Double vision issues

[Eyedentity3]

Ok I had this patient come in on Saturday that has me scratching my head. For starters here is the script.

OD +1.75 -3.25 x 002 PD28.5
OS +1.75 -3.00 x 172 PD27.0
+2.50 OU
19mm seg OU
She was put into a Zeiss GT2 3DV which means it is a compensated RX :

OD +1.67 -3.08 x 002
+2.41
Prism 0.77 @271
OS +1.69 -2.84 172
Prism 0.77 @ 270
+2.43

Ok here is the issue we made two pairs for the patient this set and also a set of Hoya tact EP40. In the Zeiss pair distance is fine but while reading she is having double vision issues. In the tact no double vision problem exist. The fit is just fine, PD's and seg are right where they are supposed to be. Both sets of lenses are 1.60 with A/R (Purecoat & EX3 respectively), 4.25 base. The Zeiss is a drillmount with polished edges. The Tact is a hide-a-bevel. If I am missing any info that would be helpful let me know.

I am thinking the compensated RX is causing the issues, but I'm not sure. Guy's and Gals thank you for being such a great resource.

Also, here is the previous script:

+1.50 -3.25 x179
+1.50 -3.25 x174
+2.50 OU

[chip anderson]

Is the vertical center "right where it's supposed to be?" Is material poly?

[Robert Martellaro]

I am thinking the compensated RX is causing the issues, but I'm not sure.

Extremely unlikely. Is the diplopia horizontal, vertical or oblique? Hard to imagine how you can induce intractable diplopia without a grevious error in prism thinning, inset, or some other major paramater. Maybe a vertical phoria on the downgaze only (might not show up on the comp glasses with more plus higher in the lens, and maybe by a shorter corridor in the old eyeglasses)? Call the prescriber if the optics are squeaky-clean.

[Arsenal]

Are the lenses aligned with each other properly? I've seen this issue before with lenses, especially in drill mounts, that are not on the same plane.

[Eyedentity3]

Everything seems to be lined up nicely. The material would be 1.60 hi-index. The diplopia is horziontal. Thank you for getting back to me.

[Jubilee]

I have seen a few patients who have either no convergence, or a larger compared to the norm, experience this issue. With the tact being an occupational style lens, the corridor is a lot larger, and more forgiving on down gaze. I suspect Robert is right in that this may be a phoria issue.

[Robert Martellaro]

Are the lenses aligned with each other properly? I've seen this issue before with lenses, especially in drill mounts, that are not on the same plane.

X-ing?

Everything seems to be lined up nicely. The material would be 1.60 hi-index. The diplopia is horziontal. Thank you for getting back to me.

Your welcome. I can't remember anyone with healthy eyes complain of diplopia under these circumstances, except for the very first few minutes of wear. Maybe she's exophoric? Try a hand held trial lens (prism). Is the fitting point center pupil? What was the old PAL design? I've been getting a lot these (vision anomalies) lately- maybe the two full moons last month are catching up with us?

[optical24/7]

Horizontal; Get a layout chart and precisely as you can mark your 180 engravings. Then put a *dot* on the DRP, PRP and center of near verification. Check for horizontal prism at each location. If it progressively increases as you go down the lens, the umbilic may have been inset/outset too much. (fabrication error).

[sharpstick777]

Robert is correct. The issue is with the old pair having too much unwanted prism, either due to too much prism thinning or a surfacing error. look at the prism in the near portion of the old pair, old pd, and I will bet you will find there was a mistake and your patient has compensated. That being said, the GT2-3D is a great lens, but with a narrow reading zone.

Extremely unlikely. Is the diplopia horizontal, vertical or oblique? Hard to imagine how you can induce intractable diplopia without a grevious error in prism thinning, inset, or some other major paramater. Maybe a vertical phoria on the downgaze only (might not show up on the comp glasses with more plus higher in the lens, and maybe by a shorter corridor in the old eyeglasses)? Call the prescriber if the optics are squeaky-clean.

[sharpstick777]

Your welcome. I can't remember anyone with healthy eyes complain of diplopia under these circumstances, except for the very first few minutes of wear. Maybe she's exophoric? Try a hand held trial lens (prism). Is the fitting point center pupil? What was the old PAL design? I've been getting a lot these (vision anomalies) lately- maybe the two full moons last month are catching up with us?

I do see it after Cataract surgery on occaision, esp with previsous hyperopes who are post surgically now slight myopes.

