I was on my way to the airport, and for a change, I had some time to spare. That’s why I decided to swing by my office (always a good idea to pop in unexpectedly once in awhile) and make an appearance. As I approached the front door I was delighted to notice we were pretty busy. So of course I jumped in and began to help a few clients. I figured I’d leave the new sales to others, and worked my way through a few deliveries and repairs.

I was about to leave, when I decided to help just one more person. “How may I help you?” I asked. The man was probably in his late 60s or early 70s. “I didn’t get these glasses here, but I’m having a lot of trouble with them. The place where I got them is no help…they said I’ll get used to them. But I’ve had ‘em nearly a month, and I’m telling you something’s wrong! Would you mind just checking to make sure they were made right? Can you help me?” 

With that, he handed me a prescription and a pair of Flat-Top 28 Transitions in a fashionable, semi-rimless frame. I said I would check and asked him to have a seat. The prescription read: “O.D. +0.25 + 2.75 x 175; O.S. Plano + 2.75 x 095; +2.50 add O.U.” The lensometer read: “O.D. +0.25 +3.00 x 173; O.S. Plano + 2.75 x 093; +2.50 add O.U.” I told him what I believed to be the truth, “Pretty darn close…almost right on the money.” 

Then I started to probe. “What exactly seems to be the problem?” He explained that he had no problem seeing at a distance. Driving and walking around was okay. The problem manifested itself at near. He said whenever he tried to read a book, he felt some strain almost right away, and that every once in awhile he was experiencing a double image. I asked him if the double image was horizontal or vertical. His puzzled look immediately told me I needed to rephrase my question. I asked him if the double images were on top of one another or side-by-side. When he told me the images were on top of one another (vertical) I thought of one thing – Anisometropia - leading to vertical imbalance at the reading level.

You should also know that there was no slab-off or reverse slab-off present in his glasses. At first glance, the prescription he presented to me would not give a reason for pause. But upon further, closer consideration, perhaps it does. Consider the power in the 90th meridian of each of these lenses. Since the add powers, and presumably the reading level is the same in both eyes, we can disregard both in our calculations. 

In the right eye, the sphere power is +0.25 and the cylinder power is +2.75 at axis 175. Since that is nearly 90-degrees away from the vertical meridian, we can presume that nearly 100% of the cylinder power is present in the vertical plane. Therefore, +3.00D of power is present in the 90th meridian of the right eye. In the left eye, there is no spherical power. The cylinder power is +2.75, but with an axis of 95 there is virtually no power present in the vertical meridian. Therefore, the effective power in the 90th meridian of the left eye is close to zero. This creates a vertical imbalance of 2.4 ^ diopters which far exceeds the minimum amount of vertical imbalance that should at least be considered for special correction. 

You could reasonably argue that an inexperienced ECP would easily place the initial spectacle order without ever considering the vertical imbalance. However, you could not convince me that the ECP should be forgiven for telling the patient to “just get used to it,” after trying it for a month, and expressing difficulty time and again. 

This type of patient and this particular scenario are ones that we, as an industry, are beginning to see quite often. With more and more Baby Boomers coming of presbyopic and cataract age, the number will only increase. This particular patient had just had an IOL (Intraocular Lens) implant in one eye, but his doctor said it might be a year or more before the other cataract was ready to be removed. What is he to do while he is waiting for the second surgery?

I explained the situation to him in language he could understand, and explained that he basically had three choices: 

1. Deal with it. Put up with the discomfort until the other cataract was removed and hopefully, the eyes would be in better optical balance. 
   

2. Replace one lens with a lens that included a slab-off (bicentric) prism. 
   

3. Get two separate pairs of glasses – one for distance and one for reading. (If he looks through the optical center of his lens he will not experience any prism)

Just so you know: He opted to keep his Flat-Top 28 Transitions and use them strictly as distance glasses, and he purchased two pairs of single-vision reading glasses – one for home and one for the car. You should also know that he is extremely happy with the current set-up. However, he is extremely disillusioned with the eye care “professional” where he initially purchased his glasses. He claims he’ll never buy his glasses from anyone other than me. That makes me happy – and proud. 

Some tips for handling anisometropia in general, and particularly in the 21st century:

• Treat post-IOL patients as if they were wearing glasses for the first time. 

• If only one eye received an IOL implant, ask how long it will be before the other eye is done. 

• Verify that the visual acuity is reasonably good in each eye. (Vertical imbalance requires two well functioning eyes in order to be problematic) 

• Check for total vertical imbalance. If it is more than 1 ^ Diopter, educate the patient as to what anisometropia is, why it wasn’t a problem before, and what the possible effects would be if it goes uncorrected in the new glasses.

• Document your discussion.

• Discuss the benefits, costs, advantages and disadvantages of slab-off prism.

• Discuss the benefits, costs, advantages and disadvantages of two pairs of single-vision glasses.

• Remember that single-vision wearers and people with very low acuity in one eye are not candidates for vertical imbalance corrections for the reasons stated above. Corrections that have 1.5 Diopters or less vertical imbalance are not candidates for anisometropic correction via slab-off prism. However, there are other techniques for these smaller imbalances that are quite effective. You would need to determine that these smaller amounts are, in fact, troublesome to the patient.

Educate or re-educate yourself to be competent in this area. Brush up on anisometropia, antimetropia, asthenoipia, diplopia, Prentice’s Rule and the resulting prismatic effect of these prescriptions. Once you brush up, you will be surprised how often you will spot vertical imbalance in prescriptions. As a result, you will be truly helping your patients to a better quality of life and increasing your revenue in the process. What could be better?