Anisometropia (a difference between the eyes of 2 Diopters or more) will require a three-pronged approach to create the ideal outcome. First, deal with the cosmetic issue of one lens being thicker than the other. Second, deal with the aniseikonia (the sick feeling) from the prescription imbalance. Third, deal with the induced prism. Here is how to do all three…
The cosmetic issue – How to minimize the thickness difference.
There are three ways to minimize the thickness difference between the two lenses in cases of anisometropia. All three should be utilized for maximum benefit.
First, be sure to use a high index lens, the higher the better keeping in mind specific gravity.
Second, order it Aspheric. We many times only consider aspheric lenses in cases of plus power lenses, but don’t underestimate its ability to make minus power lenses of different powers look more similar.
And third, probably the most important step,especially for minus power lenses, is to select the smallest frame the patient feels comfortable with. This will help as well in our Part 3 discussion (minimizing prismatic effects) since a small frame will limit how far off axis the patient can look, thereby minimizing the induced prism potential.
The anisekonia issue – How to deal with the nausea.
The sick feeling with anisometropia comes from the magnification difference between the two eyes. Therefore, we need to minimize the different image sizes the two lenses are creating. There are three things we can do to help accomplish this.
First, specify to the lab that the base curves be made equal.
Second, specify that the center thickness be made as equal as possible. This may mean that a minus power poly lens is given a 2.0 center thickness instead of a 1.0 in order to more closely match the PLUS power lens over the other eye.
Third, order the lenses aspheric. An aspheric cut to the lenses will flatten the lenses, especially plus power lenses, this will go a long way in helping to make them more equal since its effect will be more pronounced on the higher power lens, therefore a great equalizing force.
The induced prism issue – How to deal with the double vision
Due to the difference in lens thickness between the two eyes, prism is induced when the patient looks off center. If the patient is wearing a multifocal lens, the first step is to order Slab-off. However, this only solves a quarter of the problem. Slab off can only be placed in the inferior quadrant of the lens, this leaves 3 more quadrants for the patient to see double through. Many opticians only think of keeping the patient from seeing double through the reading part of the lens, but what about when the patient looks off axis in other directions (up or to the sides)? Which brings us to step 2, get the patient into the smallest frame they feel comfortable with. This will minimize the distance off-axis they can look in these other directions.
Additionally, be sure to educate the patient on the likelihood of seeing double when looking up and to the sides or they WILL be back to complain. Also, don’t overlook the possibility of contact lenses or single vision lenses in these cases. Both of these options will help the patient from looking off-axis.
Best of success in your day today!