[optical24/7]

The issue is with the old pair having too much unwanted prism, either due to too much prism thinning or a surfacing error. look at the prism in the near portion of the old pair, old pd, and I will bet you will find there was a mistake and your patient has compensated. That being said, the GT2-3D is a great lens, but with a narrow reading zone.

I wouldn't assume the old pair were made incorrectly. There should have been a complaint on pick-up of them back then. Prism thinning (shouldn't) have anything to do with horizontal diplopia since it's a vertical/cosmetic compensation. The most likely suspect is the corridor placement. If it were a simple prismatic error in fabrication, it should show up it the distance area too.

If the diplopia is at near only, horizontal, a simple check at the NRP should show unwanted in/out prism.

Most likely it's a fabrication issue.

[sharpstick777]

Prism thinning can create diplopia is some patients because even the though prism is balance (not effective binocular vision) it still impacts the muscles, just equally. However, reduce prism (or prism thinning) and its the same as carrying a heavy backpack, and now setting it down. You feel light and its hard to walk straight. Muscle memory has changed. It can take awhile for eyes to retrack after that weight has lifted, to restore muscle memory.

I wouldn't assume the old pair were made incorrectly. There should have been a complaint on pick-up of them back then. Prism thinning (shouldn't) have anything to do with horizontal diplopia since it's a vertical/cosmetic compensation. The most likely suspect is the corridor placement. If it were a simple prismatic error in fabrication, it should show up it the distance area too.

If the diplopia is at near only, horizontal, a simple check at the NRP should show unwanted in/out prism.

Most likely it's a fabrication issue.

[Robert Martellaro]

Yoked prism can influence convergence, and possibly cause fatigue, but not diplopia. I'm guessing that this is a noncomitant deviation that didn't show up with the old glasses because the old PAL design had a shorter/faster power profile.

Diplopia is a serious concern. Get the doctor involved if this isn't resolved in a reasonable amount of time (weeks, not months).

[sharpstick777]

I am referring to post cataract patients who, post surgery, experience effective near vision convergence disorder. Although their RX is dramatically improved, they experience double vision for non-neurological reasons usually in the reading only, but I have seen distance as well. Since the visual system has been tested to only tolerate about 2 D of yoked prism well, any prism more than this has the potential for non-neurological effective diplopia because progressive create effective prism away from ground OC. Since most people experience the onset of this gradually, they have adapted to it. However, post surgically, its all taken away at once. I see mostly in it patients who where spherically +-4 or more, then almost plano after surgery. I also see it in lower powers where the post surgical result is anisometropic, where previously they were either fully hyperopic or myopic. There is a lot of prism off center in a high power progressive lens that long time progressive wearers have have adapted to. If you take it away in one day, the muscles require time to adapt in some cases.

Yoked prism can influence convergence, and possibly cause fatigue, but not diplopia. I'm guessing that this is a noncomitant deviation that didn't show up with the old glasses because the old PAL design had a shorter/faster power profile.

Diplopia is a serious concern. Get the doctor involved if this isn't resolved in a reasonable amount of time (weeks, not months).

[optical24/7]

Prism thinning can create diplopia is some patients because even the though prism is balance (not effective binocular vision) it still impacts the muscles, just equally. However, reduce prism (or prism thinning) and its the same as carrying a heavy backpack, and now setting it down. You feel light and its hard to walk straight. Muscle memory has changed. It can take awhile for eyes to retrack after that weight has lifted, to restore muscle memory.

Sharp, I'm still trying to grasp how (vertical) prism thinning can effect horizontal diplopia (sans fabrication error) Can you elaborate?

[sharpstick777]

Sharp, I'm still trying to grasp how (vertical) prism thinning can effect horizontal diplopia (sans fabrication error) Can you elaborate?

Prism thinning is just one of many factors that can contribute to unwanted prism and cylinder in progressive lenses. You have heard of manufacturers talk about “Unwanted Astigmatism” in progressive lingo, it’s much more complicated than that, it’s a combination of unwanted irregular Cyl, unwanted Prism and unclear focal points that contribute to total progressive distortion.

To rewind a little, standard ST-28 bifocals are created by stacking one lens on another. It’s easy to see, we even call it an “add” for this reason. Most Progressives are created by stacking lenses too, but it’s more complex as the curves vary radially at every fraction of a degree, instead of just at 2 points, and it’s harder to visualize because we lose delineation. So let’s visualize progressives by stacking cones to illustrate a lens. So for example if we drew a lens from the front, placed the pupil near the top third, and drew a triangle down slightly nasally from that point, we will have created both a corridor, and a cone in 3 dimensions, with a reading area at the bottom, dropping down from the pupil. We can draw short lateral lines that get steeper toward the bottom to delineate the power change toward the add.

