Astigmatism is a type of refractive error in which the eye does not focus light evenly on the retina. This results in distorted or blurred vision at all distances. Other symptoms can include eyestrain, headaches, and trouble driving at night. If it occurs early in life it can result in amblyopia.
The cause of astigmatism is unclear. It is believed to be partly related to genetic factors. The underlying mechanism involves an irregular curvature of the cornea or abnormalities in the lens of the eye. Diagnosis is by an eye exam.
Three options exist for the treatment: glasses, contact lenses, and surgery. Glasses are the simplest. Contact lenses can provide a wider field of vision. Refractive surgery permanently changes the shape of the eye.
In Europe and Asia astigmatism affects between 30 and 60% of adults. People of all ages can be affected. Astigmatism was first reported by Thomas Young in 1801.
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Signs and symptoms
Although astigmatism may be asymptomatic, higher degrees of astigmatism may cause symptoms such as blurry vision, squinting, eye strain, fatigue, or headaches. Some research has pointed to the link between astigmatism and higher prevalence of migraine headaches.
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Types
Axis of the principal meridian
- Regular astigmatism - principal meridians are perpendicular.
- With-the-rule astigmatism - the vertical meridian is steepest (a rugby ball or American football lying on its side).
- Against-the-rule astigmatism - the horizontal meridian is steepest (a rugby ball or American football standing on its end).
- Oblique astigmatism - the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.
- Irregular astigmatism - principal meridians are not perpendicular.
In with-the-rule astigmatism, the eye has too much "plus" cylinder in the horizontal axis relative to the vertical axis (i.e., the eye is too "steep" along the vertical meridian relative to the horizontal meridian). This causes vertical beams of light to focus anterior to (in front of) horizontal beams of light, within the eye. This problem may be corrected using spectacles which have a "minus" cylinder placed on this horizontal axis. The effect of this will be that when a vertical beam of light in the distance travels towards the eye, the "minus" cylinder (which is placed with its axis lying horizontally - in line with the patient's excessively steep horizonal axis/vertical meridian) will cause this vertical beam of light to slightly "diverge", or "spread out vertically", before it reaches the eye. This compensates for the fact that the patient's eye converges light more powerfully in the vertical meridian than the horizontal meridian. Hopefully, after this, the eye will focus all light on the same location at the retina, and the patient's vision will be less blurred.
In against-the-rule astigmatism, a plus cylinder is added in the horizontal axis (or a minus cylinder in the vertical axis).
Axis is always recorded as an angle in degrees, between 0 and 180 degrees in a counter-clockwise direction. Both 0 and 180 degrees lie on a horizontal line at the level of the center of the pupil, and as seen by an observer, 0 lies on the right of both the eyes.
Irregular astigmatism, which is often associated with prior ocular surgery or trauma, is also a common naturally occurring condition. The two steep hemimeridians of the cornea, 180° apart in regular astigmatism, may be separated by less than 180° in irregular astigmatism (called nonorthogonal irregular astigmatism); and/or the two steep hemimeridians may be asymmetrically steep--that is, one may be significantly steeper than the other (called asymmetric irregular astigmatism). Irregular astigmatism is quantified by a vector calculation called topographic disparity.
Focus of the principal meridian
With accommodation relaxed:
- Simple astigmatism
- Simple hyperopic astigmatism - first focal line is on the retina, while the second is located behind the retina.
- Simple myopic astigmatism - first focal line is in front of the retina, while the second is on the retina.
- Compound astigmatism
- Compound hyperopic astigmatism - both focal lines are located behind the retina.
- Compound myopic astigmatism - both focal lines are located in front of the retina.
- Mixed astigmatism - focal lines are on both sides of the retina (straddling the retina)
Throughout the eye
Astigmatism, whether it is regular or irregular, is caused by some combination of external (corneal surface) and internal (posterior corneal surface, human lens, fluids, retina, and eye-brain interface) optical properties. In some people, the external optics may have the greater influence, and in other people, the internal optics may predominate. Importantly, the axes and magnitudes of external and internal astigmatism do not necessarily coincide, but it is the combination of the two that by definition determines the overall optics of the eye. The overall optics of the eye are typically expressed by a person's refraction; the contribution of the external (anterior corneal) astigmatism is measured through the use of techniques such as keratometry and corneal topography. One method analyzes vectors for planning refractive surgery such that the surgery is apportioned optimally between both the refractive and topographic components.
Diagnosis
A number of tests are used during eye examinations to determine the presence of astigmatism and to quantify its amount and axis. A Snellen chart or other eye charts may initially reveal reduced visual acuity. A keratometer may be used to measure the curvature of the steepest and flattest meridians in the cornea's front surface. Corneal topography may also be used to obtain a more accurate representation of the cornea's shape. An autorefractor or retinoscopy may provide an objective estimate of the eye's refractive error and the use of Jackson cross cylinders in a phoropter or trial frame may be used to subjectively refine those measurements. An alternative technique with the phoropter requires the use of a "clock dial" or "sunburst" chart to determine the astigmatic axis and power. A keratometer may also be used to estimate astigmatism by finding the difference in power between the two primary meridians of the cornea. Javal's rule can then be used to compute the estimate of astigmatism.
A method of astigmatism analysis by Alpins may be used to determine both how much surgical change of the cornea is needed and after surgery to determine how close treatment was to the goal.
Another rarely used refraction technique involves the use of a stenopaeic slit (a thin slit aperture) where the refraction is determined in specific meridians - this technique is particularly useful in cases where the patient has a high degree of astigmatism or in refracting patients with irregular astigmatism.
There are three primary types of astigmatism: myopic astigmatism, hyperopic astigmatism, and mixed astigmatism.
Treatment
Astigmatism may be corrected with eyeglasses, contact lenses, or refractive surgery. Various considerations involving eye health, refractive status, and lifestyle determine whether one option may be better than another. In those with keratoconus, certain contact lenses often enable patients to achieve better visual acuity than eyeglasses. Once only available in a rigid, gas-permeable form, toric lenses are now available also as soft lenses.
Laser eye surgery (LASIK and PRK) is successful in treating astigmatism.
Epidemiology
According to an American study nearly three in 10 children (28.4%) between the ages of five and 17 have astigmatism. A recent Brazilian study found that 34% of the students in one city were astigmatic. Regarding the prevalence in adults, a recent study in Bangladesh found that nearly 1 in 3 (32.4%) of those over the age of 30 had astigmatism.
A Polish study published in 2005 revealed "with-the-rule astigmatism" may lead to the onset of myopia.
A number of studies have found the prevalence of astigmatism increases with age.
History
As a student, Thomas Young discovered that he had problems with one eye in 1793. In the following years he did research on his vision problems. He presented his findings in a Bakerian Lecture in 1801.
Independent from Young, George Biddell Airy discovered the phenomenon of astigmatism on his own eye. Airy presented his observations on his own eye in February 1825 at the Cambridge Philosophical Society. Airy produced lenses to correct his vision problems by 1825, while other sources put this into 1827 when Airy obtained cylindrical lenses from an optician from Ipswich. The name for the condition was not given by Airy, but from William Whewell.
By the 1860s astigmatism was a well established concept in ophthalmology, and chapters in books described the discovery of astigmatism.
Source of the article : Wikipedia
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