A majority of people who wear contacts and glasses have some degree of anisometropia. Anisometropia is a condition where the refractive errors differ between eyes. Most eye care professionals often reserve the use of this term for cases in which the difference is significant, around 2.00 diopters or more. Another form of anisometropia is called anitmetropia and this is a condition in which the power designations for each eye differ. For example the right eye may have a plus power while the other has a minus power. The refractive error it self is not to problematic for the patient. However when spectacles are used to correct this error problems can begin to arise.
All corrective lenses displace images. In the case of a converging or magnifying lens the images are displaced towards the edges of the lens creating magnification. In the case of a diverging or minifying lens the images are displaced towards the center of the lens creating minification. The amount of displacement depends of the power of the lens. In the case of a anisometropic patient the image displacement between the right and left eyes differ greatly. As the wearer moves their eyes away from the center of the lens this displacement increases, sometimes so much so that the double vision occurs. This is more of a problem for the presbyope who needs to look away from the center of the lens to view through the reading portion of thier progressive or bifocal lens. People with distance vision and anisometropia will generaly tilt their whole head to view through the centers of thier lenses.
There are several ways to correct for this prism imbalance. A popular technique used when making the spectacle lenses is called bicentric grinding or slab off. In this case either the eye with the most minus power or least plus power has prism ground in only half of the lens, creating a visible line that bisects the lens. Another option would be that the wearer could have two sets of glasses one for reading and the other for distance. However most patients prefer to wear contact lenses. Contact lenses greatly reduce the amount of prismatic effect experienced with spectacle wear. When a contact lens wearer rotates his or her eyes the lens stays relatively stationary moving with the eye and reducing the amount of prism otherwise experienced with spectacle lenses.
The exact cause of anisometropia is not to easy to pin down. It could be congenital or be the result of trauma. Often if one eye has a less significant refractive error than the other eye it will become dominant, in fact so much so that the non dominant eye will loose much of its visual function. This condition is called amblyopia. With the loss of function this amblyopic eye often begins to wander creating a condition known as strabismus.