Cones give us something valuable: the power can vary along the lateral curves, allowing the lens to “progress”. Imagine lateral curves getting steeper as they go down, like Executive lenses stacked on top of each other. There are problems optically with cones however. Cylinder is cylinder only when centered over the pupil, and then it effectively becomes unwanted prism. The first problem with cones is that they create irregular cylinder along their outside edge, but since they are not parallel and come to a point instead, nor is that unwanted and unprescribed cyl is not centered on the pupil. It really doesn’t come to a complete focal point either. In addition cones create prism, base down in this case. Since a progressive cone will vary in power, getting stronger toward the add, that unwanted prism will actually increase the farther we get from the pupil, relative to the add power. As well, even the though lateral curves can be construed into effective power, the central, or umbilic curve cannot, it varies not optically, but based on the lateral curves, so the steeper those curves, the more inherent distortion we add.

So in a cone we have combined, unwanted prism, base down that increases according to add power. Unwanted prism laterally, that also increases based on add power, and no clear focal point, in addition to unwanted irregular cylinder, all under an umbilic curve that as it gets steeper (higher add or shorter corridor) increases distortion. I refer to all these effects combined as Total Potential Distortion, or TPD. Cones are inherently optically impure.

Since this is a “Progressive” lens, and we cannot have any lines, the curves we impose must have an opposite curve to keep the surface smooth. So in the case of our corridor cone, to achieve our requirement of smoothness or no-lines, we will place next to our first cone, 2 cones on either side of our first, but in reverse upside down, to keep all the curves “smooth”. So if our first cone created problems, these 2 adjacent cones are nightmares. We can visualize this out by adding two cones, one on each side, next to our “corridor cone” we drew earlier. But the lateral curves are now the opposite direction of our first cone.

Now we have 3 cones, so although the lens is smooth, we compound our problems optically. These two cones now have zero curves that are complimentary to the correction since the lateral curves on or corridors are reversed of our corrective curves, everything runs in the opposite direction that is useful. Our Add creates plus power, these create high minus cyl RIGHT NEXT TO OUR CORRECTIVE CURVES. This compounds the unwanted cylinder and prism effects. These are optically disastrous, creating so much unwanted prism and cyl, or what I call TPD, that they are unusable so we have our “soft” focus area, which is more appropriately called “junk”. Worse, this TPD is aggravated by higher add powers, shorter corridors, and harder lens designs (harder = steeper curves). This distortion increases geometrically, so going from a +1.00 to a +2.00 add power, can triple TPD, and from a 17 to 15 corridor in the same powers can double TPD, depending on the adaptablity of the design (most modern designs are neither hard nor soft, they adapt in different ways).

The only other item I have ever seen with these kinds of stacked inverted cones is a… fun house mirror.

So we have 3 cones, one that is not great, and 2 that are disastrous. The combined total TPD can be so high over a +1.25 Add (it increases geometrically) that it has a negative effect even on the usable areas of the lens. You will see this as “swim” or “sway” caused by the unwanted prism and cyl that varies in different locations of the lens, pulling our image in different directions at different points. You will also see this as add power and lens power increases because they inherently shrink the usable areas of the lens. Fields of view grow narrower as balancing this mess of unwanted prism becomes more difficult.

These cones are not equal, centered, or parallel, their axis moves toward the pupil, because as the eye converges it shortens the usable area nasally faster than temporally. The outside reverse cones are larger and have their own axis, and because of their angle effectively create base out prism, which increases as we go lower in the lens. As a result of this imbalance we will often end up with large amounts of unwanted base out prism through the corridor that is relative to the RX and Add power. This is one factor why high Plus patients often experience narrower reading and occasionally zero intermediate. That prism is pulling on corridor so much the virtual lens is no longer centered over their pupil.

So we have a mess of competing prisms and cyls, and we add prism thinning, it once again adds a factor that creates unwanted variable prism at various points in that lens, which increases with the spherical RX power. Prism thinning can have a similar effect to oblique cyls, its just sending light in more directions and adding to the mess we already have. Prism thinning is not centered on the pupil, nor is it centered on the corridor, a bad combination in higher powers. We have unwanted prism and cyl at various axis’s, in various powers, all over that lens that some points combine for huge amounts of unwanted distortion.

