Let's not lose sight of it: astigmatism

By Cristiano Antonino On Feb 18, 2023

Astigmatism is a condition in which there is a difference in the refractive power of the ocular diopter on the different axes, otherwise defined as meridians, and in most cases this anomaly is localized in the cornea causing visual problems

The main consequence that this condition entails is visual blurring (both near and far), if the two lines of focus are orthogonal to each other we speak of regular astigmatism, if they are not orthogonal but scissor, or discontinuous, or greater than two we speak of irregular astigmatism or high order astigmatic aberration.

The result of this asymmetry between the meridians is an ellipsoid shape, with the consequent formation of two focal points, as opposed to the sphere which has only one.

A person who does not have forms of astigmatism physiologically has a flatter horizontal corneal axis and a more curved vertical one and overall a spherical cornea: given this, when the rays of a light source pass through a normal lens, the resulting image on the retina will be punctiform since the rays will be conveyed into a single focus.

In the case of a person with more or less mild astigmatism, the cornea is instead characterized by a flattening along one of the meridians, giving the lens an elliptical shape characterized by the presence of two lines of focus.

The light rays in this case will not focus on a single point, but on lines of focus placed on different axes (called “focal lines”).

What is astigmatism and how to recognize it

Astigmatism is an ametropia, i.e. an anomaly of refraction of the eye that affects vision at any distance: the ocular system of those affected is not able to form a punctiform image of a punctiform object.

The term “astigmatism” can also be understood as that refractive condition in which there are variations in power in the different meridians of the eye.

This condition occurs when the cornea is more compressed along one of the meridians, defined as “of maximum power” and “of minimum power”.

Basically, a homocentric condition defines the sphericity of the cornea while astigmatism identifies the different radii of curvature of the meridians, which involve the refraction of the rays in equally different points.

An area is then formed within which the rays can penetrate, defined as Sturm’s Conoid.

Different types of astigmatism (such as simple, compound or mixed) are due to the position that the focal lines of the Sturm conoid occupy with respect to the retina.

However, “classic” astigmatism must be distinguished from astigmatism defined as “physiological”: in this case, the curvature of the cornea is wider along the vertical meridian but the asymmetry is compensated (also physiologically) by a modification of the globularity of the lens.

Astigmatism: the causes

Astigmatism is characterized by a strong hereditary component and usually does not appear before the age of two.

The most frequent form is the SR form (according to the rule), followed by the oblique form and the CR form (against the rule).

The diopter surfaces of the eye that lead to cases of astigmatism are the cornea and the lens.

As previously specified, in physiological cases (0.50 – 0.75 diopters) the two entities automatically compensate for the minimum asymmetries on the meridians.

The most frequent cases of astigmatism occur due to abnormalities on the outer surface of the cornea.

The causes that accompany corneal anomalies are only linked to various factors: the fundamental cause is due to corneal stiffness, but also the pressure exerted by the eyelid can lead to astigmatism.

As for the lens, this is usually associated with a modest astigmatism that oscillates between 0.50 D and 0.75 D CR.

Among the various causes of astigmatism, the main ones can therefore be summarized as follows:

high-grade astigmatism is usually congenital, and can undergo some changes in the course of life
Irregular astigmatism can arise as a result of trauma, injury or infection of the cornea
an incorrect posture of the head can lead to functional forms of oblique or against the rule astigmatism
in the years of presbyopia one may be subject to mild or against the rule astigmatism

Astigmatism: the symptoms

Each type of astigmatism has some common symptoms and others that are more specific.

For example, in mild astigmatism it is possible that the subject is asymptomatic or presents symptoms deriving from continuous accommodation (ie the persistent change of focus).

Vision in cases of mild astigmatism is almost normal, while it can also cause severe distortions in those suffering from more severe forms (greater than 1 D). In this case, patients could present a distorted vision of objects, even seeing them in an elongated form, and the disturbance will not decrease based on the distance from the subject.

