1.4.23 regular astigmatism. It can be corneal, lenticular and

1.4.23 Components of hypermetropiaAccommodation in hyperopia is of greater importance than the structural factors leading to it, because accommodation is a key dynamic factor in correcting at least part of the refractive error. Nomenclature for various components of the hypermetropia is as given below:1) Total hypermetropia is the total amount of refractive error which is estimated after complete cyclopegia with atropine. It consists of latent and manifest hypermetropia.2) Latent hypermetropia implies the amount of hypermetropia (about 1 D) which is normally corrected by the inherent tone of ciliary muscle. It is detected only by cycloplegia, can be overcome by accommodation.3) Manifest hypermetropia is the remaining portion of total hypermetropia which is not corrected by ciliary tone. It is determined by noncycloplegic refraction. It consists of two components:i. Facultative hyperopia is that which can be overcome by accommodation (AR., 1987).ii. Absolute hyperopia cannot be compensated with accommodation (MW., 1947).?1.4.3 AstigmatismAstigmatism is the type of refractive error wherein the refraction varies in different meridian. Furthermore, the rays of light entering the eye cannot converge to a point focus but form focal line image (fig. 5). There are two types of astigmatism; these are regular astigmatism and irregular astigmatism.   Astigmatism is a focus problems caused by an irregularly curved cornea in which the eye does not focus light evenly onto the retina. This can cause images to appear blurry and stretched out. It occurs when the optical power of the eye is too powerful or too weak across one meridian. Those having simple astigmatic refractive error see particular orientation as blurred, but see orientations at right angles as clear. This defect can be corrected by cylindrical lenses with refracting light more in one area of the eye than the other (MedlinePlus, November 2013), (Institute, October 2010).1.4.31 Regular astigmatism:When the refractive power changes uniformly from one meridian to another and the parallel rays of light are not focused on a point but form two focal lines. This type of error is called as regular astigmatism. It can be corneal, lenticular and retinal astigmatism.1) Corneal astigmatism is due to abnormalities in the corneal curvature. It is the most common cause of astigmatism and is usually congenital.2) Lenticular astigmatism is usually rare and is one of the following types:i. Curvatural. It is due to congenital abnormality in the lens curvature and often seen in lenticonus. ii. Positional. It is due to congenital tilting or oblique placement of lens. It can also occur in traumatic or congenital subluxation of lens.iii. Index. It is due to difference in refractive index of lens in different meridians often in patients having diabetes and cataract.3) Retinal astigmatism is due to oblique displacement of macula and is seen rarely.1.4.32 Meridian types of regular astigmatism:Depending upon axis and angle between two principal meridians, it can be further classified into following types:1. With the rule astigmatism: In this type of refractive error the two principal merdia is placed at right angle to each other but the vertical meridian is more curved than the horizontal meridian. Thus, its correction requires either concave cylinder at or near 180o or convex cylinder at or near 90o.2. Against the rule astigmatism: It is astigmatic condition in which horizontal meridian is more curved than vertical meridian. Thus, its correction requires convex cylinder at 180o or concave cylinder at 90o.3. Oblique astigmatism: It is that type astigmatic condition in which two principal meridia are not horizontal and vertical but are at right angle to each other e.g. at 45o and 135o.4. Bi-oblique astigmatism: In this type of refractive error the two principal meridia are not at right angle to each other, e.g. one may be at 40o and the other at 110o.1.4.33 Refractive types of regular astigmatism: Depending upon the position of two focal lines in relation to retina, it is further classified into following types:1) Simple astigmatism: It is the type of regular astigmatism in which on rays of light are focused on the retina in one meridian and other rays of light are focused either in front or behind the retina in the other meridian and are called as simple myopic or hypermetropic astigmatism.2) Compound astigmatism: It is that type in which the rays of light in both meridia are either focused in front of the retina or behind the retina and are called as compound myopic or hypermetropic astigmatism.3) Mixed astigmatism: It is that type in which one rays of light are focused in front of the retina in one meridian and the other rays of light are focused behind the retina in other meridian. Thus, in one meridian the eye is myopic and on the other meridian the eye is hypermetropic.1.5 ManagementAnyone can have refractive error; however refractive errors tend to run in families. Myopia is more common among Asian people. It tends to occur in young people at or before the age of puberty and increases in degree until the age of about thirty. Hypermetropia tends to appear as people age because younger people have a natural mechanism by which they adjust to it. As in presbyopia however, there comes a point at which the eye is no longer able to adjust and vision becomes blurred. Refractive error can be corrected with visual aids like eye glasses (spectacles), contact lenses, or surgery.In refractive errors the optics of the system are flawed so the image is blurred. Symptoms of poor vision and asthenopia result other symptoms may include double vision, haziness, glare or halos around bright lights, squinting, headaches and eye strain, and in childhood amblyopia can also occur making refractive errors in them more complex than in adult patients. Refractive errors cannot be prevented, but they can be diagnosed by an eye examination. Since children do not usually complain of visual difficulties, that’s why a comprehensive eye examination is very important for children. Early detection and treatment with corrective glasses, contact lenses or refractive surgery provide the very best opportunity to correct vision problems, so the child can learn to see clearly (Opubiri & Pedro-Egbe, july 2012).1. Eye glassesEye glasses of the appropriate power are the simplest and most effective way to correct most forms of refractive error (fig. 6). Various combinations of lens shape are often used according to a person’s individual measurements. An eye care professional can prescribe appropriate lenses to correct refractive error and to give the best vision. 2. Contact lensesContact lenses work by becoming the first refractive surface for light rays entering the eye, producing a more precise refraction or focus. They can be used for the various types of refractive error and are especially useful for myopia. In many cases, contact lenses provide clearer vision, a wider field of vision, and greater comfort (fig. 7). They are a safe and effective option if fitted and used properly. However, they can cause serious problems if they are not fitted and used properly. It is very important to wash your hands and clean your lenses as instructed in order to reduce the risk of infection. 3. Laser surgeryLaser or Refractive surgery can be used to re-shape the front surface of the cornea, effectively making it into a corrective lens for the particular type of refractive error. This change in eye shape restores the focusing power of the eye by allowing the light rays to focus precisely on the retina for improved vision. A special computer measures the surface contour of the eye, and the shape to be ‘carved’ into the eye is calculated. The laser is then used to vaporize the surface tissue of the cornea, resulting in a new shape (Institute, October 2010).