WHAT IS ASTIGMATISM OF THE EYE?
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 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.
Glasses or contact lenses have to correct in all directions. Uncorrected lens focusing errors reduce visual acuity and eye comfort.
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 the 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 is vision corrected ?
Apart from contact lenses and glasses to correct vision, 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 entirely correct and balance the eyes
What is optical prism?
A prism has at least two flat surfaces that form an acute angle (less than 90 degrees). Like glass or plastic, the transparent material can have various 3-dimensional shapes and have identical, parallel and flat ends or bases. Prism lenses have light-bending properties (or index of refraction) as they can displace the viewed image horizontally, vertically, or a combination of both directions.
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In relation to refractive error, exactly halfway between the red and green lines above is an equal blur in both meridians. This equates 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
Your Ophthalmologist or Optometrist evaluates focusing problems during your eye examination. Some rare 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.
The most common application for this is the treatment of strabismus.
Manufacturers can grind for prism effect or move the centre of the spectacle lens.
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 a 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 improved symptoms when they turn or tilt their head. Yoked prism can move the image away from the 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 determine 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 sometimes used. 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 tumour, high blood pressure, thyroid problems even cataracts. Visual field mapping can determine if your visual pathway is functioning as it should.