In optics, astigmatism is a lens or mirror defect in which the size and shape of an image vary for different points of focus. Light passing through different parts of an astigmatic lens, for example, is focused at different distances beyond the lens, so that the image of a point can appear variously as a short horizontal or vertical line or an ellipse. The best focus is a small circle known as the circle of least confusion.
Astigmatism in human vision
Astigmatism in human vision happens when the front part of the eye – the cornea – (or sometimes the eye lens itself) is not perfectly regular and symmetrical in shape. The astigmatism of the eye is a type of refractive error in which the eye does not focus light evenly on the retina. The light rays are not all focused at the same point. This happens because the eye does not have the same power to focus along all the meridians of the cornea, causing in the mildest cases a less sharpness of the images. This results in a distorted or blurred vision at all distances. If the cornea is more oval than round (like the back of a spoon), then light passing through it isn’t focused properly on the retina at the back of the eye. This results in a blurring of vision at all distances.
Astigmatism is a very common eye deficiency and generally easily treatable that depends on the shape of the cornea. Astigmatism worsens both distance and near vision and does not relate to the age of the patient as presbyopia does.
What are the causes of astigmatism?
Astigmatism can be caused by an alteration in the curvature of the cornea, which instead of having a normally spherical shape has an ellipsoidal profile. In this case, light rays from objects are projected unequally in various points of the retina. The astigmatic eye sees poorly both from far and near, objects may appear blurred but also doubled.
Treatment of astigmatism aims to compensate for the irregular curvature that causes blurred vision. It can be achieved with corrective lenses: glasses or contact lenses. The correct diagnosis and the best eyeglass prescription always comes from a thorough medical examination. Summary examinations often insufficiently correct this defect. Another method of correction is surgery. In the past, this defect was corrected by incisional surgery, such as radial keratotomy, which has now been superseded by the excimer laser.
What are the symptoms of astigmatism?
Depending on the type of astigmatism, the symptoms can be very different. Mild astigmatisms may be asymptomatic, or they may present with symptoms resulting from continually changing focus (accommodation) in an attempt to obtain the sharpest possible image.
Because objects are not focused equally in all meridians some parts of them are in focus, and others out of focus. For example an E may see the vertical line in focus and the horizontal lines out of focus and vice versa. The crystalline lens in some cases can partially compensate, putting in focus first the vertical lines and then the horizontal ones and let the brain to process them. In severe cases, objects may appear distorted in different ways: for example, a circle is perceived as an oval shape. The visual effort to compensate for the defect may cause headache, fatigue, burning and pain affecting the eyeballs and ciliary arch, tearing. High-grade astigmatism usually presents with blurred vision, in both distance and near vision.
Vision in astigmatism is nearly normal for mild degrees, while it is generally reduced for higher degrees and at no distance improves. Perceived objects may appear distorted, as if elongated, especially in severe astigmatism. Generally, distortion along the vertical direction is felt to be less annoying and interferes less with reading. Mild astigmatism induces asthenopic-like symptoms. To improve vision in the case of oblique astigmatism, some individuals keep their heads slightly tilted.