Astigmatism

Astigmatism is a type of refractive error in which the eye does not focus light evenly on the retina. This results in distorted or blurred vision at all distances. Other symptoms can include eyestrain, headaches, and trouble driving at night. If it occurs early in life it can result in amblyopia. The cause of astigmatism is unclear. It is believed to be partly related to genetic factors. The underlying mechanism involves an irregular curvature of the cornea or abnormalities in the lens of the eye. Diagnosis is by an eye exam. Three options exist for the treatment: glasses, contact lenses, and surgery. Glasses are the simplest. Contact lenses can provide a wider field of vision. Refractive surgery permanently changes the shape of the eye. In Europe and Asia astigmatism affects between 30 and 60% of adults. People of all ages can be affected. Astigmatism was first reported by Thomas Young in 1801. Although astigmatism may be asymptomatic, higher degrees of astigmatism may cause symptoms such as blurry vision, double vision, squinting, eye strain, fatigue, or headaches. Some research has pointed to the link between astigmatism and higher prevalence of migraine headaches. In with-the-rule astigmatism, the eye has too much 'plus' cylinder in the horizontal axis relative to the vertical axis (i.e., the eye is too 'steep' along the vertical meridian relative to the horizontal meridian). Vertical beams of light focus in front (anterior) to horizontal beams of light, in the eye. This problem may be corrected using spectacles which have a 'minus' cylinder placed on this horizontal axis. The effect of this will be that when a vertical beam of light in the distance travels towards the eye, the 'minus' cylinder (which is placed with its axis lying horizontally – in line with the patient's excessively steep horizontal axis/vertical meridian) will cause this vertical beam of light to slightly 'diverge', or 'spread out vertically', before it reaches the eye. This compensates for the fact that the patient's eye converges light more powerfully in the vertical meridian than the horizontal meridian. Hopefully, after this, the eye will focus all light on the same location at the retina, and the patient's vision will be less blurred. In against-the-rule astigmatism, a plus cylinder is added in the horizontal axis (or a minus cylinder in the vertical axis). Axis is always recorded as an angle in degrees, between 0 and 180 degrees in a counter-clockwise direction. Both 0 and 180 degrees lie on a horizontal line at the level of the center of the pupil, and as seen by an observer, 0 lies on the right of both the eyes.