Distortion along the vertical meridian is generally less annoying than distortion along the horizontal meridian.

In cases of oblique astigmatism, tilting the head seems to improve the disorder slightly.

Mild astigmatism often leads to asthenopic disorders, and therefore to a visual weakness due to excessive efforts deriving from continuous accommodation.

The symptoms common to all forms are:

eye fatigue
burning eyes
eye pain
tearing
heachache
vision reduction
double vision
blurred vision

The types of astigmatism

Astigmatism is classified differently depending on various factors.

Anatomical elements involved:

corneal astigmatism: abnormal curvature of the cornea
internal or lenticular astigmatism: alterations of the internal dioptric means of the eye (e.g. lens)

Orientation of the corneal meridian:

astigmatism according to rule or direct (more common): the curvature is greater along the vertical meridian or one close to it
against the rule or indirect astigmatism: the curvature is greater along the horizontal meridian or one close to it
oblique astigmatism

Position of the two foci in relation to the retina:

Myopic astigmatism:

simple: a focal line is short-sighted, i.e. it falls in front of the retina
compound: both focal lines are myopic, i.e. they fall in front of the retina

Hyperopic astigmatism:

simple: a focal line is farsighted, i.e. falls behind the retina
compound: both focal lines are farsighted, i.e. they fall behind the retina

Mixed astigmatism: condition in which one focal line is myopic, i.e. falls in front of the retina, and the other is hyperopic, i.e. falls behind the retina.

The degree of astigmatism is expressed in diopters:

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weak/mild astigmatism: 0 to 1 diopter
Medium astigmatism: 1 to 2 diopters
strong/high astigmatism: greater than 2 diopters

How is astigmatism diagnosed

The diagnosis must be made by an ophthalmologist, who can use various tests to highlight the presence and determine astigmatism.

These tests are:

Snellen chart: quantifies the visual acuity reduced by astigmatism
keratometer or ophthalmometer: allows you to measure the anterior curvature of the cornea in its central area
corneal topography: it allows to obtain the point-by-point mapping of the curvature of the cornea
autorefractometer or schiascope: they allow to obtain an objective measurement (without the patient’s collaboration) of the astigmatic component
subjective refraction test: it allows to obtain the correct measurement of astigmatism with the collaboration of the patient

Astigmatism: the most effective treatments

As for the diagnosis, also in this case the treatment must be personalized by the ophthalmologist.

Astigmatism can be corrected with cylindrical or toric-shaped ophthalmic lenses, as well as with gas-permeable or soft contact lenses.

Through refractive surgery, with the use of the laser, the evolution of the disorder and stage I and II keratoconus can be blocked.

However, some considerations are necessary, such as:

the effect on the retinal image
the distortion of the perceived image
the age of the person and his propensity for lenses
previous habits
the extent of the astigmatic condition
previous offset changes regarding axis and power

The coexistence of further ametropias, the degree of astigmatism and all the factors indicated above contribute to determining the best solution.

Historically, astigmatism was discovered more recently than the more widespread myopia, which is why contact lenses and special technologies for refractive surgery only appeared in the late 1990s.

Astigmatism: how to prevent it and effects on daily life

The most recent discoveries have shown how correct posture of the head and spine can prevent the onset of the disorder in subjects who are not congenitally predisposed.

In all other cases in which astigmatism is congenital or due to trauma, injury or infection, unfortunately there is no real way to prevent it.

In a mild form, the disorder does not have a significant impact on daily life also because many of the subjects affected by the disorder present asymptomatic.

In the more severe forms, the use of lenses greatly facilitates living with the disorder in a non-invasive way: it is therefore a solution recommended for people with astigmatism only and in any case of a modest degree.

For all subjects in which the problem is present in conjunction with other disorders such as myopia, hyperopia and presbyopia, or if the astigmatism is of a high degree, then it is possible to consider more invasive solutions such as refractive surgery.

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