What is Astigmatism?
In very simple terms, it means that as light enters the eye and is focused on the retina, the different portions of the eye that focus the light are not round or spherical. Therefore, the light is not focused to a clear sharp point but is focused to a linear pattern on the retina. People with astigmatism who look at lights at night without wearing any correction will see a streak coming from or around the light.
What Causes Astigmatism?
Astigmatism has a genetic component which plays a significant role in the causation of astigmatism. Astigmatism can occur from ocular surgery, ocular injury, corneal infection, growth of a pterygium into the pupil area, a variety of corneal diseases, and changes as we age. Most astigmatism is related to the shape of the cornea (clear portion of the eye that first focuses light into the eye). The cornea is shaped more like a football versus a basketball. Astigmatism can also be due to changes in how the lens focuses light onto the retina and changes in the contour of the retina itself.
Fluctuation of Vision from Astigmatism
Blurred vision from astigmatism that is not corrected with glasses or contact lenses can fluctuate significantly. The most important factor in changing vision has to do with the size of the pupil. In bright light with a small pupil, the vision may be reasonably good. In dim light with a larger pupil, the vision will be more blurred. Other causes for fluctuating vision from astigmatism is changes in the tear film, body position, accommodation, lid position, and the effects of the extraocular muscles on the eye.
Changes in Astigmatism as We Age
When we are younger, the astigmatism and steeper side of the cornea is more commonly located at ninety degrees. As we age, the astigmatism changes and is more commonly located at one hundred eighty degrees. It has been theorized that the constant pressure and blinking of the upper eyelid over the years causes the steeper side of the cornea to be located horizontally at one hundred eighty degrees.
Diurnal Changes in Corneal Shape
The amount of astigmatism and axis (direction of astigmatism in degrees) varies during the day. There is evidence that the cornea flattens while sleeping and gradually becomes steeper through the day.
People with jobs that require a lot of reading or near work may have changes in the shape of their eyes. The pressure of the lower lid on the eye over time can change the shape of the cornea.
People with thicker and tighter eyelids tend to have more astigmatism due to the pressure of the eyelids on the cornea.
As it was discussed earlier, the pupil size can affect the vision. The pupil works like the aperture in a camera, small size provides sharper focus.
Accommodation or Focusing at Near
As the lens inside the eye changes its shape to focus at near, it may not change its shape in a spherical way and therefore induce astigmatism.
In between blinking, the tear film on the surface of the cornea gradually becomes thicker inferiorly due to gravity. This can cause a change in the manner in which light is first focused on the retina.
There are a variety of factors that can affect astigmatism. In general the astigmatism and the steeper side of the cornea tends to be at ninety degrees when we are young. As we age, the horizontal cornea becomes steeper and gradually increases over time. Astigmatism can be corrected with glasses, contact lenses, or surgery.