ANISOMETROPIA

•  When refractive status of two eyes is equal, it is known as isometropia. 
• When refractive status of two eyes is not equal, it is known as anisometropia. 
• A difference of 1D in refractive status of two eyes creates a difference of 2% in size of retinal image of two eyes. 
• Up to 5% difference in retinal image size of two eyes is well tolerated. Thus an anisometropia of up to 2.5D is acceptable. 
• Anisometropia of up to 4D may be acceptable in some individuals and more than 4D creates diplopia.

Etiology

 Congenital and development anisometropia: This occurs due to differential growth of the two eyeballs. 

Acquired anisometropia 

1. Uniocular aphakia occurs after cataratoconous lens removal 

2. From trauma to the eye 

3. Due to inadvertent surgical treatment of refractive error

Vision status in anisometropia 

Binocular vision: Binocular vision is present in small degrees of anisometropia. An anisometropia of about 1.5 D to 3 D is tolerated depending upon the individual.

 Alternate vision: Alternating vision occurs when one eye is emmetropic or moderately hypermetropic and the other eye is myopic. The emmetropic or moderately hypermetropic eye is used for distance vision and the myopic eye is used for near vision. These patients are usually comfortable and never have to make an effort of either accommodation or convergence. 

Uniocular vision: When the refractive error is high in one eye compared to the other, then the high degree refractive error eye receives continuously blurred images compared to the other eye. Due to this the eye receiving blurred image is suppressed and develops amblyopia. This type of amblyopia is called anisometropic amblyopia.

Symptoms:- 1. Blurring vision 2. Diplopia 3. Eyestrain 4. Headache

Clinical types 

Simple anisometropia: In simple anisometropia one eye is emmetropic and the other eye is either myopic or hypermetropic. e.g. 6/6 vision with–2.00DS. It is known as simple myopic anisometropia. If other eye is hypermetropic e.g. 6/6 vision with +2.00DS, it is known as simple hypermetropic anisometropia.

Compound anisometropia: In compound anisometropia both eyes have refractive error. The refractive error may be hypermetropic or myopic, but one has a higher refractive error than the other. one eye requires–2.00DS and other–5.00DS) or hypermetropic (compound hypermetropic anisometropia; one eye requires +2.00DS and other +5.00DS) but one eye has higher refractive error than the other

Mixed anisometropia :When the refractive error of one eye is hypermetropic and other eye is myopic it is mixed anisometropia. This is also termed antimetropia. e.g. one eye requires–2.00DS and other eye requires + 2.00DS.

Simple astigmatic anisometropia: When one eye is normal and the other has simple myopic or hypermetropic astigmatism it is termed simple anisometropic astigmatism. 

Compound astigmatic anisometropia: When both eyes are astigmatic but of unequal degree, it is called compound astigmatic anisometropia. 

Mixed astigmatic anisometropia: When one eye has hypermetropic astigmatism and the other eye has myopic astigmatism.

Diagnosis:-

The patient is diagnosed with retinoscopy. 

He may complain of defective vision, headache, and other eyestrain symptoms.

Clinical test 

The visual status is assessed by using either FRIEND test or Worth’s Four Dot test.

Treatment:-

1. Spectacles:- The corrective spectacles can be tolerated up to a maximum difference of 4D. 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. • Phakic Refractive Lenses (PRL) and Refractive Lens Exchange (RLE) are quite useful in very high degree anisometropia.