Eye Disorder / Refractive Disorder
A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity. An eye that has no refractive error when viewing a distant object is said to have emmetropia or be emmetropic. An eye that has a refractive error when viewing a distant object is said to have ametropia or be ametropic. Refractive errors are frequently categorized as spherical errors and cylindrical errors: Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. people with refraction error frequently have blurry vision. When the optics are too powerful for the length of the eyeball (this can arise from a cornea with too much curvature or an eyeball that is too long), one has myopia. When the optics are too weak for the length of the eyeball (this can arise from a cornea with not enough curvature or an eyeball that is too short), one has hyperopia. Cylindrical errors occur when the optical power of the eye is too powerful or too weak across one meridian. It is as if the overall lens tends towards a cylindrical shape along that meridian. The angle along which the cylinder is placed is known as the axis of the cylinder, while 90 degrees away from the axis is known as the meridian of the cylinder. people with a simple astigmatic refractive error see contours of a particular orientation as blurred, but see contours with orientations at right angles as clear. When one has a cylindrical error, one has astigmatism. The common refractive disorders are described below.
Myopia (Nearsightedness) Myopia is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed. In myopia or nearsightedness, people have more difficulty seeing distant objects as clearly as near objects. In myopia, the problem is that the eye is too long and images focus in front of the retina instead of on the retina. AAO says myopia is usually inherited and is often discovered in children under age 12, often during routine eye exams.Eye care professionals most commonly correct myopia through the use of corrective lenses, such as glasses or contact lenses. It may also be corrected by refractive surgery, but this does have many risks and side effects. The corrective lenses have a negative optical power (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.
Hyperopia Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short or the lens cannot become round enough), causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance. As an object moves toward the eye, the eye must increase its optical power to keep the image in focus on the retina. If the power of the cornea and lens is insufficient, as in hyperopia, the image will appear blurred. The causes of hyperopia are typically genetic and involve an eye that is too short or a cornea that is too flat, so that images focus at a point behind the retina. people with hyperopia can usually see distant objects well, but have trouble focusing on nearby objects. Various eye care professionals, including ophthalmologists, optometrists, orthoptists, and opticians, are involved in the treatment and management of hyperopia. At the conclusion of an eye examination, an eye doctor may provide the patient with an eyeglass prescription for corrective lenses. Minor amounts of hyperopia are sometimes left uncorrected. However, larger amounts may be corrected with convex lenses in eyeglasses or contact lenses. Convex lenses have a positive dioptric value, which causes the light to focus closer than its normal range.
Asigmatism 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. There are two types of astigmatism: 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. Regular astigmatism arising from either the cornea or crystalline lens can be corrected by a toric lens. Astigmatism causes difficulties in seeing fine detail, and in some cases vertical lines (e.g., walls) may appear to the patient to be tilted. The astigmatic optics of the human eye can often be corrected by spectacles, hard contact lenses or contact lenses that have a compensating optic, cylindrical lens (i.e. a lens that has different radii of curvature in different planes).
presbyopia presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. presbyopia's exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the lens's curvature from continual growth and loss of power of the ciliary muscles (the muscles that bend and straighten the lens) have also been postulated as its cause. Like gray hair and wrinkles, presbyopia is a symptom caused by the natural course of aging. The first signs of presbyopia--eyestrain, difficulty seeing in dim light, problems focusing on small objects and/or fine print--are usually first noticed between the ages of 40-50. The presbyopia of the human eye can often be corrected by spectacles, hard contact lenses or contact lenses.