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.
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, and refractive
surgery.
Types Based on axis of the principal meridians
Regular astigmatism – principal meridians are perpendicular.
With-the-rule astigmatism – the vertical meridian is steepest.
Against-the-rule astigmatism – the horizontal meridian is steepest.
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.
Adding a minus cylinder in the horizontal axis makes the horizontal axis "steeper" which
makes both axes equally "steep".
In against-the-rule astigmatism, a plus cylinder is added in the horizontal 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
Astigmatism throughout the eye Astigmatism, whether it is regular or irregular,
is caused by some combination of external and internal 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 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 Symptoms
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.
Tests 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 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.
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.
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.
Laser eye surgery is successful in treating astigmatism.
Corneal incisions if properly placed can correct astigmatism.
These techniques include Mini Asymmetric Radial Keratotomy, Astigmatic Keratotomy and Limbal
relaxing incision.
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 is
known as ocular residual astigmatism, and is expressed in diopters.
ORA is the astigmatism in the eye not attributable to the anterior corneal surface.
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.
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 and refractive 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.
An overview of LASIK complications can be found in the main entry for LASIK.
About 7% of patients preoperatively have an ORA that could result in an increased post-LASIK
astigmatism.
Epidemiology According to an American study published in
Archives of Ophthalmology, nearly three in 10 children 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 of those over the age of 30 had astigmatism.
A recent Polish study 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.
See also Eyeglass prescription
Refractive surgery Lens
Ophthalmology Optometry
Optician References
External links Astigmatism at DMOZ