The cornea on the outer part of the eye and the lens within the eyeball are responsible for bending light (refraction) so that it focuses on the most sensitive part of the retina. The sharper the image cast on this area, the clearer of vision. The more common vision problems are associated with refractive errors – this means that the light is not bent optimally to cast a sharp image on the eye. Most people are well versed with two common refractive errors that causes myopia (nearsightedness) and hyperopia (farsightedness). Astigmatism is another refractive eye disorder that is less well understood by many yet it is one of the more common refractive errors.
What is astigmatism?
Astigmatism is a refractive error due to an uneven curvature (roundness) of the cornea thereby leading to a number of visual problems. Almost 9 out of 10 people have astigmatism, however, it is to such a slight degree that it usually does not create a major vision problem. Astigmatism is a bit more complex refractive error that myopia and hyperopia both in the mechanism causing the visual disturbance and the nature of the vision problem. This can vary from blurred vision to problems viewing certain colors and even double vision (diplopia). Astigmatism may arise concurrently with myopia or hyperopia thereby complicating the refractive error even further.
How does astigmatism occur?
A sharp image has to be cast on the retina for clear vision. Light rays are refracted (bent) mainly by the cornea and lens to achieve this. The lens can change it curvature and therefore its ability to refract light. The cornea, however, cannot alter its shape at will and has to be evenly rounded (symmetrical) in order for all rays of light to be bent to corresponding degrees and meet at the focal point.
In astigmatism, the cornea is not evenly curved and therefore the light is not refracted evenly. Parts of the cornea may be more curved while others less so meaning that the rays of light do not bend to meet at focal point that will fall on the most sensitive part of the retina. With other refractive errors, the lens of the eye can compensate to some degree. It does this by changing its curvature with the help of the ciliary muscles. By contracting and relaxing, these muscles allow for a change in tension on the lens and therefore a variation of the curvature. However, the lens curvature is changed almost equally in each plane. It can therefore not accommodate for a greater or lesser degree of refraction in one plan compared to another and help to compensate for the visual disturbance in astigmatism.
Types of Astigmatism
Normally the different segments of the corneal curvature are the same. These segments are known as principal meridians and lie at a 90 degree angle to its the neighboring meridians.The irregularity of the cornea means that the principal meridians are not the same and/or the meridian does not lie at a 90 degree angle (perpendicular) to the surrounding principal meridians.The abnormalities in the meridian arrangement gives rise to the different types of astigmatism.
Astigmatism can be classified as simple, compound or mixed. The simple and compound types can be further subdivided into hyperopic or myopic depending on how the vision is affected. In order to enable vision, the light rays must meet at a single point known as the focal point. So the light rays (focal lines) entering through one principal meridian should be bent to the same degree as a corresponding focal line entering through another meridian.
In simple astigmatism, one light ray (focal line) falls perfectly on the retina while the corresponding light ray (focal line) which should be bent to the the same degree to meet it at the focal point instead falls short of the retina (myopic) or behind it (hyperopic). With compound astigmatism both light rays (focal lines) fall either short of the retina (myopic) or behind the retina (hyperopic). In mixed astigmatism, neither focal line strikes the most sensitive part of the retina accurately with one line falling short and the other falling behind the retina.
Therefore the different types of astigmatism are :
- Simple hyperopic astigmatism
- Simple myopic astigmatism
- Compound hyperopic astigmatism
- Compound myopic astigmatism
- Mixed astigmatism
Causes of Astigmatism
Astigmatism is a result of an irregular curvature of the cornea. It seems to run in families and is more likely to occur in premature babies or babies with low birthweight. It is possibly due to incomplete development of the cornea in fetal life. Uneven curvature of the cornea can arise secondary to many conditions affecting the eye. This may include eye trauma and surgery to the eye. Another less common cause of acquired astigmatism (develops during life) is a condition known as keratoconus.
Keratoconus is the gradual thinning and outward bulging of the cornea which distorts the degree of refraction (bending) of light rays entering the eye. It is a progressive disorder that can develop over years. The exact cause of kertoconus is unclear and it is believed to be due to multiple causes that collectively cause this abnormality. There may be a familial risk and keratoconus is also more frequently seen in people with allergies and in genetic defects such as Down syndrome.
Signs and Symptoms
Astigmatism is a common condition and may be minor to the degree that no disturbance in vision may be noted. When present, the visual disturbance is most commonly blurred vision, distortion in vision sometimes more to one side and even double vision in severe cases. At times there may be a difficulty seeing one color in contrast to another. Astigmatism can vary to the extent that both a combination of both near and far vision are distorted sometimes to different degrees. It therefore makes astigmatism a more complex refractive error.
The other signs and symptoms are common to both hyperopia and myopia and often resolve once the refractive error is corrected. This is a result of eyestrain and includes :
- Eye discomfort, pain or burning sensation
- Light sensitivity (photophobia)