2. Causes of Astigmatism
1) General causes of Astigmatism
Astigmatism can arise from both the cornea and the crystalline lens, but most symptoms happen from the cornea. Distorted cornea shaped like a rugby ball, hinders the light rays from bending to a single focus point, and each light rays forms image in different location on the surface of the retina.
Usual astigmatism-caused image formations display a difference about 90 degree in light ray's direction. Such situation in which the longitudinal direction of minimum bending rate, and the maximum bending rate forms a right angle is called regular astigmatism. Most people have astigmatism of 0.5 to 1.25 D(Diopter). Astigmatism sometimes arises from surgeries in cornea, or crystalline lens, and also from cornea cicatrix(scars that remain on the cornea after minor damage to the cornea). These acquired astigmatism is called irregular astigmatism.
In short, Astigmatism can be classified as regular and irregular. Regular astigmatism in which two axes of distorted light rays form a right angle, refer to most of the conditions and is fairly easy to remedy. Irregular astigmatism's light rays form random degrees and happen from damages to the cornea. The irregular surface of the cornea generates random directions for incoming rays, and makes it harder people to alleviate irregular astigmatism symptoms.
As seen from above, since most astigmatism arises from issue with the cornea, total astigmatism rate almost equals the rate of astigmatism caused by irregular cornea. The subtraction of cornea astigmatism from the total astigmatism level is called the residual level of astigmatism. The residual level is mostly too low to cause a significant trouble with vision, but may sometimes exacerbate the vision correction through rigid-gas permeable lens(RPG). Most residual levels of asgimatism are assumed to be caused from crystalline lens.Although there are no clear causes discovered for astigmatism, the symptom can be inherited. Conditions such as blepharoptosis, in which the eyelid pushes on the eye and inner eyebrow keep touching the surface of the eye, can also cause certain levels of astigmatism.
Besides from naturally caused astigmatism, damages in cornea or surgical operations on the organ can also cause astigmatism. Pterygium can also cause astigmatism for elderlies.
2) Conical Cornea
Although it is very rare, Conical Cornea could also be one of the conditions that cause severe astigmatism.
Most astigmatism's conditions can be exacerbated along with myopia as perosn's eyeball grows. However, when a person's period of growth ends, the conditions also stops advancing to worse levels. If the astigmatism keeps on exacerbating, Conical Cornea could be one explanation.
Conical Cornea refers to a condition in which the central part of the cornea steadily keeps becoming thinner, and shapes the cornea into protruding conical shapes. The condition is reported for 50 to 230 people in every 100,000 people, with the frequency of reports increasing. However, the increased frequency of report does not necessarily indicate that Conical Cornea became more rampant, but rather indicated advanced technology of discovering patients with high risks of Conical Cornea during tests for bending rate remedies.
Conical Cornea often start from adolescence mostly in both eyes, but often display different progress rates in each eye. 15% of patients have been reported to have conical cornea in only one of their eyes.
Conical Cornea causes irregular astigmatism, as the cornea becomes thinner and starts protruding. The condition progresses for roughly 10 to 20 years and stops suddenly with different rate of exacerbation. Some sever cases require cornea transplants to remedy vision.
The causes of conical cornea is not well known. 6~8% are reported as inherited conditions, and since the condition accompanies collagen anomaly disease, mental retardation, and allergic symptoms such as atopic conjunctivitis, we assume that anomalies in collagens and constant physical contact to the eye causes such symptom.
If conical cornea keeps progressing, the patient's eye will display Fleischer's ring with light brown colors that is caused from pigmentation of iron substances, and various vertical lines will be found on inner cells of the cornea. Since the protruded part of the cornea's center usually inclines downward, the corneal topographic pattern test will display red protruded regions usually on the downward side of the cornea.
Conical Cornea causes blurry vision from astigmatism, blurry light, double images, stimulated and dazzling vision. The acknowledged cure is only corneal transplant, despite various new experiments such as riboflavin and plastic rings are introduced. However, the new methods are more focused on preventing exacerbation rather than curing the symptom.
More common method of correcting vision is using glasses, if not, RPG contacts. Vision correction using contacts can be classified as non-spherical contacts lenses and conical cornea-exclusive contact lenses.
As corneal correction surgeries advance, the conical cornea has been the center of attention. Patients who have conical cornea cannot undergo those surgeries such as LASIK or LASEK due to potential risks of corneal ectasia from excimer laser, and the constant progress of conical cornea itself.