A study of aniseikonia and Knapp's law using a projection space eikonometer

Knapp's Law, which states that anisometropias due to varying abnormal axial lengths between eyes would not result in inequality in relative retinal image size, provided the correcting spectacle lens was placed at the far point of the eye, has been shown to fall short in clinical practice in several studies using "direct comparison eikonometry". To test these findings using space eikonometry and to further elucidate this Law's clinical applicability, the following study was conducted.

Thirteen patients with suspected axial anisometropia of at least 4 Diopters were identified, selected and examined. Cycloplegic refraction, A-scan ultrasonic ocular biometry and Essilor Projection Space Eikonometry were performed.

Ten of the thirteen patients had their anisometropia due primarily to ocular axial length differences. Of these ten, seven (70%) had detectable levels of aniseikonia and 3 (30%) demonstrated no aniseikonia. The other three patients whose aniso-metropia was due to combined axial and refractive components, all had aniseikonia.

As a geometric optics theory, Knapp's Law stands on its own merits. However, in clinical practice, reduction in retinal element density in high myopia limits its applicability. Such patients often do have significant aniseikonia which can produce ocular referable complaints and/or interfere with binocular vision.