ANISEIKONIA OCCURS when there is a disparity in perceived image size or shape. Symptoms of aniseikonia include diplopia, headaches, dizziness, disorientation, and eye strain. Aniseikonia is often created by an optical difference in prescriptions between the eyes, otherwise known as optical aniseikonia or anisometropia.

Aniseikonia may also occur when posterior ocular damage causes axial elongation from compression or stretching of the retina or macula (Okamoto et al, 2017). For example, scleral buckle surgery or an epiretinal membrane (Figure 1) may cause symptomatic retinal aniseikonia. Interestingly, unilateral functional vision deprivation (such as congenital ptosis) or cataract in children causes axial elongation, thus also leading to aniseikonia (Zhu et al, 2019).

Magnification is dependent on lens shape and power. Both shape and power factors rely on lens thickness, back- and front-surface lens power, and vertex distance from the pupil. This explains why image size differences are minimized in contact lenses as compared to spectacles. Nevertheless, aniseikonia can still be induced in wearers of contact and scleral lenses if the devices are fitted with asymmetric parameters. To reduce contact or scleral lens-induced aniseikonia, consider prescribing lenses that are similar in center thickness, tear lens reservoir size or corneal vault, and lens power (Figure 2).

Consider a 32-year-old Hispanic male who had a history of iatrogenic corneal ectasia in the right eye and corneal transplant and pseudophakia in the left eye. As a current scleral lens wearer, he noted discomfort due to a smaller image size in his right eye as compared to his left.

The patient presented wearing 48.00D base curve with –8.00D power in the right eye and a 42.00D base curve with +3.00D power in the left eye. The patient’s right eye was refitted into an oblate design scleral lens with a 42.00D base curve and –2.00D power. Upon biomicroscopy, the lenses measured more equal in vault over the cornea. With this refit, the patient noted less eyestrain and better comfort.

Aniseikonia is a source of visual disturbance and discomfort. Specialty lenses such as sclerals may inadvertently induce aniseikonia. Careful attention to fit and design parameters is advised to limit differences in image size for patients wearing scleral lenses. Protecting the cornea and restoring vision are primary, but to maximize the benefits of specialty lenses one must appreciate how the parameters affect the visual experience. CLS

Refererences

1. Okamoto F, Sugiura Y, Okamoto Y, Hiraoka T, Oshika T. Aniseikonia in various retinal disorders. Graefes Arch Clin Exp Ophthalmol. 2017 Jun;255:1063-1071.
2. Zhu X, He W, Kraus CL, et al. Presence of Posterior Staphyloma in Congenital Cataract Children. Curr Eye Res, 2019 Dec;44:1319-1324.