by
Mamisoa Andriantafika
In the first part of this article, we saw what astigmatism really is: an eye whose optical power varies with direction, with two focal points instead of one. We also saw that it can be regular or irregular, and that it comes from the cornea, from the crystalline lens, or from both at once.
That leaves the question every patient cares about: how is it corrected? There is not one single method but a whole range of solutions, from simple glasses to intraocular lenses. Here is how to make sense of them.
The treatment depends in particular on the degree of astigmatism, on whether it is regular or irregular, on any associated myopia or hyperopia, on age, on the state of the cornea and the crystalline lens, and finally on the patient's visual expectations.
| Method | Principle | Typical situation |
|---|---|---|
| Glasses | Sphero-cylindrical lens | Regular astigmatism, any age |
| Soft toric contact lenses | Lens with a stabilised cylinder and axis | Significant regular astigmatism |
| Rigid and scleral lenses | A new regular optical surface in front of the cornea | Irregular astigmatism, keratoconus |
| LASIK / PRK | Laser reshaping of the cornea | Regular astigmatism, healthy cornea, stable adult |
| Toric ICL | Implant in front of the preserved crystalline lens | High myopia with astigmatism |
| Toric IOL (cataract, PRELEX) | The crystalline lens is replaced by a corrective implant | Cataract, or presbyopia in selected indications |
The right treatment does not depend only on the number of dioptres, but on the origin and geometry of the defect.
A cylindrical lens provides a different power along each of its meridians, exactly the opposite of the eye's defect.
Glasses are the simplest solution. A cylindrical or sphero-cylindrical lens provides a different power along its meridians, and so compensates for the eye's astigmatism. They generally work very well for regular astigmatism.
One point is worth knowing. When significant astigmatism is corrected for the first time, or when its axis changes considerably, a short period of adaptation may be needed. Some people temporarily perceive distorted lines or an altered sense of distances. This phenomenon is normal and usually fades within a few days.
Classic soft contact lenses can correct myopia or hyperopia. To correct significant astigmatism, they generally need to be toric: they then have a cylindrical power and an axis.
The difficulty is that this axis must stay correctly orientated on the eye. Toric lenses are therefore designed to settle into a set position. They are an excellent solution for many regular astigmatisms.
For some irregular astigmatisms, particularly those linked to keratoconus, glasses become insufficient. A rigid gas permeable lens can then create a new regular optical surface in front of the cornea.
A scleral lens bridges over the cornea: the fluid beneath the lens neutralises its irregularities.
Scleral lenses go even further. They rest on the sclera, the white of the eye, and bridge over the cornea. The space between the lens and the cornea is filled with fluid, which neutralises much of the irregularity of the corneal surface. They are now a major tool of visual rehabilitation in irregular corneas.
The laser does not remove the same amount of tissue in every meridian: it corrects the sphere and the cylinder at the same time.
When the astigmatism is regular and the cornea is suitable, laser refractive surgery can change its shape directly. That is the principle of LASIK and PRK, photorefractive keratectomy.
The laser does not remove the same amount of tissue along every meridian. The treatment is calculated to modify the curvature of the cornea differentially, and to reduce the sphere and the cylinder at the same time. In appropriate indications, LASIK and PRK can therefore correct myopia, hyperopia and astigmatism.
But the amount of astigmatism is not the only criterion. Before refractive surgery, the regularity of the cornea, its thickness, its topography, the stability of the refraction, the ocular surface and the potential risk of ectasia must also be analysed. We covered these criteria in detail in our article on eligibility for refractive surgery.
A cornea showing irregular or suspicious astigmatism is not simply "astigmatism that is harder to treat". It may be a situation in which conventional refractive surgery is not indicated. In certain specific cases, topography-guided treatments can be considered, but their indication must be assessed individually.
Correcting astigmatism does not necessarily have to be done on the cornea. Another option is to place a lens inside the eye while keeping the natural crystalline lens. That is the principle of the ICL, or phakic implant.
In its toric version, the implant itself carries a cylindrical correction. This solution can be considered in certain patients with high myopia combined with astigmatism, when laser correction of the cornea is not the best option.