Of course in low adds, this distortion is fairly low so we have fewer problems, so as patients grow older they adapt to this distortion incrementally over a period of 15 years. Ever wonder why long time progressive wearers sometimes have trouble adapting to new, even Free-form lenses? Their brains have gotten used to the distortion, or unwanted prism where it is. Ever wonder why Hyperopes and oblique cyls have a harder time getting used to progressives? Both complicated the direction and scope of TDP. This is why occasionally post cataract patients who have adapted to unwanted cyl and prism over the course of 20 years, can have a hard time adapting when that all goes away in one day. Its like carrying a large heavy backpack for 20 years, you will feel so much lighter but your muscles will have to regain their memory before you can walk again. It can occasionally result in non-neurological near diplopia.

Of course, 100% Free-form lenses with Backside Adds inherently reduce (not eliminate) TPD because when we move the Add to the back, we move the “Progressive” or transitional curves along with it. On a front side all lens our curves go from Convex for the front side distance, steep concave for our first reverse cone, then back to steep convex for our add power, then back to steep concave for our second cone, then back to convex for the distance. On a back-side add lens we start at flat concave for the distance, moderate concave for our progressive cone, and then flat concave for the add, back to moderat concave, then steeper concave. ALL the lateral curves are concave, not a mix of convex and concave. As a result, with a backside add we still have cones, but those cones diverge at a much slower rate than a front-side add. All the curves remain closer to the corrective curves of that lens, for that patient. As a result we have less TPD, less swim, and wider fields of view in most back-side add lenses, inherently.

[sharpstick777]

Sharp, I'm still trying to grasp how (vertical) prism thinning can effect horizontal diplopia (sans fabrication error) Can you elaborate?

Sharp, I'm still trying to grasp how (vertical) prism thinning can effect horizontal diplopia (sans fabrication error) Can you elaborate?

Prism thinning is just one of many factors that can contribute to unwanted prism and cylinder in progressive lenses. You have heard of manufacturers talk about “Unwanted Astigmatism” in progressive lingo, it’s much more complicated than that, it’s a combination of unwanted irregular Cyl, unwanted Prism and unclear focal points that contribute to total progressive distortion.

To rewind a little, standard ST-28 bifocals are created by stacking one lens on another. It’s easy to see, we even call it an “add” for this reason. Most Progressives are created by stacking lenses too, but it’s more complex as the curves vary radially at every fraction of a degree, instead of just at 2 points, and it’s harder to visualize because we lose delineation. So let’s visualize progressives by stacking cones to illustrate a lens. So for example if we drew a lens from the front, placed the pupil near the top third, and drew a triangle down slightly nasally from that point, we will have created both a corridor, and a cone in 3 dimensions, with a reading area at the bottom, dropping down from the pupil. We can draw short lateral lines that get steeper toward the bottom to delineate the power change toward the add.

Cones give us something valuable: the power can vary along the lateral curves, allowing the lens to “progress”. Imagine lateral curves getting steeper as they go down, like Executive lenses stacked on top of each other. There are problems optically with cones however. Cylinder is cylinder only when centered over the pupil, and then it effectively becomes unwanted prism. The first problem with cones is that they create irregular cylinder along their outside edge, but since they are not parallel and come to a point instead, nor is that unwanted and unprescribed cyl is not centered on the pupil. It really doesn’t come to a complete focal point either. In addition cones create prism, base down in this case. Since a progressive cone will vary in power, getting stronger toward the add, that unwanted prism will actually increase the farther we get from the pupil, relative to the add power. As well, even the though lateral curves can be construed into effective power, the central, or umbilic curve cannot, it varies not optically, but based on the lateral curves, so the steeper those curves, the more inherent distortion we add.

So in a cone we have combined, unwanted prism, base down that increases according to add power. Unwanted prism laterally, that also increases based on add power, and no clear focal point, in addition to unwanted irregular cylinder, all under an umbilic curve that as it gets steeper (higher add or shorter corridor) increases distortion. I refer to all these effects combined as Total Potential Distortion, or TPD. Cones are inherently optically impure.

Since this is a “Progressive” lens, and we cannot have any lines, the curves we impose must have an opposite curve (with a front side add) to keep the surface smooth. So in the case of our corridor cone, to achieve our requirement of smoothness or no-lines, we will place next to our first cone, 2 cones on either side of our first, but in reverse upside down, to keep all the curves “smooth”. So if our first cone created problems, these 2 adjacent cones are nightmares. We can visualize this out by adding two cones, one on each side, next to our “corridor cone” we drew earlier. But the lateral curves are now the opposite direction of our first cone.

