Mamisoa ANDRIANTAFIKA, MD FEBO
Eye Specialist
Brussels, Belgium

Astigmatism

Left, original picture with Niagara Falls in the background. Right, picture seen by an astigmatic patient.

Vision of an astigmatic patient

Definition of astigmatism

Regular astigmatism in ophthalmology is optic aberration causing picture focalisation in two distinct points determined by two perpendicular horizontal and vertical planes.

More simply said,a deformation along an axis of one or more interfaces (cornea or lens) is approximated in two perpendicular planes of focalisation.

Therefore, the image has two focals which can be in front or behind the retina, defining different types of astigmatism: myopic, hyperopic or mixte.

One can see an image distorded along a certain axis. Depending of the type and intensity of astigmatism, the overall image will be blurry more sensibly in far or closer distance. Symptoms can be those of an hyperopic patient or a myopic patient added with the distorded image. But generally, they are less sensible to distance because the value of astigmatism is usually lower than hyperopia or myopia. Moreoften, it causes a uniformed blurry image. The eye will try to accommodate to find the clearer image, without success. This can cause visual fatigue and headache usually at the end of the day.

Generally, astigmatism is caused by an axial distortion of cornea and/or lens that can be compared to the distortion of a baseball ball into a football ball.

Correction of astigmatism

To correct regular astigmatism, it is necessary to move one plane to focus at the same point the other does, and then move them together with a spheric correction to make them focus on retina. To move the focus along one axis, we use a cylindric glass or toric lens .

If astigmatism is caused completly by the cornea, we can use a spheric semi-rigid contact lens. Irregularity due to the cornea will be corrected by the anterior side of the contact lens, and the irregular interface between lens and cornea will be completed by tears. The refraction of the image will then be perfect.

Finally, astigmatism can be corrected by surgery. We can reshape the cornea e.g with laser, insert a toric lens inside the eye or remplace the eye lens also by a toric lens.

Some points about astigmatism

Astigmatism can be irregular and cannot therefore be simplified by two perpendicular planes. Generally irregular astigmatism is caused by a disease of the cornea. A traumatism or corneal infection causing a scar can lead to a localized distortion of the cornea. A keratoconus, which is a corneal dystrophy (structural anomaly) usually causes an inferior bump of the cornea.

If interfaces keep being transparent, use of a semi-rigid contact lens can restore a better vision. Some surgery procedures depending of the severity of the pathology causing astigmatism can be performed, up to a corneal graft.