is a vision condition characterized by the inability, at any distance, to focus on the retina the image of a dot when you are watching at a point object.
The word comes from the Greek “a-stigma”: “a”=without and “stigma”=dot, spot, point.
In the astigmatic eye the light rays cannot be focused on a specific point of the retina, where instead a complex image is formed by two perpendicular lines, called “focal”, separated by a focal gap.
In most cases astigmatism is caused by an imperfection in the curvature of the cornea (external astigmatism), but there can be also a form of “internal astigmatism” caused by imperfections in the crystalline lens or in the retina.
Corneal astigmatism can be regular or irregular. In regular astigmatism, the principal meridians are 90 degrees apart (perpendicular to each other). In irregular astigmatism, the principal meridians are not perpendicular.
For most, the condition is regular corneal astigmatism, which gives the front surface of the eye an oval shape.
Irregular astigmatism can result from an eye injury that has caused scarring on the cornea, from certain types of eye surgery or from keratoconus, a disease that causes a gradual thinning of the cornea.
Astigmatism is often present at birth and may occur in combination with nearsightedness or farsightedness.
Astigmatism causes blurred vision both at near and far distance. The symptoms are related to the magnitude and type of astigmatism. When astigmatism is regular and mild, the patient can suffer from a slight vision worsening while when astigmatism is medium or high or in cases of irregular or mixed forms, the visual problems are often associated with accommodative asthenopia (eye strain).
The astigmatic eye tries to focus the images through accommodation in order to get a clear vision, but the same accommodative efforts may cause asthenopic symptoms, such as eye strain, headache, red eyes and blepharitis.
Diagnosis and Treatment
Astigmatism is diagnosed through a comprehensive eye examination that may include visual acuity, keratometry/topography and refraction.
Most astigmatism can be corrected with spectacles or contact lenses.
In the last decades many surgical treatments have been proposed to correct astigmatism when high and seriously hindering the quality of vision.
Astigmatism is very common before and after cataract surgery. Astigmatism can be corrected simultaneously with cataract surgery thanks to the implantation of a toric IOL. A toric IOL is a modification of a standard IOL (used in almost all cataract surgery) that has the needed astigmatism correction built into the lens. Toric IOLs have special markers on the peripheral parts of the lens that enable the surgeon to see the orientation of the astigmatism correction in the lens. Once the toric IOL is implanted in the eye, the surgeon then rotates the lens, so the astigmatism correction is properly aligned for best results.