WHAT IS ASTIGMATISM OF THE EYE?

What is astigmatism?

Astigmatism is a  common vision problem due to the surface of the eye and lens causing blurred vision,requiring corrective laser surgery,glasses,soft or rigid contact lenses Different types of astigmatism cause refractive errors. The eye focus problem of astigmatism is due to the  shape of  your cornea and lens ( corneal astigmatism and lenticular astigmatism) resulting in a non point point focus. If the red and green lines above reduce  to a point focus  clearer and more comfortable vision results. Accordingly refractive surgery,contact lenses and spectacle correction can also correct any eye focus problem you may have.

WHAT CAUSES AN EYE FOCUS PROBLEM ?

Astigmatism can be genetically inherited and develop in one or both eyes. In addition,eye strain,eye disease and eye rubbing can alter the shape of the eye. Even your tear film can alter the quality of your vision. 

In relation to refractive error exactly halfway between the red and green lines above is  where there is equal blur in both meridians.This is equal to the spherical equivalent, or best vision sphere. If the astigmatism is corrected the cross over point or focus can be on the retina. By contrast in myopia or short sightedness the focus is in front. In hyperopia or long sightedness the focus occurs behind the retina 
Focusing problems are evaluated by your  Ophthalmologist or Optometrist during your eye examination. Some irregular types such as keratoconus, can worsen over time. Excessive eye rubbing and some forms of eye disease or eye injury  can also make some eye focusing problems worse. There are different front surface types of  corneal astigmatism, and lenticular types of refractive error requiring correction by glasses,contact lens or surgery.

WHAT IS AN OPTICAL PRESCRIPTION?

A spectacle prescription contains the terms  “Sphere”, “Cyl”, and “Axis”. The positive sphere number (with a plus sign) indicates hyperopia and negative numbers (with a minus sign) indicate myopia.  The most common axis location on  the cornea or lens  is 180 degrees or so . This is referred to as “with the rule ”  or if around 90 degrees as “against the rule ” .Also the oblique type occurs outside this range. The irregular type  occurs when the meridians are not 90 degrees apart. 

HOW COMMON ARE EYE FOCUS PROBLEMS ?

astigmatism effects

Glasses or contact lenses have to correct in all directions.  Uncorrected  lens focusing errors  reduce visual acuity and eye comfort.

SEE MORE ON VISUAL ACUITY

How is vision corrected ?

Apart from contact lenses and glasses to correct vision, special contact lenses to reshape  the cornea can be used. Also  laser eye surgery techniques such as  LASIK, ASLA and SMILE can rectify some optical problems.
Sometimes optical errors can be overcome when the eye’s natural lens is removed and replaced with an artificial lens. However glasses or contact lenses may still be required to fully correct and balance the eyes 

WHAT IS PRISM ?

A prism is an object made up of a transparent material, like glass or plastic, that has at least two flat surfaces that form an acute angle (less than 90 degrees) They can have a variety of 3 dimensional shapes such as cylinderical, Triangular, quadralateral, pentagonnal, hexagonal, septagonal octogonal, etc sometimes descibed as polygonal.They are defined as having identical,parallel and falt ends or bases.

Prism lenses are made from thin pieces of the optical material used in prescription eye glasses. As they have light bending properties (or index of refraction) they can displace the viewed image horizontally, vertically, or a combination of both directions. The most common application for this is the treatment of strabismus.

Prism can be ground into lenses during manufacture or, a prism effect can also be achieved by moving the center of the spectacle lens off the visual axis.

By moving the image in front of the deviated eye, double vision can be avoided and comfortable binocular vision can be achieved. Other applications include yoked prism where the image is shifted an equal amount in each eye. This is useful when someone has a visual field defect on the same side of each eye
Individuals with nystagmus, Duane’s retraction syndrome, 4th Nerve Palsy, and other eye movement disorders experience an improvement in their symptoms when they turn or tilt their head. Yoked prism can move the image away from primary gaze without the need for a constant head tilt or turn

Prism correction is measured in prism dioptres. A prescription that specifies prism correction will also specify the “base”. The base is the thickest part of the lens and is opposite from the apex. Light will be bent towards the base and the image will be shifted towards the apex. In an eyeglass prescription, the base is typically specified as up, down, in, or out, but left and right are also used sometimes. Whether a patient needs this type of correction can be determined by a variety of methods.

Prism dioptres are represented by the Greek symbol delta (Δ). A prism of power 1Δ would produce 1 unit of displacement for an object held 100 units from the prism. Thus a prism of 1Δ would produce 1 cm visible displacement at 100 cm, or 1 meter.
In cases of strabismus your eyes can turn in (esotropic) or out (exotropic). They may also turn up or turn down. The eyes may even turn vertically and laterally at the same time.

This may result in double vision which can affect your lifestyle and mobility.

Heterophoria refers to the tendency of the eye to turn although it mostly pulls itself back in alignment with the other eye in order to avoid double vision.
This results in headaches, fatigue while reading and eye strain.

In some cases diplopia can result from diabetes, a brain tumor, high-blood pressure,thyroid problems even cataracts. Visual field mapping can determine if your visual pathway is functioning as it should.

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