REFRACTIVE ERRORS
Last Updated : 1/4/2011
Jayesh N Doctor
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Normal vision means child is able to appreciate an object at a certain distance in its different proportions and colours.

When the baby is born, the two eyes don't move together and there is

absence of fixation to the light

.

Around three months of age, the baby starts fixating the eyes on the light and around six to eight months of age both the eyes start moving together. Eyes start following the light source in different directions. When the eyes don't move together it will look as if the baby has developed a squint.

Initially in earlier age the eyes are usually Hypermetropic (far-sighted) which gradually over a period of time changes into normal eyes. At times, the eyeball may enlarge more and the child may develop myopia (near-sightedness).

Normal eye:

Here, the parallel rays of light are focused on the retina and child is able to see clearly which is many a times described as 6/6 or 20/20 vision. In other words also described as emmetropia.

Hypermetropia:

In this condition, the eyeball is shorter in length and the parallel rays of light are focused behind the retina leading to clear vision for distant object but has to strain for near. When the strain is more a child may develop convergent squint. This may occasionally lead to Amblyopia (lazy eye). Many a time convergent squint may be corrected by giving plus number spectacles (glasses) to the child.

Myopia:

this condition due to elongated eyeball size the parallel rays of light are focused in front of the retina leading to blurred vision for distant objects. Child may face symptoms like glare and photophobia and may develop divergent squint. Giving minus number glasses can treat myopia.

Astigmatism:

In this condition, the curvature of the front surface of the eye known as cornea is different in different meridians (axes). There may be a higher power in one meridian compared to the other. Due to this unevenness of the cornea, the rays of light are focused unevenly on the retina, resulting in a distorted image formation. For example: A circular image appears oval. The child may try to see objects by tilting the head, squinting the eyes or by blinking excessively.

Nystagmus:

Here, there is an involuntary oscillation of both the eyes associated with a refractive error. It is generally associated with albinism (absence of pigment melanin). Patient may face photophobia (intolerance to light) which can be treated by prescribing tinted glasses correcting the refractive error also.

My husband and I have glasses. What are the chances that my child will get glasses?
At birth practically all eyes are hypermetropic (farsighted) to the extent of 2.5 to 3.0 diopters and as the growth of the body proceeds the axial length increases until when adolescence is passed. The eye should normally be emmetropic (without any number). As a matter of fact it is found that in over 50% of the population emmetropia is not reached, some degree of hypermetropia persists. On the other hand, the eye may become myopic (short sighted). It is generally genetic (i.e. hereditary). If both the parents are wearing glasses then there are high chances of child getting glasses. To rule out whether a child needs glasses or not, regular eye check up at definite intervals is advisable.

What are the

various therapies available to treat refractive errors

?

As we have seen that the basic problem for a person with a refractive error (i.e. eye number) is that the focused image of the seen object is not formed at the plane of the retina. So the basic principle of treatment is to shift the focused image on the retina to see the object perfectly clear. To do this there are different methods used by the ophthalmologist.

Note: only the following methods of treating the refractive errors are effective. No sort of exercise or medicine can cure the refractive error.

1.

Optical treatment:

This is the simplest & most commonly used method. Refractive errors are corrected by the use of glasses or contact lenses.
Glasses:
- For Hypermetropia, Convex or Plus(+) number glasses are used.
- For Myopia, Concave or Minus(-) number glasses are used.
- For Astigmatism, Cylindrical glasses are used.
Contact lenses: This is the next most widely used method. Contact lenses are of various types:
- Hard lenses.
- Semi Soft Lenses
• Regular semi soft lenses
• Extended wear semi soft lenses
- Soft lenses
• Regular soft
• Daily disposables
• Cosmetic soft lenses

2.

Surgical treatment:

It is recommended after the age of 18 years. The eye number has to be stable for at least 6 months. The various surgeries that can be performed are:

Radial keratotomy (RK): It decreases myopia by flattening the cornea by making multiple radial incisions of varying depth. The following complications may occur:
• Accidental corneal perforation
• Vision threatening complications like infection
• Rupture of eye globe by trauma due to weakened scar
• Overcorrection
• Undercorrection
• Unstable vision.

Astigmatic keratotomy: It is similar to RK and is used in treatment of cases of astigmatism. Here only one meridian/axis is cut to make a more curved axis flatter.

PRK: PRK with Excimer laser involves the reshaping of the outer surface of the cornea. The range of correction is from -1.0 to - 6.0 D and +1.0 to +4.0 D. The disadvantage is that it has a longer visual recovery period.

LASIK: This is the latest and the safest procedure.
Principle: A thin layer of cornea is cut like a flap with a precise cutting instrument. This clear flap is lifted up and folded upon to the top eye. Excimer laser is applied over the internal part of the cornea to reshape it and the flap is placed back in its original position where it heals into place with no stitches.
Range of correction: -1.0 to -18.0 D and +1.0 to +8.0D. It corrects astigmatism upto +/- 4.0 D. Its advantage is that visual recovery is very rapid. Within one or two days most patients see clearly.



Contributor Information and Disclosures

Jayesh N Doctor
Ophthalmic Surgeon, Doctor Eye Institute, Mumbai, India


First Created : 1/4/2001

References

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