At least a third of people have some degree of astigmatism.
HEALTH Often occurring in conjunction with other vision defects like myopia or hyperopia, astigmatism is a refractive error caused by an irregularly shaped cornea or lens (lenticular). Instead of being perfectly spherical, the eye is more oval. Light is refracted or focused on multiple points, resulting in blurry vision at any distance. 
Astigmatism may be hereditary since it is often present at birth. Prevalence of astigmatism can increase with age and result from eye injury or surgery. In optometry and ophthalmology, the vertical (tangential) and horizontal (sagittal) planes or meridians interfere with distinguishing fine details.  The difficulty with reading can be understood by the description of an optometrist: It's as if the horizontal and vertical lines are at two different distances. For this reason, some sufferers rotate or angle reading material in an attempt to better bring variable planes into focus.
Horizontal or Vertical?
"Sagittal" actually means "vertical." But it refers to horizontal astigmatism since vertical planes are out of focus. Your optometrist can perform a thorough examination and provide a prescription to correct vision. If you are reading this on a desktop computer, there is a quick test to see if you have either tangential or sagittal astigmatism:
- Without corrective lenses, look at the URI in your browser window. It's the field with the web link that begins: "http://..."
- Focusing only on the first two slashes, get close enough to the display so that they are sharp. Then slowly move your head back until they blur or double.
- Rotate your head to the left so that your eyes are nearly parallel to the lines. If slashes sharpen, it suggests tangential astigmatism.
- Now rotate your head to the right about the same angle as the slashes and begin moving your head back a little. If the lines become sharper, you may have sagittal astigmatism. If the number of slashes multiplies, it supports the prior tangential conclusion.
A catalyst for astigmatism can be a rare condition called keratoconus. This progressive thinning of the cornea results in severe astigmatism resulting in poor vision that cannot be clearly corrected with spectacles. Some keratoconus improvement is possible with contact lenses but may eventually require cornea transplantation. Simple astigmatism is often corrected with glasses, contacts or refractive surgery.
Various vision related anatomical posters are available at Store.ClincialPosters.com. The more artistic Astigmatism poster depicted on this page is available in two sizes and in discounted bulk quantities. For futher information about astigmatism online, visit Astigmatism Guide with nearly three dozen online articles. 
- Prevalence of refractive error in Bangladeshi adults. ophsource.org
- Astigmatism. aoa.org
- Astigmatism (eye). wikipedia.org
- Astigmatism Guide. .astigmatismguide.co.uk