Anisometropia

Anisometropia is the difference between the refractive power of the two eyes. This is often due to one eye having a slightly different shape or size from the other causing asymmetric curvature (astigmatism), asymmetric farsightedness (hyperopia) or asymmetric nearsightedness (myopia).

Usually small amount of anisometropia is common and of not much concern. A difference of 1 D in two eyes causes a 2% difference in the size of the two retinal images. A difference up to 5% in retinal images of the two eyes is well tolerated. In other words, an anisometropia up to 2.5 D is well tolerated and that between 2.5 and 4.0 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.

Causes of Anisometropia

1. Congenital and developmental anisometropia occurs due to differential growth of the two eyeballs.
2. Acquired anisometropia may occur under the following conditions:
   • Uniocular aphakia after removal of cataractous lens
   • lens Implantation of IOL of wrong power
   • Inadvertent surgical treatment of refractive error
   • Trauma to the eye
   • Keratoplasty in one eye

Types of Anisometropia

1. Simple anisometropia: In this case, one eye is normal (emmetropic) and the other eye is either myopic (simple myopic anisometropia) or hypermetropic (simple hypermetropic anisometropia).
2. Compound anisometropia: In this case, both eyes are either hypermetropic (compound hyper metropic anisometropia) or myopic (compound myopic anisometropia), but one eye is having higher refractive error than the other.
3. Mixed anisometropia: In this case, 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.
6. Mixed astigmatic anisometropia: In this case, one eye has hypermetropic astigmatism and the other has myopic astigmatism.

Investigation and Diagnosis

1. Retinoscopic examination in patients with defective vision is used to make diagnosis.
2. Testing for state of binocular vision may be done by either FRIEND test or Worth’s four-dot test.

I. FRIEND test

In this test, the letters F, I, N are written in green and R, E, D in red colour. It is incorporated in the Snellen’s vision box. The patient is made to sit at a distance of 6m after wearing diplopia goggles with red glass in front of right eye and green in front of the left eye and is asked to read these letters. Results are interpreted as:

• In the presence of binocular single vision, the patient will read FRIEND at once.
• In the presence of uniocular vision, the patient will persistently read either FIN or RED.
• In the presence of alternate vision, the patient will read FIN at one time and RED at other time.

II. Worth’s four- dot test

In this test, the patient wears goggles with red lens in front of the right eye and green lens in front of the left eye and views a box with four lights: one red, two green and one white.

Interpretation is as below:

• If the patient sees all the four lights in the absence of manifest squint, he or she has normal binocular single vision.
• In abnormal retinal correspondence, the patient sees four lights even in the presence of a manifest squint.
• If the patient sees all the four lights in the absence of manifest squint, he or she has normal binocular single vision.
• In abnormal retinal correspondence, the patient sees four lights even in the presence of a manifest squint.
• When the patient sees only two red lights, it indicates left suppression.
• If the patient sees only three green lights, he or she has right suppression.
• When he or she sees three green lights and two red lights, alternately, it indicates presence of alternating suppression.
• If the patient sees five lights (two red and three green), he or she has diplopia.

Treatment

1. Glasses: The first step is correcting the difference between the eyes with glasses (or contact lenses in certain cases). This may be all the brain needs to start using both eyes together, but the glasses/contact(s) must be worn consistently as instructed. If the vision in the “lazy” eye has not adequately improved with the glasses/contact(s) alone, you will need to force the brain to pay attention to this eye so that vision improves. This can be done by covering or patching the stronger eye, using a drop to blur the stronger eye, or by filters over the glasses.

2. Contact lenses: These are advised for higher degrees of anisometropia. Contact lenses may be very useful in young children with high anisometropia, who might otherwise become amblyopic on the side of the more ametropic eye.

3. Other modalities of treatment include:

• Intraocular lens implantation for uniocular aphakia.
• Refractive corneal surgery for unilateral myopia,astigmatism and hypermetropia.
• Phakic refractive lenses (PRLs) are good option for refractive error of 4 to 10D.
• Refractive lens exchange (RLE) is a better option for high refractive error of >10D.

Note:

• In children, efforts should be made to fully correct the anisometropia to prevent anisometropic amblyopia.
• In adults with amblyopia, under correction of more ametropic eye may be required to avoid ocular discomfort.

Reference: Theory and practice of optics and refraction.
Image Reference : https://www.seevividly.com/info/Lazy_Eye/Amblyopia/Anisometropia