Hypermetropia, hyperopia
Defective vision is most common caused by ammetropia (errors of refraction). The use of a simple “pinhole” made in a piece of card will help to determine whether or not there is a refractive error. The vision will improve unless the refractive error is extremely large. Most infants are hyperopic, probably because the axial length of their eyeballs is too short. Consequently, hyperopia decreases with growth. It occurs when the length of the eyeball, the curvature of the cornea and the power of the unaccomodated lens all are appropriately developed for focusing collimated length on the retina.
In hyperopia (also known as hypermetropia) (farsightedness) parallel light rays, coming practically from 5-6 meters or more, with accommodation relaxed, are focused behind the retina. The hyperopic eye can see objects that are near. In hyperopia, accommodative effort will bring distant objects into focus by increasing the power of the lens, so the person uses his accommodation that is reserved for near objects, if the degree of hyperopia is more than this accommodation, the person cannot see far objects also. In addition, without its accommodation a hyperopic eye cannot have clear vision at any distance (neither far, nor near). In high degrees of hyperopia patients complain about decreased far and particularly near vision. In low degrees of hyperopia, patients complain when working longer at near distance (unclear image, pain in the eyes, forehead and headache).
Hyperopia (Hypermetropia) (far sightedness): the optical power is too low so parallel rays of light converge towards a point behind the retina (when the eye is at rest). Causes are;
- decrease in anterior posterior diameter of the globe= axial hypermetropia
- decrease in curvature of the cornea= curvature hypermetropia
- decrease in refractive index of the lens= index hypermetropia. Physiologically in old age, pathologically in diabetics under treatment.
- Positional=posteriorly placed crystalline lens. Can occur as congenital anomaly or is a result of trauma or disease
- Absence of lens such seen as in aphakia
Types of hyperopia According to degree:
- Weak up to (+) 2Dpt,
- Medium from (+) 2 up to (+) 5dpt
- High more than (+) 5 Dpt
By clinical manifestation:
- Manifest= remaining part of total hypermetropia. It is corrected by accommodation and convex lens. Consist of facultative hyperopia (corrected by patient accommodative effort) and absolute hyperopia (residual part not corrected by patients accommodative effort)