Can I adjust a patient's glasses rx to equal the rx of severe pantoscopic tilt?

[jonovan]

A patient has keratoconus in both eyes and is around 20/200 with his glasses (he sees around 20/40 with sclerals, but he still likes wearing his glasses a fair amount). He says the letters appear "doubled" with his glasses, but if he tilts his glasses 45 degrees (top out, bottom in) he sees better; letters are less doubled. It doesn't actually improve his visual acuity; just reduces the doubled image.

I've refracted him in both the phoropter and a trial frame for an extended period of time, but nothing I've gotten has been as good as him tilting the lenses. He's around -10.00-4.00x020 in both eyes (with multiple different rx's over the years by different docs, but he thought this was about the best), and obviously with keratoconus, it's hard to nail down a precise rx. I've tried changing sph, cyl, and axis, either a lot (+/-3.00 sph and cyl, 360 degrees on the axes; the auto-refractor thought his axes should be closer to 90 but he hated that, and I'm not nearly good enough at retinoscopy to figure out anything through the massive scissoring) or a little (+/-1.00, 10-20 degrees on the axes). Tweaking the axes around 10 degrees helped the most, but it's still not anywhere near as good as the tilt. I even had him try to adjust the axes himself by having him spin the trial frame dial, but he comes out around the same axes.

So, my question is, is it possible to change his prescription straight-on to be as good as his prescription tilted 45 degrees, or is the tilt instead inducing some kind of irregular cylinder or higher-order aberration that I'll never be able to equal?

I'd like to know if I should tell him it's as good as I can get it, or if I should bring him back and trial frame refract him some more.

Thanks for your thoughts. :)

[Lensman11]

Put the glasses in a lensometer and tilt the same as he does take a reading, that what he is looking thru. It might be more than just Rx change he is creating bizarre prism with that high of an Rx. Try the Rx in trial frame and add yoked prism. Keep us posted as to your results.

[optical24/7]

Right off the top of my head, with that much panto, the lens is going to be moved out from normal wear position, increasing vertex, thus dropping the minus power from ground/prescribed. It should also induce yoked, base up prism and the tilt will move the cyl axis up.

I also think it is going to be difficult to “Rx” the weird light refraction that much tilt is creating.

There are smarter folks here, maybe they can add to this.

[drk]

1. With irregular astigmatism all regular refracting methods are invalidated. In other words, whatever works, works. Therefore the question is a valid one, and not a slop-fest.
2. Tilting on the 180 meridian is going to certainly add minus cyl x 180, and I'm pretty sure it's going to add plus to the vertical meridian. The "working assumption" axis of 20 is WTR so of course there's a little resultant action going on, but you can't decipher that.
3. I don't think you're going to get much accuracy/repeatability by tilting a trial frame in a lensometer (although that's better than nothing). I'd just present more minus cyl x 180 until it doesn't help, and then introduce some plus power into the vertical meridian to see if he likes that (you have to combine trial lenses to get that effect).

[Kwill212]

2. Tilting on the 180 meridian is going to certainly add minus cyl x 180, and I'm pretty sure it's going to add plus to the vertical meridian. The "working assumption" axis of 20 is WTR so of course there's a little resultant action going on, but you can't decipher that.

Pantoscopic tilt is going to add MINUS sphere power.

[drk]

Pantoscopic tilt is going to add MINUS sphere power.

http://www.medrounds.org/optics-review/2006/05/40.html#:~:text=The%20change%20in%20power%20of,%2F3%20sin2a) .

(Pull quote for the TLDR crowd.)

"An under corrected myope will therefore be able to obtain better distance acuity by tilting his glasses. For example, the effect of tilting a –10.00 diopter lens 10 degrees along the horizontal axis results in an optical correction of –10.10 –0.31 x 180 which gives a spherical equivalent of –10.25D. If the same lens is tilted 30 degrees, the resultant effective optical correction is –10.83 –3.33 x 180 with a spherical equivalent of –12.50 diopters. This is why an under-corrected myope tilts their spectacles to attain better distance vision."

So, no, I'm wrong. There is no plus power induced in the vertical meridian. Maybe some minor minus. But it's mostly minus cyl with the axis the same as the axis of tilt.

And I'm mostly right and you're mostly wrong. So I win.

You lose!

MUHAHAHAHAHAHAHAHA!!

[Kwill212]

uh, which part am I mostly wrong about??? If you mean I didn't say anything about the cylinder, I left that out because you got it right the first time. I was just correcting the sphere portion for others that may come along and read this.


Panto increases sphere power and induces cylinder power, with the same sign as the sphere power, at axis 180.

[optical24/7]

I’ve not researched extreme panto (45 degrees), but I would think vertex would be affected ( at an extreme angle)..No?..

[Kwill212]

I’ve not researched extreme panto (45 degrees), but I would think vertex would be affected ( at an extreme angle)..No?..

Seems like it would. This situation I think is beyond calculable with equations and rules. 45 of tilt is going to induce like 15 diopters of cyl., probably 10 or so mm of vertex chance, I would guess a bunch of yoked prism, distort the image like crazy(this might be the winner). Extreme tilt of this power is maybe squishing the image so much vertically it takes away some of his "doubled" letters.