. However, larger amounts require correction with eyeglasses or contact lenses. These will adjust the incoming light so that the images will focus exactly on the retina at the back of the eye. Refractive surgery procedures such as LASIK can also be considered to permanently correct astigmatism by reshaping the cornea.
The full series of LASIK Vision Correction Explained tutorials are Tutorial 1, Tutorial 2, Tutorial 3, Tutorial 4, Tutorial 5, Tutorial 6,Tutorial 7, Tutorial 8, Tutorial 9, Tutorial 10, Tutorial 11 and Tutorial 12.