Astigmatism is an eye disorder, which is suffered by nearly two-third of the world population. This way, astigmatism has been the most common eye refractive error that affects how someone perceives an object in front of their eyes. In astigmatism-suffering individuals, the cornea is aspherical in shape, so that the incoming rays are converged into multiple focal points instead of merely one. When the focal points are more than one, the perceived objects are blurred and cannot be seen clearly and properly as a perfectly converged object. Astigmatism influences both the objects in distance as well as those up close.
Types of astigmatism
There are some types of astigmatism, although all these types share a similar underlying risk factor triggering the occurrence of astigmatism, which is the irregular shape of the cornea that causes incoming rays create more than one focal points, causing unclear or blurry vision. Types of astigmatism may differ among affected individuals. Based on focus of the principal meridians, astigmatism is grouped into these primary types:
Myopic astigmatism refers to the eye condition, with which light from outside the eye falls into two focal points. In myopic astigmatism, the meridians of the eye are nearsighted. A meridian itself is a curve that bisects a sphere, such as an eyeball. With myopic astigmatism, light rays are brought to a focus in front of the retina. As a result, individuals with myopic astigmatism tend to have problem perceiving objects far from the eyes. When an individual is suffering from compound myopic astigmatism, both focal lines are located in front of the retina.
In individuals with hyperopic astigmatism, the light rays are brought to a focus behind the retina. This form of astigmatism also causes one hyperopic meridian and another one without any refractive error. As a result, individuals suffering from hyperopic astigmatism have a problem on focusing their vision to the objects up close to the eye. People may also develop compound hyperopic astigmatism, in which both focal lines are located behind the retina.
With this type of astigmatism, light rays come to two focal points, one of which is in front of the retina, while another one is behind the retina. As a result, the person suffering from this eye disorder experience the symptoms of both nearsightedness and farsightedness at the same time. Because of this, it is really difficult for a person with mixed astigmatism to gain clear vision.
In addition to these three primary types of astigmatism, this vision disorder is also classified based on axis of the principal meridians. Thus, the types of astigmatism include:
- Regular astigmatism, in which the principal meridians are perpendicular. This is the most common type of astigmatism suffered by children. In children, with-the-rule regular astigmatism is frequently suffered by children. This is the type of astigmatism wherein the vertical meridian is steepest. On the other hand, against-the-rule regular astigmatism is more common. This is where the horizontal meridian is steepest. These explain why children with astigmatism commonly require a minus cylinder, while the elderly usually need the plus cylinder. Adding a minus cylinder in the horizontal axis makes the vertical axis less steep, while adding a plus cylinder in the horizontal axis makes it less steeper, so that both axes are equally steep.
- Irregular astigmatism, in which principal meridians are not perpendicular. This is a quite uncommon type of astigmatism, which cannot be fixed with traditional astigmatism treatments, such as glasses. As a result of not perpendicular meridians in the patient’s eye, individuals suffering from irregular astigmatism often perceive double images—called also as ghost image. In other cases, starburst that results from this image distortion may also occur.
Severe astigmatism can affect central vision. In this type of astigmatism, blurriness makes it difficult for the patient to focus on text or an object. As the affected eye undergo this focusing difficulties, more severe astigmatism may be accompanied by severe headache, fatigue, nausea, or dizziness. Astigmatism suffered by children may also develop during lifetime and thus, get worse. Therefore, when certain symptoms of nearsightedness or farsightedness are observed, it is very important to get the affected eye checked.