What is hypermetropia or farsightedness? What is a possible reason for hypermetropia? How is hypermetropia corrected?
Hypermetropia or farsightedness is the defect of vision in which the human eye can see distant objects clearly but is unable to see nearby objects clearly.
In this case, the image of a nearby object would fall behind the retina instead of on the retina.
In hypermetropia, the cornea is flattened, or the axial length is too short. Therefore, the images do not focus when they reach the retina. A hypermetropic eye must increase its lens power to bring distant objects in focus on the retina for clear vision. This requires contraction of the ciliary muscle, and therefore, the far-sighted eye is never at rest and works even to see near objects crystal clearly. Hypermetropic corrections add positive focusing power to the eye for clear crystal vision.
Possible reasons for hypermetropia :
(1) Curvature of the cornea and the eye lens decreases. Hence, the converging power of the eye lens becomes less.
(2) The distance between the lens and retina decreases ( relative to Norma's eye ), and the focal length of the eye lens become very large due to the flattening of the eyeball.
How is Hypermetropia corrected? Hypermetropia is corrected using a suitable convex lens. A convex lens of proper focal length is chosen to produce the required convergence of the lens. Light rays are converged by the convex lens before they appear to the eye lens. Hence, after the converging action of the eye lens, the image is formed on the retina for clear crystal vision.
Small hypermetropia can be corrected by voluntary accommodation. Even high errors of refraction may be corrected by this way, but convex lenses can be required if symptoms are not relieved.
Accommodation:
The degree of hypermetropia is corrected by the accommodative effort, which is known as facultative hypermetropia. The remaining uncorrected hypermetropia is called absolute hypermetropia. When total hypermetropia after abolishing voluntary accommodation is known as manifest hypermetropia, the accommodative effort cannot be sustained with growing age, and the hypermetropia becomes absolute till the effort of lenses fails to correct any hypermetropic eye for clear vision. Thus the facultative hypermetropia becomes abolished, and there remains no difference between absolute hypermetropia and manifest hypermetropia.
Some of the hypermetropias are corrected with the help of the ciliary muscles' inherent tone, which is called latent hypermetropia. The degree of latent hypermetropia is high in young persons and becomes less with older age. Complete cycloplegia can abolish latent hypermetropia. The refractive error estimated under the complete cycloplegia is called total hypermetropia.
When hypermetropia is fully corrected:
The hypermetropia is fully corrected, but due to sustained accommodative efforts, the patient develops asthenopic symptoms.
Asthenopia (eyestrain).
Frontal or frontotemporal headache.
Watering.
Mildaversion to light.
By these symptoms, we can say that hypermetropia is fully corrected.
When hypermetropia is not fully corrected:
When hypermetropia is not fully corrected by the voluntary accommodative efforts, then the patient complaint of defective vision of more for near than distance due to this accommodative effort. Patient present with:
Asthenopia.
Defective vision more for near.
By these symptoms, we say that hypermetropia eye is not fully corrected.
The severity of hypermetropia:
These hypermetropia eyes are classified as below -
Low: Up to + 2 D.
Moderate: From + 2.25 to 5 D.
High: More than + 5 D, rarely exceeds 6 – 7 D, which is equivalent to a 2 mm shortening of the optic axis. Individual cases of much higher degrees, without any other associated anomaly, have been recorded.
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