Unlike LASIK or PRK, the ICL does not reshape the cornea. It adds a new optical element inside the eye. Toric ICLs are specifically designed to correct myopia and astigmatism at the same time.
The ICL does not reshape the cornea: it adds a corrective lens inside the eye.
Once the crystalline lens has been removed, all that matters is the corneal astigmatism that will remain after the operation.
During cataract surgery, the natural crystalline lens is removed. The component of astigmatism coming from that lens therefore disappears with it. The situation is very different from a simple glasses prescription: the surgeon must above all anticipate the corneal astigmatism that will remain after the operation, as well as any changes induced by the surgery itself.
If this astigmatism is significant, a toric intraocular lens can be used. This implant has different powers along its axes, and must be positioned at an orientation calculated before the operation. This calculation relies on precise biometry, which the Anterion provides by combining the corneal measurements and the biometric data of the eye.
Toric intraocular lenses are today an effective and predictable method. Comparative studies show a greater reduction in residual astigmatism and better spectacle independence than with non-toric implants, even when the latter are combined with limbal relaxing incisions.
Toric intraocular lenses exist in different categories: monofocal, extended depth of focus in some ranges, multifocal or trifocal in others. Correcting astigmatism and correcting presbyopia nevertheless remain two distinct objectives, which must be considered separately when choosing the implant.
PRELEX, for Presbyopic Lens Exchange, belongs to the same family as refractive lens exchange, or Refractive Lens Exchange (RLE). Technically, the operation resembles cataract surgery, with the difference that the crystalline lens is still sufficiently clear: it is removed for mainly refractive reasons, then replaced with an intraocular lens.
In an astigmatic patient, this implant can be toric. Depending on the indications, it can also incorporate a strategy for correcting presbyopia. Astigmatism can therefore be treated at the same time as myopia, hyperopia and, where appropriate, presbyopia.
It is important to understand, however, that a lens exchange is intraocular surgery. In a young person who still has good accommodation and a clear crystalline lens, removing it solely to correct astigmatism is generally not the first choice. The choice between corneal laser, phakic implant and lens surgery therefore depends greatly on age and on the overall refractive situation.
Corneal laser, phakic implant or lens exchange: age and the overall refractive situation guide the choice.
In progressive keratoconus, the aim of cross-linking is first to stabilise the cornea, not to eliminate a cylinder figure.
Sometimes, but the reasoning is completely different. The cause of the irregularity must first be treated or stabilised.
In progressive keratoconus, for example, the aim of corneal cross-linking is above all to stabilise the cornea and reduce the risk of progression. It is not simply a treatment designed to remove a cylinder figure.
Depending on the situation, visual rehabilitation can then rely on glasses, on rigid or scleral lenses, and in some cases on various specialised surgical techniques. Irregular astigmatism must therefore be regarded as a problem of the quality and geometry of the optical surface, and not just as a cylinder value.
Yes. The cornea and the crystalline lens are not perfectly fixed structures. Astigmatism can evolve gradually with age: its power can change, but so can its axis. The crystalline lens can also alter the eye's total astigmatism over time, particularly as it ages or as a cataract develops.
On the other hand, the appearance of irregular astigmatism, or its rapid increase, particularly in a young person, may warrant investigation of the cornea. An ectasia such as keratoconus must then be excluded.
Astigmatism that increases rapidly in a young person warrants investigation of the cornea.
Two patients each with 2 dioptres of astigmatism can in reality present completely different situations. In the first, it may be regular corneal astigmatism perfectly corrected with glasses. In the second, the cylinder may be partially compensated by the crystalline lens. In a third, the same value measured at refraction may conceal an irregular cornea.
This is why the choice of a correction never depends solely on the number of dioptres. For glasses, the refraction is generally the main information. For refractive surgery, the geometry of the cornea must be understood. For a toric ICL, the correction provided by the implant must be planned taking the whole optical system into account. And during cataract surgery or refractive lens exchange, the astigmatism that will persist once the natural crystalline lens has been removed must be anticipated.
That, in the end, is what makes astigmatism so particular: it is not only an amount of optical defect. It is also a direction, an origin and a geometry. And it is this combination that determines the best way to correct it.
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