Introduction to Hypermetropia
Hypermetropia (hyperopia) or farsightedness is the refractive state of the eye where in parallel rays of light coming from infinity are focused behind the retina with accommodation being at rest.
Aetiologically, depending upon the mechanism of production, hypermetropia may be axial, curvatural, index, positional and due to absence of lens.
- Axial hypermetropia is the commonest form. In this condition, the total refractive power of the eye is normal but there is an axial shortening of eyeball. About 1mm shortening of the anteroposterior diameter of the eye results in 3D of hypermetropia.
- Curvatural hypermetropia is the condition in which the curvature of cornea, lens or both is flatter than the normal. Which results in a decrease in the refractive power of eye.About 1mm increase in radius of curvature results in 6D of hypermetropia.
- Index hypermetropia occurs due to change in refractive index of the lens in old age.It may also occur in diabetics under treatment.
- Positional hypermetropia results from posteriorly placed crystalline lens.
- Absence of crystalline lens either congenital or acquired ( following surgical removal or posterior dislocation) leads to aphakia- a condition of high hypermetropia.
Clinical Features (Symptoms) of Hypermetropia
In patients with hypermetropia, the symptoms vary depending upon the age of the patient and the degree of refractive error.These can be grouped as under:
- Asymptomatic: A small amount of refractive error in young patients is usually corrected by mild accommodative effort,without producing any symptom.
- Asthenopic symptoms: At times the hypermetropia is fully corrected but due to sustained accommodative efforts the patient develops asthenopic symptoms.These include:
- Tiredness of eyes
- Frontal or frontotemporal headache
- Mild photophobia
- Defective vision with asthenopic symptoms: When the amount of hypermetropia is such that it is not fully corrected by the voluntary accommodative efforts, then the patient complains of defective vision more for near than distance associated with asthenopic symptoms due to sustained accommodative efforts.
- Defective vision only: When the amount of hypermetropia is high,the patients usually do not accommodate (especially adults) and there occurs marked defective vision for near and distance.
- The effect of ageing on vision: There occurs a progressive loss of accommodative power with ageing, thus moving the eye from latent and facultative hypermetropia to greater degrees of absolute hyperopia. There occurs progressive defective vision.
- Intermittent sudden blurring of vision may occur due to spasm of accommodation including pseudomyopia. Such a condition is detected by cycloplegic refraction which reveals the underlying hyperopia.
- Crossed eye sensation: Some patients may feel that their eyes are crossing without any diplopia. It occurs due to excessive accommodation.
Signs of Hypermetropia
- Visual acuity varies with the degree of hypermetropia and power of accommodation. In patients with low degree of refractive error, visual acuity may be normal.
- Size of the eyeball may be normal or may appear small as a whole.
- Cornea may be slightly smaller than the normal.
- Anterior chamber is comparatively shallow and the angle is narrow.
- Fundus examination may reveal optic disc which may look small and more vascular with ill-defined margins and even simulate papillitis.
- A-scan ultrasonography (biometry) may reveal a short anteroposterior length of the eyeball.
Treatment of Hypermetropia
A. Optical treatment
Basic principle of treatment is to prescribe convex lenses, so that, the light rays are brought to focus on the retina.
Fundamental rules for prescribing glasses in hypermetropia include the following:
- Total amount of hypermetropia should always be discovered by performing refraction under complete cycloplegia.
- If the total manifest refractive error is small, e.g, 1D or less, correction is given only if the patient is symptomatic.
- The spherical correction given should be comfortably acceptable to the patient. However, the astigmatism should be fully corrected.
- Children younger than 4 years who require hypermetropic correction can usually accept the full cycloplegic measurement. Once a child reaches school age, consider reducing the plus for the refractive prescription by about one-third, but the child is not required to accommodate more than 2.5D continually for the distance.
- The older children may not accept full cycloplegic refraction because of blur at distance. So, always first under-correct and prescribe the glasses that the child accepts comfortably. Gradually increase the spherical correction at 6-month interval till the patient accepts manifest hypermetropia.
- If there is associated exophoria, the hyperopia should be undercorrected by 1-2D .
- In the presence of accommodative convergent squint, full correction should be given at the first setting.
- If there is associated amblyopia, full correction with occlusion therapy should be started.
- It is important to remember that hypermetropia may diminish with the growth of the child. So, refraction should be carried out every 6 months, and if necessary, the correction should be reduced.
- For adults, give the manifest correction. Correct for infinity rather than for 6 meter distance in the examination room.
Modes of prescription of convex lenses
- Spectacles are most comfortable, safe and easy method of correcting hypermetropia.
- Contact lenses are indicated in unilateral hypermetropia. For cosmetic reasons, contact lenses should be prescribed once the prescription has stabilized; Otherwise, they may have to be changed many times.
B. Surgical Treatment
Surgical treatments of hypermetropia includes:
- Conductive keratoplasty (CK)
- Thermal laser keratoplasty (TLK)
- Hyperopic LASIK or PRK or their other variants
- Phakic IOLs (ICL), and
- Refractive lens exchange (RLE)
Reference : Hypermetropia – Wikipedia