Astigmatism: What It Is and Correction

it is not very clear and distorted both from a distance and up close.This happens due to the fact that the images are focused on two different planes of the retina, one anterior and one posterior, rather than converging at the same point. Astigmatism is caused by a defect in the curvature of the cornea and, less often, by the irregularity of the lens.

Astigmatism: what is it all about?

In "astigmatism - unlike what happens in myopia," hyperopia and presbyopia - the curvature of the cornea is not the same in all its points (called meridians), but varies.

Shutterstock

This variation means that the cornea is not perfectly spherical; consequently, the power of the ocular diopter is not the same on all meridians. From all this derive vision problems (visual blurring) of objects observed both from near and from afar.

It should be noted, however, that there is an astigmatism that is defined as physiological - of about 0.50 diopters - due to the fact that in the normal subject the curvature of the cornea on the vertical meridian is slightly wider than that on the horizontal meridian. This physiological astigmatism is compensated by a change, also physiological, in the globosity of the lens.

If we consider a light source whose rays pass through a normal positive spherical lens, which has an equal power along all the meridians, the image that will form on the retina will be point-like. When, on the other hand, the two curvatures (vertical and horizontal meridian) are made markedly different, the ocular system will have two foci, placed on two different planes, and the light rays will therefore focus not on a point but on two lines perpendicular to each other, called focal lines. This is astigmatism .

Shutterstock

Therefore, in a system that is called homocentric, the meridians all have the same radius of curvature (neglecting physiological astigmatism) and the refracted rays converge in a single point. In an astigmatic system, however, the meridians have different radii of curvature and the refracted rays converge at equally different points.
Astigmatism is said:

Adjust if along the same meridian the curvature is always the same;
Irregular if the curvature changes.

If the most curved meridian is the vertical one or one very close to it, astigmatism is called second rule; if it is the opposite, it is called against the rule.

Classification

Astigmatism can be classified according to the position that the two focal lines have with respect to the retina:

Compound myopic astigmatism: Both meridians are myopic, so the two focal lines fall in front of the retina.
Simple myopic astigmatism: one meridian is emmetropie and the other is myopic, so one focal line falls on the retina and the other in front of it.
Mixed astigmatism: the two main meridians are one myopic and the other farsighted, whereby one focal line falls in front of the retina and the other behind.
Simple hypermetropic astigmatism: one meridian is emmetropie and the other is hypermetropic, whereby one focal line falls on the retina and the other behind.
Compound hypermetropic astigmatism: Both main meridians are farsighted and the two focal lines fall behind the retina.

they consist of a decrease in vision both from far and near. In particular, visual sharpness is affected: astigmatic subjects experience, in fact, an annoying blurring of what is observed.

Astigmatism: associated symptoms and disorders

Frequent is the association with accommodative asthenopia (headache, ocular burns, lacrimation, blurred vision, conjunctivitis, eye pain following prolonged close work).

For further information: Astigmatism Symptoms , and which have the ability to modify the power of only one of the meridians - the pathological one - leaving all the others intact. Surgical treatment by laser is also possible, of course, after consultation with the specialist doctor. For severe astigmatism, implantation of toric intraocular lenses can be used.