Astigmatism

What is astigmatism?

In astigmatism, unlike what happens in myopia, hypermetropia and presbyopia, the curvature of the cornea is not equal in all its points, called meridians, but varies, so that the cornea is not perfectly spherical; consequently, the power of the ocular diopter is not the same on all meridians. The result is vision problems (visual blurring) of objects observed both from near and far.

There is an astigmatism that is defined as physiological, of about 0.50 dioptres, 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 modification, also physiological, of the globularity of the crystalline 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 instead the two curvatures (vertical and horizontal meridian) become markedly different, the ocular system will have two focuses, placed on two different planes, and the light rays will therefore be in focus not on a point but on two perpendicular lines. at the other, called focal lines . This is astigmatism.

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, on the other hand, the meridians have different curvature radii and the refracted rays converge in equally different points.

Astigmatism is said to be regular 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 him, astigmatism is called the second rule, if it is the opposite, it is called the counter rule .

Astigmatisms 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 hypermetropic, so a focal line falls in front of the retina and the other behind
• Simple hypermetropic astigmatism: one meridian is emmetropie and the other hypermetropic, so a focal line falls on the retina and the other behind
• Compound hypermetropic astigmatism: both main meridians are hypermetropic and the two focal lines fall behind the retina.

Causes

The causes of astigmatism are related to deformation of the cornea or deformation of the lens.

The accommodation in the astigmatic cannot modify the extent of the vice, but can only place one of the two focal lines on the retina.

Symptoms

To learn more: Astigmatism symptoms

The symptoms of astigmatism consist in a loss of sight both from far and near. The association with accommodative asthenias (headache, eye burns, watery eyes, blurred vision, conjunctivitis, eye pain following prolonged close work) is frequent.

Even the symptomatology does not concern both visual acuity and the appearance of manifestations forming part of accommodative asthenopia.

Correction of astigmatism

It is implemented through the use of cylindrical lenses called toric lenses, both in the form of glasses and contact lenses, and which have the ability to modify the power of only one of the meridians, the pathological one, leaving all the others intact. The use of contact lenses and surgical laser treatment is also possible.