Astigmatism (eye)

# Overview
Astigmatism is a defect of the eye, where vision is blurred by an irregularly shaped cornea.  The cornea, instead of being shaped like a sphere, is ellipsoidal (like an egg) and reduces the cornea's ability to focus light.  Astigmatism is a refractive error of the eye in which there is a difference in degree of refraction in 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 (sagittal) plane, but not in front of the retina in the vertical (tangential) plane.  Astigmatism causes difficulties in seeing fine detail, and in some cases vertical lines (e.g., walls) may appear to the patient to be leaning over. The effects of astigmatism of the eye can often be corrected by prosthetic lenses with a cylindrical lens (i.e. a lens that has different radii of curvature in different planes), contact lenses, or refractive surgery.

# Types

## Based on axis of the principal meridians
- Regular astigmatism  principal meridians are perpendicular
With-the-rule astigmatism  axis lies between 0 and 30 or 150 and 180 degrees
Against-the-rule astigmatism  axis lies between 60 and 120 degrees
Oblique astigmatism  axis lies between 30 and 60 or 120 and 150 degrees
- With-the-rule astigmatism  axis lies between 0 and 30 or 150 and 180 degrees
- Against-the-rule astigmatism  axis lies between 60 and 120 degrees
- Oblique astigmatism  axis lies between 30 and 60 or 120 and 150 degrees
- Irregular astigmatism  principal meridians are not perpendicular
Also known as Murdoch Syndrome (Ref: glastonbury Medics)
Axis is always recorded as an angle in degrees, between 0 and 180 degrees in a counter-clockwise direction. 0 and 180 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 eyes.  Although it is unproven, there remain proponents of the theory that astigmatism allows a greater palette of colors to reach the brain.

## Based on focus of the principal meridians
- Simple astigmatism
Simple hyperopic astigmatism  retina coincides with first focal line
Simple myopic astigmatism  retina coincides with second focal line
- Simple hyperopic astigmatism  retina coincides with first focal line
- Simple myopic astigmatism  retina coincides with second focal line
- Compound astigmatism
Compound hyperopic astigmatism  both focal lines are in front of the retina
Compound myopic astigmatism  both focal lines are behind the retina
- Compound hyperopic astigmatism  both focal lines are in front of the retina
- Compound myopic astigmatism  both focal lines are behind the retina
- Mixed astigmatism  focal lines are on both sides of the retina (straddling the retina)

# Prevalence
According to an American study published in Archives of Ophthalmology, nearly 3 in 10 children between the ages of 5 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 recent Polish study revealed that "with-the-rule astigmatism" may lead to the onset of myopia.
A number of studies have found that the prevalence of astigmatism increases with age.

# Diagnosis

## Symptoms
Although mild astigmatism may be asymptomatic, higher amounts of astigmatism may cause symptoms such as blurry vision, squinting, asthenopia, fatigue, or headaches.

## Signs and tests
There are a number of tests used by ophthalmologists and optometrists during eye examinations to determine the presence of astigmatism and to quantify the amount and axis of the astigmatism. 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. A corneal topographer 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 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.
Another refraction technique that is rarely used 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.

# Treatment
Astigmatism may be corrected with eyeglasses, contact lenses, or refractive surgery. Various considerations involving ocular health, refractive status, and lifestyle frequently determine whether one option may be better than another. In those with keratoconus, rigid gas permeable contact lenses often enable patients to achieve better visual acuities than eyeglasses.  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.