by
Mamisoa Andriantafika
Astigmatism is extremely common. Yet when a patient reads "cylinder -1.25 at 90°" on their prescription, they rarely understand what it actually means. Astigmatism is often described by saying that the eye is "shaped like a rugby ball rather than a football". The image is easy to picture, but it does not really explain the defect.
Astigmatism is above all an optical problem: the power of the eye is not the same in every direction. Light therefore does not converge in the same way depending on its orientation, and the image of a point no longer forms a perfect point on the retina. That is precisely where the word "astigmatism" comes from. In this first part, we will look at the defect itself: its origin, its symptoms and its different forms. The second part, to be published on 24 August 2026, will cover the ways of correcting it.
The word comes from Greek. The prefix a- means "without". The word stigma means "point", "mark" or "spot". Astigmatism can therefore be roughly translated as "absence of a point".
The choice of word is particularly apt from an optical point of view. In a normal optical system, rays coming from a point should ideally converge towards another point. In an astigmatic system, they do not all converge at the same place.
The history of this defect is interesting. Thomas Young had already described the phenomenon at the beginning of the 19th century. The British astronomer George Biddell Airy then studied his own astigmatism and, in 1825, described its correction using a cylindrical lens. The term "astigmatism" was suggested to him a few years later by William Whewell, a British scientist known for coining several scientific terms. The first printed uses of the word appear in the mid-19th century.
Astigmatism literally means "absence of a point": the image of a point no longer forms a point.
An astigmatic eye does not have a single focal point, but two principal focal points at different distances.
To form a sharp image, the eye must focus light onto the retina. The cornea provides a large part of the eye's optical power. The crystalline lens, located behind the iris, completes this focusing.
In an eye without significant astigmatism, the optical power is approximately the same whichever meridian you examine. In an astigmatic eye, it varies with direction. Imagine, for example, that the cornea is slightly more curved vertically than horizontally. It will not focus vertical lines and horizontal lines in the same way.
In the classic case of regular astigmatism, two principal perpendicular meridians can be defined: one meridian with the strongest optical power, and another, at 90° to it, with the weakest.
Instead of a single focal point, there are then two principal focal points at different distances. Between the two lies a zone called Sturm's conoid. You obviously do not need to know about Sturm's conoid to wear your glasses. This representation does, however, explain why astigmatism can give an impression of blur, doubling or stretched outlines.
The symptoms depend on the degree of astigmatism, its axis, the patient's age and any other associated optical defects. Mild astigmatism can be perfectly well tolerated. More marked astigmatism, on the other hand, can show itself in several ways.
| Sign | How it shows itself |
|---|---|
| Blurred vision | At distance and near alike, depending on the axis and power of the defect |
| Imprecise outlines | An impression of doubling or of a "shadow" around letters |
| Visual fatigue | After prolonged reading or a day in front of screens |
| Difficulty with small print | Confusion between letters or numbers of similar shape |
| Headaches | Sometimes, during prolonged visual effort |
In children, significant uncorrected astigmatism can encourage amblyopia.
In children, significant uncorrected astigmatism deserves particular attention. An image that remains blurred throughout the period of visual development can encourage amblyopia, in other words an eye that never learns to see sharply. This is one of the reasons why an ophthalmological examination, supplemented if necessary by an orthoptic and neurovisual assessment, is so valuable in children. Astigmatism is one of the common refractive errors that can be corrected with glasses, contact lenses or, in some adults, surgery.
The axis on your prescription does not indicate where the eye is looking. It describes the orientation of the cylinder that corrects the defect.
Yes. It is in fact very common. Myopia, hyperopia and astigmatism describe different properties of the optical system. An eye can therefore be myopic and astigmatic, hyperopic and astigmatic, or astigmatic without any significant myopia or hyperopia.
That is what a glasses prescription with a sphere, a cylinder and an axis expresses. Take an example: -2.00 (-1.00 at 90°). Here the value -2.00 corresponds mainly to the myopia. The cylinder of -1.00 corresponds to the amount of astigmatism being corrected, and 90° indicates its orientation.
The axis therefore does not mean that the eye "looks at 90°". It simply describes the orientation of the cylinder needed to correct the optical defect.
Knowing how to read these three figures changes the way you look at your prescription. The sphere tells the story of the myopia or hyperopia. The cylinder and the axis tell the story of the astigmatism. Two prescriptions with the same "total" can therefore describe very different eyes.
This distinction matters far more than it seems. In particular, it determines whether the astigmatism can easily be corrected with glasses.