Now we have 3 cones, so although the lens is smooth, we compound our problems optically. These two cones now have zero curves that are complimentary to the correction since the lateral curves on or corridors are reversed of our corrective curves, everything runs in the opposite direction that is useful. Our Add creates plus power, these create high minus cyl RIGHT NEXT TO OUR CORRECTIVE CURVES. This compounds the unwanted cylinder and prism effects. These are optically disastrous, creating so much unwanted prism and cyl, or what I call TPD, that they are unusable so we have our “soft” focus area, which is more appropriately called “junk”. Worse, this TPD is aggravated by higher add powers, shorter corridors, and harder lens designs (harder = steeper curves). This distortion increases geometrically, so going from a +1.00 to a +2.00 add power, can triple TPD, and from a 17 to 15 corridor in the same powers can double TPD, depending on the adaptablity of the design (most modern designs are neither hard nor soft, they adapt in different ways).

The only other item I have ever seen with these kinds of stacked inverted cones is a… fun house mirror.

So we have 3 cones, one that is not great, and 2 that are disastrous. The combined total TPD can be so high over a +1.25 Add (it increases geometrically) that it has a negative effect even on the usable areas of the lens. You will see this as “swim” or “sway” caused by the unwanted prism and cyl that varies in different locations of the lens, pulling our image in different directions at different points. You will also see this as add power and lens power increases because they inherently shrink the usable areas of the lens. Fields of view grow narrower as balancing this mess of unwanted prism becomes more difficult.

These cones are not equal, centered, or parallel, their axis moves toward the pupil, because as the eye converges it shortens the usable area nasally faster than temporally. The outside reverse cones are larger and have their own axis, and because of their angle effectively create base out prism, which increases as we go lower in the lens. As a result of this imbalance we will often end up with large amounts of unwanted base out prism through the corridor that is relative to the RX and Add power. This is one factor why high Plus patients often experience narrower reading and occasionally zero intermediate. That prism is pulling on corridor so much the virtual lens is no longer centered over their pupil.

So we have a mess of competing prisms and cyls, and we add prism thinning, it once again adds a factor that creates unwanted variable prism at various points in that lens, which increases with the spherical RX power. Prism thinning can have a similar effect to oblique cyls, its just sending light in more directions and adding to the mess we already have. Prism thinning is not centered on the pupil, nor is it centered on the corridor, a bad combination in higher powers. We have unwanted prism and cyl at various axis’s, in various powers, all over that lens that some points combine for huge amounts of unwanted distortion.

Of course in low adds, this distortion is fairly low so we have fewer problems, so as patients grow older they adapt to this distortion incrementally over a period of 15 years. Ever wonder why long time progressive wearers sometimes have trouble adapting to new, even Free-form lenses? Their brains have gotten used to the distortion, or unwanted prism where it is. Ever wonder why Hyperopes and oblique cyls have a harder time getting used to progressives? Both complicated the direction and scope of TDP. This is why occasionally post cataract patients who have adapted to unwanted cyl and prism over the course of 20 years, can have a hard time adapting when that all goes away in one day. Its like carrying a large heavy backpack for 20 years, you will feel so much lighter but your muscles will have to regain their memory before you can walk again. It can occasionally result in non-neurological near diplopia.

Of course, 100% Free-form lenses with Backside Adds inherently reduce (not eliminate) TPD because when we move the Add to the back, we move the “Progressive” or transitional curves along with it. On a front side all lens our curves go from Convex for the front side distance, steep concave for our first reverse cone, then back to steep convex for our add power, then back to steep concave for our second cone, then back to convex for the distance. On a back-side add lens we start at flat concave for the distance, moderate concave for our progressive cone, and then flat concave for the add, back to moderat concave, then steeper concave. ALL the lateral curves are concave, not a mix of convex and concave. As a result, with a backside add we still have cones, but those cones diverge at a much slower rate than a front-side add, the lateral curves stay closer to the RX corrective curves. All the curves remain closer to the corrective curves of that lens, for that patient. As a result we have less TPD, less swim, and wider fields of view in most back-side add lenses, inherently.

[sharpstick777]

As Jim Sheedy demonstrated in one of his studies, yoked prism can cause a loss of VA. Most people can tolerate up to 2D, but between 2 and 4 diopters it become subjective. People will respond differently. Over 4D of prism, will result in loss of VA for most patients.