Anisometropic Refraction

Anisometropia represents the condition in which a patient’s refractive error significantly differs between the two eyes. To what degree the term significant implies is unclear, as the threshold of inequality resulting in adverse symptoms is unique to any given individual. Most commonly, a refractive difference of 2.00D is considered significant in defining the presence of anisometropia. 

Fig:- Anisometropia

The amount of difference between the refractive error of two eyes is at least 2.00D which may cause aniseikonia and also anisophoria. Result is significant adaptation problem, symptoms are headache, eyeache, tearing, decreased reading stamina, vertigo, spatial distortion, meridional aniseikonia in case of astigmatism. However, some patients may be asymptomatic. Unfortunately, the asymmetric spectacle lenses that are prescribed in an attempt to equalize visual acuity create their own asymmetrical effects on magnification, and a discrepancy between the retinal image sizes between the two eyes is the final consequence.

Fig:- Aniseikonia

Aniseikonia (the perception of an image size difference between the two eyes) can create significant adaptation problems and prove to be a management challenge.

These patients may complain of headache, eyeache, tearing, and decreased reading stamina. Patients may also experience vertigo and spatial distortions, sometimes so severe that steps and curbs may appear tilted. Asymmetric astigmatism may result in meridional aniseikonia creating similar symptoms. With commonly available instruments it is difficult to measure aniseikonia. The difficulty in solving the aniseikonia problem starts with the inability to exactly quantify its degree. 

Fig :- Space Eikonometer

The Space Eikonometer, an instrument that was designed to measure small amounts of aniseikonia, is no longer manufactured, though there is some newly available computer software, an approximate estimate of 1% image size difference per diopter of anisometropia is taken as reasonable measurement. 5% may probably be taken as extreme tolerance limit. The worst side is patient tends to suppress the image in one eye if the refractive error is great which may lead to amblyopia in suppressed eye. Contact lenses are successful modality of treatment. Prism may be used to treat secondary muscular imbalance. Base curve, thickness of the lens and vertex distance changes may also help.