Aniseikonia

Causes

Retinal image size is influenced by the object's size and position, corrective lenses, and the eye's own optics including refractive power and axial length. Aniseikonia may arise naturally or be induced by correcting refractive errors, particularly anisometropia (significantly different refractive errors between eyes) or antimetropia (myopia in one eye and hyperopia in the other).

Meridional aniseikonia occurs when refractive differences are limited to one meridian, as in astigmatism. Historically, aphakia—absence of the crystalline lens due to cataract removal—caused significant anisometropia and aniseikonia, especially when only one eye was operated on; today, intraocular lens implantation during cataract surgery largely prevents this. Retinal aniseikonia can result from forward displacement, stretching, or edema of the retina.

Diagnosis

A simple demonstration of aniseikonia involves holding a near target, such as a pen or finger, about six inches directly in front of one eye. The person alternately closes each eye and notices that the target appears larger to the eye it is directly in front of. When viewed with both eyes, a small amount of aniseikonia is present, explained by relative distance magnification and asymmetrical convergence.

Treatment

Optical aniseikonia from anisometropia can be addressed with spectacles, contact lenses, or refractive corneal surgeries. Spectacle correction modifies optical magnification by adjusting parameters such as base curve, vertex distance, and center thickness; lenses designed for this purpose are called iseikonic lenses. Contact lenses often produce less difference in retinal image size and offer a wider field of vision; a combination of contact lenses and glasses can create a weak telescope system to eliminate magnification differences.

The optimal design depends on cost, cosmetic factors, and patient tolerance of contact lenses. Refractive surgeries also reduce optical image size differences. Aniseikonia due to uniocular aphakia is best corrected surgically with intraocular lens implantation, while retinal aniseikonia is treated by addressing the underlying retinal disease. Before designing optics, the aniseikonia must be measured.

When the disparity is astigmatic and non-uniform, images may appear wider, taller, or diagonally different. Field-dependent aniseikonia, as seen with epiretinal membrane or retinal detachment, cannot be fully corrected with standard lenses, though partial correction often improves visual comfort.

Did You Know?

• Asthenopic symptoms alone may occur even if image size difference is less than 7%.
• If magnification difference becomes excessive, it can cause diplopia, suppression, disorientation, eyestrain, headache, and dizziness and balance disorders.