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Tuesday, March 13, 2012

What is ANISOMETROPIA

The optical state with equal refraction in the two eyes is termed isometropia. When the total refraction of the two eyes is unequal the condition is called anisometropia. Small degree of anisometropia is of no concern. A difference of 1 D in two eyes causes a 2 percent difference in the size of the two retinal images. A difference up to 5 percent in retinal images of two eyes is well tolerated. In other words, an anisometropia up to 2.5 is well tolerated and that between 2.5 and 4 D can be tolerated depending upon the   individual sensitivity. However, if it is more than 4 D, it is not tolerated and is a matter of concern.
Etiology
1. Congenital and developmental anisometropia occurs due to differential growth of the two
eyeballs.

2. Acquired anisometropia may occur due to uniocular aphakia after removal of cataractous
lens or due to implantation of IOL of wrong power.
Clinical types
1. Simple anisometropia. In this, one eye is normal (emmetropic) and the other either myopic (simple myopic anisometropia) or hypermetropic (simple hypermetropic anisometropia).

2. Compound anisometropia. wherein both eyes are either hypermetropic (compound hypermetropic anisometropia) or myopic (compound myopic anisometropia), but one eye is having higher refractive error than the other.

3. Mixed anisometropia. In this, one eye is myopic and the other is hypermetropic. This is also
called antimetropia.
4. Simple astigmatic anisometropia. When one eye is normal and the other has either simple myopic or hypermetropic astigmatism.
5. Compound astigmatic anisometropia. When both eyes are astigmatic but of unequal degree.

Status of binocular vision in anisometropia
Three possibilities are there:
1. Binocular single vision is present in small degree of anisometropia (less than 3).
2. Uniocular vision. When refractive error in one eye is of high degree, that eye is suppressed and develops anisometropic amblyopia. Thus, the patient has only uniocular vision.
3. Alternate vision occurs when one eye is hypermetropic and the other myopic. The hypermetropic eye is used for distant vision and myopic for near.
Diagnosis
It is made after retinoscopic examination in patients with defective vision.
Treatment
1. Spectacles. The corrective spectacles can be tolerated up to a maximum difference of 4 D. After that there occurs diplopia.
2. Contact lenses are advised for higher degrees of
anisometropia.
3. Aniseikonic glasses are also available, but their
clinical results are often disappointing.
4. Other modalities of treatment include:

  •  Intraocular lens implantation for uniocular aphakia.
  •  Refractive corneal surgery for unilateral high myopia, astigmatism and hypermetropia.
  •  Removal of clear crystalline lens for unilateral very high myopia (Fucala's operation).

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