In regular astigmatism, the two principal meridians are approximately perpendicular. The optical defect can then generally be described by a relatively simple combination of sphere, cylinder and axis. This is the astigmatism usually corrected with glasses or toric contact lenses. Several orientations are distinguished.
| Form | Steepest meridian | Particular feature |
|---|---|---|
| With-the-rule (direct) | Vertical | A common configuration in young people |
| Against-the-rule (inverse) | Horizontal | Its proportion tends to increase with age |
| Oblique | Oblique orientations | Same optical principles, intermediate axes |
These three forms remain regular astigmatisms as long as the optical geometry stays sufficiently regular and the two principal meridians are well defined.
Regular or irregular: this is the distinction that decides whether glasses will be enough.
Faced with an irregular cornea, endlessly changing the cylinder in the glasses does not solve the problem.
The situation is different when the power of the cornea varies in a complex way within a single meridian, or when the geometry of the cornea becomes asymmetrical. A simple sphero-cylindrical correction is then no longer enough to reproduce this optical surface correctly. This is known as irregular astigmatism.
| Possible cause | Mechanism |
|---|---|
| Keratoconus | The cornea progressively thins and deforms |
| Other corneal ectasia | Progressive deformation of a weakened cornea |
| Corneal scar | An opaque or distorted area disturbs the surface |
| Corneal dystrophies | Certain diseases alter the regularity of the cornea |
| Trauma | A wound or residual deformation |
| Large pterygium | The membrane growing over the cornea distorts it |
| Previous corneal surgery | A graft, incisions or laser can leave an irregular surface |
| Very irregular ocular surface | An unstable tear film degrades the first optical surface |
Keratoconus is a classic example: the cornea progressively thins and deforms, which causes both myopia and increasingly irregular astigmatism. In this context, continually changing the cylinder in the glasses does not necessarily solve the problem. You need to understand why the cornea is irregular. Corneal topography or tomography may then be necessary.
Classic keratometry essentially measures the curvature of certain zones of the cornea. Corneal topography goes much further: it produces a genuine map of the corneal surface.
Tomography also makes it possible to study the geometry in three dimensions. It analyses in particular the anterior and posterior surfaces of the cornea, as well as its pachymetry, in other words its thickness. We carry out this examination with the Anterion, an anterior segment optical coherence tomographer, which gathers these different measurements during a single examination.
These examinations play a particularly important role when keratoconus, irregular astigmatism or ectasia is suspected, and when refractive surgery is being considered. They make it possible to understand the structure of the optical defect rather than simply measuring the cylinder needed in front of the eye.
Topography does not measure a single figure: it draws the map of your cornea.
On a topographic map, regular astigmatism draws a symmetrical "bow tie". A keratoconus, by contrast, draws a localised, asymmetrical area of steepening. It is this difference in pattern, far more than the cylinder value, that guides management.
The cylinder in your glasses does not tell the whole story of your cornea: the crystalline lens can reinforce or compensate for corneal astigmatism.
This is probably one of the most important notions to understand. When a prescription reads, for example, -1.50 D of cylinder at 100°, it describes the overall optical result of the eye. It does not necessarily mean that the cornea has exactly 1.50 dioptres of astigmatism at 100°. Why? Because the cornea is not the only optical element of the eye.
Corneal astigmatism comes from the shape of the cornea. It has two components of its own: the anterior surface and the posterior surface. The anterior surface plays the main role, but the posterior surface is not optically neutral. Modern tomography devices, such as the Anterion, measure both surfaces to estimate the total corneal astigmatism.
The crystalline lens can also produce astigmatism: this is internal astigmatism. Its effect can reinforce the corneal astigmatism, but it can also partially compensate for it. This is one of the reasons why the cylinder measured during refraction and the cylinder measured on the cornea can differ.
Refractive astigmatism, or total ocular astigmatism, is ultimately what is observed when the whole optical system works together: cornea, crystalline lens and the other optical components of the eye. It is mainly this result that is measured during refraction to determine a glasses correction.
It is therefore perfectly possible to have 1.50 D of corneal astigmatism but only 0.75 D of cylinder in the glasses. The reverse is also possible. And since astigmatism has both a power and an axis, you cannot simply subtract two figures: precise comparisons require vector calculations. This distinction becomes particularly important when planning refractive surgery or a toric intraocular lens.
Let us recap. Astigmatism is not a disease but a geometry: an optical power that varies with direction. It can be regular or irregular, corneal or internal, isolated or combined with myopia or hyperopia. Two patients each with "2 dioptres of astigmatism" can in reality present completely different situations.
That is precisely why the next question has no single answer: do you need glasses, toric contact lenses, laser surgery, an ICL implant or a toric intraocular lens during cataract surgery? The choice depends on everything we have just seen: the regularity of the cornea, the origin of the cylinder and the overall refractive situation.
That is the subject of the second part of this article, to be published on Monday 24 August 2026: from glasses to scleral lenses, from LASIK to toric intraocular lenses, we will review all the ways of correcting astigmatism, and above all how to choose the right one.
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