Induced Prism

**mlm** · 08-07-2004, 09:12 PM

Our clinic has an interesting job right now, and I wanted to get some thoughts from everyone about what happened.

A young woman came in wanting to purchase a new pair of glasses using a current prescription (less than 1 year old) from Switzerland.

OD -9.50 -0.50 x180
OS -9.00 -0.50 x180

Her current pair is a metal, oval frame. She chose a plastic, rectangular frame for the new pair. She says the current pair is Zeiss 1.9, so we went with the same material. We had the lab do all the edging (rather than doing the work in-house), and she picked up the new glasses last week.

Early this week, she came back in tears, saying that she couldn't judge the depth of stairs and she was walking into door frames. So, we neutralised both sets of glasses. While both pairs have the same powers, there were a few oddities:

Old glasses have a 2.5 base curve, and the left lens has 0.75 prism base up.
New glasses have a 1.25 base curve, and the left lens has 0.25 prism base down.

There is no indication for prism in the prescription, so we're assuming it was done by accident originally (although it's hard to tell because the glasses are older than the Rx). Regardless, she's used to it now and can't accept her vision with the new glasses. So, a couple of questions:

- The new glasses should not have had any prism either. Is there something about the lens material that makes it difficult to not create prism? Or was she just really unlucky?

- We are remaking the glasses to match as close as possible to the old lenses. However, should we be putting in the 0.75 prism? (The OD said to do it; I'm just wondering how correct that is.)

(By the way, I'm putting this out here merely for my own education. People much better then me are already solving this one.

)

**Rich R** · 08-07-2004, 11:22 PM

I don't believe the prism would be a factor, but I could be wrong..

When you say .25 base down prism, that's not bad at all, the tolreance is 1mm or 1.00 prism allowed.
I would suspect more the base curve or lens material.

Good luck.
Rich R.

**mlm** · 08-08-2004, 01:14 AM

I figured that the 0.25 prism wouldn't mean much on its own, but would it make a difference because of the previous 0.75 prism?

**mrba** · 08-08-2004, 02:41 AM

OD -9.50 -0.50 x180
OS -9.00 -0.50 x180

When checking for prism check the rt eye in the lensometer first because it has more power. The .25 you are finding may not be there, and if it is, it's nuthin! if she is seeing double images it is the prism, if not, then it probably isn't.

Old glasses have a 2.5 base curve, and the left lens has 0.75 prism base up.
New glasses have a 1.25 base curve, and the left lens has 0.25 prism base down.

If you could include net direction of prism that would be helpful. We are assuming they are opposing directions per eye, but assumptions can be dangerous.

As far as base curves go, since the new pair is flatter, I would be inclined to say the patient isn't BC sensitive.

However, you may want to double check OC's and Vertex. It could be that the vertex is changing the power just enouph to be wierd, and the OC's could be off just enouph to cause distortion when looking down at stairs. It might also be lens thickness, in that thinner lens centers are more distortion prone.

These lenses are glass, so I don't think distortion is it either!

Early this week, she came back in tears, saying that she couldn't judge the depth of stairs and she was walking into door frames.

Hmmm, change the base curves, match OC'c and PD's with the old pair, and give .5 vertical (cut her from the .75 she likes).

It's probably all in her head.

**drk** · 08-08-2004, 08:44 PM

Base curve. Look no further.

**mlm** · 08-08-2004, 09:51 PM

Originally Posted by mrba

When checking for prism check the rt eye in the lensometer first because it has more power. The .25 you are finding may not be there, and if it is, it's nuthin! if she is seeing double images it is the prism, if not, then it probably isn't.

If you could include net direction of prism that would be helpful. We are assuming they are opposing directions per eye, but assumptions can be dangerous.

I should clarify. I don't work in the clinic's eyeglass dispensary & lab, so I told the story as best I can. With these small amounts of prism, how can you tell if the prism is really in the left lens only? I had actually wondered if we were just assuming that.

As far as base curves go, since the new pair is flatter, I would be inclined to say the patient isn't BC sensitive.

So going flatter isn't as disorienting as going steeper? I'm not sure if anyone thought to check the original base curve before ordering the new glasses, but should that have been done? I've always been under the impression that it's best to let the lab decide the proper base curve for a given lens.

However, you may want to double check OC's and Vertex. It could be that the vertex is changing the power just enouph to be wierd, and the OC's could be off just enouph to cause distortion when looking down at stairs. It might also be lens thickness, in that thinner lens centers are more distortion prone.

Hmmm...I hadn't thought of that.

Hmmm, change the base curves, match OC'c and PD's with the old pair, and give .5 vertical (cut her from the .75 she likes).

So we should put prism there that's not prescribed? I can understand the reasoning behind it, but it seems like we're just maintaining someone else's mistake. If this wasn't such an expensive lens in the first place, I would be tempted to match everything but take away the prism to see what would happen.

It's probably all in her head.

That thought had occurred to me too.:)

**Texas Ranger** · 08-09-2004, 09:40 AM

When you flatten the base curve, you are also shortening the vertex distance; the new lenses also have a full diopter of vertical imbalance; yes her old ones had 0.75, but the extra 0.25, combined with the flatter base, was too much for her brain to handle...some folks are quite sensitive in the moderate to high powers, and having two pair that are supposed to be a "duplication", then they can nit pick them to death...they just want to be able to wear both pair without the hassles...they're going to be made over, right? so how do they need to be made so they work the second time? And, assuming that the lab knows what the best base curve for an rx is, well that's okay, unless the pt has more than one pair of glasses, and they are the same or nearly the same rx.

**Spexvet** · 08-09-2004, 05:05 PM

I agree - duplicate everything. That includes what everybody has mentioned, and pantoscopic tilt and face form curve.

**ikon44** · 08-10-2004, 01:16 AM

have you carried out a fixation disparity test with her looking through both old and new glasses, this would determine how much of a factor the vertical prism is
in causing the problem.

in my experience, switching from a metal to a plastic frame , may also be an issue, the plastic frame may be sitting lower on her face, causing her to look
through a part of the lens she is not used to.the pantoscopic tilt will again usually be very different, i would get her to look down at the ground with the
new ones will adjusting the tilt angle to see if the distortion changes.

A remake may not be neccesary if you can adjust the frame.however a "bench rest" may be indicated, since she is so convinced the specs are no good.

**mlm** · 08-10-2004, 11:14 PM

Originally Posted by ikon44

have you carried out a fixation disparity test with her looking through both old and new glasses, this would determine how much of a factor the vertical prism is
in causing the problem.
...
A remake may not be neccesary if you can adjust the frame.however a "bench rest" may be indicated, since she is so convinced the specs are no good.

Please forgive my ignorance. What's a fixation disparity test?

We're already in the process of doing a remake, and the optician has instructed the lab to duplicate everything including the 0.75 prism. I really just put out this story so that I can understand what went wrong and how could we have prevented it from happening. Thanks to everyone for your imput!

**Robert Martellaro** · 08-11-2004, 01:28 PM

A diopter of VI will not cause the symptoms you described. Call Zeiss and ask for the proper BC for this script in the 1.9 index. Zeiss makes this lens up to a total minus of 20.00D with a plano BC so I would guess that the 2.50 BC might be the correct curve. Increasing the panto will make the floor look flatter and improve the perspective with steps and stairs. Check the vertical OC position. The glasses are probably riding lower than expected due to the zyl bridge. Make sure that the vertical OC is about 3-5mm below the pupil. If it's more than 5mm the distance and peripheral acuity will be compromised.

Robert