|
With Halloween approaching, I felt it appropriate to discuss something that sounds a little scary: Anisometropia. After all, some of us probably go all day without uttering a seven (count ‘em, seven!) syllable word.
As challenging as the word appears, if we break down its four Greek elements, we find it’s not so mysterious after all. “An” (not), “iso” (same), “metr” (measure), “opia” (eye). Therefore, we can define anisometropia as a condition in which the two eyes have different refractive errors. From a practical point of view, this condition should be something eye care professionals should consider taking corrective measures for if the patient meets three criteria; the key word being “consider.”
First, the patient is wearing some type of multifocal. Second, the patient must be experiencing fairly good visual acuity in both eyes with the new prescription. And third, there must be 1.5 diopters or more of vertical imbalance at near. The first two criteria are easy to determine, the third requires that we remember a couple of things we learned in Opticianry 101 – Prentice’s Rule, and how to determine the total power in the 90th (vertical) meridian. Once it is determined if there is a significant enough vertical imbalance, the ECP must then determine the best way to correct for it.
This issue faces us more than we realize because sometimes just a quick glance at the prescription does not enable us to notice the imbalance. Second, as more Baby Boomers require IOL (intralocular lens) surgery – one eye at a time – the complaints surrounding anisometropia increase proportionally. Perhaps the best way to delve further into this issue is to review Prentice’s Rule, how to determine the power in the vertical meridian, and then applying that information to a couple of case studies.
Prentice’s Rule: One way of describing an ophthalmic lens is as curved surfaces made up of short straight lines, each angled a bit more than the next, which means that we could also describe a lens as a series of prisms with their apices or bases angled toward the OC (optical center). A plus lens’ prisms have their bases oriented toward its OC – which is why a plus lens is thicker in the middle and thinner toward its periphery. A minus lens’ prisms have their apices oriented toward its OC – which is why a minus lens is thinner in the middle and thicker toward its periphery. When looking through the OC, a patient experiences no prismatic effect. As the eye deviates from the OC, prism occurs. While there are several different ways to express it, the most practical form of Prentice’s Rule is: P = dD/10, where P = the amount of prism at a given point on the lens; d = the distance of that given point from the optical center of the lens; and D = the total dioptric power of the lens.
Remember, for the sake of these case studies (as in real life) we will only be considering this effect in the 90th meridian with multifocal wearers who have good VA (visual acuity) in both eyes. Why? Anisometropia is not a problem with single vision users, since regardless of the difference in lens power, all they need to do is physically move their heads (as opposed to just their eyes) so that they are always looking through both OCs, thus not experiencing a prismatic effect. Anisometropia is also not a problem if the corrected VA is significantly lower in one eye. Why? The weaker eye is sublimated, and the good eye “takes over” most of the work.
For example, what amount of prism is a wearer experiencing if the optical center is moved 5mm out (toward the temple area) on a -6.50 lens? Using the formula:
P = 5 x 6.50 = 32.5 / 10 = 3.25^. The OC is out. Since it is a minus lens the apex is where the OC is, and the base is in the opposite direction. So the answer is
3.25^ base in.
That formula is straightforward and simple with a spherical correction. If cylindrical power is also present, determining the power in the 90th meridian is just a little bit trickier. With a handy cheat sheet, it becomes almost a treat. Therefore, consider the following reference chart:
|
0 degrees away from the axis of the Rx, there is 0 times the cylinder power.
30 degrees away from the axis of the Rx there is .25 times the cylinder power.
45 degrees away from the axis of the Rx there is .50 times the cylinder power.
60 degrees away from the axis of the Rx there is .75 times the cylinder power.
90 degrees away from the axis of the Rx there is 1 times the cylinder power. |
Now, let’s consider another example. OD: +1.75-2.50 x 135; add +2.00. OS: -4.00-2.50 x 180; add +2.00. We will assume that the drop when reading is consistent for both eyes, so that is not a factor. We need only determine the imbalance, by determining the power in the 90th meridian using the above information and formula. The right eye’s cylinder is 45-degrees away from axis 90, so the cylindrical power is -1.25. Added to the +1.75 sphere, the power in the 90th meridian is +.50D. Add +2.00 and the power when reading is +2.50D. The left eye’s cylinder power is 90-degrees away from axis 90, so the cylindrical power is 100%. -2.50 added to -4.00 means the total power in the 90th meridian is -6.50D. Add +2.00 and the power when reading is -4.50D. Assuming a 10mm drop to read, the patient will experience
2.5^base up OD, and 4.5^ base down OS, for a total imbalance of
7^. Assuming the corrected VA in both eyes is 20/20, some correction for the imbalance must be considered. If not corrected, the patient will experience major problems resulting from dissimilar image sizes and superimposed images.
Years ago, opticians might consider using dissimilar segment styles to neutralize the prismatic effect, but these days not so much. Additionally, with as much imbalance as is present above, the OC properties of dissimilar segments would not be enough to overcome such a significant imbalance. If you asked, most ECPs would say that the best or only way to correct for this amount of imbalance is by “slabbing off” the lens. While the methods of applying this kind of bicentric grind to a lens are varied and quite exacting, its computation is best left to lab professionals. However, in the example above, a slab off would not be indicated, as it is only effective up to 6 diopters of imbalance.
So…in the case listed above there is only one solution. What is it? The only practical solution for the patient’s problem is single-vision reading glasses. If the patient wears single-vision readers and simply glances down to read, she will still experience problems. That is why the eye care professional must instruct the patient to lower her head, so that she is reading through optical centers of the lenses at all times. Although the patient may still feel some discomfort due to the magnification of the right eye and the minification of the left eye, it seems to be the best, if not only, practical solution.
With these types of issues increasing every day, hopefully ECPs on the front lines will revisit all of the challenges related to these issues. The goal for this article was to pique your curiosity and motivate you to reconsider your approaches in dealing with them.
|
