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Astigmatism


What is Astigmatism?

In medical terminology, imperfection in the curvature of the cornea is called astigmatism, literally translated as "without spot", which already provides an idea of its effects on vision. A normal-sighted human eye focuses the incoming light so that it falls on a single point on the retina. This creates a sharp image that is transmitted to the brain. If the shape of the cornea is altered and deviates from its optimal curvature, its refractive power changes. Incoming horizontal light is then refracted differently than incoming vertical light due to the uneven curvature. As a result, the light is no longer focused on a single point on the retina. Two different planes are created in which the light is refracted, resulting in a blurred image.1-3 A point is therefore not depicted as a point, but as a line.

It is estimated that astigmatism affects about 40% of adults worldwide. This makes Astigmatism the most common refractive error of the human eye.4

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What causes Astigmatism?

Astigmatism can have different causes. First of all, it is important to know that even normal-sighted individuals show imperfection in the curvature of the cornea in the course of the day, depending on the pressure of the eyelids on the eyeball, pupil dynamics, eye rotation, tear film, and last but not least age. Up to a certain degree, Astigmatism is considered "normal".5

  • Genetic factors
  • High myopia
  • Corneal disease (e.g. keratoconus)
  • Surgical interventions

How does Astigmatism manifest itself?

Mild forms of irregular corneal curvature often remain unnoticed. Only with increasing severity do patients notice fuzzy or blurred vision. Visual impairment is perceived equally at near, intermediate, and far distances. Headaches and eye pain may occur as a result.

How is Astigmatism diagnosed?

If Astigmatism is suspected, your eye specialist has a variety of examination methods at their disposal:

  • Corneal topography: This imaging procedure with the help of a so-called keratograph is used to visualise and analyse the corneal surface.2
  • Corneal tomography: This imaging technique provides information about the posterior cornea and corneal thickness in addition to information on the corneal surface. This therefore allows a more precise analysis by means of a three-dimensional representation of the entire cornea.2

Both a power and an axis adjustment are made to determine Astigmatism. The following examination modalities are available for this purpose:

  • Objective refraction: An infrared image is projected onto the ocular fundus, while the patient is looking into what is called an autorefractor. At the same time visual acuity is measured. If the image is not sharp, different lenses are successively positioned in front of the eye until a sharp image is registered.10
  • Subjective refraction: The patient looks at an eye chart, while the eye specialist holds different corrective lenses in front of the eye. The patient states which correction gives the sharpest image.10 The axial position of the Astigmatism is then determined using a cross cylinder.

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How is Astigmatism treated?

The aim of correction of Astigmatism is to bring the two focus lines back to one point. There are various methods available for this purpose.

If the refractive error and angle of astigmatism are known, the refractive error can be corrected optically with a visual aid. This requires cylindrical lenses, so-called toric lenses. These lenses have different effects in different axial positions and are marked with the abbreviation "cyl". Appropriately corrected (toric) contact lenses may also be used. In the case of an irregular corneal curvature, compensation is possible via rigid (hard) contact lenses. The tear film evens out the irregularities between the cornea and the contact lens. 2,3,10

Apart from optical correction, there is also the possibility of laser treatment. Here, the defective corneal curvature can be evened out by abrasions on the surface of the eye.2

References

1. Walter P, Plange N. Basiswissen Augenheilkunde. Springer Berlin Heidelberg 2016.

2. Grehn F. Augenheilkunde. Springer Berlin Heidelberg 2019.

3. Woltsche N, Werkl P, Posch-Pertl L, Ardjomand N, Frings A. Astigmatismus. Der Ophthalmologe: Zeitschrift der Deutschen Ophthalmologischen Gesellschaft 2019; 116: 293-304.

4. Hashemi H, Fotouhi A, Yekta A, Pakzad R, Ostadimoghaddam H, Khabazkhoob M. Global and regional estimates of prevalence of refractive errors: Systematic review and meta-analysis. Journal of current ophthalmology 2018; 30: 3-22.

5. Mohammadi S-F, Tahvildari M, Z-Mehrjadi H. Physiology of Astigmatism. InTech, Rijeka.

6. Sanfilippo PG, Hewitt AW, Hammond CJ, Mackey DA. The Heritability of Ocular Traits. Survey of Ophthalmology 2010; 55: 561-583.

7. Sanfilippo PG, Hewitt AW, Hammond CJ, Mackey DA. The Heritability of Ocular Traits. Survey of Ophthalmology 2010; 55: 561-583.

8. Read SA, Collins MJ, Carney LG. A Review of Astigmatism and Its Possible Genesis. Clinical & experimental optometry 2007; 90.

9. Asimellis G, Kaufman E (Hrsg. 2018). Keratoconus. StatPearls, Treasure Island, 2018.

10. Deutsche Ophthalmologische Gesellschaft (DOG) und Berufsverband der Augenärzte Deutschlands e.V. (BVA). Leitlinie - Empfehlung zur optischen Korrektur von Refraktionsfehlern: Brille 2011: 1-9.