An astigmatism is usually due to an imperfection in the curvature of the cornea, resulting in perception of objects as oblong and blurred. Sometimes an astigmatism can result from an irregular shaped lens inside the eye. Astigmatism is quite common and frequently occurs along with nearsightedness and farsightedness. It is usually in both eyes but may be found just in one eye.
Like other visual conditions, astigmatism is not necessarily a fixed entity. In a study on musicians around symphony season, researchers noted that the musicians held their eyes and head in a tilted posture for long periods of time. Corneal changes occurred seasonally in a high percentage of the musicians, resulting in astigmatism. The researchers attributed these changes to the fact that the musicians' eyes tilted to adapt to their body distortion to try to maintain balance, causing the chronic straining of some eye muscles and relaxing of others. There was a near 100% correlation between the instruments played, the corresponding posture necessary to play the instruments, and the predicted astigmatism. This is an example of "function affecting structure."
Anatomy of Astigmatism
The cornea is a transparent dome-shaped cover of the front part of the eye. It is normally round-shaped. When a cornea is astigmatic, it is shaped more like a football (egg-shaped), with a steeper, more-curved portion, and a flatter, less-curved portion. This does not allow light to be focused evenly on the retina.
The steeper portion of the cornea focuses light more than the flatter portion, causing two different focal points within the eye. Depending on which side of the cornea is steeper, the focus may be on the retina, but also in front of the retina, behind the retina, or the two focus points may straddle the retina.
Types of Astigmatism
Astigmatisms are described relative to the shape of the cornea.
- Regular. The midlines (meridians) are perpendicular.
- With-the-rule. The vertical meridian is the steepest, like a football on its side.
- Against-the-rule. The horizontal meridian is the steepest.
- Oblique. The steepest curve is between 120 and 150 degrees and 30 and 60 degrees.
- Irregular. The principal meridians are not perpendicular.
Astigmatisms are also described in terms of where the focus falls on the retina.
Simple astigmatism can cause hyperopic vision (farsighted) or myopic vision (nearsighted) with one focus point on the retina, and the other either behind or in front of the retina, respectively.
Compound astigmatism. Both focus points are either behind (hyperopic) or in front of (myopic) the retina.
Mixed astigmatism. One focus point is in front of the retina, and the other is behind it.
Signs and Symptoms
- Blurred vision or distorted vision at near, distant, or both.
- Eyestrain, such as headaches, blurred vision, eye redness, eye fatigue, and/or twitching.
- Distorted line perception, such as when straight lines appear crooked, or lines appear clearer in one direction than the other.
- Monocular double vision is double vision when only one eye is open.
- Migraine headaches may be experienced.
Causes and Risk Factors
Genetics. The cause may be inherited, which may include other related eye conditions, such as keratoconus.
Environment. Temporary or seasonal astigmatism may result from working for long hours with one's head at an angle. This can often be corrected naturally.
Corneal conditions. In less common cases, astigmatism can be due to corneal conditions, such as keratoconus. Scarring or thinning of the cornea creates a risk for astigmatism.
IVR injection. Intravitreal injection of ranibizumab for wet AMD may cause astigmatism.
Intraocular lens implant. Astigmatism may become more severe after cataract surgery when a standard monofocal IOL is implanted.
Pterygium. Corneal tissue growth may cause astigmatism; it may eventually cover the cornea.
Excessive near- or far-sightedness.
Glasses and contacts. If astigmatism interferes with vision, or causes eyestrain, glasses are the safest method of correction. Contact lenses are an option, but they are not always appropriate.
LASIK surgery is also another option. A technique known as topography-guided excimer laser surgery has been found to be effective for those with irregular astigmatism by making it possible to fine-tune LASIK surgery for meridian irregularities.
Toric lens. During cataract surgery, toric intraocular lenses, instead of traditional spherical lenses, may be implanted to correct astigmatism. Toric lenses are a relatively new option. In the past, intraocular lenses could not correct astigmatism, although corneal incisions during cataract surgery (limbal relaxing incisions) could sometimes help. Toric lenses take into account meridian steepness, and accordingly, apply very precise corrections. These lenses have to be placed quite precisely during installation. In addition, both monofocal and multifocal toric lenses are available for those with low levels of astigmatism who wish to improve their reading vision.
Vision is not static and changes over time. Even without any pathology, vision tends to weaken as we age, as our eye muscles weaken. The eye lens becomes less flexible, and our ability to break down nutrients and deliver them to our eyes becomes compromised. Much can be done to maintain healthy vision, however, through diet, regular exercise (including daily eye exercises), and targeted supplementation. These can significantly help reduce the risk of future eye disease.
Nutritional supplements such as lutein, astaxanthin, or vitamin D3, may be of value if the astigmatism is combined with retinal weakness involving macular pigment,1, 2 photoreceptors, or other eye conditions.
Protect your eyes from the damaging effect of blue light and UV light from the sun.3
Some research suggests that natural light exposure may counter-balance too much indoor/close up work causing strain on the eyes. Even on an overcast day, exposure to natural light for 2–3 hours may be helpful.
Manage chronic stress. Research has shown that long-term tension contributes to serious disease, which factors in to the causes of eye conditions.
Develop a regular exercise routine. A minimum of 20–30 minutes of vigorous walking, swimming, etc. five days a week, for example, is excellent for eye and overall health.
Computer users. If you are a computer user, review our information on computer eye syndrome to learn how to minimize eyestrain.
See these eye disease prevention pointers.
1. Landrum, J.T., Bone, R.A., Joa, H., Kilburn, M.D., Moore, L.L., et al. (1997). A one year study of the macular pigment: the effect of 140 days of a lutein supplement. Exp Eye Res, Jul;65(1):57-62.
2. Tong, N., Zhang, W., Zhang, Z., Gong, Y., Wooten, B., et al. (2013). Inverse relationship between macular pigment optical density and axial length in Chinese subjects with myopia. Graefes Arch Clin Exp Ophthalmol, Jun;251(6):1495-500.
3. Piermarocchi, S., Saviano, S., Parisi, V., Tedeschi, M., Panozzo, G., et al. (2012). Carotenoids in Age-related Maculopathy Italian Study (CARMIS): two-year results of a randomized study. Eur J Ophthalmol, Mar-Apr;22(2):216-25.