|Classification and external resources|
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. The two types of astigmatism are regular and irregular. Irregular astigmatism is often caused by a corneal scar or scattering in the crystalline lens, and cannot be corrected by standard spectacle lenses, but can be corrected by contact lenses. The more common regular astigmatism arising from either the cornea or crystalline lens can be corrected by eyeglasses or toric lenses. A 'toric' surface resembles a section of the surface of a Rugby ball or a doughnut where there are two regular radii, one smaller than the other one. This optical shape gives rise to astigmatism in the eye.
The refractive error of the astigmatic eye stems from a difference in degree of curvature refraction of the two different meridians (i.e., the eye has different focal points in different planes). For example, the image may be clearly focused on the retina in the horizontal plane, but not in the vertical plane. Astigmatism causes difficulties in seeing fine detail resulting in blurred vision. Three options exist for the treatment of astigmatism: spectacles, contact lenses (either hard contact lenses or toric contact lenses), and refractive surgery.
Based on axis of the principal meridians
- 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, a minus cylinder is placed in the horizontal axis to correct the refractive error (or a plus cylinder in the vertical axis). Adding a minus cylinder in the horizontal axis makes the horizontal axis "steeper" (or better: makes the vertical axis "less steep") which makes both axes equally "steep". In against-the-rule astigmatism, a plus cylinder is added in the horizontal axis (or a minus cylinder in the vertical axis).
Children tend to have with-the-rule astigmatism and elderly people tend to have against-the-rule astigmatism.
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 centre of the pupil, and as seen by an observer, 0 lies on the right of both the eyes.
Based on focus of the principal meridians
With accommodation relaxed:
- Simple astigmatism
- Simple hyperopic astigmatism – first focal line is on 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)
Astigmatism 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.
Although mild astigmatism may be asymptomatic, higher degrees of astigmatism may cause symptoms such as blurry vision, squinting, asthenopia, fatigue, or headaches. Some research has pointed to the link between astigmatism and higher prevalence of migraine headaches.
A number of tests are used by ophthalmologists and optometrists 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.
Another rarely used refraction technique involves the use of a stenopaic 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.
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. If the astigmatism is caused by a problem such as deformation of the eyeball due to a chalazion, treating the underlying cause will resolve the astigmatism.
Corneal incisions if properly placed can correct astigmatism. These techniques include Mini Asymmetric Radial Keratotomy (M.A.R.K.), Astigmatic Keratotomy (AK) and Limbal relaxing incision (LRI). However these techniques are used less often than laser-performed ones.
Ocular residual astigmatism
A method of planning and analyzing astigmatism results in cataract/intraocular lens, corneal, and refractive surgery involves a vector analysis approach. The vectorial difference between the corneal astigmatism and the refractive cylinder (at the corneal plane) is known as ocular residual astigmatism (ORA), and is expressed in diopters. ORA is the astigmatism in the eye not attributable to the anterior corneal surface. (Note—ORA is distinct from what is sometimes called residual astigmatism or surgical residual astigmatism, which is the astigmatism remaining after surgery.) ORA is also called intraocular, lenticular, or noncorneal astigmatism. ORA is the minimal amount of astigmatism that can remain in the overall optical system of the eye.
The most commonly performed refractive surgical techniques, such as LASIK, change the shape of the anterior corneal surface. When the refractive cylinder differs from the corneal astigmatism in magnitude and/or orientation, there will be some astigmatism remaining postoperatively regardless of how perfectly the LASIK was executed. If the laser treatment is based solely on refractive parameters, as is customary, then all the ORA will remain on the cornea (90° away from the calculated ORA axis, because it is neutralizing the ORA). If the laser treatment is based completely on corneal parameters, then the ORA will remain in the spectacle refraction postoperatively. It is intuitively obvious in that case that people with little or no ORA will have better resulting vision than people with higher degrees of ORA. Studies have confirmed this—LASIK is significantly less effective in correcting astigmatism when astigmatism is mainly located in the internal optics; and conversely, the efficacy of LASIK is significantly higher in people whose astigmatism is located mainly on the anterior corneal surface.
The accompanying figures are polar and double-angle vector diagrams from the same patient, whose axes of topographic (SimK) and refractive (R) astigmatism are significantly different. The ORA is the vectorial difference between these measurements.
The concept of ORA is of fundamental importance to refractive surgeons and their patients, as ORA exposes the impossibility of obtaining optimal results by concentrating only on the shape of the anterior corneal surface or the manifest refractive cylinder. The optics of the entire eye must be taken into account when planning refractive surgery.
According to an American study published in Archives of Ophthalmology, 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 number of studies have found the prevalence of astigmatism increases with age.
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