Astigmatism: definition, symptoms, and treatment

General overview: mechanism and symptoms

The cornea is an avascular, transparent tissue located at the front of the eye. It provides two-thirds of the eye’s refractive power and serves as an anatomical barrier protecting the eye from external aggressions. Its shape is typically that of a segment of a sphere, with a constant radius of curvature. When this is not the case, astigmatism occurs: the cornea loses its constant curvature and becomes more oval-shaped, resembling a rugby ball.

A patient with astigmatism will experience blurred vision both at near and far distances. In some cases, vision may also be distorted along a specific axis. This condition can cause mild eye fatigue and headaches. Astigmatism is a frequent refractive error, affecting 85% of the population to varying degrees. It can be isolated (simple astigmatism) but is often associated with myopia or hyperopia (compound astigmatism).

Although most cases are corneal in origin, astigmatism can sometimes be caused by deformation of the lens.

Degrees of astigmatism

Astigmatism is common, but it is often mild and may not require correction. Individuals with mild astigmatism tend to squint to see clearly, exerting pressure on the cornea with their eyelids, which slightly deforms the cornea and naturally corrects the astigmatism.

However, 25% of the population has astigmatism greater than 1 diopter, which necessitates clinical management.

Glasses and contact lenses

Correction with glasses or contact lenses is common. However, manufacturing lenses for astigmatism is complex: they must account for all corneal irregularities and do not provide uniform correction across their surface. Round frames should be avoided, as lens rotation can immediately lead to blurred vision.

When corrective devices are chosen, regular follow-up and frequent adjustment of the lenses are necessary.

Laser refractive surgery

Refractive surgery is a safe and effective option for astigmatism correction. The principle is to reshape the cornea using a laser. There are two main techniques: LASIK
and PRK.

In both cases, the laser is applied to the corneal stroma, the intermediate layer beneath the epithelium and in front of the endothelium. LASIK and PRK differ mainly in how access to the stroma is achieved: either by creating a stromal flap or by gently peeling the epithelium. LASIK works on the deeper stromal layers, whereas PRK is a more superficial technique.

The choice of surgical technique depends on the patient’s corneal morphology. If the cornea is thin (less than 500 micrometers), PRK is recommended.

Phakic intraocular lens implantation

Astigmatism is not always suitable for laser refractive surgery. In such cases, the implantation of a phakic intraocular lens may be recommended.

This procedure involves placing a custom-made, flexible lens in front of the natural lens, between the iris and the crystalline lens, through a small incision. The lens unfolds into place and provides the necessary correction.

This is an “additive” surgery: no ocular structures are removed. Additionally, the implantation avoids the risk of dry eye sometimes observed after laser refractive surgery.

The results are generally excellent and stable over time. However, in less than 1% of cases, the implant may not provide the desired correction and must be removed and replaced with a more suitable lens.

Cataract surgery

During cataract surgery, astigmatism can be corrected using so-called “toric” intraocular lenses.

What is orthokeratology, and can it effectively treat astigmatism?

Opinions remain highly divided regarding orthokeratology, and this treatment should be approached with caution. The principle is to temporarily reshape the cornea by wearing rigid contact lenses overnight, theoretically allowing for improved vision during the day due to the mechanically induced deformation.

Proponents claim that these “night lenses,” made of rigid material, act as molds and that their effect on corneal shape can last 16 to 18 hours. According to supporters, this technique could be a universal solution, treating myopia, hyperopia, astigmatism, and even presbyopia.

However, there are well-established contraindications, including systemic diseases such as diabetes or arthritis, insufficient or irregular sleep, certain allergies, large pupil diameter, severe dry eye, and conditions like keratoconus.

Beyond the still-debated efficacy, it is important to highlight the risks of orthokeratology, particularly infection, corneal ulceration, and the potential onset of keratoconus.

What is internal astigmatism?

While astigmatism is most often due to deformation of the anterior corneal surface (corneal astigmatism), this is not always the case. More rarely, the condition may be caused partially or entirely by a defect in the curvature of the posterior corneal surface or a deformation of the crystalline lens (either its anterior or posterior part). In these cases, it is referred to as internal astigmatism.

Total astigmatism is the sum of corneal and internal astigmatism. When both phenomena are present in the same patient, this is termed “mixed astigmatism”.