Aniseikonia is a condition in which the client suffers from differences in image sizes between the two eyes. This is usually manifested as space perception difficulties, fusion problems, asthenophia, eye strain, fatigue, reading difficulty and diplopia. Aniseikonia can appear in unilateral aphakia, meridional anisometropia, anisophoria, after monocular cataract surgery and in clients with different density of photoreceptor on the retinas. Bracket test assesses the client’s retinal images on every eye separately but under binocular conditions. Vertical and horizontal phorias can be also assessed in this test.
Equipment and set-up:
Put a red glass filter in front of the client´s right eye and the green filter in front of the left eye over the correction for the near. The test is held at 40 cm. Show the client this test and ask them about the position of the red bracket in relation to the green one. The middle circle is a central stimulus in case of fusion problems. If the brackets are not aligned, ask the client to touch one bracket and to elongate or contract the bracket so both are aligned. The programme will show you the percentage of the difference of the retinal images of both eyes, horizontally and vertically. When vertical bracket test is finished, double click it to assess the horizontal side as well.
Interpreting the results:
The result is given in percentages. Firstly, any possible heterophoria should be corrected first. A few possible ways to correct retinal image size:
– Size lenses which are afocal and produce small amount of magnification,
– Contact lenses in cases of refractive anisometropia,
– Spectacles in cases of axial anisometropia,
– Different vertex power for left and right lens in spectacles (increasing the distance in hypermetropes and reducing the distance in myopes),
– Combination of contact lenses and spectacles,
– Increasing the lens thickness for the eye with smaller image,
– Decreasing the refractive index of the lens material.