Eye muscles is not realted to astigmatic, instead of that, the irregular shape of cornea or lens can cause astigmatic. 1. If the cornea does not curve perfectly - if one half is flatter or steeper than the other - the light that hits it will not refract properly and the retina at the back of the eye will receive an imperfect image. The person will have blurred vision from that eye - astigmatism.2. The lens has variations in its curvature, rather than having a perfect curve, causing images to reach the back of the eye (retina) imperfectly. Most patients with lenticular astigmatism have a normally shaped cornea - the defect is only in the curvature of the lens. Source:http://www.medicalnewstoday.com/articles/158810.php.
Astigmatism is very common. Some experts believe that almost everyone
has a degree of astigmatism, often from birth, which may remain the same
throughout life. It is said that the tendency to develop astigmatism
(not astigmatism in itself) is inherited and that therefore some people are more prone to develop astigmatism than others.
Some cases of astigmatism are caused by problems in the lens of the
eye. Minor variations in the curvature of the lens can produce minor
degrees of astigmatism (lenticular astigmatism).
In these patients, the cornea is usually normal in shape. Infants, as a
group, have the least amount of astigmatism. Astigmatism may increase
during childhood, as the eye is developing. Source: http://www.eyerobics.com.au/astigmatism.html
Astigmatic is caused by the shape change of cornea.Usually the shape or cornea is in smooth hemisphere and can focus the lights that come into our eyes so that it can reach the retina.But when the shape of cornea changes,the lights can't be focused and dispersed in all sides,we can't see things clearly,thus the astigmatism develops.