Do I have Astigmatism?
As we start off the New Year we will tackle one of the most misunderstood topics in vision care….astigmatism. If we break down the meaning of the word, the “a-” at the beginning means “without” and “-stigma” means “point.” Astigmatism, therefore, is a refractive error that keeps light from coming into focus at a single point on the retina (the retina is a very important structure inside the eye that acts like a projector screen). So, to follow the analogy, having astigmatism means the “projector” is out of focus. In most cases, astigmatism is not related to eye disease or eye health and can be treated with the same methods as myopia (nearsightedness) and hyperopia (farsightedness).
There are two causes of astigmatism: corneal and lenticular:
The cornea is the clear dome-shaped structure at the front of the eye. When light enters the eye, the cornea is the very first surface in our eyes to focus or bend the light. Corneal astigmatism occurs when the cornea is not a perfectly spherical shape, and therefore, the light is bent unevenly. After the light passes through the cornea, it is also focused by a second structure -the crystalline lens (natural lens inside the eye, which we are born with). Lenticular astigmatism is a result of an irregular shape of the crystalline lens, which can also cause the light to be focused unevenly onto the retina.
For now, let us focus our discussion on the more common cause of unfocused or blurry vision – corneal astigmatism. When the cornea is spherical in shape – like the shape of a basketball – it will cause light that enters the eye through any part of the cornea to be focused evenly, leading to a single point on the retina. If the corneal curvature is not spherical, however, light is focused unevenly, depending on where it passes through the cornea. A cornea with astigmatism is curved more like a football than a basketball – has a longer (or flatter) curve from top to bottom and a shorter (or steeper) curve from side to side. Most corneas are actually similar to this shape, with a steeper shape in one direction and a flatter shape in another direction. The amount of difference between the curvatures of these two directions is called the cylinder power (how much astigmatism a person has). The orientation of the curves is called the axis. This orientation can be recorded like clock hours or a degrees marker on a compass, with the 3 o’clock equivalent to 0 degrees, 9 o’clock equivalent to 180 degrees, and 12 o’clock position at 90 degrees. For instance, if the football is placed in an up and down position, at 12’o’clock as described above, the axis is considered 90 degrees. If the football is tilted halfway between 12 o’clock and 3 o’clock, the axis would be 45 degrees.
Symptoms of astigmatism may include blurred or distorted vision and will likely affect vision at all ranges (distance and near). It can also cause eyestrain and headaches in some patients. The degree of symptoms is directly related to the difference in curvature or the amount of astigmatism.
Classification: Regular Astigmatism vs. Irregular Astigmatism
Regular astigmatism is common and occurs when the steep corneal axis and flat corneal axis are oriented exactly 90 degrees away from each other. This type of astigmatism can simply be corrected with glasses or contact lenses. Irregular astigmatism is less common and more difficult to treat. It usually is the result of a dry ocular surface, trauma, scarring, corneal transplants, or eye diseases such as keratoconus or pellucid marginal degeneration.
3 Types of Astigmatism
Astigmatism can be combined with other refractive errors – either myopia (nearsightedness) or hyperopia (farsightedness). An individual can have both nearsightedness and astigmatism…we call this myopic astigmatism. In myopic astigmatism, the curvature in both orientations cause the light to focus in front of the retina. Likewise if someone has farsightedness and astigmatism it is called hyperopic astigmatism and the focal points are behind the retina. If the light is focused both in front and behind the retina it is called mixed astigmatism.
Now that we have working knowledge of astigmatism, we will continue this discussion in next month with a look at how astigmatism is diagnosed and treated.
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