Nystagmus fitting

**Hayde** · 03-06-2024, 10:08 AM

Hello all,

I'm expecting an outside Rx today for a potential fitting for a Px self reporting Nystagmus (presumably horizontal.) I'd value the input of those here who have some experience with it. Also you engineering minded, please feel free to peruse my thinking and critique:

I use both pupilometer and PD stick for every patient (to catch those secretly converging in the widget.)

I've been advised the better results using a pupilometer is binocular; occlusion tends to exacerbate the motion of the single eye.

With the stick, it occurs to me gauging the outside boundary of the iris is particularly prone to parallax--and unlike other patients the curvature of the eye may play into the math correction if it's darting out far enough.

The null point seems an attractive place to establish a binocular value, which suggests to me the best place to measure is in normal position squared up bridge-to-bridge with the patient and do monocular parallax calculation for the outer eye if necessary.

I'm also told a NPD measurement is more stable, from which I can reverse engineer a DPD.

I'm tempted to try everything for the sake of data capture and hope I end up with a valid (or at least statistically favorable) answer after the fact. If y'all care to volunteer angles and techniques I can do while he's here, please feel free.

Would I be correct in inducing this Px historically favors executive BF for the sake of a wider seg that captures his gaze more reliably?

Edit to add: Just to be explicit--my thought is to gauge the edges of iris at the boundaries of horizontal motion--with the tentative assumption that the ideal PD placement is in the center of the values.

**drk** · 03-06-2024, 11:23 AM

Good questions.

First lens design. I would agree that a wider, more prominent seg is smart.

Re: centering the optics...
a. If high power lens, it's for the better central optics.
b. I kinda doubt binocular vision is applicable, so centering the optics for no prism is probably not a primary concern.
c. Maybe a low abbe value if higher power lens just to hedge in case you miss?

As to nystagmus, my experience is that no two are alike. Not all have null points. I guess it's good to have them follow a pen to see if it settles down. They'd probably know. Pretty neat idea to try a converged measurement. Even if it's 33 cm, you could work out the math.

Some do go wild when occluded ("occlusional nystagmus").

Some have slow beat to one direction and fast beat to the other...ugh.

These folks have reduced acuity, so know that it could be about 20/80 or worse. So extreme accuracy is not possible. For that matter, neither is the refraction/prescription!

I agree with you: I would probably line my face up with the patient eye to eye and maybe mark the left-most and right-most position of each eye and put the O.C. in the center of the range.

**Loki** · 03-06-2024, 11:35 AM

Agree with drk, maybe also measure canthus to canthus between eyes just to verify your other measurements.

**Uncle Fester** · 03-06-2024, 12:55 PM

"Are you comfortable with your current glasses?"

"Well I measure something somewhat different and want to see if we can make things better."

"This may or may not help but you have to promise not to give up on this change too quickly."

"And I will exchange to your old measurements at no charge."

"BUT- There is no refund!"

Manage expectations all you lurkers and be up front when you are out on a limb!!!

**AngeHamm** · 03-06-2024, 04:17 PM

Originally Posted by Uncle Fester

Manage expectations all you lurkers and be up front when you are out on a limb!!!

Managing expectations is the most underrated skill in this business.

**KrystleClear** · 03-07-2024, 03:07 PM

I swear I just read, while studying, that you are supposed to occlude while taking pds for nystagmus.

If they have their old glasses, I would see if you can mark them up with the lensometer and roughly see what PD was used last time, to compare. I guess it depends on whether they are happy with them or not.