FIX AND FIT

Troubleshooting Anisometropia

Understanding how to help patients with this difficulty can make you an optical star

One of the integral roles of the eyewear dispenser is troubleshooting problems that patients may have with their new glasses. Many patients have difficulty putting into words what they are experiencing, so it’s up to us to translate and correct the problem at hand. One such problem is vertical imbalance at the near point.

WHAT

The problem is caused by the prismatic effect that is present in any prescription lenses when the eyes look anywhere but the optical center. It becomes an issue when there is a considerable difference between the OU Rx and OS Rx.

WHO

This issue is most often noticed when patients get their first pair of bifocals. Single vision wearers may experience double vision or eye strain when gazing in various directions, but they can reduce or eliminate it by turning their head to position the optical centers in line with the object being viewed.

Bifocals cause the eyes to look through an area 9mm or more from the optical center of the distance portion of the lens, which creates a certain amount of prism at the near point. Some patients don’t experience the effects of anisometropia. Instead, their brains suppress the image from one eye.

The most common situation where we find this problem is when the patient has cataract surgery on one eye. Usually the eye is corrected to close to a plano Rx while the other eye may still require a fair amount of correction. This can create a rather dramatic change for the patient depending upon the amount of difference.

HOW

Those bothered by this prismatic imbalance can be helped with a slab-off: a prism ground on the lower portion of one lens to counteract the prism that would normally be at that point.

We can calculate the amount of vertical imbalance and order a slab-off; however, it is usually unnecessary to specify the amount to a lab since their computers will automatically calculate the amount.

Another possible solution may be something like the Shaw lens, which addresses this problem (shawlens.com) and may be suitable for such patients.

This is precisely why certification programs require dispensers to be aware of the lens power at a given meridian, or the amount of prism caused at a particular position through the lens.

A line that extends across the lens at the top of the segment indicates the lens is a slab-off

The slab-off incorporates base-up prism at near only, countering the usual base-down prism this lens would have and balancing the image with the other lens

When troubleshooting lens problems, understanding this concept is invaluable. Even more invaluable is applying that understanding to create a lens solution that will solve your patient’s problem.

—Alex Yoho, ABOM