On occasion we have patients who come in with a prescription showing a significant power difference between the right and left eyes. Sometimes this variance is rather significant, around four diopters, or less significant, at around two diopters. This condition is called anisometropia, and is often congenital but can also be caused by cataracts or trauma.
Antimetropia is a form of anisometropia and is a condition in which one eye needs a plus power while the other requires a negative power. These conditions cause several problems when correction is attempted with eyeglasses. As the eye looks through a lens at its visual axis, there is no prismatic effect, therefore there is no image displacement. As the eye rotates and moves away from the visual axis prismatic effect occurs causing image displacement.. In an anisometropic patient this displacement is not equal, causing diplopia, which makes fusion difficult. Another problem encountered when anisometropia is corrected with spectacle lenses is aniseikonia.
Aniseikonia is a difference in image size between the right and left lens caused by the difference in magnification or minimization from the lenses. Aniseikonia is corrected with an iseikonic lens. It should be noted that anisometropic patients tend to lean more towards the use of contact lenses. Contact lenses tend to be more stationary on the eye, moving with it so that prismatic effect is almost nonexistent.
To correct this diplopia, a form of lens fabrication called bicentric grinding, or slab off is used. Slab-off is a technique in which the base-up prism is ground on half the lens in either the most minus or least plus lens. The finished product looks as if a “slab” or section of the lens has been removed, hence the name slab-off.. Not every one with anisometropia needs slab-off. There are those that can easily adapt to the power difference with no problem, However some patients cannot fuse the images together. Hyperopic patients tend to show symptoms more so than myopic patients; symptoms include eye-ache, headache and blurring.
Prebyopic patients tend to need slab-off more so than non presbyopes. A non-presbyopic patient can learn to tilt their whole head downwards, keeping vision close to the optical center of the lens, thus minimizing the amount of prismatic effect. Presbyopic patients on the other hand, must view much further down the lens in order to make use of the reading segment. The process of bicentric grinding can be done on both single vision lenses and multifocals, including trifocals and progressive lenses. Slab-off or bicentric grinding is much more common on lined multifocals, but care must be used in the case of trifocals. The visble line that bisects the lens horizontally must line up between the reading and the intermediate section of the seg, not the top line like in bifocals. 1/29/2008