Astigmatism

The focusing of images is done by the surface of the cornea and also by the lens, the transparent lens placed behind the iris. Both contribute to focusing the image on the retina. A normal cornea has a symmetrically curved surface. When the cornea is not symmetrical, we speak of astigmatism.

Astigmatism can run in families and is often associated with other refractive defects, hyperopia and myopia. Generally astigmatism is a refractive defect that is born with us and is also stable throughout life or changes little.
It can, however, increase or manifest itself in older age due to the pressure of the upper eyelid on the eye, which becomes a little droopier.

When the cornea and lens are perfectly spherical in shape, all the refracted light is focused and forms a clear image on the retina (macula).
If however, the surfaces are not symmetrical, the image is blurred, in which case we speak of astigmatism.
Astigmatism can be regular, the cornea instead of having a spherical shape has an oval shape.

Types of astigmatism:

• Corneal, if the asymmetry of curvature is in the cornea;
• Lenticular, if it's in the lens.

Usually both exist concomitantly, with corneal astigmatism always being the one we evaluate. When the astigmatism is regular, only the usual measurement (keratometry with corneal topography) is sufficient for a prescription for spectacles or contact lenses.

When surgery is involved, it is also important to associate aberrometry, which allows the study of the wavefront and the distortions suffered by the light entering the eye and forming the image on the retina, associated with corneal and lenticular astigmatism.
This will allow a personalised correction, introduced in the laser algorithm, which will improve the quality of vision.

What happens with astigmatism is that the image becomes partially blurred, creating a blurry zone and a focused zone. This distorts the image, which becomes blurred with shadows or even duplicated.

The symptoms are generally discomfort and visual fatigue (asthenopia) and headache (cephalea). Children, in whom it goes unnoticed and who have never seen otherwise, are more prone to *such symptoms. Pre-school screening is very important so that refractive and other problems with significant repercussions on their development and learning can be identified.

In irregular astigmatism the cornea has various deformations, making vision very blurred. This is the case with keratoconus, a congenital corneal disease, mostly bilateral, that manifests itself in adolescence or later until the fourth decade of life, with a variable incidence of about 1:2000. About 5 to 8% of cases are familial. It may be isolated or associated with connective tissue diseases.

Corneal topography, in addition to clinical signs, is fundamental for the diagnosis and its characterisation. Also the diagnosis of subclinical keratoconus is of extreme importance in cases of candidates for refractive surgery.

Initially it is possible, with special (rigid) contact lenses, to maintain satisfactory visual acuity. The conical shape of the cornea makes adaptation very difficult. The evolution of the corneal cone deforms its shape more and more and consequently distorts the images. Surgery may then be necessary to improve the patient's clinical situation.

In some cases, it is possible to perform a less invasive surgery by inserting one or two corneal rings (rigid rings made of biocompatible material) in the thickness of the cornea, which can, at least partially, control the very high and irregular astigmatism resulting from the deformation and stabilise the cornea. Subsequently, corneal crosslinking is sometimes used. This technique consists of applying a drug (riboflavin) to the surface of the cornea. Light is then shone onto the cornea for a period of time,
ultraviolet light is then applied for a certain period of time. The reaction makes the corneal collagen stiffen and stabilises the deformation.

When the thickness of the cornea is very thin and severely deformed, transplant surgery is necessary not only to improve vision but also to avoid the risk of rupture due to the extreme thinness of the corneal tissue.

There are other degenerative corneal diseases, but they are much rarer.
Another situation similar to keratoconus is corneal ectasia, a possible complication of refractive surgery or laser surgery. It is similar in all aspects, including treatment. It is a rare complication nowadays. It results from previous weakness of the collagen fibres of the cornea. By removing tissue to correct ametropia, the whole structure is weakened. Over time and due to intraocular pressure the cornea deforms and vision deteriorates.

The study prior to the excimer laser surgery is extremely important in order to avoid situations that will manifest themselves later with visual losses that can sometimes be accentuated.
Corneal topography is important in these cases, as it allows the curvature of the cornea and its irregularities to be evaluated in a single examination, and therefore regular or irregular astigmatism. It rules out the existence of subclinical keratoconus, a case that prevents LASIK surgery. The topography also allows evaluating the aberrations, the thickness of the cornea and the depth of the anterior chamber, which is important to evaluate the possibility of implanting an intraocular lens, if necessary.

In short, astigmatism can be corrected, whether regular or irregular, using the various techniques available to us today.

Correction in these cases can be done with glasses. However, if the defect is high and unilateral, it is sometimes difficult to bear the difference in graduation between the two eyes (anisometropia). Toric contact lenses can be used to correct astigmatism.

Refractive surgery by LASIK is another possibility. As mentioned above, astigmatism is often associated with myopia and hyperopia. The latter are the usual reason for consultation for this type of surgery. Therefore the concomitant astigmatism will also have to be resolved.

A high degree of astigmatism associated with myopia may invalidate laser surgery. It is always possible in this case to propose a phakic lens surgery, using a toric lens. We give this name to lenses for the correction of astigmatism, whether they are spectacles, contact or intraocular.